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Carmagen was retained by a US-based Refiner to conduct a “Cold EyesReview” of their Cat Feed Hydrotreater (CFHT) reactor fouling and

pressure drop problems. High reactor Delta Pressure (DP) buildup requiredseveral premature shutdowns. In any large refinery, a premature shutdown toclean a unit or replace/skim reactor catalyst can cost hundreds of thousands ofdollars per day in lost revenues and expenses.

This unit processes about 50% coker products in the feed. It wasdesigned to operate at 45kB/D, and revamped several years ago to process70kB/D without adding preheat exchangers or paralleling the two-seriesreactors. Prior to the most recent cycle, DP was limiting across the first bed;therefore, the refinery removed the inter-bed quench and re-distributorassembly from the first reactor R1, but the unit DP problems persisted. Therewere attendant flow and temperature maldistribution problems in the reactor,with excessive DP in R2 and a hot spot in R1. Carmagen recommendedalternative solutions and general recommendations to eliminate thoseproblems. They included modifying the bed grading scheme with fewergradings and without the use of “virtual scale” baskets, reinstalling the firstreactor inter-bed assembly and including proprietary vapor and liquid bypasstubes for the top bed, reconfiguring the reactors for parallel operation andeliminating the multiple grading scheme, inspecting and leak testing theexisting distributor trays, and evaluating alternate disposition of coker naphthaand light coker distillate feeds. Implementing appropriate items above wouldbe necessary to achieve a target 3-year run.

The major fouling precursors are entrained small solid particles in thefiltered feed (smaller than 25 microns), plus dissolved chemical componentsthat react to form smaller and larger insoluble solid reaction products from ironnaphthenates, diolefins, and other reactive cyclic and aromatics with alkenesubstituents. Reactors loaded with the equivalent of 1/16 inch cylindrical- orshaped-catalyst extrudates will pass particles smaller than 150-200 microns,depending on the catalyst compaction, shape and length distribution.However, when the concentration of <25 microns solids gets too high, theywill also accumulate and cause DP in the catalyst bed, particularly in pockets

Fouling and Plugging CauseHigh Reactor Pressure

Drop and PrematureShutdown in

Hydroprocessing UnitsBy Joseph J. Kociscin

Continued on Page 5

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E-mail: carmagen@carmagen.com • Web Site: http://www.carmagen.com

JULY 2004

1 Fouling and Plugging Cause High ReactorPressure Drop and Premature Shutdown in Hydroprocessing Units

2 Flange Joint Assembly and Bolt-up Procedures

3 Flange Bolt Tightening Methods

4 All About Nickel Alloy Welding Electrodes

In the Next Issue:• Common Causes of

Piping Vibration• Common Causes of

Flange Leakage

The Carmagen Engineering Report © ispublished periodically by our staff andpresents information and viewpoints onengineering topics relevant to thehydrocarbon processing industry. While thecontents of The Carmagen EngineeringReport © have been carefully reviewed,Carmagen Engineering, Inc. does notwarrant it to be free of errors or omissions.Some back issues are available and may berequested while supplies last.

Editor ..................................... Lori CarucciWriters .......... Carmagen Engineering Staff

We welcome your comments andsuggestions for future editions. Pleasesend them to bmesa@carmagen.com.

All materials within this newsletter arecopyrighted by Carmagen Engineering, Inc.and cannot be used without the approval ofCarmagen Engineering, Inc.

The CARMAGEN ENGINEERING Report© 2

This article summarizes good flange assembly practicesthat may be used to help develop detailed flange bolt-

up procedures. Controlled-torque bolt-up procedures arethe methods of choice for achieving reliable bolt-up ofmost flanged joints in typical process plant services.However, the specific flange bolt-up procedure used (e.g.,hammer and wrench, controlled-torque, stud tensioner,etc.) depends on flange service and design conditions.

❖ Check studs, nuts, and flange/nut contact surfaces forcleanliness and burrs. Clean them using a wire brush.

❖ Check flange nut bearing surfaces. Clean the flangenut contact surfaces around the entire bolt circle usinga wire brush. Use a brush with stainless steel bristleson alloy surfaces. Ensure that these contact faces arefree of scratches, dirt, scale, burrs, and otherprotrusions. Remove defects by grinding.

❖ Uniformly lubricate the stud and nut threads on allcontact surfaces, including the nut bearing surface thatcontacts the flange.

❖ Check condition of flange faces. Clean gasket seatingsurface on flange face using a wire brush. Ensure thatthe surface is free from scratches, dirt, scale, remnantsof old gaskets, and other protrusions. For flanges thatare used in typical and critical services, ensure that theflange face at the OD is parallel to the gasket contactface by measuring the gap between them using a steelruler. The gap should not exceed 0.010 in. (0.25 mm)at any point.

❖ Check flange-to-flange alignment to verify that it iswithin the specified tolerances. Excessive flangemisalignment (especially lack of parallelism) increasesthe likelihood of in-service leakage.

❖ Check flange faces for proper gasket insertion gap.The gap between flanges should be just sufficient toallow for gasket insertion. Excessive gap will result inneeding to force the flanges together, which willincrease the likelihood of in-service leakage.

❖ Install studs in the lower half of the flange to supportthe gasket when it is inserted.

❖ Inspect the gasket to ensure that it is in accordancewith the specification and free from defects.

❖ Insert the gasket between the flanges and ensure itsproper placement (i.e., centering in the joint), takingcare not to damage the gasket. If necessary to usesomething to hold the gasket in place, a light spray ofadhesive can be used. Alternatively, thin cellophane or

masking tape may be used on the outside edge of thegasket, with enough material protruding to allowremoval during the initial tightening process. Tapeshould be located to avoid contacting the flangeface/gasket seating surfaces since this could provide aleak path during operation.

❖ Install remaining studs and nuts and ensure that thereis complete thread engagement in both nuts. Usehardened steel washers if the studs are 1-1/2 in. (38mm) diameter and above if the studs are to be torqued.If a bolt tensioner will be used, the stud shouldprotrude beyond the nut by at least one (1) boltdiameter on one side only in order to permitattachment of the tensioner head.

❖ Mark bolting sequence numbers and reference boltlocations on the flange OD. Use a criss-cross bolttightening sequence (like tightening the wheel on acar).

❖ In all cases, tightening should proceed in stages (i.e.,not to the maximum stud stress at one time), andproceed in a criss-cross pattern. This helps ensureuniform stud load and gasket compression around theflange circumference.

The use of appropriate flange assembly and bolt-upprocedures will eliminate many flange leakage problems.Go “back to the basics” first when dealing with suchproblems. �

Flange Joint Assembly and Bolt-up Procedures

By Vincent A. Carucci

The CARMAGEN ENGINEERING Report© 3

The selection of the proper flange bolt tightening techniquerequires experience and good engineering judgment. The

successful application of any technique also requires

qualification of both the tools that will be used and the crew whowill do the work. The following table summarizes the mostcommonly used flange bolt tightening techniques.

Abolt-up procedure should be developed for each techniqueto be used, even the “old reliable” manual wrench and

hammer. A separate procedure should be developed for eachmanufacturer and model of hydraulic or pneumatic tools thatmay be used.

In concept, a hydraulic torque wrench is simply aconventional wrench which has been modified such that ahydraulic cylinder pushes on the end of the wrench handle.

Hydraulic bolt tensioners employ a high-pressure hydrauliccylinder that attaches to the stud that is to be tensioned, and thenstretches the stud directly. The nut is then tightened by handusing a short bar or bevel gear arrangement. When the hydraulicpressure is released, the load is then transferred to the nut, whichthen maintains load on the stud and compression of the gasket.

When a torque wrench is used for flange bolt-up, it isnecessary to have an approximate torque value that must beapplied to achieve the required preload stress for standard size

bolts. Many unknowns, including the amount and type oflubrication, affect the actual torque needed to obtain the desiredbolt stress. The only reliable way to determine bolt stress is tomeasure bolt elongation during procedure qualification, and thento adjust the torque values as necessary, to achieve the desiredbolt preload stress.

The majority of flange joints in process plants areassembled based on achieving a 50 ksi (345 MPa) averagepreload stress using ASTM A193/A193M Grade B7 or B16bolts. Lower bolt stresses are appropriate for some combinationsof flange type, rating, and bolting material. This is necessary toavoid damage to the flange, gaskets, or bolts (e.g., ring jointflanges, flange rating Classes 900 and higher, austenitic stainlesssteel bolts, etc.).

Selecting the appropriate bolt-up method, and havingtrained crews using documented bolt-up procedures, will helpachieve leak-free flanged joints. �

Flange Bolt Tightening MethodsBy Vincent A. Carucci

MethodManual Wrench

Impact Wrench

Hammer Wrench

Hydraulic Torque Wrench

Manual Beam and Gear- Assisted Torque Wrench

Hydraulic Bolt Tensioner

ProsReadily available, easy to use

Fast, easy to use

• Can tighten larger studs than manual wrenches

• Readily available

• Cost effective

• Reasonable level and uniformity of preload achievable

Readily available, easy to use

• Most accurate andachieves uniformpreload

• Eliminates gallingproblems

Cons• Wide variation in stud prestress

• Limited to relatively small diameter studs [≤ 1 in. (25 mm)]

• May overstress small studs

• Final torque level not adjustable and may vary

Prestress depends on accessibility of stud andexperience of mechanic

Higher initial purchase cost than conventionalwrenches

May be difficult to use when clearances are small

• Different set of tensioner heads required foreach stud size

• Sometimes not enough clearance around nutsto install tensioner heads

• Must remember to order longer studs to permitattachment of tensioner heads

• Not useful for hot bolting since hydraulic sealscan rapidly overheat

The CARMAGEN ENGINEERING Report© 4

When discussing Ni alloy filler metals frequently used forpiping and pressure vessels by refineries, chemical plants

and power plants, one must start with dissimilar joints, such aswelding 300 series austenitic stainless steels to carbon and lowalloy steels, and also consider weld overlay operations. Initially,type E310 (~25%Cr-20%Ni) electrodes were used for suchapplications. They were easy to use, had weldor appeal and theamount of dilution was not very critical. However, many suchwelds failed in service since the inherent micro-fissuring of thesefully austenitic deposits propagated into cracks when subjectedto thermal stresses caused by the large differences of Coefficientsof Thermal Expansion (CTEs). By replacing the E310 withE309 (~23%Cr-13%Ni), a stainless steel with some ferrite, themicro-fissuring problem was reduced or even eliminated. Thesejoints are more dilution sensitive. Since they also retain the largedifferences in CTEs, users were concerned with high stressesand possible thermal fatigue along the ferritic to austenitic steelfusion line when the weld was subjected to a heat treatingoperation and/or to high temperature (>320ºC [>600ºF]) service.

The introduction of 600 Series Ni-alloy filler metals(~72%Ni, 15%Cr & 8%Fe), which are also known asINCONEL®, have a CTE about halfway between ferritic andaustenitic steels. This reduces the thermal stresses by dividingthem between two fusion lines. They are also less sensitive todilution problems and micro-fissuring. In the discussion of theseNi-alloys, trade name designations and AWS/ASMEClassification have been used. When the latter start with an “E,”they refer to coated electrodes for the SMAW process; whenthey start with “ER,” they refer to rods and bare wire used forinert gas and submerged are welding processes.

The use of Inconel filler materials started with INCO-WELD-A® (ENiCrFe-2) and INCO-ROD-A®, now calledINCONEL® 92 (ERNiCrFe-6). While both of these materialsfulfilled their intended objective, both presented some newproblems. The coated electrode had little weldor appeal since itspuddle was not very easy to control; this has since beencontrolled by a modification of the secondary chemicals and theintroduction of INCONEL® 182 (ENiCrFe-3). The compositionof the INCONEL® 92 bare wire made the deposit subject to agehardening when exposed to heat treatment or servicetemperatures ≥700ºC [1300ºF], which increased strength butdecreased ductility. For most applications, this wire has beenreplaced by INCONEL® 82 (ERNiCr-3), which does not ageharden.

Weld deposits containing high Ni to Cr ratios are moresusceptible to sulfur corrosion when subjected to temperatures>370ºC [>700ºF]. This ratio and the risk of sulfur corrosionhave been lowered by selecting alloys that contain more Crand/or some Mo such as Alloy 671 with ~44% Cr (ERNiCr-4)and Alloy 625 with ~22%Cr & 9%Mo (ENiCrMo-3) and(ERNiCrMo-3). However, at the present time, the Alloy 671 hasonly been AWS/ASME classified as a bare wire filler metal andthe Alloy 625 filler metals should not be used for service attemperatures >540ºC [>1000ºF], since the deposits tend to

embrittle with time. For applications up to 1000ºC [1830ºF],Alloy 617 with Co addition (ENiCrCoMo-1 & ERNiCrCoMo-1)has been developed.

Over 60 Ni-alloy filler metals have been classified byAWS/ASME and more are pending. Many are designed to meetspecific or special requirements that do not usually apply topiping and pressure vessels. In addition to some of the 600series alloys, two other types are of concern when dealing withrefineries, chemical plants and utilities.

800 Series Ni-alloys, also known as INCOLOYs®

(~33%Ni, 21%Cr & Fe), have a number of base metalapplications. Since AWS/ASME has not classified a matchingfiller metal, these alloys are usually welded with one of theINCONELs®, which are quite compatible. However, in Europe anumber of Alloy 800 type filler metals have been developed andare accredited by some regulatory agencies.

400 Series Ni-alloys, also known as MONELs®

(~ 65%Ni & 30%Cu) are provided with matching filler metals.After years of development, we are now using the 7thcomposition (ENiCu-7 & ERNiCu-7) to weld Alloy 400 to itselfand to steels and other nickel alloys. However, here we mustprovide a word of caution. One supplier uses the term MONEL®

for two quite different alloys. In addition to Ni-Cu alloymentioned above, this supplier also uses it for a copper nickelalloy (~70%Cu & 30%Ni) which AWS/ASME classifies asECuNi and ERCuNi. To prevent mix-ups, it is suggested to usethe applicable AWS/ASME classification, or renaming the fillermetals “NiCu-MONEL” and “CuNi-MONEL.” �

All About Nickel Alloy Welding ElectrodesBy Harry W. Ebert, PE, FAWS

TYPICAL or NOMINAL CHEMICAL COMPOSITION

of FILLER METALSmentioned in this article

FILLER METALS % Ni % Cr % Mo % Other

E & ER309 13 23 – –

E & ER310 20 25 – –

ENiCrFe-2 65 15 – Mn : 2

ERNiCrFe-6 65 15 – Ti : 3

ENiCrFe-3 65 15 – Mn : 7

ERNiCr-3 65 20 – –

ERNiCr-4 55 44 – –

E & ERNiCrMo-3 65 22 9 –

E & ERNiCrCoMo-1 50 23 9 Co : 12

E & ERNiCu-7 65 – – Cu : 30

E & ERCuNi 30 – – Cu : 70

The CARMAGEN ENGINEERING Report© 5

Continued from Page 1: Fouling and Plugging Cause ...

of higher density or lower void fraction than the bulkcatalyst average.

In addition to the fouling and plugging work, wewere asked to conduct a technical comparison of newdistributor trays being considered to replace the existingbubble cap trays. During the last 10 years, manyhundreds of commercial sieve distributor trays, tube andchimney distributor trays were replaced with the newgeneration distributor trays, which resulted in activitycredits from 10 to 35%. Similar significant credits werealso achieved when old leaky trays were repaired.Vendors and licensors of the new generation vapor/liquiddistributor trays take advantage of the kinetic energy ofthe vapor (or treat gas) flowing through the tray tubes ornozzles. Various devices are used (e.g., the spiral mixing

type in the VENDOR A design, the entrainment or liquidlifting type of “U-Tubes” in the VENDOR B design, andthe downcomer tubes with holes and slots to generate amist of small droplets that sprays out of the nozzle as inthe VENDOR C and VENDOR D designs, and the pigtailspray nozzle tray arrangement in a second VENDOR Cdesign). In my opinion, these vapor-assisted, liquiddistributors are superior to the simple multiple short andlong tube designs, the sieve tray designs, and the reverseflow bubble cap design. However, the new and someexisting reverse flow bubble cap designs, with a smallerpitch, seem to be doing quite well. In many cases, theredoes not appear to be any big performance differenceamong the new tray designs, but the cost difference couldbe significant. �