select expansion joints properly

8/22/2019 Select Expansion Joints Properly

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j!Select expansion joints properly1lCorract selection and installation ofJl expansion joints will add years to theirf service life. Dick Pengelly, Vokes Ltd., Guildford, EnglandI! T,\.ELVE PERCENT IS the generally accepted figure of, capil:t! expenditure for piping systems in the hydrocarbonl processing industry. Expansion joints are used extensively! to provide flexibility for vibration and thennal expansion., Proper selection and installation of expansion joints willl add years to their service life.; Historically, problems due to thermal expansion were! solved by the use of loops. Earl: systems developed into! more sophisticated standard design loop systems. Furtherl developments led to the use of bellows expansion jointsj ncc,[)Orating multi-ply designs.! ~ h l t i - p l y construction is the best technical compromise1 between the requirements of pressure containment (thick

wa][ shallow convolution) and high flexibility (thin walldeep convolution). Th e force required to deflect a convolution increases with the cube of the thickness. A number of individual thinner walls provides the strengthrequired for pressure containment without sacrificingflexibility.Although bellows are used in "sealed" types of applicatic;1s, such as valves, this article concentrates on thermalexpansion absorbers (pipe line and vessel applications)and flexible connectors (exhaust systems).Flexible connectors applied in the past to both flexiblehose and expansion joints. In this article we includernanifolds, interconnecting and uptake bellows (Fig. 2)and gas turbine exhaust units. Cnits within this rangeare generally designed for low pressure, medium/highlernperatme applications, e.g. 1 K g / e m ~ at 450 C, althou:!: the 300 series stainless steel convolution materialsene:::J.lly used in this application can operate successfully'.lp to a maximum temperature oi 650 C.

The multi-ply design is widely used in flexible connectors providing the following features:...._The stability of multi-ply construction absorbs lowarnplitude high frequency vibr:J.tion, associated with these

flvnRocARBON PRocEssiNG :\farch 1978

applications, an d eliminates "vibration in sympathy"which leads to fatigue failure in single-ply thick wall units...... Lateral movement across the axis of the unit can bebetter absorbed. This is particularly important in applications on exhaust engine manifolds where misalignmenton assembly, particularly at the engine refurbishing stage,is a common occurrence...,.. Th e multi-ply design provides low flexure stress withinthe convolution resulting in extended life.Piping/vessel applications. Stock units range fromsimple single axial bellows 32mm-300mm N.B. for heatingand ventilating, to 4.5m dia. units for steel works furnacegas mams.Basic types of expansion joints range from simplesingle units to complex restrained units suitable for combined movements. Pressure vessels and heat exchangersgenerally use simple unrestrained single units to absorbdifferential expansion/contraction between shells. As anexample, one such unit uses an expansion joint 1,800mmin diameter with design conditions of 37 K g j c m ~ at335 C. In this example the shell at the expansion jointis approximately 35mm thick. The complete expansionjoint assembly with end fittings is pressure tested at50 K g j c m ~ at the expansion joint manufacturers' works.This test will be repeated on the final assembled vessel,including expansion joints.This assembly, although the simplest type of expansionjoint, requires maximum attention to all details duringmanufacture, assembly, testing an d installation as thecomplete vessel is basically built around the convolutionsection. Failure or malfunction of the expansion jointwould most likely result m a major strip down of thecomplete vesseL

End fittings ha\e to be designed to suit customermating equipment whether the unit is supplied in flangedor pipe-ended form. Final assembly of the unit betweenthe end fittings is generally left to the expansion jointsupplier who will fit standard design convolutions withinthe overall frame work wherever possible. Standard convolutions are theoretically designed by computer andtested on destruction fatigue rigs to ensure safety. Designs are compared with established standards within the

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SELECT EXPANSION JOINTS PROPERLYindustry, e.g. Esso, Kellogg, EJMA etc., as customers generally refer to these standards.Theoretical fatigue life requirements, requested bycustomers, are considered against theoreticaljpractical results to give standard unit movement. A simple calculation then gives the permissible movement for the required life cycles. In this way cyclic life variations ofany magnitude can be designed. Basically designs showing life cycles in excess of 1,000,000 are termed infinitelife.Selection. Expansion joint suppliers prefer to discussapplications at a very early stage of system design, priorto the finalization of full piping runs, vessel siting, p1inthpositions, anchor positions, etc. Early planning can resultin greater use of standard units offering advantageousprice and delivery. Early planning also insures that customdesigned units, such as the pressure balanced type shownin Fig. 1, are delivered when needed.

Th e specified cyclic life of an expansion joint must notbe an assumption. Investigations of plant operation anddesign life can usually provide reasonably accurate figures.Theoretical life cycle requirements and safety factors for

all expansion joints should be listed by the piping/stressengineers. In this way the supplier can recommend theuse of standard design convolutions where possible.

Examples are readily available where an "estimated"life requirement has been increased at each stage of plantdesign resulting in an unpractical an d costly specification.Discussion with experts can reduce this possibility. Twostandards, hinged or gimbal units, can replace an unwieldyTwin-Tied unit with a saving in transport cost. Fo r largediameter exhaust ducting two standard single units canoften replace a very long Twin.

Assuming that the basic information has been assessed,(diameter, pressure, temperature, media cyclic life, etc.)the selection of convolution material and type can begin.First consideration should be given to the general type ofunit to be used (i.e. restrained or unrestrained) . Thisconsideration is determined by a number of factors bu tthe main one is working pressure.

Fig. 1-Axial pressure balanced units designed to fit into atight system where space is restricted.

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Historically, externally reinforced comolutions wereused in medium/high pressure applications, but the useof this specific type of convolution is now restricted dueto the added complication of relying on rings positionedin the base of the convolution to resist the pressure withinthe system.

Examples of unit types catagorized by pressu;-(' areshown in Fig. 2, Table 1. .\ s a rule expansion j o i n ~operating at pressures below 2 K g j c m ~ are of the twinunrestrained type to accept combined axial and latera}movement. The Twin-Tied type units of two or three tiebars can be used for working pressures up to 10 Kg/cm2.Hinged and gimbal type units are used above this pressure. It is important to note that the Twin-Tied/Two-Tiebar unit has the added feature of allowing slight angulation (approximately 3 ) of the end flange in additirll1 tocombined lateral movement. This can be part:n; Ltrlyimportant in systems having elbows other than 90Cl orwhere the system angles slightly (i.e. the base of a tank).

Consideration of siting anchors is of importance as thiswill determine thermal movement an d divides the systeminto a number of simpler pipe runs. Normally majoritems of equipment such as pumps, compressors, heatexchangers, turbines etc. will be main anchors. It isessential that the movement, and therefore the !nJ.uing,is not "pushed" tm,ards these as they are the majorconnections to be protected within the system. Vnrestrained units tend to be cheaper than restrained ones.Restrained units do have the ad\antage of reducing thenumber of anchors and guides so the overall system maybe Jess expensive even though the individual expansionjoints are higher priced.In taking the basic steps described, systems will generally fall into a fairly small range. Involving e:-::J:msionjoint manufacturers at all stages of selection is bcne:lcialto a project.Materials. The. 300 e r i ~ of Austenitic stainless steelshave long b e ~ ~ a c ~ ~ p t ~ ~ f " ~ ~ forming convolution sections.Individual manufacturers choose the 321, 316 or 304types of material. This range, however, is prone to pittingand oorrosion c r a c k i n ~ ~ t e d . ~ r a ; t u r e : ; whenused in chio;iae or 'sulfur laden environments therefore it is essential to consider not only the medi: p:1'singthrough the expansion joint, but also the atrn,,sphcrraround the system.

Fortunately other materials are available for use inthese conditions, each with varying temperature limitations. lncoloy 825 is suitable for this environment up toa temperature of 4SOO C, and Incoloy 800 'is suitable fortemperatures around 800 C depending on working pressure an d movement.

I t is possible, in certain applications, to reduce pittingcorrosion by keeping the convolution temperature above t in adewpoint. Steam tracers can be used to mama .minimum continuous temperature. The additional cost !Soften offset by less use of expensive corrosion resistantmaterials.

March 1978 srr:GHYDROCARBON PROCES

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Fig. 2-Expansion joint types.

Installation techniques. All units arnvmg at a siteshould be fitted with external convolution protectionshrouds an d temporary sizing bars, whether suppliedfree length or cold set. End fittings should also be coveredto prevent entry of dirt.

Unloading of equipment should be done with care toensure that slings are not positioned around the convolutions or the protection shrouds. It is advisable to designpermanent lifting lugs, holes on flanges or restraintcarriers for units weighing in excess of 60 Kg. Thiswill reduce bad slinging and mishandling by slingingfrom tie bars. Ample labelling should be placed on allunits detailing lifting positions, temporary sizing bars,design drawing number, serial number an d the manufacturers name, address and telephone number.

I t is essential that full up-dated drawings of the expansion joints are held on site for easy reference. Th emanufacturer should be advised of any changes in siteoperating conditions that could effect expansion joints.All units should be installed with the convolutions protected an d the temporary sizing bars in place.

Not more than one unrestrained unit should be positioned between two anchor points. This rule virtuallyapplies without exception an d ensures that the systemdoes not become unstable due to pressure thrust on elbows

CommentsType of Normal Application (All units manufactured from series 300 stainless steels willexpansion pressure normally operate f rom- 160 to + 650 C and unitsjoint range manufactured from lnco/oy 825 from -160 to+ 450 C.)Axial

Twin

Two tierbartwin tied

Three tiebartwin tiedHinged

Gimbal

Pressurebalanced

A Full vacuum to Straight pipe runs, exhaust40 kg/em' depend- systems. Axial movement only.

B

cD

EF

G

H

ing on joint diameter

Full vacuum to2 kg/em'






Low pressure systems using acombination of axial and lateralmovement. Exhaust lines, lowpressure heating lines.Tank settlement and bulge andpipe growth. Lateral movementwith angulation of end flangesto absorb combinedmovements.Similar to two tie bar unit butsuitable for lateral movementonly. Tank settlement andbulge and pipe growth.Normally used in pairs orthrees on higher pressuresystems where guiding has tobe minimal.Used in pairs or threes on highpressure systems. Suitable forangular movement on anyplane.Fitted generally on pipe elbowsto balance both pressure thrustand spring rate. Particularlyuseful in conjunction withdelicate equipment such asturbines.

HYDROCARBON PROCESSING March 1978

This is the simplist type of expansion joint but other than withexhaust low pressure (max. 2 kg/em') applications to use itcorrectly it must have the pipe anchored and guided either sideof the unit. Anchors must be substantial enough to withstandthe axial spring rate and pressure end thrust. For examplea 150 mm NB bellows compressed to full movement at 21 kgproduces an anchor load of approx. 6 tonnes.Twin bellows can absorb lateral movement with a limited amountof axial movement. This type of unit transfers the pressurethrust into the pipework system and suitable guides areessential. Where la rge twin units are used, some additionalsupport of the center tube may be necessary.This is the cheapest form of restrained unit. Lateral movementwith angulation of end flanges to absorb combined movements.Pressure thrust contained within the tie bars with vacuumloadings controlled by the limit tubes on tie bars. Anchors aretherefore generally reduced or eliminated.These units absorb lateral movemen t on two planes but with noangulation of end flanges.

Suitable for angular movement in one plane only. These arevery robust units with hinges in double shear.

Particularly advantageous where combined large movementsare present as spacing of units can reduce angulation andtherefore increase life. Both gimbal and hinged units havelimit stops fitted, thereby eliminating the need fo r intermediateanchors.This type of unit has main advantages in tight pipework systemswhere combined movements are present and terminal loadshave to be kept low.

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II SELECT EXPANSION JOINTS PROPERLYetc; and that convolutions with lower spring rate are notsubjected to more than design movement.Anchors should be designed with a safety factor toaccept total loading due to the following:

1. The force required to compress or extend the convolution the calculated amount (i.e. spring rate timesmovement)

2. System working pressure times expansion joint effective cross section area (i.e. pressure thrust)3. Frictional resistance due to the total system guides4. Centrifugal thrust due to th e flow of media when the

anchor is situated at a pipe elbow.Generally items 1 and 2 are by far the largest. Inaddition to the above, anchors might have to be designed

to accept additional stresses such as shock loads, windloads, pipe/vessel weights and system pressure tests.Accurate guiding of the systems (Fig. 3) is essentialespecially where unrestrained units are used. Expansionjoint manufacturers can recommend a variety of simpleand planar guides. Each manufacturer will have specificrecommendations regarding position of these guides, bu tas a general rule, guides should allow sufficient clearancebetween pipe and fixtures to ensure positive guiding without excessive frictional loads.Position of pipe guides close to the expansion jointis most critical. The first guide should be located amaximum of four pipe diameters from the expansionjoint and the second a maximum of 14 pipe diametersfrom the first. Intermediate guides should be positionedwithin the maximum distances recommended by theexpansion joint manufacturer. Expansion joints shouldbe positioned at least four pipe diameters from valves,dampers, reducers etc. and not be used to correct pipemisalignment unless specifically designed to do so.Next is the installation of the expansion joint with thefollowing precautions: Protect convolutions from weld spatter by use of heatresistant materials or anti-splatter grease

Fig. 3-Wel l guided unrestrained single expansion joint show-ing the first guides close to the expansion joint.

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Fig. 4-Multi-Tie Bar tw:n unit fitted into a petrochemica' flaregas system.

Check that the flow arrm, marked on the unit coin.cides with the direction of the main pipe line flow Check that the insulation is chloride free and it doesnot come in contact with comolutions. This is pre\enteclby the fitting of external protection shrouds Check that the system is correctly filled and \cntcclin order to remove all air from the line before c o n t i n u i n ~site pressure tests Check that the convolution does not come in contactwith paint on the pipe end as this can cause corrosion. Zir11embrittlernent of stainless steel operating at ele\ated tcnrperatures is a known hazard.

After installat ion, th e unit should be chrckcdthoroughly for damage, particularly in the conmlutionarea. I f checks are satisfactory, the temporary in,tall.rtionbars can be removed.Cause of failures. Th e maJonty of site failures ocrurwithin the first few months of installation. A few of tl11causes are: Corrosion due to use of incorrect convolution material.resulting in cracking Incorrect installation, either due to inadequat': :L:rdwrsguides or th e unit being i m ~ a l l e d , due to pi;in;.: crro;.,with torsional twist. This will drastically reduce tile eye::life of any expansion joint Site damage to the comolution, another reason f,:the fitting of production shrouds

d. l Vibration in the system which has not been pre 1cte-c:Once these problems ha\'e been overcome, a multi-?:;bellows expansion joint normally gives long m ; ; i n t e n a n - ~

free life.The object of economic plant design should g e n ~ r a ~

be to design equipment for full plant life whether 11 .three or thirty years. Expansion joints are no e x c e p t ! ~ ~an d will give satisfactory life in the majority o.f a p P ~cations if the basic rules, mentioned in this arocle, .l l11obeyed.

March 1978 HYDROCARBON PRoct:ssJ!(':

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select expansion joints properly

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