50450799 piping stress analysis bechtel
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PIPING STRESS ANALYSIS
DESIGN MANUAL
STRESS GROUP
BECHTEL INC.
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I 1//79 ~ ~R ~mON J W}S -
REV. DA= -N ~R REVISION BY ~R
aE~ Coff. OES. GUIDE NO. -“PLANTDESIGN AND PIPING
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TABLE OF CONTENTS
1. GENERAL
2. DRAWING DISTRIBUTION AND PROCEDURES
3. INITIAL PIPING STUDIES
4. STRESS RELIEVED VESSELS AND PIPING
5. REVIEW OF CRITICAL PIPING
5.1 Pumps
5.2 Compressors
5.3 Turbines
5.4 Ai rfans
5.5 Heaters
5.6 Buried Piping
5.7 Cryogenic & Low Temperature Piping
6. STANDARDIZATION OF APPROACH TO PIPING PROBLEMS
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
Allowable Pipe Spans
Allowable Pipe Overhang
Pipe Guide Spacing
Instrument Strong Back Flexibility
In-Line Pumps
Expansion Loop Design
Pipe Anchors
Stacked Exchangers
Off Plot Pipeways
7. MISCELLANEOUS AND SPECIAL PROBLEMS
7.01 Slug Flow
7.02 Mitered Elbows
7.03 Tee Connections
7.04 Injection Connections
7.C5 Heater Coil Decoking
7.c6 Catalyst Regeneration
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7.07
7.08
7.09
7.10
7.11
7.12
7.13
7.14
7.15
7.16
7.17
7.18
7.19
7.20
7.21
7.22
.7.23
Reformer
Cold Spr
Blowdown
TABLE OF CONTENTS (CONT’D.)
Furnace Pigtail Design
ng
Systems
Field Checkout
Soot Blowers
Settlement and Frost Heave
Ambient Temperature Effect on Bare Piping
Control Valve Piping
Hydrotest of Large Low Pressure Piping
Pipe Supports
Tank Field Piping
Steam Trace and Steam Trap Piping
Plastic Piping
Rotations, Reactions and Stresses atNozzle Connections to Vessels
Bowing of Pipe
Compressor Bottle Support
Tank Nozzle Movements Due to Pressureand Temperature
8. PIPING STRESS ANALYSIS WORK CHECK LIST
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1. GENERAL
This Design Guide is intended to aid stress personnel infollowing approved procedures and techniques to complete
their work on an assigned project.
Although it is reco~nded that the standards be followed
closely, individual thought and sound engineering judge-
ment must be used at all times.
In reviewing piping isometrics, mdels or drawings, the
Stress Analyst should keep in mind that the aesthetic de-sign of the piping systems is the responsibility of the
piping design groups and therefore he should review them froma stress and support standpoint only. Exceptions to theabove should only be made when a situation ridiculously
improper or a large econamic saving is involved, keepingin mind lost time in making revisions and their affect on
schedules.
All piping systems reviewed by the Piping Stress Analysis Group
shall be considered for all the “Design Conditions” as listedunder Section 301 of the Code for Pressure Piping ANSI 631.3,
latest ~, or other applicable codes. As a general rule
most computer analyses of piping should include only the effectsof thermal expansion, restraints and effects of support, anchor
and terminal movements. Effects of dead load on a well supported
system are generally small. Other effects are to be studied by
special calculations only when engineering judgement deems them
to be possibly severe.
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2. DRAWING DISTRIBUTION & PROCEDURES
The following normal procedures may be adjusted for particular
projects or office locations to suit the special conditions andrequirements of those projects and locations.
2.1 The assigned Stress Analyst shall confer with the PressureVessel Job Supervisor and indicate his preference of draw-ings which should be distributed to him. These drawingsshould generally be plot plans, P&lD’s, paving and grading,underground piping, pipeway stanchions, line designation
tables, basic data, flow diagrams, piping drawings andpiping isometrics. When vessel drawings and structuraldrawings are included, the filing of drawings becomes amajor problem. In fact, much filing would be avoided ifP&lD’s and paving grading drawings were not included. Thisjudgment is left to each individual.
2.2 The routing of piping isometrics between the Plant DesignGroup and the stress group has been standardized to increase
efficiency of all groups concerned and to reduce the amountof paper handling. Isometrics will be referred to as iso’s infurther discussions. The presently adopted procedure for isodistribution on model led jobs is:
a) After isometrics are drawn up and checked within the PlantDesign Group and are ready to issue for construction, a
print of each together with a transmittal list shall besent to the Stress Analyst one week before date to beissued for construction.
b) The Stress Analyst then places a design data stamp (SeeStandard C-701) on all iso’s except those which can be
approved for stress by inspection without specific designdata. The stamped iso’s should then be filled in withthe necessary design data from piping specifications andline design tables. An efficient and acceptable method ofrecording the expansion temperatures is to ~reDare a list
of maximum “exp’i temperatures for eachas shown in the Line Designation Table
1A (instr. air)------100oF RW (raw
UA (util. air)-------lOO°F Cw (coo
N (nitrogen) ---------lOO°F LS (1OW
DW (drinking water)--lOO°F MS (reed
particular servicei.e.:
water)----------- 100oF
ing water)------- l2OoF
press.steam) --40# sat stm temp.
press. steam)-150# “ II It
PW (potable water)---lOO°F HS (high press. steam)-60()# “ “ “
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c)
d)
e)
But process lines require individual temperature assignmentfrom the line tables.
Likewise, a list can be prepared for pipe specifications whichare repeated often that are of carbon steel and the same schedule.
Alloy spec.’s and their schedules should be specially listed forease of identification.
The iso’s are then reviewed at the models and passed by judgmentas much as possible, leaving only a few to verfiy by computercalculation. All iso’s passed by inspection should be markedup with support designations during the review of each iso.
This in general will be the most efficient operation exceptwhere a group of iso’s must be immediately released by the
Plant Design Group for prompt delivery to the fabricator tomeet a schedule.
After all the iso’s listed on a particular transmittal have beenreviewed, those which can be field-supported, or require nosupports, or which can be supported by whollv standard supportdetails, ”are indicated on the” transmi~tal
the iso itself with the designations FTS,
The Plant Design Group can stamp the origwithout need of their passing through the
Technicians. will be retained by The Plan
,and the blue pr”int of
respectively.
nal iso’s accordinglypipe support groups.
Design Group for thepurpose of assigning proper designations to the-l’STD” supports
required on every iso. This should bxpedite the ~paration ofiso’s to be issued for construction on the Rev. O issue.
All other iso’s are checked off on the original transmittal asbeing approved for stress with an engineered support designation
~except where a flexibility change or calculation is needed.The symbol HFS indicating IiHold For !jtressli will be tagged on the
transmittal~posite the iso involved. Two copies of the trans-mittal with the above notations should then be given to the PlantQesign Supervisor.
All iso’s as they are approved by the Stress Analyst, should beinitialed on the tracing by the Stress Analyst or his designated
alternate. Where iso’s require a calculation, the tracingshould be detained by the Plant Design Group until the Stress
Analyst finalizes his study of them. The Stress Analyst shouldassign the highest priority to finalizing these iso’s.
IAetI isols are verified as satisfactory by calculation, the Plant
Design Group should be imediatel:~ notified for its release. Andif iso’s require a revision, the print shouId be marked up withthe required change and a copv ~f the print should be given to the?Iant Design Group. After the iso revisions have been made, a
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new print should be again issued to the Stress Analyst for final
review. If the iso is correct the Stress Analyst will initialthe tracing as approved.
All prints marked up by the Stress Analyst with the support require-
ment symbol ~ are then turned over to the Support Group. if iso’sare stamped for review of critical support details, the pipe
support designer must return the iso and support details to the
Stress Analyst who, upon approval of the detail, initials thestamped area on the iso.
The Support Group then adds the “PSt’ numbers and locations tothe iso tracing and initials the tracing. The tracing is thenreturned to the Plant Design Supervisor for issue.
[f after an iso is issued for construction, the Plant Design Groupmakes a revision to the piping, it is the responsibility of the
Plant Design Supervisor to stop the support group from furtherwork on the iso and reclaim the print marked up by the Stress
Analyst. we Piping Supervisor then reissues the iso and theoriginally marked up print to the Stress Analyst who reviews the
iso for further approval and sJpport mark-up. Where pipingrevisions are judged insignificant by the Plant Design Supervisors,
(i.e. not affecting flexibility or support of the system) the isois then just reissued for construction, by-passing the Stress
Group.
If piping isometric numbers are revised by the Plant Design Group,a cross reference list of new numbers versus old numbers must beprovided to the Stress Group to keep records straight. To keepbetter control of isols marked up by the Stress Group, the plant
Design and Support Groups should also keep a check list of iso’sreceived.
The stress markups are then kept in alphabetical and numericalorder in special long binders by the R“ipe Support Group forreference.
k) When the job is complete the marked isometrics are returned to the---0
Stress Analyst whb keeps them close at hand for approximately:
1 year, then files them in storage.w
* 2.3 A sepia of all orthographic drawings of piping on-plot or off-~ plot should be issued to the project Stress kalyst prior to
being issued for construction.t The sepia shall be stamped and
u distributed per Standard C-702 upon stress review completion.kw The Stress Analyst shall convert sepias of the piping L drawings
into stress STR drawings and maintain a drawing control of all%a STR drawings per Standard C-703.
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3. INITIAL PIPING STUDIES
3.1 Study preliminary plot plan and pipeway layouts for troublesomearrangements.
a) Indicate pump placements which will aid in achieving flexiblepiping arrangements. Avoid placing pumps directly opposite
annecting equipment.
b) Estimate the number and position of pipeway expansion loops forsteam, condensate and other long, high-temperature systems.
c) Keep mvements in steam lines to generally 4 inches or a maximum
of 6 inches by judicious number and location of loops. Determine
the loop size to help in positioning the header in the pipewayto avoid large overhangs o? the necessity of auxiliary means ofsupporting loops. Design mests of loops as early as possibleand give exact layout to Plant Design Group. Expansion movements,insulation thickness, effect of cold spring and extra clearanceshould all be included. Generally keep a minimumof l+ to 2“extra clearance from adjacent piping or other obstructions forworst case of design temperatures or differential pipe mvements.
3.2 Review preliminary alloy piping isometrics or layouts by inspec-tion for material commitment. Generally this is done to avoidlarge differences between material coimnitment and final purchase
of alloy pipe and fittings required; therefore, an exact analysisshould not be made. Retain the preliminary study for comparisonwith the final iso to be issued-for-construct ion At this time
many iso’s can still be passed for stress by inspection, but it
is recomnded that piping to pumps, compressors and possiblyheaters, exchangers or reactors when high reactions are suspected,should be run as a formal calculation on the computer.
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4. STRESS RELIEVED VESSELS AND PIPING
4.1 The Pressure Vessel Job Supervisor will provide a list of allstress relieved vessels on the job and all established dates from
the fabricator for stress relief of each particular vessel.These dates will be marked on tags put on the vessel mdels by
the vessel department. Normlly the model should be completedand “checked” a minimum of (6) weeks ahead of the stress reiiefdate. This gives the stress analyst and support group (2) weeks
to complete their work and get details sent to the fabricator(4) weeks prior to actual stress relief.
4.2 It is very important that the Plant Design Supervisor remind allhis designers that the piping should not be revised thereafter.
If the change must be made, the revision has to be coordinatedwith the vessel fabricator immediately to avoid serious problems
such as re-stress relieving and deiay in delivery.
4.3 Piping requiring stress relief generally is drawn up and issued
to the shop together with the required pipe supports which areto be welded on and stress relieved with the pipe. Occasionally,support details are held up for one reason or another and fail
to reach the shop in time. The supports must then be welded to
the pipe in the field. Welding of supports to stress relievedpiping in the field is to be avoided. The stress relief kitsare not only costly in themselves (sometimes amounting to several
hundred dollars) but require many manhours for their installation,application and removal. Stress relief must still be appliedwhere process reasons dictate (i.e. stress corrosion or other),
but for PI material, non-pressure parts or external attachmentsare not required by A.N.S. I. Code to be stress relieved as longas the throat of the attachment filiet does not exceed 3/4”.For any questions regarding welding of supports to stress re-lieved pipe refer to the general welding instructions for pipe
supports.
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5. REVIEW OF CRITICAL PIPING
The following equipment & cond tions invo ving critical pipingdiscussed within eachrequire special treatment, and are briefly
classification.
5.1 PUMPS
5.101
5.102
5.103
5.104
Pumps, turbines and compressors have common sources of
concern. The greatest concern is for keeping proper
alignment of the pumps and compressors in relation to
their turbine or motor drivers. Improper alignmentcauses hot bearings with resulting wear and/or seriousvibration. Reactions to the cast steel nozzle and
casing structure is generally of secondary concern.Whenever the casings are made of cast iron the
allowable loadings should be reduced 25%.
Acceptable loadings on most centrifugal and rotarypumps which are base, frame, flange or centerline
mounted, are shown in Standard C-705. When the
loadings are higher than permissible every effortshould be made to meet the allowable loadings byincreasing the flexibility of the piping systemrather than employing expansion joints.
Standard C-721 (4 sheets) shows some common configura-tions of pump piping. The tabIes accompanying thevarious figures show the maximum operating temperature
of the system without overstressing the pipe. When
the maximum allowable temperature is greater than150°F$ the system is OK for 300°F steamout or steam
tracing.
Piping reactions on in-line, deep well, vertical framemounted, reciprocating pumps, heavy barrel type, or
other specialized pumps must be reviewed on anindividual basis. The primary rule regarding any
piping system to pumps is that the allowable stress of
the pipe at the nozzle must not be exceeded, and thatreactions in lbs should generally not exceed 150 x thenozzle diameter in inches or that permitted by thepump manufacturer in loadings published on his vendor
prints, or by agreement, or per specifications.
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5.105 In-1ine pumps should be capable of withstanding equivalent pipeallowable stress based on the minimum nozzle size and re-duced to material allowable stress for the cast body. These
pumps should be supported by the adjoin-ing piping only,except, where the horsepower of the pump exceeds 75H.P.,
the pump itself should also be supporte.d_on_..~_p.ier. SeeStandard C -729. Generally, none of these suppo-rts re-quire bolting, in fact, if the pump can SI ide it providesrel ief for thermal expansion. (Refer Par. 6.05).
5.106 Deep well pumps generally have a cylindrical plate steelcasing which is flanged and bolted to a concrete founda-
tion. Loadings to nozzles of this type of equipment arelimited to the allowable stresses of the pipe and/or casing.
5.107 Pump piping can be designed to twice the normal allowable. . . ... . . ... .. ___“Stress as”pti~ Standard” C“-70”5 Wh”en’ considering steam-out or.. . . . .u~set St earn trace temperatures. When the pump and/or piping,is being stea~out,
—-..—. ...the ~mp is not runniff~ and therefore--.<. ..=— ..— .—.. .- ...
misalignment does ;~~ate.. . - . .. . ...— ______ —..—— -.—-. -
5.108 Pressure rating of pumps is indicative of ability of pumpcasing and supports to withstand piping reactions. As the
pu~ pressure rating is increased, it is built ~re
sturdily; it has heavier walls, weighs mre and is more
stable with sturdier supports. Naturally, therefore, it
can withstand higher piping reactions.
5.109 Where pumps are top suction and/or top discharge, the onlymanner of removing eccentric loads on the pumps would be ‘from beams above. For pumps handling hot materials the
piping should be spring supported to beams above. Therefore,for ease of supporting pump piping in this case, the pumpshould be located under the stanchion struts, (i.e. those
running parallel to and on each side of the pipeway).
5.110 Whenever possible the pump suction lines should be supported
to a concrete pad extension of the pump foundation. Where
this is not possible, beams should be embedded in thefoundation and projected out the sides or front far enough
to support the piping under the vertical riser. In the
case of plants located in regions of frost heave, thesebeams must adequately clear the maximum estimated heave of
the area slab. Where differential vertical expansion of
the pump versus the piping permits, the supports discussedabove should be solid, sliding type supports. Spring sup-ports should only be used when this vertical differentialexpansion is high or questionable.
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5.2 COMPRESSORS
wLIn
The types of compressors usually found in refineries and
chemical plants are as foliows:
Paragraph
Centrifugal, Rotary and Screw 5.21
Reciprocating 5.22
In-Line 5.23
Blowers and Fans (Below 1 psig EAP) 5.24
The allowable loadings, methods of calculating them, typesof support, and piping design considerations for each of theabove compressor types, are discussed individually in the
paragraphs noted.
5.21 Centrifugal, Rotary and Screw Type Compressors
Allowable loads on centrifugal compressors shall be
covered by Bechtel Standard specifications. These specifi-cations shall state that the equipment shall be designed towithstand the following external loadings:
Vertical Component
The allowable vertical reaction from combined forces, and mo-
ments due to all piping connections, or to any one pipingconnection (either upward or downward) at any support pointshall be at least one half the dead weight reaction of theco~ressor at the support point.
Horizontal Transverse ComDonent
The allowable horizontal reaction from combined forces.and mments due to all piping connections, or to any indivi-
dual piping connection, in a horizontal transverse direction
at any support point shall be at least one third the totaldead weight reaction of tbe compressor at the support point.
Axial Comoonent
The allowable axial force from combined axial forces of allpiping connections, or axial force of any one piping con-nection, in an axial direction on the compressor casing shall
be at least one-sixth the compressor weight.
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b)
c)
5.22
a)
b)
For calculation preparation set up the individual systems
connected to the compressor casing and support structures .as indicated in Standard C-724, 725 & 726, or by some otherequivalent system. To avoid moment restraints, allrestraints used should be simple couples. The layout of
the problem and the subsequent computer run should bebased on the coordinate system as shown in the Standards.
Generally, centrifugal compressors are not sources of seriousvibration and therefore, the piping is given only a cursory
review for resonance. Large frameworks of free standingpipe or Iarge overhangs should be snubbed to prevent largeamplitude vibration.
Piping to centrifugal compressors need not have a machinedspool piece to makeup the last connection to the compressor.
For years the construction department has displayed thecapability to mate flanges by bringing misaligned piping
into prcper position by the heat and quench method. How-ever, where cold spring is employed the field should’ beinstructed carefully as to the proper procedure to produce
the results desired.
Reciprocating Compressors
Piping reactions on reciprocating compressors are not cri-
tical from the standpoint of misalignment of equipment, butdue to piping vibrations, the piping stresses should not
crowd the allowable stress range. Although higher stresses
can be allowed at the nozzle than for centrifugal compressors,
it is not unreasonable to keep axial and shear forces withinthose shown in Standard .C-705, and as a conservative rule,keep stresses to within twice those permitted for turbinesin the same standard.
Generally there is no need to combine pipe system loadingsfor reciprocating compressors as was required for centrifugal
compressors since piping is usually small and reactions arenegligible relative to the sturdy equipment. In fact, most
piping systems to this type of equipment can be reviewed byinspection.
Vibration is a rather serious problem within piping to recip-
rocating compressors. The piping generally should be guided.,held down and possibly restrained with hydraulic type vibrationsnubbers when unsupported lengths or spans fall into the rangeof the first or second harmonic of the compressor operatingfrequency..-
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c) Pulsating compressor discharge requires that special
cylindrical Ilbottlesll be designed to prevent sur9e vibra-
tion. These bottles are often large diameter and heavy.
Therefore, to reduce the possibility of a fatigue failurebetween the discharge bottle nozzle and the cylinder headnozzle, the dead load of the bottle should be supportedby elastic supports as described in paragraph 7.22.Sometimes the compressor manufacturer recomnds a wedge
type solid support. These have been widely used but don’taliow any room for error of installation. The wedges have
to be adjusted when the compressor is at operating tempera-ture. For upset temperatures the wedge type may be danger-
ous since no further expansion can be absorbed. Bechtel
Refinery Division practice usually avoids using wedge
supports. Suction bottles can utilize solid supports since
suction temperatures vary negligibly. I5.23 In-Line Compressors
Misali~ment of in-line compressors obviously is no problem,
since their driver is bolted to their casing. Permissible
loadings on their nozzles can approach the allowable of thepiping system, but should be reduced by the allowable stressfor the cast material of the equipment when nozzle and pipe
thicknesses are comparable. \lhether the in-line compressor issupported or’ not depends on ability of piping to support it.The analyst must be sure to take vibration into consideration.
5.24 Blowers and Fans
Due to the possible light weight construction of this type
of equipment the allowable nozzle load tables should not beused. The vendors prints should be examined for clues rela-
tive to strength and manner of supports, and/or other pertinentdata. If no allowable loads are published, the intake and/or
discharge lines might require impregnated cloth or neoprene
expansion joints. This type of joint is banded on to the
exterior of the adjacent pipes with suitable small gap betweenthe pipe elements. Generally, the sheet should have a slight
circumferential bulge between bands to absorb tensile movements.Generally, piping or ducts to blowers and fans are large and
thin walled, requiring direct routing. These may require
expansion joints made of rubber or stainless steel and berectangular, oval or circular in shape. Allowable loadings
on this type of equipment are based on engineering judgement,
since allowable are not usually published or known by the
manufacturer.
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5.25 TO reduce operating reactions from piping to compressors the
most generally used methods are to employ cold spring or byincreasing the flexibility of the piping. Expansion joints
are virtually forbidden since they suffer from vibrationfatigue.
If a system is to be cold sprung it should follow the rules
of the ANSI B31.3. The cold spring should be located at a
convenient place in the system, generally a flanged connec-tion or a field weld. See Standard C-723 for cold springnotations.
It is important that no rotation at the welded joint is per-
mitted to assure that proper counter moments are built intothe system. Instructions on this procedure should be sent
to the field for critical systems. Where cold spring is in-effective or impractical, the piping should be rerouted toimprove its flexibility.
5.3 TURBINES
Centrifugal turbines with pedestal, base or flange muntings,are the only types considered herein.
5.31
5.32-
5*33
Flange mounted steam turbines are used as in-line pump dri-
vers and are therefore not misalignedby piping reactions.Piping stresses can approach the mximum piping allowable
except where cast iron casings are encountered, then thestresses should be reviewed considering the lower allowablestress of cast iron.
Piping reactions on pedestal and base munted centrifugalturbines are governed by two conditions. First, if the tur-bine is a pump driver and is single stage, the allowableloadings as noted in Standard C-705 should apply. Secondly,
where the turbine is multi-staged or is used as the driver
to a compressor, the allowable loads will be in accordancewith the Bechtel Standard Turbine Drive Specification aspreviously described under Centrifugal Compressors.
For preparation of calculations to verify loading conditionson the turbine, use the procedure as outlined under paragraph5.21(alfor centrifugal compressors.
5.4 AIRFANS
Airfan heat exchangers have gained widespread popularity and use
over the last several years. At least three major problems
confront the piping stress analyst.
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5.41 First, where the inlet and outlet header boxes have two or
mre nozzles per unit, a difference in expansion exists bet-
ween it and the attached pipe header. For years many suchunits have been connected together using only fitting makeupwith no apparent ill effects. (Very similar to cylindri-cal exchangers being connected by their nozzles being boltedtogether directly.) Therefore, a practical standard is need-
ed for determining when additional flexibility is requiredand how to compute it. Standard C-717 suggests that fittingmakeup is tolerable until the difference in horizontal expan-sion between the nozzles of the pipe header and header box
exceeds l/1611. This applies to either the inlet or dischargesides but not when several units are joined together and theinlet and discharge nozzles are at the same end. Where the
expansion difference exceeds I/1611 use the formuia indicatedto compute length “~” required between manifolds.
5.42 Secondairfan
header
channeheader
overall expansion of the pipe header joining several
units together must be accommodated by allowing thebox to slide on its clip supports within the unit side-
supports. Normally the gap between each end of thebox and the support channel should be s/16” or mre.
This is now generally accepted and appears in Bechtel s~eci-
fications issued to manufacturers who are to bid on the jobs.Where mre than s/161’ mvement is required, the pipe header
can be cold sprung, as shown on Standard c-718, pulling theunits together as much as 5/1611, whereby the permissibleexpansion can be increased from 2 x s/16° or 5/811 at each
end of the units to 2 x 5/8’ or 1 l/4° total for the overalllength of all units connected together.
5.43 Thirdly, where inlet and outlet piping are at the same endof the airfan units, extra flexibility of the out”let pipingis generally required and should be routed as shown on Stan-dard c-718 or in some equivalent manner. External pipingloads affecting the equipment nozzles additionally should
conform approximately to those loadings published by eachmanufacturer.
‘ Another manner in which difference in expansion between inlet
and outlet pipe headers can be absorbed is by requesting theairfan manufacturer to supply horizontally spl it header boxes
that slip individually to absorb the difference in movements.This method would generally permit fitting makeup between thepipe header and the header boxes for both inlet and outlet
connections even though both are located at the same end of
-the airfan.
5.5 HEATERS
Early in the design of a plant, specifications are drawn up andmaterial requisitions are prepared regarding the types of heaters
to be used. It is at this stage that the stress group should
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5.5 (Cent’d.)
confer with the project engineers regarding support requirementsof external piping to the heaters. The material requisitionshould state that it will be the responsibility of the heatermanufacturer to provide adequate platform framing or other means
toaccmodate all external piping loads of the inlet and/oroutlet piping.
5.51
5.52
5.53
5.54
A preliminary piping load estimate should be sent to the
selected manufacturer for completion of his platform design.
Unless this is done at an early stage, it might prove costly
to arrange for piping to be supported to the heater afterthe design and/or fabrication is completed.
In general, piping to the heaters should first be studiedfor inherent flexibility without alteration of heater inter-
nal supports or openings into the heater. If the proposed
piping is either overstressed or creates unacceptable, highreactions on the heater nozzles, then either the piping shouldbe rerouted to produce a desired flexibility or the heatermanufacturer should be requested to absorb some reasonablelateral mvement of the heater tubes. This ~vement may re-
quire some alteration of the tube support castings on hori-zontal , rectangular (box type) heaters and some possible
enlargement of the openings to either the horizontal or ver-
tical (cylindrical) heaters.
When a horizontal, rectangular heater is being used, theradiant and convection section tubing is generally anchored
(axially only) at the front of the heater with allowableloadings indicated. Where the manufacturer does not indicatean anchor, he should be requested to add an anchor to allnozzles and submit their allowable reactions. It is better
to have the piping anchored and the mvements therefore con-trolled rather than to let systems float and be in doubt as
to ultimate movements. In some cases, such as heaters used
in amnia plants, the heater tubes are anchored internal Iy,whereby large movements are indicated at the nozzle and are
imposed on the external piping. By judicious Iocation of
equipment these movements can be counteracted by expansionof external piping.
Cylindrical heaters (axis vertical) have their tube coilsrunning vertically. They can be supported either at the top
or the bottom of the tubes. The tubes are guided periodi-
cally to the heater shell. The inlet and outlet nozzles
generally hang free, being supported to the adjacent tubethrough the 180° return bend at one of the ends. Therefore,
these tubes can be moved laterally in a horizontal plane, torel ieve external piping stresses and reactions if necessary.
D.G. C-5
Page 16 of51-
I
@‘/5.54
5.55
5.56
5.57
(Cent’d. )
But the manufacturer must be agreeable to the particular re-lief movements requested. If the piping is amply flexible,no modifications are necessary by the heater manufacturer,
but the reactions on the nozzle must be reasonable. Theseallowable loadings as indicated on their drawings generallyare 500 to 1000 pounds.
When considering the design of piping to cylindrical heaters
the location of the tube supports can be critical. When
top supported, with inlet and outlet nozzles at the bottomof the heater, large vertical mvements occur and are im-posed on the external piping below. This may require costlyadditional pipe for flexibility and the use of expensive
constant load spring supports.
If the tubes are top supported with inlet and outlet nozzlesat the top, then the external piping can be supported to the
platforms or shell at the same level as the tube supports.
This would reduce the need for constant load spring supportsbut external piping flexibility is still required betweenthe heater and other equipment or the pipeway. When the
tubes are supported at the bottom and the nozzles are at
either the bottom or the top, the need for external pipingflexibility or constant load spring supports can both be mini-mized.
Additional care must be used when considering 2 phase flow
in heater piping. The inlet will generally be 100% liquidat .50 to .85 specific gravity but the material in the outletwill vary from the inlet liquid density to a nearly 100%
vapor flow. This creates special support problems and thedifferential load must be minimized on connecting piping by
pre-setting springs for an intermediate Ioad condition.
5.6 BURlED PIPING
Buried piping, regardless of depth of burial or soil in whichit is buried, has the tendency to expand or contract with temper-
ature changes whether from flow temperatures or surrounding soil
temperature changes. The total change in length it undergoes
depends on the restraint of the soil both from friction and pas-sive resistance.
5.61 Computing Growth of Buried Pipe
A reasonable approach to calculating buried pipe mvements is
based on resistance to mvement from soil friction in a rect-angular load pattern as shown in Standard c-716. This has
been found to be slightly unconservative by roughly 20% since
D.G. C-5
Page 17 of 51
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EL2wLIn
—
5.61
5.62
5.63
5.64
(Con’ t.)
cyclic expansion and cooling tend to increase end movements.
The choice of a proper coefficient of soil friction is of
great importance since the value can vary from .4 to greaterthan 1.0.
Results of Jackinq Tests
From Jacking tests made by P.G.&E. Co. (see Sept. 1933 issueof ‘Western Gas”) on 37’-4” length of 22” pipe with 2’-6’Iof cover (assume average cohesionless soil) showed a soilfriction of 0.40 psi or closely a co-efficient of frictionof 0.4.
Method of Restraining Expansion of Buried Pipinq
At corners (right angle turns) of buried piping systems,
large expansions might cause a failure at the elbow, due
to restricted flexibility , or similarly at branch connec-
tion of underground header.
For small temperature changes the system can be fullyrestrained to prevent the above failures. Methods ofproviding full restraint are by anchoring the pipe with
concrete blocks which encircle the pipe or by deadmenwith struts attached to the pipe. (Standard c-728. ) Alsothe line can be fully restrained using very large bends in
the pipe through the principle of hoop compression. (SeeStandard C-71O.)
Stress Analysis of Buried Pipinq Systems
5.641 General
AS in above ground piping systems, thermal expansionstresses are induced in buried piping systms when
the temperature of the systems changes. However, the
thermal stress condition of buried piping systems ismuch more complex than that of above ground piping
systems due to restriction of the piping movement bythe surrounding soil. The stress level in the pipedepends on the temperature change, pipe size, piping
configuration, soil characteristics, depth of burial,skin friction, operating pressure, etc. For a longstraight buried pipe under temperature change, thethermal expansion of the middle portion is completely
restrained by the soil friction and only the end
portions, generally a few hundred feet long, show somemovement. See the Sample Problem, as herein afterreferred to, of Standard 735, Page 2. The length
of the end portions, which expand under partial
D.C. C-5
18 of51
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i
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5.641 (Con’t.)
—
restraint, and the resulting end movements may be
calculated by the formula shown in Standardc-716 and is shown on Page 2 of the Sample Problem.
Buried piping systems under temperature change maybe moved laterally near the bends and branch connec-
tions. It is assumed that the pipe moves againsta soil spring and the maximum spring force is equal
to the passive soil resistance. A buried piping
systm may be analyzed for thermal expansion efforts
to include soil friction and soil resistance by thepiping flexibility program ME632 or ME 101.
5.642 Input Data Preparation
a) Dimensions of Calculation Model
b)
After determining the length of the partiallyrestrained portions of the buried pipe system,the calculation model can be set up as shown on
Page 4 of the Sample Problem. As can be seen,only 8001 of the 5000’ run of the complete system,as shown on Page 1 of the Sample Problem isincluded in the model since the remainder is
totally restrained. To achieve the 3.49” deflec-
tion of Data Point 33 either the anchor at Data
Point 80 can be moved in the “-X” direction oran equivalent rate of expansion can be appliedto the 8001 length to produce the same result.
Actually, the length of the partially restrainedrun of pipe as calculated by Design Guide c-716does not include the soil resistance on the pipeat right angles to the main run as shown by DataPoints 8 through 33. A more accurate result maybe obtained by a rerun with a new partially
restrained length, Data Point 33 through 80,including the lateral soil resistance on Data
Points 2 through 33.
Soil Resistance “Sprinqs”
A buried piping system under temperature changemoves against a soil spring force (subgradereaction) which has a limiting value equal to
the passive soil resistance. [t is found fromtests that the buried pipe moves against thesoil a certain amount or displacement beforedeveloping a maximum passive soil resistance.
This displacement depends on the soil propertyand the depth of the burial. From the FoundationEngineering Handbook, the displacement is about
0.05 H for sand and 0.10 H for clay, where H is
the depth of the burial to the bottom of thepipe in inches.
I
D.G. C-519 Ofsl; I
,.,
&.:
— 5.642 (Con’ t.)
In the absence of the subgrade reaction datafor the jobsite, the displacement of 0.03 H hasbeen used as the necessary movement to developa passive state and it is used generally forconservatism. The soil spring constant KS iscalculated as follows:
KS =Passive Soil Resistance (Standard C-710)(#
‘ft)or #/in./ft..03H (in.)
The soil springs are treated as translationalrestraints with a flexibility of KA #/in. from
Ks x length of pipe affected. The restraints arespaced such that the passive soil resistance
on the pipe is adequately represented. Ingeneral a closer restraint spacing is requiredfor the area where high stresses and movements
are anticipated. However, the spacing shouldnot be closer than two times the pipe diameter.
Since the soil resistance must not be higherthan the passive soil resistance on the pipe,the analysis shall be carried out by a trial-and-error method. More than one computer runmay be needed to obtain a satisfactory answer.
The forces of the soil spring restraints fromthe computer result must be below the passivesoil resistance. Othetwise, the soil springrestraints must be changed to the restraintswith constant force whose magnitude is equal
to the passive soil resistance. Another computerrun with the new restraints should be made until
no restraint reactions frm computer result aresignificantly higher than the passive soil
resistance.
5.7 CRYOGENIC AND LOW TEMPERATURE PIPING
Cryogenic piping is understood gene~ally to include the rangeof operating temperatures from -150 F to absolute zero (-459.4°F).
Cryogenic piping is more critical than nomal refrigeration andother low temperature piping for several reasons. Greater carein design is required to prevent water vapor from entering theinsulating media where it would freeze and cause an insulationbreakdwn. Special anchors and supports also are required to
prevent low temperatures from affecting carbon steel support beamsand causing brittle fracture. The stress analyst’s responsibilitycovers thermal construction and design basis for supports, guides
and anchors, etc. Project engineering shall specify the insulationand vapor barrier requirements.
—D.G. C-5
20. of 5]
I ‘.,,-
5.71 Support Desiqn
Special saddles have been designed within the stress groupfor cradling the insulating media. It was found that for24I’ pipe containing LNG (liquid methane at -258°F) a 6“
thickness of polyurethane (density 2 lbs/cu ft) or foamglass is usually required for insulation. At supportpoints, a higher density of the polyurethane has beenused instead of low density polyurethane or foamglass
because of better abrasion and shear resistance. The
limit of 1% deformation under dead load is a reasonable
criteria to determine the proper density of the poly-urethane block. A recent installation required 7#/cu ft
density for a 24’I pipe and supports spaced up to 26I
apart. The cost of polyurethane increases with density,so it is suggested that a practical minimum be arrived
at. See Standard C-727 for a recommended saddle design.Saddle supports have been either clamped to the insulatingmedia or cemented to the insulating block with polyurethaneelastomer, both have been found to work satisfactorily.
Likewise, the special support block of insulation betweenthe saddle support and the pipe has satisfactorily beencemented to the pipe itself to ensure movement of thesupport with the piping system. Until feedback fromoperating plants or engineering design proves otherwise,all support blocks should be cemented to the pipe with
Ad~prene or its equivalent, #2050 Adhesive (PolyurethaneElastomer), by CFR Division of the Upjohn Company or other
suit~ble compound. The above adhesive has been tested to-423 F (liquid hydrogen) by the research division of one
of the aircraft companies and found to maintain itsadhesive qualities at those low temperatures.
5.72 Anchor and Guide Details
Wherever it is felt that below freezing temperatures canaffect support, guide or anchor members constructed of
carbon steel stressed to values of 5000 psi or greater,special details should be provided to insure that the
structural members are not detrimentally affected by
those temperatures. Special carbon steels or alloy steelsshould be used having proper impact value where tempera-
tures dictate. See Standard C-727 for recommended anchorand guide designs.
5.73 Reduction of Friction at Support Surfaces
Whenever the anchor forces or frictional forces at supportsmight prove detrimental to the system’s design, specialsliding or roller supports should be provided. Te~lon
slide plates bonded to the under surface of the pipesaddle support channel have been successfully used. These
D G, C-521 of51
I ,,
5.74
slide plates bear on a similar slide plate bonded to a
metal plate which is tack welded to the support beam.The overall thickness of the two slide plates cmonly
is 7/1611 total. Their usefulness does have temperaturelimitations which vary with each manufacturer.
Flexibility Design of the Pipinq System
The materials used in Cryogenic piping systems increasein strength as the system gets colder and brittlefracture is avoided by the proper selection of special
materials of construction. Therefore, conservatively,the same allowable bending stress is permitted as if thesystem was at 100°F. To absorb the contraction of the
piping system, the first consideration should be to useexpansion loops or offsets. Where this is not possible,bellows type expansion joints should be utilized in tandem
within a minimum offset in the piping. The use of thebellows type in direct extension or compression should
be avoided but are not prohibited. Bellows expansionjoints must be very carefully protected from icing upand ultimately being crushed. This is their main draw-back. Other methods of absorbing the systems contractionare as follows:
a) Jacketed piping with internal axial expansion joints.This may require expansion joints periodically inthe external jacket pipe, if the system has long runs.
This system incorporates insulation in the jacket space.
b) In very special cases, the line might be prestressed
to absorb contraction. This requires no expansion
joints but suffers from large expansion forces andrequires very special installation procedures. The
use of hydraulic jacks or liquid gas cooldown mightbe employed.
—
22 Ofs]
D.G. C-5
I
6. STANDARDIZATION OF APPROACH TO PIPING PROBLEMS
6.01
a)
b)
c)
6.o2
.
6.03
Allowable Pipe Spans
The spans in Standard L-518 are limited by longitudinal bendingstress or a midspan deflection which has proven accept-
able from past experience, whichever governs. Although the spansare the maximum allowable, they are limited to a practical span for
general pipeway use of 20 to 25 feet, These and other limitationsare explained in the Standard itself.
Where it is impractical or very costly to install special stanchions
for support of small line branches from the pipeway headers or the
support of long runs of very small piping, consideration should be
given to suppo~ting the lines fcantilevered structural mebers
between larger lines. Normally
piping system is not a good pol
Occasionally groups of very sma
injection lines, can be banded I
of the ~rouD as a whole reduces
om a single large diameter
welded on, or by a trapeze
the supporting of pipe tocy and should be generally
1 diameter piping, such as
ogether whereby the moment
header by
beam hungany other
avoided.
chemical
of inertiathe bending stress or deflection of-.
the system to a permissible amount.
Allowable Pipe Overhang
At turning points of pipeway stanchions, the supported piping systems
have varying lengths of pipe overrunning the last support beam and rise
up or turn down to join similar “overhangs” of piping from the adjacentpipeway at right angles to the first one. These overhangs withincertain limitations are permissible without support. But, when the
overhang is such that stress or deflection limitations are exceeded(See Standard L-532) then. the overhanq requires a special support.
Dummy legs welded to the piping elbow and extended until it crossesthe next stanchion beam is the most corrnnonly used method of supporting
the overhang. Essentially, it supports the system by extending thepipe as a “beam” across two supports. (See Standard L-539 for dummylegs required.) Where the dummy leg becomes too long, special beamsshould be added to the stanchion to support the overhang.
Pipe Guide Spacinq
Pipe guides are used for several purposes. They keep lines essentially
straight for good general appearance, or they prevent buckling due tohigh axial loads from friction or expansion loop forces. Guides canalso be used to react against lateral line connections thereby con-stituting an anchor for the branch pipe. When anchoring branch
piping by this method the guides are placed on the main header at
the beams on adjacent stanchion column lines. The lateral reactionis taken by “bead’ action of the 20’ to 25’ pipe span. Under high
loads the stress or deflection of the pipe should be checked.
D.G. C-5
Page23 of51
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6.04
u
a) Guide spacing varies for the different areas of application. Onvessels, guide spacing is reduced from those permitted in on-plot
or off-plot piping. This is due to higher wind loads with in-creased elevation and load limitations of the various guide detailsused. See the pipe support manual for these allowable spacings,
b) On-plot and off-plot guide spacings could be essentially the same
except that within a guide range for any pipe size, it is pre-ferable to use the low side of the range for on-plot pipewaysand to use the high side of the range for off-plot pipeways. Thereason for this is that on-plot piping, being more critical in
nature due to branch connections, should have a more conservativedesign.
c) The suggested guide space ranges are:
Line Size Guide Space Ranqe
2,1 40 ‘ - 501
3,I 40 ‘ - 51)1
4, I 40 ‘ - 601
6, I 60 I - 801
8, I 80 I - 100’
10” 100’ - 120’
12” 120’ - 150’
14” 120’ - 150’
16” 150’ - 200’
18” 150’ - 200’
20” 200’ Max.
24I I II II
The guide space ranges are a general rule and in situations wherehigh axial loads exist these guide spacings should be reduced,
after checking for buckling in column action.
Instrument Stronq Back Flexibility
a) During normal operation instrument strong backs heat up with theattached vessel and since no differential expansion exists between
the two there is no flexibility problem. But , if some faulty
operation develops within the instruments, the block valves at thevessel nozzles can be shut and the instruments removed for repair.The strong back at this time cools down to atiient temperatures.At this time there is a differential expansion that exists between
the strong back and the vessel. Unless the nozzles or the offsets in
the piping to the strong back are flexible enough a failure couldoccur in the vessel nozzle or in the strong back proper and connect-ing instruments. Standard C-706 has been developed to give the
!.G. C-5Page24 of 51
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b)
Piping Designer a reasonable approach in providing flexibility
in the system before it is reviewed by the Stress Group.
These systems should be approved by the Stress Group by checkingwith the above standard.
Support of Instrument Stronq Back
Where long strong backs are offset and “Christmas Trees” are hung
from vessel nozzles there is a need of supporting these assemblies
to the vessel shell or platforms. Generally this is done by
inspection without taking time to go into lengthy calculations.If in doubt, add a support, always taking notice of affects of
differential expansion between supports and nozzle connections.
6.05 In-Line Pumps
As a general rule, in-line pumps exceeding 75HP shou}d be
supported on a foundation regardless of whether the piping is supportedseparately or not. On pumps of this size, base fianges may or may notbe provided, but this need not dictate that flanged pumps be bolted
down. If sliding is required, provide base plates and either eliminatebolting or add notes to pertinent drawings or isometrics to adjust nuts
hand tight. Sleeves may possibly be used to assure that nuts will notbear tightly on flanges. The stress analyst should note that holes
are to be oversized or slotted to allow for movement required. Pumps
smaller than 75 H.P. may be supported to the adjacent piping. See
Standard C-729 for suggested support techniques.
6.06 Expansion Loop Desiqn
The design of expansion loops for pipeways or any pipe system has been
programmed to produce a book of “Loop Tables”. These tables enable astress analyst to closely design by inspection a loop to any desired
stress or reaction force. A complete description of the method used to
arrive at a design is found within the Design Guide C-3.
a) A design pad (form 149) is available for recording all pertinent
information regarding the design and location of the expansion
loop. To arrive at the minimum sized expansion loop required,
the maximum allowable stress for the piping system has to bedetermined from the limitations in the code on the material at the
operating temperature. The actual size of the expansion loop is
equal to or greater than the minimum loop size to fit properly ona supporting media. The spring constant and the resulting bending
force within the system are then tabulated on the design pad forreference.
D.G. C-5
Page25 of 51
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—6.07 Pipe Anchors
.-
The anchors described herein are for above ground piping. Anchors
are used to direct the expansions or contractions of piping systemsand thereby prevent interferences with other piping or structures,
and/or control reactions to attached equipment. The reactions atanchors are taken by support beams made of braced or unbraced struc-
tural steel or precast concrete. These anchor reactions shall beplaced on an ozalid of the pipeway, specifically reduced for use by
the stress group, and a print of it passed on to the structural groupfor review of their stanchion design.
a) It is suggested that these anchor loads be calculated and faith-fully tabulated on a form for later reference. The client upon
occasion has requested these loadings, therefore, the tabulationmay be very important. The individual stress analyst may compute
them one by one, as he comes to an anchor tentatively and compute
all the loadings at once when the piping is finally completed.Standard calculation sheets are available for these anchor cal-culations (form 149).
b) The calculation sheet for expansion loop data and anchor force
determination does not include a listing of every item for tab-
ulation but covers key items for final summation to obtain theanchor force. It is suggested that the auxiliary sheet of pipeweights (form 188) will be used by the stress analyst to mentally
add up incremental weights for a particular system under “Wt”.The coefficient of friction to be commonly used for steel on steel
shall be 0.2 unless special surfaces are applied or additionalfactor of safety is desired. Where piping is supported on round
bars the coefficient of friction should be raised to 0.3.
c) Anchor loads for buried piping can be computed by forqulas re-
comnded in the section on “Buried Piping”. (par.5.6)
d) When computing anchor loads for above ground piping, the loadings
on each side of the anchor generally tend to balance out to some
degree, In some cases a long run of piping will be anchored near
the center of the run just to prevent gradual creeping of thesystem. The frictional force on each side of such an anchor may
theoretically balance or cancel out. The load to assign to su$h
an anchor should never be less than 25% of the frictional force
from one side alone.
6.08 Stacked Exchangers
a) When exchangers are stacked it is customary to use radial nozzles
directly connecting the two channel sections and the two shells.The hotter shell expands more than its adjacent shell and tends
to be constrained by the inter-connections. Some deformation of
the nozzles takes place and when the temperature difference andresulting stresses are large enough they can cause a failure inservice. This failure not only results in a plant shutdown but
could be the cause of a disastrous fire or explosion.
D.G. C-5Page26 of51
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io.
b) Standard C-715 has been established to give analysts a common
approach in reviewing the problem. As can be seen, when the
difference between mean temperatures of the adjoining shellsis greater than 100°F some provision should be made to add flex-ibility to the nozzle connections. These studies should be madeearly in the job such that nozzle orientations can be correctedbefore fabrication is started. Nozzle and piping arrangements
to improve flexibility are shown on the Standard.
c) To reduce movements of piping from exchangers leading into unitpipeways, hot exchangers should be anchored at the support closest
to the pipeway. The exchanger expansion tends to cancel the ex-pansion of the connected piping and its affect on pipeway clear-
ances. Where cooling water from below grade is connected to the
channel end of the exchanger the exchanger should be anchored tothe support closest to the channel end. For a dimensional guidesee Standard Drawing L-512.
6.o9 off plot Pipeways
6.091 General
a) Prior to the design and layout of offplot pipeways the project”
stress analyst should meet with the offplot project engineer todiscuss and establish proper temperatures for the expansion de-
sign of offplot piping. The temperature range shall be realistic,and it shall include reasonable atiient temperature variations at
the job site, but unless the client insists, remote upset condi-
tions should not be stipulated.
b) Review should be made with Project Engineer and the Client,
regarding the use of steam-out. Usually offplot piping is not
steamed out and is therefore not designed for that conditio~Normal operating and maximum or upset design temperatures should
be listed for all lines on the offplot line designation tableswhich should be completed prior to making the stress studies.
c) Methods of absorbing pipe expansion should be reviewed with the
offplot project engineer to see if the client might have restrictionson the use of expansion joints or couplings, etc.
d) When the design conditions have been established and if no formalmemorandum has been issued by the project engineer, the stressanalyst should prepare a memorandum covering all final decisionsand issue it to both the Chief Pressure Vessel Engineer and the
Offplot Project Engineer, who should be requested to transmit such
information to the client for information and record.
D.G. C-5
Page~7 of 51
I .
6.092
a)
b)
c)
d)
e)
f)
9)
Expansion Studies
The design approach to offplot piping should not be as strin-gent as that for on-plot piping, therefore systems should bedesigned up to the maximum stress allowed by ANSI code for the
upset condition except where reactions dictate otherwise.Additional lengths may be required to nest loops or use common
supports. In some cases where only few stress cycles may occur,Article S-1 of ASME Section Vlll, Div.2, L)esignb~edon Fati gue
%alysis might be employed. This criteria allows up to3 times theallowable stress intensity for secondary stresses, thereby
permitting close to 60,000 psi for A106 or A53 GRB pipe materials.
Tt,e expansion review of offplot piping is essentially a clearancecheck of pipes as they move relative to one another or whether
they interfere with the structural appurtenances ~f sleepers or
stanchions. See Standard C-720 for other than 90 corner move-
ments.
The tie-in temperature used as the calculation basis should
consider the specific time of year for plant construction. AISO
care should be exercised to consider the clearances and stressesfor both expansion and contraction of all adjacent piping systems.
Except in the vicinity of offplot pump manifolds or other equip-ment limiting reactions or stresses, the systems should be allowed
to expand up to a practical limitation of 12” at a corner of thepipeway or at each leg of expansion loops. This means that ex-
pansion loops should normally absorb up to 24” of expansion.
Support shoes for insulated piping in pipeways now are ordered
in two standard sizes, 18” & 30”. The 1811 shoe permits 6’I and
the 30” permits 12° of movement each way from their ~ with 3“of overhang for assurance that the system won’t hang up on thesupport. This 3“ overhancj is a standard allowance to be used atany support after maximum movement of an insulated piping system.
As an aid to the pipe support design group, the supports at whichpipe expansions exceed 6“ and 12” should be noted to assure that
shoes of proper lengths are assigned to each support.
Design of pipe guides and anchors is covered in paragraphs6.03 and 6.07.
Cold spring of systems should be avoided unless absolutely nec-
essary to reduce reactions at equipment or provide necessary
clearance. See Standard C-723 for method of noting cold spring.
Branch piping from the offplot pipeways leading into diked tank
fields must be reviewed for restriction of lateral movement due to
small clearance in the sleeve buried in the dike. Sometims the
pipe is just coated, wrapped and buried in the dike which there-
fore permits negligible lateral movement. Anci~ors may therefore
be required close to these branch connections to protect them
page 28 of51D.G. C-5
I
.—
against excessive lateral movement. Expansion of thesebranches whether from dike sleeves or pump manifolds can
be allowed to deflect the headers laterally, therefore,
the guides in the headers should be located far enough
apart to keep reactions back to the pumps or sleeve sealsto a reasonably low value. Axial movement of thesebranches is generally prevented by either the burial of
the pipe in the dike or by a link seal between the sleeveand the pipe on the tank side of the dike. Piping withinthe diked area is described in paragraph 7.17 on “Tank
Field Piping.i’
6.093 Pump Manifolds
Pump manifolds can be quite complicated and “tight” butwhen near ambient operating temperatures the expansionmovements are usually small. Such movements can bedirected away from the pumps if anchors and restraints
are properly located. See Standard C-730 for an exampleof a properly anchored system. Offsets in the branchesto the pumps should be avoided wherever possible.
“Normally no offsets are requir~d in these branches onsystems at temperatures of ~50 F or less. Wheretemperatures exceed say 150 F, then offsets in the
branches to the pumps may be required to improve flexi-bility and reduce reactions on the pumps. Where the
suction or discharge lines leading to or from the pumpsare eccentric by several feet from the pump centerline
it may not only be required to support the overhang, but
al SO restrain movement in the axial direction of thebranch pipe. Several support-restraints of this typeare shown in Standard c-731.
wkm
D.G. C-5
Page 2~of 51
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7. MISCELLANEOUS AND SPECIAL PROBLEMS
--0b .\0.
7.01
a)
b)
7.02
a)
b)
Slug Flow
In two-phase gas-liquid flows where the phases are unevenlydistributed and pass through a restriction, such as a valveor an expanded section, or a turn such as an elbow, there is avariable force exerted on the containing walls. This variableforce creates impact loadings on the guides .and supports of
the system which must be adequately accounted for in theirdesign. The magnitude of this variable force cannot be accuratelyevaluated due to the complexity of the flow. Therefore thedesign loads of the supports and guides should include an addi-
tional design factor which might be classed as an impact factor.
in lieu of some definitely calculable factor it is suggestedthat this impact factor be 3.0 times the weight of a slug ofliquid that might pass separately through the pipe as approxi-
mated by the Project Process Engineer.
These variable flow conditions also can affect the systemdetrimentally by setting up severe vibrations. The systemshould be carefully reviewed with this in mind and if necessary
hydraulic type shock absorbers should be utilized to prevent
large amplitude vibrations. Within one of our recent refinery
projects slug flow caused large diameter piping to vibratecontinuously and though the amplitude was small (1/811 peak to
peak) it resulted in a failureata 301’ diameter tee connection.
The tee connection was reinforced considerably and a hydraulicdamper was used to reduce the amplitude of vibration. Although
guides and hydraulic struts can reduce slug flow effects, best
results are obtained in reducing slug flow internally or
routing pipe to permit a more uniform and smooth flow.
Mitered Elbws
Mitered elbows are used many times in low pressure piping systemsfor economy since the cost of welded or seamless elbows becomes
prohibitive in the larger size pipes. Two of the drawbacks
to mitered ells are high stress concentrations and poorerflow characteristics. As more pieces are used to mab Up a metered
ell flexibility increases and stress intensifications decrease.In other wrds the mbility of a smooth e’
For flexibility studell must be replacedlity. The method to
ter with more pieces approaches the flexi-bow of the same radius.
es using some q?mputer Programs the mitered Iby an equivalent elbow with the same flexibi- Ibe used in obtaining the equivalent elbow is j
shown in Standard C-709. In most of the more recently ideveloped programs the mitered elbow is handled auto~ticallY. i
I—
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7.02
c) Before using miters in a flexibility calculation, theyshould be checked for permissible pressure by the formulasin paragraph 304.2.3 of the ANSI B31.3 Code.
Page 31 of 51
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7.03 Tee Connections
.—
a) The history of failures in piping systems points to teeconnections as being particularly vulnerable. Tee connec-tions have high hoop stress patterns around them which arenon-uniform and involve stress raisers or intensificationfactors. Vibration causes cyclic stresses which may below in magnitude, but can be troublesome when actingthrough medium to high frequency. When piping systemsare studied by the computer flexibility program, careshould be taken to always include the stress intensifica-tion factor at all tee connections. If the specificcomputer program doesn’t have this capability, then add
them manually to the output. Although no strict rule canbe given regarding allowable stresses at tee connections
in vibrating systems, good engineering judgment shoulddictate that the analyst use less than the maximum
allowable stress.
b)’ The stress concentration factors for tee connections havebeen calculated by a computer program and the resulting
values have been plotted on graphs for easy reference.
The graphs are shown in Standards C-711,712,713, and 714.They give stress factors for unreinforced tees, teesreinforced with pads both equal to the header thickness
and 1.5 times the header thickness and for forged tees.
It is suggested that all questionable stress levels attee connections in flexibility calculations include theproper stress factor read from one of the graphs pro-vided. An accuracy of 2 decimal places is sufficient.
When calling for a reinforcing pad the minimum width shallbe 0.2 x branch outside diameter, which results in equal
stress at both the crotch and the outside of the reinforc-ing pad.
7.04 Injection Connections
Whenever piping connections, involving injections of nearambient temperature fluids, are made into piping ~ystemsoperating at elevated temperatures, say above 500 F, a
critical stress condition exists at the nozzle connectionwhether reinforced or not. Failures have already beenbrought to our attention. To avoid the sudden transitionof temperature change the small pipe should first enter alarger nozzle at a blind flange or weld cap attached to its
end. This should be brought to the attention of the ProjectEngineer who in turn should locate these for review by the
Project Stress Analyst. A standard will be developed tocover this problem.
—
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7.05 Heater Coil Decokinq
—
a) During normal operations of most heaters a layer of coke
gradually builds up on the inner wall of heater tubes. As the
thickness of the layer increases the firing rate must also beincreased. This increase in firing rate results in an increasein the tube wall temperature and could eventually exceed the
maximum allowable temperature for the stress level in the tube.
To prevent this condition the coke must be removed periodicallyor whenever operating conditions indicate excessive coke build-up.
b) The coke is removed by using a steam-air or thermal decoking
method. This method involves heating the tubes first and then
passing steam through them at a specific mass velocity andthen introducing a mixture of steam and air. The effluent is
water quenched and discharged to the sewer, and the gas isvented from the quench drum stack. The temperature affectingthe external decoking mainfold and effluent piping duringthis time is generall ~ 1000°F. Flue gas temperatures gener-ally reach 400 to 450 F.
c) Supports for piping to the heaters must be properly locatedand designed for the disengagement of both normal piping and
the connection to decoking effluent or steam piping.
7.o6 Catalyst Regeneration
Catalyst regeneration is required periodically in CatalyticReformer Plants to reactivate the catalyst for efficientplant operation. At the time of regeneration the hydrocarbons
are stripped or burned off of the catalyst and the entiresystem is made as free as possible of residual hydrocarbons.
This is done by heating the system to normally greater thanoperating temperatures by nitrogen until a specific temperaturelevel is reached and oxygen is then gradually added in small
volumes further increasing the temperature. This burns off
the hydrocarbons and is continued until a system gas analysisshows that little hydrocarbon remains. Regeneration tempera-
tures usually read as high as 900 to 1000°F for a periodlasting several days.
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Obviously these increased temperatures create an additional
condition within the external piping system that must beaccounted for in both piping flexibility and support.
7.07 Reformer Furnace Pigtail Design
The piping connections of both the inlet and outlet piping
to the vertical tubes of Reformer Furnaces are subjected toboth vertical expansions of the tubes and the horizontal
expansions of the inlet and outlet collection headers. Thelarge vertical expansions require that the outlet collection
header be spring supported to move up and down with the
furnace tubes. These furnace tubes are generally supportedsolidly near their base and expand upward several inches.
The piping connections from the furnace tubes to these
collection headers are called pigtails because of theirdesign shape. Unfortunately there is a space limitation andthese pigtails are somewhat restricted in their flexibility.In the analysis of any pigtail dead load stresses of theloop must be considered along with stress concentration fac-
tors at tee connections.
7.o8 Cold Spring
a) Cold Springing of piping systems originally was utilized toreduce stresses and reactions in piping systems, and to
equalize somewhat the displacements of piping about aneutral axis, or reduce interferences.
b) The modern piping code no longer permits the reduction ofstress, as such, but permits allowable stresses within a“stress range”. Therefore, if a system is cold sprung, a
certain portion of the stress range is already utilized andonly the remainder is permitted for the expansion beyond
the amount of cold spring.
c) Since cold springing is an additional operation for thefield to complete, it is suggested that cold springing shouldonly be requested where it is critical to the systems design.For example, piping to turbines or compressors might require
cold spring to reduce reactions to an acceptable level.
--D.G. C-5
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d)
7.09
a)
b)
Generally, in pipeways, the design of expansion loops shouldnot involve cold springing although the loops may still bedesigned right up to the maximum allowable stress. Other
systems between columns, exchangers, drums, or connectionsto other piping should not be cold sprung unless absolutelynecessary to avoid the additional operation. It has beenfound that cold spring notations have been overlooked or
cold springing has been improperly applied in the field, un-less great care has been taken to flag and describe the
manner in which it is to be applied. If equipment must beprotected by cold springing of its piping systems and themanner of procedure of cold springing is felt to be particularlyimportant, the Stress AnaIyst should write step by step pro-cedures and send them to the Field Engineer in charge.
Typical cold spring notations are shown on Standard C-723.
Blowdown Systems
Blowdown piping as a general rule operates at low pressureswith medium to high temperatures (i.e. 300°F - 1000°F) and
close to 10~~ vapor. The main headers are usually largediameter pipes up to as much as 4 or S feet in diameter.
The systems become operative upon sudden release of vaporsfrom safety relief valves and therefore are subject to
sudden surges of gas flow. This tends to set up large am-plitude vibrations or shaking of the system. To protectagainst the piping from bouncing off supports or damaging
adjacent equipment, hold down guides should be judiciously
located throughtout the system. The system should be amplyanchored to direct its thermal movements and where movements
are too large to absorb within the inherent offsets of thepiping, loops, or offsets with tandem expansion joints, are
recommended. Direct axial expansion joints are undesirable-/-
because of large anchor forces required to contain the.-., .
system. There is no limit to the total expansion which the....
above devices can take except that sound engineering judgement
shall be applied to limiting anchor forces, lengths ofsupport saddles, and spacing required to other pipe or equip- . .
ment.
Branch connections, expanding thermally between relief valve
and the blowdown headers, may require the addition of flexible
offsets to absorb such movements. The allowable stress of the
pipe at the connection to the relief valve should be limitedto prevent any distortion at the valve which would render itinoperative. As a rule of thumb the resultant bending stressat the connection should be kept below a maximum of 10,000 psi.
..-, - .=.,
~:’g:;i.:;.~;r~f,
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—
Large weight reactions should be removed from the reliefvalve by use of spring supports or equivalent. Welding ofgussets from the valve discharge pipe to the valve inlet nozzleas a solid support is not recommended and shall not be usedon systems exceeding 150°F.
c) When gas flow through blowdown systems has a velocity at teeconnections above .2 Mach, the pipe wall for at least 5diameters on each side of the tee connections should be in-creased in thickness to prevent cracking by ovaling vibrations.The branch pipe should likewise be increased in thickness for
a short distance back from the tee.
d)
7.10
a)
The reactive forces resulting from the discharge of reliefvalves can be computed from the following formulas:
F = g.84w/ *M (for closed system, from API RP520-Jan. 1963) ~
F= : v,
Where F =
w.
K =
T =
M =
9=
V1 =
Pl,Pa =
Al =
+ (P1-Pa)Al (for vented system,from ANSI B31. l,Appendix 11)
“reactive force at valve outlet, lbs.
mass flow rate, lb./see.
ratio of specific heats, Cp/Cv
absolution temperature at inlet, ‘R = ‘F + 46o
molecular weight of gas or vapor
gravitational constant = 32.2 ft./sec2
exit velocity, ft./see.
exit and atmospheric pressure, psi
flow area, in.2
Field Checkout
Field Checking has become an important part of the Project Stress
Analyst’s responsibility. Errors in the Field due to omission orimproper interpretation of design drawings have necessitated that
critical piping”be reviewed just prior to 14nit startup. A moanlist should be developed at the field covering any items yet to be
completed by the construction department (to cover possible omiss-
ions) and to itemize in detail any corrections or modificationsrequired on any support or piping installations where the design
intent was not met. Exceptions may be made where the system asinstalled will function adequately and every effort should be
made to avoid requesting corrections unless there is danger of
failure of some component of the system. The moan list shouldbecome part of a report which is then given to the Job Superin-
tendent and the Supervising Field Engineer.
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B.07.17 Tank Field Pipinq
a) The piping in tank fields is subject to several special design
considerations such as:
Tank settlementEarthquake movements
Containment within diked areas
b) Where tank settlement is a problem the first pipe support shouldbe located, say, 20 feet away from the nozzle and be of an adjust-able type. Adjustable supports can be made up of wood blocklayers 1/2 inch or I inch thick that can be retracted as the
tank settles. Where adjustable supports do not fit into thedesign, flexible couplings or joints can be used in a tandemarrangement. A tandem unit involves a length of pipe with aflexible connector at each end which absorbs deflections byangulating the unit at right angles to the axis of the pipe.
c) Earthquake movements can be accommodated by providing a pipeoffset at the tank. This offset can be used for both settle-ment and earthquake movements. The routing of a long linewithout offsets directly connected to a tank nozzle should beavoided.
.
d) Piping routed between and anchored in dikes of a tank fieldgenerally requires either loops or offsets to absorb itsexpansion and contraction, even though only affected by ambient
temperature changes. The burial of the pipe in the dike pro-vides sufficient restraint generally to anchor the pipe. Where
sleeves are used, a link is generally provided between the pipeand the sleeve.
7.I8 Steam Trace and Steam Trap Pipinq
w
a) Steam trace piping details are provided to the field by an
Engineering Standard L-521. A problem arises in the connect-
ions from the steam headers to the steam traced pipe. The
expansions of the steam header requires that the interconnectingbranch pipe to the steam traced pipe be of sufficient flex-ibility to absorb the deflection without failure. Also the
clearance of the branch pipe to other piping or structures mustbe considered. The location of these steam trace branches is
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wLm
b) On return to the main office, the Stress Analyst should make abrief trip report to the Project Engineering Manager, and sendcopies of the report with the moan list attached to the ProjectSuperintendent, the Supervising Field Engineer, the Unit Project
Engineer, and the Chief Vessel-Stress Engineer. S= Standard
C-704.
c) A typical check list of items for field review might include:
1) Clearances between piping systems or between critical
piping and structural members or any equipment. Thisincludes revi- of critical cold springing.
2) Sufficient overhang of pipe support shoes on beams
to allow for maximum pipe movement.
3) Movement of piping as affecting instrument or electricalconnections.
4) Spring supports adjusted to proper loadings and stops
removed after hydro test of system.
5) Pipe anchors located and installed correctly.
6) Expansion joint assemblies installed properly including
orientation of hinges or tie rods, if any. Sizing bars
to be removed.
7) Critical Piping:
Steam lines, including Turbine piping.
Reactor piping.
Furnace Transfer lines.
Blowdown Systems.
Compressor Piping.Pump Piping.
Hot process piping (generally over 350°F).Cryogenic and refrigeration piping.Steam trace connections.
7.11 Soot Blowers
a) In today’s high performance steam generators, “Controlled
Cleanliness” of horizontal and pendant tube surfaces must bemaintained to assure proper heat absorption and optimum steam
temperatures. Soot blowers are needed for the specific pur-
pose of cleaning tubes in the convection section of heatersand boilers. The soot blowers are constructed of long hoodedframes which support horizontal lances up to 24’ long.
D.G. C-5
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The lance (or femle pipe) is extended into the heater orboiler convection section by retracting it from its internal,Ima]ell feeder PiPe*
The soot blower assembly is fed by air orsteam at a flanged nozzle 1S’ or 20’ out from the wall of theconvection section. It is not rigidly held at this point butcan be moved laterally a small amount (say III +) and evenslightly rotated.
b) If several units are to be connected together by a common steam
header, the above movements can normally be tolerated. Thesoot blower frame is supported at each end to the platformstructures of the heater or boiler.
7.12 Settlement and Frost Heave
a) Differential settlement between pieces of equipment, or structuresand equipment, can induce damaging reactions or stresses toboth piping and the equipment to which it is attached.
b) It is essential that specific settlement or heave deflectionsare obtained frm the structural department for critical
locations such as around pumps, tanks, and at all vessels andcolumns. These deflections must then be incorporated intothe design analysis of all affected piping. Where thesedeflections cannot be easily absorbed it may require that pipe
supports be extended below the frost line or that piles bedriven to prevent settlement.
C) TO avoid special piles for foundations, pump piping may besupported to the pump foundation itself by extending a portion
of the foundation under the piping. Also, a beam can be em-bedded into the pump foundation with a short section cantilevered
out to support the eccentric pipe system. This cantileversection should be sufficiently above the grade slab so thatanticipated frost heave will not affect it. Support lugs may
be cinch anchored into the side of the foundation. The methodof supporting the pump piping must therefore be agreed uponearly in design stages of the plant.
d) At plant sites where frost heave is a problem, the support of
piping manifolds alongside exchangers can be supported tostructural members fastened to the sonotube or pier supportsof the exchanger itself rather than provide deep separatefoundations for the piping separately.
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I . .
_&:e)
7.13
a)
b)
7.14
a)
Where piping systems below grade are subject to settlementpiled supports should be provided to prevent detrimentaldeflections of branch piping to pumps or other equipment.
Deep burial of these headers are required in areas affectedby frost heave.
Ambient Temperature Effect on Bare Pipin~
Empty piping in long pipeways can be greatly affected by atmos-pheric temperature (ambient) changes. Stagnant systems in100-l lO°F temperatures can reach effective wall temperatures
of 130° to 140°F depending on ~i pe surface coloration or cover-ing. It is suggested that 130 F minimum be used for the hightemperature design of systems affected by ambient changes only.
For the contraction of systems below the tie-in temperaturethe basic design data of the locale should be reviewed to de-termine the minimum temperatures that the systems will besubjected to. It is very important that contraction from atie-in temperature be considered when checking clearances, or
designing expansion joints with limit stops or internal sleeves.
Severe failures in systems have already occurred where thiswas not properly accounted for.
The tie-in temperature should realistically be chosen for thetime of the year of installation and the locale of the plant.
For example, if piping is to be installed in Alaska during thewinter months the tie-in temperature might range from below
zero to freezing (32°F), whereas piping installed in Libyain the winter months would range from 40° to 80°F. For longpipeways this can result in a considerable difference in ex-pansion movements.
Control Valve Pipinq
Piping to control valves or let down valves are subject tovibration which sometimes reaches dangerous ampl itudes ordestructive frequencies. In general the connecting pipingsystems should be guided whenever possible to eliminatelarge amplitude vibrations. But, where sonic vibrations occurwith high energy input, the pipe tends to oval,or wave patternsdevelop circumferentially dictating that rigid attachments
should be avoided since failure at points of rigidity on thepipe wall will generally occur.
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.
! ‘-
b)
7.15
a)
b)
7.16
a)
b)
c)
It is the responsibility of the Control Systems Group to flagsystems with those critical tendencies for special study and
corrective design. When the stress anaIyst is confronted with
this type of problem he should contact the Control Systems
Specialist for the proper solution to the problem.
Hydrotest of Larqe Low Pressure Pipinq
The design of supports for large diameter piping systems canbe greatly affected by whether or not the system will be filledwith a liquid, since the filled weight can be many times the empty
weight. Therefore in the early stages of plant design it isextremely important to get agreement with the client and ourconstruction department on the basis of support design oflarge diameter piping systems normally handling gas flow.
If a hydrotest is imperative then the structural group mustdesign supports for liquid load. If the system will be airtested, or by similar alternate gas test, then al I partiesconcerned must agree in writing in order to protect Bechtel ’sinterest, and avoid design checking or modifications near
job completion.
Ring girders or thick saddle plates may be required at supportpoints for hydrotested systems.
Pipe Supports
When marking up piping isometrics or drawings for requiredsupports, the list of Standard Support Symbols as shown on
Standard c-707 should be utilized. This will help thesupport group to interpret the markings of each stress analystin a commonly understood fashion.
Elaborate, highly detailed, and non-standard supports should
be avoided. Supports should be as simple as possible.
Some basic precepts on where, when and how to support Iare:
1) Avoid supporting one pipe to another except for sma
utility lines being routed to off plot facilities a
large line high above grade.
iping
1ongside a
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w
2)
3)
4)
5)
Occasionally individual branch lines of small diameterare routed between the pipeways and process vessels at an
excessive span and at elevations in excess of 10’ above
grade. In cases like this, the support to a larger linenearby is acceptable.
Spring supports should be specified only when important tothe safe or proper design of a piping system. A greatnumber of spring supports are often rather casually de-signed into the piping in a plant. Upon closer inspection,however, it will usually be found that the system could bedesigned with solid supports. The use of shims in pre-springing pipe will permit minor expansion movements.
Maintenance Supports should not be provided unless requiredby clients specifications. During plant shutdowns anysystem that is to be repaired can be temporarily shored up.
Piping to vertical vessels that are flanged at the vesselnozzle should be provided with a bracket support. This ismore for installation and maintainance generally than for
stress purposes. Non-flanged piping to vessels may not re-quire these bracket supports, stress permitting.
Where rigid guides or struts would restrict the free expansion
of a piping system in such a way as to affect it detrimentally,the system should be guided with truck shock absorbers (see
support group details of acceptable units) or in the case oflarge, critical piping the more specialized hydraulic
cylinders should be installed. (i.e. Bergen, Grinnell, Barco,
Harpak or equivalent types). Piping expansion occurs generallyat a slow enough rate to permit the gradual adjustment within
the hydraulic unit.
d) Spring Supports
1) Spring supports, when properly used, fulfi]] a verY importantneed in the support of piping systems. However, they should
not be used indiscriminately or as an easy solution for thesupport of piping which is affected by vertical eX~JansiOnS
or other mechanical movements. For proper installation
procedures for the construction department a structural
standard M504 is available for their use.
D.G. C-5
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I
2) Solid supports are usually practical when the support lug
on the inlet or outlet piping is at the proper elevation tobalance out vertical expansion of the equipment and its pedestalsupports. In other cases as a generai rule, it is better tomake a calculation if it will prove that a solid support is
acceptable. if calculation time does not exceed about four hoursit is probably worth making the calculation in order to eliminate
the spring supports.
3) When spring supports are used in pump systems the stress
analyst must review the effect of the spring reaction on thesystem based on a spring preset reaction which has beencalculated for a liquid filled system. Prior to start-up thisreaction is applied to an empty pipe system. If the liquidweight portion of the reaction can’t be tolerated by the pumpor piping, the spring may have to be preset at some value between
the full and half full pipe weight reactions. When the liquid
weight affect is intolerable for even a 50% weight change, thepiping will have to be rerouted to provide an acceptable design.
See Specification M-504 for field installation instructions for
spring supports and ensure that design of spring supported
systems is consistent with the requirements of that specification.
4) When to Use Spring Supports
If vertical expansions or mechanical movements (imposed on a
piping system restricted by solid type supports) result inintolerable stresses or reactions, then spring supports may berequired. Spring supports permit the piping system’s flexibility
to be used to absorb system movements within tolerable limits;they must be used on hot piping systems adjacent to pumps, turbines
and compressors when solid supports cannot be tolerated.Wherever variable spring hangers are used, the stress analyst must
check to assure that the total variation in support effect doesnot result in harmful stresses and forces within the piping system.
Otherwise constant spring supports or counter weight supportsshould be considered. Generally for non-critical systems, varia-
tion of support force up to ~ 25% and movements up to 3“ may be
allowed.
5) How to Avoid Using Spring Supports
If a support is not adjacent to a piece of rotating equipment or
some other similarly delicate apparatus, a piping system subjected
to 3/8” or less movement might well be shimmed at supports after
the system has been completely welded in place or bolted up. Ifthe flexibility of the piping permits, and the dead load of the pipewill not keep the expanded system on its supports, the use of
nominal shims, from 1/8” up to 1/2” thick, should be utilized
rather than specify spring supports.
D.G. C-5Page 42 of 51
I
left up to the field forces and therefore it is the duty of the
Stress Analyst during Field Check to review these connections.
b) Steam trace branches and condensate return lines are often banded
together in plants located in freezing climates. Obviously differ-ential expansions of the steam and condensate headers maydetrimentally affect these connections. Therefore, a horizontalloop must be extended from the smaller condensate branch beforerejoining the steam line for banding together. The extended loop isseparately insulated and is allowed to cantilever out unsupportedexcept by the banding upon return to the steam line. Drip legs ofsteam headers have been routed directly to a stanchion beam or otherstructural member and clamped tightly to a fixed support permittingno movement at all. Standard drawing for drip leg details, L-519,
has been revised to alert the field of this problem.
7.19 Plastic Piping
a) Because of the need within refineries and chemical plants for pipingto carry alkalies or acids, various metals are used in the fabrica-tion of special piping for this use. Some of the materials usedare rubber lined or glass lined steel pipe, and solid plastic orreinforced plastic pipe, usually known as reinforced thermo-settingresin pipe, filament wound, either hand laid, bag molded, or cast.In the case of lined steel pipe, its flexibility and support are
similar to unlined pipe. But, where plastic or reinforced plastic
pipe is used the support and flexibility requirements should closelyfollow the recommendations of the specific manufacturer. AS a
guidance, refer to ASME Code Case N115-1.
b) it ha”s been noted that different manufacturers of PVC (Poly VinylChloride) pipe recommend different methods of supporting and restrain-
ing the systems. The Stress Analyst is urged to consider expansion
and and contraction forces and stresses in systems before agreeingto totally restrain the systems with thrust blocks as recommended by
one of the plastic manufacturers. In fact, all manufacturers agree
on the cemented joints as being equal to or better in strength thanthe pipe itself. Therefore allow the free expansion of the system
normally with the suggestion that the field be notified to exercisegreat care in installing the cemented joints for complete adequacy.
Also, the manufacturers allowable support spans should not be exceeded.
c) Because of the considerably lower values of Young’s modulus ofelasticity (1.5 to 1.0 x 106) of the plastic materials, the pressureelongation of the pipe line may be a significant factor in theflexibility or displacement stress analysis of FRP pipes. To take
this into consideration, an equivalent coefficient of expansionthat will include the pressure strain effect should be used in theBechtel piping stress programs.
Page 44 of 51
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7.20 Rotation, Reactions and Stresses at Nozzle Connections to Vessels
a) Most piping systems connected to shel 1s of columns, exchangers
drums and tanks are analyzed conservatively without consider-ing the rotational rel ief afforded at the nozzle connections.
Generally, as long as the stresses in the piping and loadingson other attached equipment are within allowable limits, the
systems as a whole is deemed acceptable. However, when thereactions on the shell nozzle appear high, then the Engineering
Design Guide C-1 “Local Stresses in Cylindrical Shells due to
External Loadings” may be used to approximate the vessel stressesdue to the fixed end reactions. If this stress is too high, acalculation can be made employing the spring constant of thenozzle attachment whereby the reduced loadings on the vessel
shell may be acceptable when compared to allowable in Guide
c-1.
If a system is obviously very tight, the spring constant “K”of any nozzIe attachment should be evaluated from the Standard
C-722 and incorporated in the calculation from the beginning.
b) If two vessels are interconnected by radial nozzles, such as
stacked exchangers, and the shells are at different temperatures,the difference in longitudinal expansion must be absorbed mostlyby a rotation of the joined nozzles at each shell connection.(i.e., nozzle rotation = differential expansion ~ total nozzlelength). See Standard C-715 for condition requiring thisevaluation and for the procedure to be followed.
c) The spring constants of Standard C-722 can also be used to find
the deflection or rotation of pipe supports (i.e., cantileversor brackets, etc.) attached to shells, due to the flexibilityof the shell under the applied loads and moments.
d) Storage tanks present a unique problem involving rotation and
deflection of shell nozzles close to the tank bottom duringfilling of the tank. These movements affect attached piping
II
and should therefore be considered when locating external pipe 4I supports or routing the pipe itself in a proper manner. See
C-732 and C-733 for design criteria.
“o-C0
7.21 Bowinq of Pipe
.
*a) Bowing in piping systems is due to unequal heating of the pipe
Ewall from side to side along its length. This type of bowing
&is unrelated to column instability from compressive axial loads.
Luk As one side of a pipe becomes hotter than the opposite, itsVI longitudinal elements expand more than those of the colder sideu and bowing occurs.2
.D.G. C-5
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b) Bowing may occur when:
1. Hot or cold fluid flows in partially filled pipes.
2. Sun’s radiation heats the tops of large empty pipeslaying close to the frozen ground.
3. Unequal surface heating of furnace tubes.
4. LNG in partially filled loading lines.
5. Channeling occurs in tubes filled with packing.
6. -The burner flames in a furnace are not equally dis-
tributed across the tube diameter.
c) If a piping system is not restrained and is consideredweightless, bowing does not induce stresses in the pipe.
Weight, friction and restraint, however, will induce stressesin the pipe, and the restraints may be subjected to veryhigh reactions.
d) Bowing can usually be tolerated when it is of only short dura-
tion. If bowing is considered detrimental, and it is notpossible to improve the uniformity of the temperature in thepipe, then external restraints must be designed and provided.
7.22 Compressor Bottle Support Ia) As explained in paragraph 5.22 (c), there is a need for an
elastic support for compressor bottle to allow for vertical
expansion downward from the cylinder support level. From
design data for rubber bearing pads, a design procedure has
been set up to properly size bearing pads. The pads can be
placed between the support lug or saddle on the compressorbottle and the load adjustment plate underneath. This load iadjustment plate is supported by four or more bolts embedded Iin a concrete pier. The plate is suspended about 3 inches !
above the top of the concrete pier to allow for tuning of the
support to inhibit vibration.
The design procedure to size the bearing pad and the adjust- 1ment plate is explained herein and the rubber bearing pad
f
physical data is shown on Std. Dwg. C-719. The support
assembly detail is a structural standard. The size of the
rubber bearing pad in the detail shall be determined by the IPiping Stress Group by completing Form No. 70 of the !
Pressure Vessel Standards. I
The suction bottles resting on the compressor cylinders needi
no support, except for eccentric or overhanging portions oftI
the bottles, since the load is in compression. The dischargebottles hang from the cylinders putting tension into thenozzles which, under constant vibration, are more likely tofail. See page 47 for Bearing Pad size calculation procedure.
ID.G. C-5
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Bearinq Pad Size (See Forms 70 and 365)
CO]. No. Description
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(lo)
(11)
(12)
(13)
(14)
Support No.
Wt. = total wt. of compressor bottle, lb.
AexP = thermal movement, vertical down from top of supportfor cylinder heat to bottom of compr. bottle
t = pad thickness, inch = 45 x(3)
L = pad length, inches = 1/3 channel depth
W = pad width i riches = channel flange width
AL = load area, in02 = (5) x (6)
AB = bulge area, in.2 =2x[(5) + (6)] x (4)
Shape Factor = (7) - (8)
E, psi, from C-719
R = weight on each pad, lb. = (2) s 4
Awt(11) x (4)
= deflection due to weight, in. = (7) x (10)
Strain = (3) + (12)~ x ‘00%”
If this > 15%, make new trial
using larger t or larger pad size and higherdurometer no.
*= {w x 100%. If this > 15%, make new trial
Uwt \l&J
using larger t or smaller pad size and lower
durometer
Bottom Plate Thickness “T” See Form 365
Design the bottom plate as a simple beam of cross-section “C” x “T”with supports spaced “F” apart and with a central concentrated loadequal to “2R” [2 x (11) from above].
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!7.23 Tank Nozzle Movements Due to Pressure and Temperature 1
I
With the advent of larqer diameter and taller storaae tanks. a ,
problem of shell defo~ation close to the bottan of-tank due toI
product storage pressure has been magnified. Under pressure thetank wall will stretch and will move radially outward if unre- 1
I
strained. At the juncture of this shell to the tank bottom thepressure creates a shear load which tends to stretch the tankbottom. This stretch is negligible compared to the shell radial
distortion, therefore, the shell is nearly totally restrainedat its juncture to the tank bottom plate. From this point avertical section would show that the shell gradually follows
an elastic curve to a point closely equal to 1.56#~ above thetank bottom where the radial deformation is equal to PR2. Any
z )
nozzle on the tank located in this bulge area will exhibit both !a downward rotation and an outward defection. This results ina bending and shifting of the piping system connected to thenozzle which must be accommodated by its inherent flexibilityconsidering all restraints acting on the piping system especially
the location of the first pipe support adjacent to the tank.
Where tank settlement is also involved adjustable supports orcouplings can be employed as described in Section 7.17. Whennecessary to study the nozzle rotation effect on external piping
to the tank refer to Engineering Standards C-732 and 733 for the ,conservative values of both rotation and deflection and input
them into a flexibility calculation. Differences in expansionof the tank shell and tank bottom, which is reacted on byfriction and may have minor buckling effects, are considered 1
negligible.
--
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8.PIPING STRESS ANALYSIS WORK CHECK LIST
The following list of work items is provided as a check list forProject Stress Analysts. It lists items essentially in their
chronological sequence as they will occur on a project and is intendedto draw attention to critical items, some of which must be reviewedand pre-planned at specific stages of a job in order to avoid delays
and changes in other engineering work.(Ref. Par. No.)
8.o1 Desiqn Data
Obtain from the Project Engineer:
a)
b)
c)
d)
8.o2
8.o3
8.o4
a)
b)
8.o5
a)
b)
Basic design data for job site, i.e., Wind Loads, E. Q. Loading,
Temperature Variations, etc.
Steam-out temp. - Proj. Eng. must issue memo to all Unit Engrs.(6.09)
Steam-trace temp. -Proj. Eng. must issue memo to all Unit Engrs.
Settlement or frost heave criteria. This is very i~~~~~~nt(7.12) “
Drawing Distribution
See that name is on Distribution of Documents, schedules, etc.for items required for stress work.
(2.1)
Initial Pipinq Studies (3.0)
Alloy Pipinq (3.2.)
Give preliminary approval for material commitment.
Make final studies so that detailed supports can beissued for shop 1S0’s.
Stress Relieved Vessels & Pipinq (4.0)
Obtain dates from the Pressure Vessel Supervisor forscheduled shop stress relief of each vessel.
Complete stress studies of piping and send 1S0’s toSupport Group six weeks ahead of scheduled ISU issued-for-construction date. Coordinate this with the Piping
Supervisor.
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8.06
a)
b)
c)
d)
8.07
a)
b)
8.08
a)
w
b)
8.09
a)
b)
c)
Heaters
The Piping Stress AnalystProject Engineer & Heater
(Ref. Par. No.)
(5.5)
shall arrange a meeting with the
Specialist and supply all infor-mation required on the heater bid specification, including
the following:
Anchor nozzles - Yes or No.
Nozzle Movement - Amount and Direction.
Support of tubes - Top or Bottom -Effect on external piping.
Need for brackets on heater shell for pipe supports and plat-
forms, etc. (Loads, details, etc.).
Compressors & Their Turbine Drivers (5.2 & 5.3)
Check with equipment specialists to assure that Mfrs. agreeto our specified loading conditions, as related to equipmentdead load.
Send piping 1S0’s with support locations to Mfr. with requestfor an analog vibration study for each reciprocating compressor.
w (5.1)
Locate large bottom out pumps with respect to vessels togive best arrangements for flexible pipe configuration.
Avoid direct piping from equipment to pump.
LooPs in Pipew~ (6.06)
Place loops in headers to limit their expansion or affect on
branches to turbines, pumps or compressors.
Locate off-plot pipeway loops as soon as possible as an aideto the Construction Dept. for field “fill-in” work. (6.09)
.
Locate and size anchors and loops for systems which are to beused during construction (steam and other utilities) when
requested. (6.07)
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8.10
a)
b)
8.11
8.12
8.13
8.14
a)
b)
.
(Ref. Par. No. )
Ai rfans (5.4)
Check with Project Engineer to assure that the specificationsrequi re a lateral movement tolerance of 5/1611 min.
Acquaint pipe designers with flexibility requirements whenseveral units are joined together.
Auxiliary Pipe Stanchions
Establish all additional auxiliary stanchions or specialsupports requiring piles or foundations, as soon as possible -when sufficient branch piping is model led, so that fieldcrews can complete pile driving operations and advance tolater operations without concern for the need for additionalpiling in an area.
Tank Field Pipinq
(7.17)
Field Checking
(7.10)
Special I’-?cign. CriteriaJ
When complete thermal cycles within a piping system exceed 7000and the expansion stress anywhere within the system exceeds1.25 Sc, the overstressed section requires full examination inaccordance with 336.5.1 (b) (2) (ANSI B31.3-1973 ED)
,
For piping in cold climates it is important to see that thoseconstructed from carbon, low alloy and high alloy steels are
not stressed higher than 6000 psi based on a combination oflongitudinal stresses due to pressure, dead load and displacement ‘
strains. The operating pressure should be no greater than 15%of the maximumtemperature beimpact test is
design pr%ssure at that time nor should thebelow -50 F. If any of the above are exceeded an ;required . (See paragraph 323.2.2)
!
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