7/27/2019 wj0909-38

1/4

During the planned testing and in-spection of one of the Saudi

Aramco refineries, fine radialcracks were observed around the circum-ference of the channel flange face of theeffluent/feed heat exchangers of the hy-drocracker unit. The hydrocracker plant

in one of the Saudi Aramco refineries has13 high-pressure effluent/feed heat ex-changers. Ten of the 13 exchangers arestacked in pairs. These exchangers are theshell and tube type. Operating tempera-ture varies from 500 to 800F for the tubeside and 300 to 700F for the shell side.The operating pressure varies from 2150to 2190 lb/in.2 for the tube side and from2230 to 2290 lb/in.2 for the shell side. Theshell side fluid is vacuum gas oi l (VGO)for 11 exchangers and hydrogen for theremaining two. The tube side fluid is amixture of VGO and hydrogen gas for all13 exchangers. These heat exchangers aremade of either 1.25%Cr-0.5%Mo or2.25%Cr-1%Mo steel. The tubesheet,channel, and channel flanges are clad with347 stainless steel. Wall thickness of theshell varies from 68 to 136 mm and thethickness of the channel flange at the rim

varies from 175 to 225 mm. These flangeswere supplied in the quenched and tem-pered condition.

The channel flange is clad at the gas-

ket face area with 309L stainless steel asa barrier layer followed by 347 stainlesssteel. A diaphragm plate (gasket) madeof 321 stainless steel or chrome-moly steelis fillet welded on the stainless steelcladding. On top of the gasket, the chan-nel cover is bolted to the channel flange.

This arrangement is shown in Fig. 1. Thepurpose of the stainless steel cladding isto avoid welding of the diaphragm platedirectly to the air-hardenable, low-alloysteel flanges as this would require post-

weld heat treatment every time the di-aphragm is removed and rewelded. De-tail designing and fabrication of these ex-changers is by Belleli, Italy.

New exchangers were received in 1997with welded diaphragms installed by themanufacturer. In 1998, during precom-missioning, some of the exchangers devel-oped minor internal leaks. This necessi-tated removal of welded diaphragm gas-kets. Due to the nonavailability of an au-tomatic cutting machine, a manual grind-ing machine was used for the removal ofthe gaskets. Improper grinding resultedin numerous deep grinding grooves closeto the gasket facings of some of the ex-changers. Due to frequent shutdown andstart-up cycles, these exchangers begandeveloping leaks in the welded diaphragmareas. Frequent gasket removal caused the

outside of the diaphragm cladding to be-come irregular and subsequent installa-tion required a larger fillet weld. Thismade subsequent removal more difficult.

It was planned in 2005 to repair theabove-described grooves during a 2007shutdown.

Cracks on the Gasket Face

On removing the gasket, cracks wereseen on the edges of the stainless steel cladgasket face. On machining, several cracksappeared on the machined surface. Thecracks were in a radial direction andspread all around the circumference ofthe channel flange face. Some of thecracks were as long as 125 mm and ex-

tended deep into the stainless steelcladding. The cracks are shown in Fig. 2.Several tests were done but the cause ofthe cracks could not be ascertained con-clusively. Chemical analyses were done onthe chips taken at various depths and theresults are shown in Table 1. Delta ferritecould not be calculated from the chemi-cal analyses because the chips could notbe analyzed for carbon and silicon. Deltaferrite on the cladding was not directlymeasured but the morphology of thecracks is not suggestive of solidificationcracking due to low ferrite. Hence the pos-sibility of these cracks resulting from lowferrite in the weld metal and being pres-ent since the exchangers were first manu-factured is ruled out.

Several replicas were taken in the lo-cation of the cracks, and it was seen thatthe cracks were both transgranular andintergranular. The replica shown in Fig. 3reveals the intergranular nature of crack-ing. The replica shown in Fig. 4 showstransgranular cracks with extensive

SEPTEMBER 200938

Repairing Cracksin Refinery Heat

Exchangers

SANYASI RAO (sannyasi.rao@aramco.com), DENNIS C. NIEMEYER (dennis.niemeyer@aramco.com), and NABEEL S.AL-BANNAI (nabeel.bannai@aramco.com) are with Saudi Aramco, Dhahran, Saudi Arabia.

In-situ postweld heat treating requires

considerable planning to develop adetailed procedure and determine the

number of power sources required

BY SANYASI RAO, DENNIS C. NIEMEYER, AND NABEEL S. AL-BANNAI

7/27/2019 wj0909-38

2/4

39WELDING JOURNAL

branching suggestive of environmentalcracking. This crack morphology could beconsistent with chloride stress corrosioncracking or caustic stress corrosion crack-ing. Although chloride concentration ofthe stream is low, concentration of chlo-rides is possible at the crevice between thegasket and flange. Similarly, caustic con-centration can result from neutralization

by soda ash washing that is done to pre-vent polythionic acid stress corrosioncracking. Crack surfaces were not ana-lyzed to detect the presence of sodium andchloride ions. However, cracks were foundpredominantly underneath the stainlesssteel cladding that is not exposed to anyenvironment.

Hard phases like martensite are knownto form in the unmixed zone at the inter-face of low-alloy steel and austenitic stain-less steel cladding. The presence of hardphases in addition to a difference in coef-ficient of thermal expansion between thelow-alloy steel and austenitic stainlesssteel cladding could be the cause of cracks.

Weld Repair

Although a detailed fitness-for-serviceevaluation was not carried out, it was feltthat the size, number, and distribution ofcracks were too severe to leave the cracksunrepaired. Hence, it was decided to re-pair all the cracks. Several initial repair at-tempts were made that resulted in crack-ing both in the weld metal and heat-affected zone Figs. 5, 6. The final repairsequence that was established follows.

Dehydrogenation

Because these heat exchangers hadbeen in hydrogen service, the channelflanges of the exchangers were first dehy-drogenated by soaking them at 350 to400C for about 4 h. This operation tookseveral hours due to the heavy thickness

of the equipment (rim of the flange variesfrom 175 to 225 mm thick).

Complete Removal of the Cracksand Original Cladding

After dehydrogenation, the entire weldcladding was removed by in-situ machin-ing Fig. 7. Complete removal of cracks

was assured by dye penetrant test. It wasfound that if a weld was deposited overthe original cracks, the cracking couldpropagate through the new cladding.

Welding Procedure

The welding process selected wasshielded metal arc welding (SMAW). Theinitial welding process used was gas tung-sten arc welding (GTAW), but the SMAWappeared to be less susceptible to crack-ing in this case and was more appropriatefor cladding. The welding electrode rec-ommended for all of the layers was ENi-CrFe-3. The preheat used was 200C. Nointerruption in welding or preheat waspermitted until the completion of thecladding. Postheating was recommendedat 350C for 2 h after completion of weld-

ing and during any interruption in weld-ing. It was required to cover the flange

with an insulating blanket after postheat-ing. Postheating and covering with an in-sulating blanket after postheating was rec-ommended to prevent hydrogen-induced cracking. The PWHT cycle was

kept unchanged. However, it was man-dated to carry out PWHT after comple-tion of the barrier layer without allowingthe job to cool below 200C (preheat tem-perature). The rest of the layers were tobe welded after completion of PWHT and

without any preheating as welding was nowto be done on the Inconel barrier layerand not on the chrome-moly steel. Rec-ommended PWHT temperature was690C, which is 30C below the temperingtemperature of the flange. This was essen-tial to prevent any significant changes inmechanical properties. Dye penetrant test-ing was required after PWHT and before

Fig. 2 Cracks on the stainless steelcladding on the flange face.

Fig. 1 Channel flange design.Fig. 4 Replica showing branching cracks(200 ).

Fig. 3 Replica showing intergranularmode of cracking (200 ).

Fig. 5 Crack in the weld during initialrepair.

Fig. 6 Crack in the heat-affected zoneduring initial repair.

7/27/2019 wj0909-38

3/4

SEPTEMBER 200940

depositing further layers. This was to en-sure that there were no cracks before pro-ceeding with the second layer. Welding

with this procedure did not result in anycracking on the weld or heat-affected zone.

The welders found it difficult not toweld on the chrome-moly steel when de-positing the second layer of cladding.Since welding directly on the chrome-moly steel required re-postweld heattreating, the WPS was revised to complete

welding all layers before carrying outPWHT. Moreover, all the layers were tobe welded with the same preheat (200C).This would ensure that any welding onchrome-moly steel at any stage would be

with the required preheat and it wouldcertainly undergo PWHT.

Welding as per the above procedure

did not result in any cracking and the same

procedure was employed on all theexchangers.

Postweld Heat Treatment

The chrome-moly channel flanges hada thickness of 175 to 225 mm at the rim.

Any welding on this thickness requiresPWHT per the requirements of the

ASME Boiler and Pressure Vessel Code ,Section VIII Division 1. However, in-situPWHT of heavy wall thickness is difficultand requires considerable planning. Al-though cracks were not anticipated, re-pair of grooves made by grinding requiredPWHT. Hence, the PWHT procedure wasprepared well in advance. The PWHTprocedure required heating only on thechannel by placing heating elements

around the rim of the flange as shown in

Fig. 8. For the five pairs of stacked ex-changers, it was required that PWHT becarried out simultaneously in order to

allow simultaneous expansion and con-traction. Interconnecting pipe betweenthe two exchangers was also prepared forheating. The temperature was to be mon-itored during PWHT and, if required, it

was to be heated.During the course of repair, it was re-

alized that the PWHT procedure was notadequate to ensure structural integrity ofthe channel and associated piping. Al-though the primary aim of PWHT is toheat treat the flange face, confining theheating to the rim of the flange as shownin Fig. 8 and with the channel being con-

structed of a heavy wall thickness, thechannel would experience a high magni-tude of thermal stresses. Moreover, thetwo heavy nozzles on the channel, as canbe seen in Fig. 8, would experience a tem-perature gradient across the nozzles,

which would result in bending stresses onthe nozzles and the connecting piping.This can lead to distortion and cracks. Inorder to overcome the problem, the fol-lowing major changes were made to thePWHT procedure Fig. 9:

1. Two more heating zones were addedto the procedure. Two 300-mm-wide

bands of heating elements that centeredon the outer edges of the two nozzles wereadded. The temperature of these twobands was required to be maintainedat about 75C lower than the PWHTtemperature.

2. Heating of the two nozzles by wrap-ping the heating element around them wasadded. The temperature of these twobands was also required to be about 75Clower than the PWHT temperature.

Due to additional heating require-ments, four to six power sources of 50/65kVA each were required for the PWHTof one exchanger. Since the stacked ex-

changers were to be subjected to PWHT

Fig. 7 In-situ machining. Fig. 8 Heating arrangement for preheating and PWHT.

Fig. 9 Modified heating arrangement for PWHT.

7/27/2019 wj0909-38

4/4

simultaneously, 8 to 12 power sourceswere required at a time. Moreover, if pre-heating and welding of other exchangers

was not to be held up while PWHT wasbeing done for the completed exchangers,more power sources would be required.

The two nozzles on the channel werecalculated to move 10-mm vertically andlaterally back, also by 10 mm. The follow-ing further precautions were necessary toensure structural integrity of the exchang-ers during in-situ PWHT:

1. Bolts for the front and middle sad-dles were loosened to allow movement ofthe exchangers due to thermal expansion.

2. Pipe anchors for channel outletpipes, wherever present, were cut andremoved.

3. Because hard refractory insulationon the channel outlet pipe was touchinga concrete pillar, the insulation was re-moved to allow free movement of the pipeduring PWHT.

4. One of the exchangers had a 12.5-

mm-thick 321 stainless steel pass partition(PP) plate welded to the chrome-molychannel. It was calculated that due to thedifference in the coefficient of thermal ex-pansion between the chrome-moly chan-nel and stainless steel PP plate, the PPplate would expand about 12 mm morethan the channel. This would result in dis-tortion of the PP plate. Hence, a 20-mm-

wide slot was cut at the center along thewidth of the PP plate toward the fixedtubesheet. This was repaired by weldingafter completion of PWHT.

Conclusion

Any extensive in-situ repair of heavywall thickness equipment made of air-hardenable steels is difficult and time

consuming. A detailed welding proceduremust be prepared for the repair. It mustallow for a dehydrogenation treatment ifapplicable. In-situ PWHT requires con-siderable advanced planning with respectto development of a detailed procedureand number of power sources required.The PWHT procedure must be designedto avoid steep thermal gradients and

allow for free movement of the equip-ment during heating and cooling. All ex-isting restraints must be identified andeliminated to ensure structural integrityof the equipment.

References

1. Bagdasarian, A. J., and Truax, D. J.Chloride stress corrosion cracking ofaustenitic stainless steels in hydroprocess-ing units.Material Performance, Paper No.97501.

2. NACE RP0170, Protection ofAustenitic Stainless Steels and Other

Austenitic Alloys from Polythionic AcidStress Corrosion Cracking during Shutdown

of Refinery Equipment.3. API RP 571, Damage Mechanisms

Affecting Fixed Equipment in the RefiningIndustry.

4. NACE RP0296, Guidelines for De-tection, Repair, and Mitigation of Cracking

of Existing Petroleum Refinery Pressure Ves-sels in Wet H2S Environments.

5. Welding Handbook, 8th Edition, Vol.4. 1998. Materials and Application Part2. Miami, Fla.: American Welding Soci-ety.

6. ASME Boiler and Pressure Vessel

Code, Section VIII, Division 1, AppendixR, Preheating. New York, N.Y.: ASME.

Acknowledgments

The authors would like to thank SteveKim, Don Hixon, and Mohammed S.Mashouf for their help extended during

these repairs and collecting plant data.

Reprints are ideal for:

I PR Materials and Media Kits

I Direct Mail Enclosures

I Trade Shows/Promotional EventsI Conferences/Speaking Engagements

I Recruitment and Training Packages

I Customer and ProspectCommunications/ Presentations

REPRINTS

For additional information, please contactFosteReprints, the official reprint provider

for Welding Journal.Email: sales@fostereprints.com

or call 866-879-9144

SM

Reprints of Welding Journal are a simple way

to put information directly into the hands of

your target audience. Having been featured in a

well-respected publication adds the credibility

of a third-party endorsement to your message.

Take Advantage of your

Editorial Exposure