1996: Failure, Repair and Replacement of Waste Heat Boiler...The waste heat boiler (WHB) (E-208A/B) installed downstream of the secondary reformer is a double compartment design (namely,

Failure, Repair and Replacement ofWaste Heat Boiler

Waste heat boiler failure, subsequent repairs to enhance the reliability of the unit, and designimprovements made to the new WHB are addressed.

Shahid Ahmed and Zaheer AnwarFauji Fertilizer Co. Ltd., Goth Machhi, Pakistan

Introduction

The waste heat boiler (WHB) (E-208A/B)installed downstream of the secondary reformeris a double compartment design (namely, A and

B), separated by an intermediate chamber. An internalbypass control system in compartment B maintains aconstant outlet temperature. The boiler in horizontallayout is a fire tube type with natural circulation andconnected with an overhead steam drum through asystem of risers and downcomers. The design incorpo-rates thin stiffened tubesheets with a tube to tube-sheets weld joint with a full penetration configuration.The inlet channel, E-208A inlet tubesheet, and cylin-drical surface of the intermediate chamber, exposed toprocess gas, are refractory lined (Figures 1-3).

Problem Description

The WHB was commissioned in 1982 and providedreliable service until 1990. No defect or leakage wasever found in E-208A during annual turnaround

inspections and hydrostatic tests. However, on E-208B, one tube weld was found leaking in a hydrostat-ic test during the 1984 turnaround inspection. Thistube weld along with a ligament crack kept on reap-pearing in subsequent inspections but without affect-ing the WHB reliability and performance (Table 1).

In January 1990, there was a total power failure atthe plant interrupting the boiler feed water supply tothe WHB. The plant tripped on steam drum low levelsecurity, and as part of the trip logic, protection steamopened to the air preheat coil in the primary reformerconvection section. This protection steam carried theheat from the secondary reformer catalyst to the WHB.The steam dram pressure control valve was on manualmode (because of some mechanical problem) andcould not be closed immediately. Consequently, thesteam generation continued even after the tripping andlead to the total loss of level in the steam drum. Theloss of water level in the WHB could not be ascer-tained at that time, because the level measuringdevices for the system are installed on the overheadsteam dram. There was no immediate indication of the

AMMONIA TECHNICAL MANUAL 100 1997

damage done to the WHB, but subsequent tube totubesheet weld failures in compartment A confirmedthe water level loss in the WHB.

Repair History

From the boiler feed water failure in January 1990to the replacement of the boiler in October 1994, wehad four shutdowns and two turnarounds of extendeddurations to repair the WHB. These failures resulted ina production loss of 91 days.

The first WHB leakage was attended in April 1990,three months after the boiler feed water failure. Thesecond repair on the WHB was carried out during theMarch 1991 turnaround. The second repair lasted onlyfour months and* the plant had to be shut down in July1991 to attend to the WHB (Table 2).

Nature of defects

The defects observed during the repairs were:• Visible cracks in the tube to tubesheet welds and

ligaments (Figure 4).• Microcracks on the tube to tubesheet weld, tube

holes, ligaments and welding lips (observed with a wetfluorescent magnetic particle test - WFMPT).

• Seepage and leakage from the tube welds duringhydrostatic tests.

Repair methods

The repair methods used to rectify the above men-tioned defects were:

• Exploratory grinding and rewelding.• Replacement of affected tubes with new ones.• Rehabilitation of tubes by adding new pieces of

about 400-500 mm in length. (The inlet channelrefractory design would not allow the complete pullout of the tubes located at the periphery of the tube-sheet.)

The repeated failures indicated a precarious condi-tion of the WHB and the situation demanded a clear-cut plan for future improvement of WHB reliability.

Reliability Improvement Measures

In-house discussions, experiences of other plantswith WHB leakages and repairs, and inputs fromHaldor Tops0e and the original equipment manufac-turer lead us to consider the following options forimproving WHB reliability:

Option 1: Partial retaking. Retubing and rehabilita-tion of the upper portion of the tubesheet (about 100-150 tubes).

Option 2: Retubing and partial replacement oftubesheet. Retubing and replacement of affected seg-ment of the tubesheet.

Option 3: WHB replacement. Replacement of E-208 A or E-208A and B.

The rationale for consideration of options 1 and 2were: (1) all tube weld failures experienced until thattime were confined to the upper portion of thetubesheet; (2) the upper portion of the tubesheet wasthe most susceptible area hi the case of a partial levelloss in the boiler.

The partial retubing of the WHB was subject to themetallurgical health of the tubesheets material. Toascertain the health of the tubesheets, a hardness testand a metallography replica (from an independentagency) were carried out.

Hardness measurements on the tubesheet surfaceswere within the range of 130-175 HB, typical for 13CrMo44 material, corresponding to the strength of 440-590 N/mm*.

Metallographie analysis

A replica examination on both of the tubesheets, oncracked ligaments, and in bores was carried out. Themicrostracture observed in all the positions consistedof ferrite and pearlite with some degree of spher-oidization but only to an extent that could be expectedfrom the material in service for the last ten years. Theconclusive statement of the report was that themicrostructure of the steel had not been seriouslyaffected, and there were no signs of severe overheatinghaving occurred in the partial dry run of the boiler.

Two tube samples (removed from both ends of E-208A) were also analyzed, but no evidence of metal-lurgical damage was found.


OUT

Figure 1. WHB E-208A/B.

BEFORE WELDING

TUBE BORE DIA«, j *t.S

.TUBE SHEET lOCOUM

AFTER WELDING

Figure 2. Inlet tubesneet E-208A. Figure 3. Tube-tubesheet joint.


Table 1. Repair History E 208 A

DATE

APR. 1990

TEB. 1991

TURNAROUND

HJL. 1991

FEBJMAR. 1992

TURNAROUND

»KC. 1992

OCT. 1993

OBSERVATIONS

- LEAKING TUBEWELDS.

- MICROCRACKS.


- MtCROCRACKS.


- MICROCRACKS.


- MICROCRACKS.

- LEAK! NO TUBEWELDS.

- MICROCRACKS.


- MICROCRACKS.

DETECTIONMETHOD

- VISUAL

- WFMPT

- HYDRO-STATIC TEST

- WFMPr

- VISUAL

- HYDRO-STATIC TEST

-WFMPT

- VISUAL.

- HYDRO-STATIC TEST

- WFMPT

- VISUAL.

- WFMPT

- VISUAL.

- HYDRO-STATIC TEST

- WFMPT

LOCATION

1M.ETTUBESHEET

12

01

05

-

06

01

12

08

15

05

01

32

OUTLETTUBESHEET

-

-

07

01

23

13

09

57

03

40

05

07

01

NO, OF TUBES

REPLACED

4

02

51

15

31

REHABILITAT™

g

50

16

20

NO. OFTUBE

WELDS

REPAIRED

01

08

3»

01

04

PRODUCTIONLOSS

NO. OF DAYS

13

23

16

33

20

21

REMARKS

FIRST REPAIR

SECONDREPAIR

THIRDREPAIR

FOURTHRF.PAIR

FIFTH REPAIR

SIXTHREPAIR

Table 2. Repair History E 208B

DATE

1984TURNAROUND

1985TURNAROUND

1986TURNAROUND

1987TURNAROUND

1988TURNAROUND

1989TURNAROUND

APR. 1990

FF.B. 1991TURNAROUND

JUL. 1991

FEB./MAR. 1992TURNAROUND

DEC. 1992

OBSERVATIONS

LEAKING TUBEWELD

- WATERMARKS

- NO DEFECT

- NO DEFECT

. WATERMARKS

- MICRO CRACKS

- CRACK

(12 MM LONG)

- MICRO CRACKS

- MICRO CRACKS

- MICRO CRACKS

- MICRO CRACKS

DETECTION

METHOD

HYDRO-STATIC TKST

DPT

DPT

DPT

HYDRO-STATIC TEST

DPT

DPT

WFMPT

WFMPT

WFMPT

WFMPT

LOCATION

INLETTUBESHEET

-

-

-

-

-

-

•

*

-

01

OUTLETTL'BESHCET

01

01

-

-

-

-

02

03

03

NO. OF TUBES

REPLACED

•

-

-

-

-

-

"

-

-

REHABILITATED

-

-

-

-

•

-

03

-

NO. OFTUBE

WELDSREPAIRED

01

01

•

-

-

01

01

02

03

-

01

REMARKS

ONE TUBE WELD REPAIRED SiLIGAMENT FOUND WITH MICROCRACKS. (REPAIRED IN 1984).

-

-

NO LEAKAGE OBSERVEDDURING HYDROSTATIC TEST.

TUBE WELD St. LIGAMENTREPAIRED IN TA-1 985.

TUBE WELD REPAIRED IN 1989

-

ALL THE THREE TUBESPLUGGED (REPAIRED IN FEB. 91AND APR. 90).

-

LIGAMENT WITH MICROCRACKS REPAIRED.

M&CWO!tK«?lll5T1IJX>C


CRACK IN TUBE TU TÜBËSHËËT WELP

LIGAMENT CRACK M TOSESHEET

Figure 4. Visible cracks in the tube to tubesheetwelds and ligaments.

Microcracks phenomenon

The other major concern in partial retubing was theappearance of microcracks before and sometimes afterthe repair work, observed with a wet fluorescent mag-netic particle test. The nature of these cracks can becategorized as:

• Cracks in the narrow ligament between the tubeholes.

• Cracks in the bores and welding lips perpendicularto the tubesheet surface.

• Transverse cracks in the tube to tubesheet welds.• Circumferential cracks in the tube to tubesheet

welds.The replicas of the cracked area showed minor

cracks parallel to the major crack. All the minor crackswere characterized as being intergranular cracks. Allthe observed cracks were located in the repair weldedarea, and no crack was found in the base metal.

These microcracks were attributed to the presence ofhydrogen in the tubesheet material. During welding,this entrapped hydrogen was released and entered intothe weld deposits resulting in the appearance ofcracks. Based on this hypothesis, prolonged heating(at 200°C) of the weld joint and the surrounding areawas attempted without success (that is, the appearanceof microcracks could not be eliminated).

Welding procedure

The welding procedure was reviewed and the postweld heat treatment was eliminated after taking hard-ness measurements on the weld deposits, which werefound within the specified range for the material.

The reported results of replica metallography andhardness test on the tubesheets promised a reliablerepair with a partial retubing option. Based on thisanalysis, Option 2 was dropped. The action on theWHB replacement was deferred until the outcome ofthe partial retubing option.

Partial retubing of WHB

The partial retubing of the WHB was carried outduring the February-March 1992 turnaround. A total of101 tubes in the upper portion of the tubesheet werereplaced. The promised reliable repair with the partial


SECONDARY(REFORMER

SUPPORTCONCRETE

FOUNDATION

WHB&SEC.REFORMERWELD JOINT

H-600X800

SUPPORT

Figure 5. Waste heat boiler layout.

retubing option lasted only nine months, and the WHBwas off-line in December 1992 for repair.

This leakage prompted us to realize that reliablerepair of the WEBB cannot be ensured and replacementof the WHB is imperative. The deferred option of theWHB replacement was revived, and actions for pro-curement of the new WHB E-208A/B were initiated.

The last leakage on the WHB was experienced inOctober 1993, before eventual replacement of the unitone year later.

Conclusion

With concerted efforts and repair experience, wewere able to increase the service factor (between fail-ures) from 4 months to 11 months, however, we couldnot restore the boiler to its original reliability.

The factors affecting the reliable repair can be sum-marized as:

• The back of the tubesheets (exposed to water) wasinaccessible for inspection and metallographic analy-sis and, hence, the exact condition and health of thetubesheet material could not be determined.

• Over a period of time, the leakages and cracks on

tube welds, initially confined to the upper portion ofthe tubesheet, started to shift down and come up to thecenter of the tubesheet during repairs. Subsequentrepairs affected the tube welds, which were not prob-lematic in the initial stage. Furthermore, certain tubewelds exhibited repeated failures. The complete stressrelieving/heat treatment of the tubesheet and the tubewelds requires further investigations and attention toavoid previously mentioned problems.

Design Improvements in New WHBSelection

It was decided that the replacement should incorpo-rate design improvements to overcome shortcomingsof the existing WHB and prepare for future require-ments. The following criteria were defined:

(1) Lower heat flux at the inlet of the firstexchanger.

(2) Adequate surface area for future update.(3) Lower outlet temperature to harvest benefits of a

new generation of high activity HTS catalyst.(4) Modification of the inlet channel refractory

design to allow complete pullout of the tubes located


Figure 6. Field constraints in WHB removal andinstallation.

at the periphery of the tubesheet.

WHB Replacement

The replacement of the WHB was carried out duringthe September-October 1994 turnaround. The new unitwas put into operation following a 29-day plant shut-down, one day less than the planned duration. It was aunique experience for us, as in-house facilities andexpertise were employed to accomplish the job.Furthermore, the maneuverability of equipment with-out disturbing or dismantling the existing surroundingequipment and structure was a feat in itself. Theequipment weighed 171 tons, and was 18,688 mm inlength with a diameter of about 2,500 mm, within thegiven field constraints.

WHB Layout

The WHB, horizontal in configuration, rests on twoflexible supports (with the WHB centerline at 2.5 mabove the ground). One support is in the form of aspring box, set up on a concrete foundation (at an ele-vation of 630 mm from the ground). The other sup-port, a pivot type, is in H-shaped construction, stem-ming from the ground level and inclined to an angle of6 deg (in cold state). These special supports are meantto take the .high thermal growth of the WHB at operat-ing parameters.

The WHB is connected to an overhead steam dram(at a 17 m elevation from the ground level) through 16risers and downcomers (Part A of the WHB has six ris-ers and six downcomers, and of the remaining risersand downcomers, two are connected to the B part ofthe WHB).

The inlet channel of the WHB is welded to the outletnozzle of the secondary reformer, and the WHB outletconnection is a flanged one.

The WHB is surrounded by columns supporting theoverhead steam drum and the secondary reformer(Figures 5-7).

Job Scope

The WHB replacement job was comprised of thefollowing major activities:


Figure 7. Field constraints in WHB removal andinstallation.

9 Cutting of risers and downcomers piping joints, theinlet channel weld joint, and the drain and vent lines.

» Dismantling of the springbox and pivot support.« Removal of the old WHB.* Placement of the new WHB and alignment.9 Welding of the inlet channel weld joint.• Modification and welding of risers and downcom-

ers, and drain and vent piping (the new boiler was oflarger diameters, and orientation of certain nozzles onthe new unit was different from the old one).

To appreciate the magnitu4e of the job scope, thedata in Table 3 will be helpful.

Job Planning

The replacement of equipment like a WHB, in anoperational plant, with all the field constraints around,requires a significantly different approach, because thestrategic installation order followed during the planterection phase (for such heavy equipment) is no longeravailable.

Field study

To aid the field study and to get a better appreciationof constraints, a 1:20 scaled mockup of the WHB,complete with its risers and downcomers, surroundingequipment, structure, and foundations was made. Withthe help of this mockup, a complete drill for WHBremoval and installation (under the given field con-straints) was conducted. The findings of this exercisewere:

» The existing arrangements of the overhead steamdrum, risers and downcomers piping, and structurearound the WHB would not allow the deployment of acrane for the WHB removal and installation job.

» The given field constraints allowed the WHBremoval and installation from one direction only (westside) by a sliding and rolling mechanism.

» Complete removal and installation of the WHB,following the linear path (from the WHB's existingposition) were not possible, as WHB downcomer noz-zles and lugs (welded to the WHB for pivot supports)interfered with horizontal and vertical portions of thetransfer line, respectively. To remove these obstaclesfrom the WHB's pathway, a transverse movement of


16 FOR PIVOTSUPPORT \

<fS9

MLl

LUG INTERFERENCEWITH TRANSEFER -

llf

!111

II

DVE WHB TILL -JGS PASSES t

TRANSFER LINE X_ ^1-600X300 H-600X600^

/'

'1

•̂

51

4WE UIUR IM

/rtAv1

\

/3--

COLUMNS 1-600X300' |TRANSEVERSETION BY

300 MM MOVE OUT WHB

Figure 8. Field constraints in WHB removal and installation.

REMOVAL

CONCRETEFOUNDATION

V WHB

V1000

SUPPORTBEAM

RETAINER BOX «•ROLLER SKID DOLLY

\ STEEL PLATE

Figure 9. WHB replacement arrangement.


Table 3. WHB Data

Weight (ton)Length (mm)Diameter max. (mm)

CUTTING/WELDING JOB

Inlet channel weld jointRisers/downcomers weld jointsDrain/vent piping weld joints

OLD WHB16118,3202,270

SIZE

0 1,570x35 mm thick0 244.5 x 17.5 mm thick076.1 x 8 mm thick

NEW WHB17118,6882,558

NO. OF JOINTS

015010

300 mm had to be given to the WHB. However, themaximum transverse movement available for the lugsto pass the columns 1-600 x 300 was 100 mm (seeFigure 8).

To negotiate these obstacles, the WHB was firstmoved (from its existing position) in the linear direc-tion until the lugs had cleared off the columns 1-600 x300. With this movement, the downcomer nozzleswould still remain clear off the interfering horizontalportion of the transfer line. Then, the WHB wasmoved in a transverse direction by 300 mm (to avoidthe interference of lugs with the vertical portion of thetransfer line). Then, the path was clear for the WHBremoval.

For removal and installation of the WHB, twooptions were considered:

Option 1: Sliding in and out of the WHB on the steelskid: sliding in and out of the WHB on the steel skid(with its top surface finished with Teflon pads) andusing hydraulic rams to push and pull the WHB on theskid.

Option 2: Roll in and out of the WHB with rollerskid dollies: moving hi and out of the WHB, loadedonto the roller skid dollies and using chain hoists forpulling.

Option 1 was submitted by an outside agency,

whereas Option 2 was worked out by our own team.The two options were evaluated for the time and costinvolved to carry out the job. Rolling in and out of theWHB with roller skid dollies was found more feasible.

To economize the cost involved in making a steelskid about 40 MTR long, an altogether differentmethod was conceived (Figure 9):

* Roller skid dollies could be moved on 25 mm thicksteel plates were laid on the concrete floor.

9 Instead of a skid, support beams could be providedbetween WHB saddles and roller skid dollies.

» To keep the dollies aligned during the movement,retainer boxes could be welded to the support beams.

Special tools

To execute the job, the requirement of special tools(not available to us) was assessed and included:

(1) 04 roller skid dollies of 100 ton capacity each,and 02 of 50 ton capacity each.

(2) 04 hydraulic rams of 100 ton capacity each,complete with electric pump and accessories.

(3) Pipe beveling machine for preparation andbeveling of risers and downcomers, 100 pipe ends of0 244.5 x 17.5 mm thick.

(4) Gas cutting machine for cutting of the WHB and


the secondary reformer weld joint of 0 1,570 x 35 mmthick.

Inspection of weld joints

Inspection was planned of weld joints by ultrasonictesting (UT). This was to save time as radiographyinterrupts the ongoing activities at a site.

Rearrangements for Job Execution

• Procured the special tools (as mentioned earlier).• Fabricated saddles for jacking up and down the

new WHB (available on the old unit but not furnishedon the new WHB).

• Fabricated support beams (of different sizes to suitthe typical space availability in the field) and retainerboxes.

• Placed new WHB in front of the old one to be usedas a dead weight for pulling out (the old unit).

• Resurfaced the uneven concrete flooring (on theWHB path).

• Reinforced columns H-600 x 600 and 1-600 x 300to be used as dead weight for pulling in the new WHB.

Field test

To complete the job preparations in every respectand to test the viability of the whole arrangement, afield test was conducted. The new WHB was mountedonto the roller skid dollies and the load was moved onsteel plates using chain hoists. The arrangement wasfound to be trouble-free and the WHB was movedsmoothly onto the steel surface.

Conclusion

The WHB replacement job was successfully execut-ed using the following plan. However, there was oneelement of surprise when the concrete floor areabeneath the WHB (about 8-10 MTR in length) yieldedunder the load. The plant specifications clearly indi-cated that this floor area was strong enough to take therequired loading. This problem of the floor sinkingwas rectified by laying thicker steel plates (60 mmthick plates available at the site) on the affected area.

The method described for WHB replacement is safe,reliable, and cost-effective for maneuvering heavyloads to small distances in areas with field constraints.

DISCUSSION

T. L. Huurdeman, DSM Fertilizers: You had a lot ofproblems with the full penetration welds in the boiler.We had a similar boiler with the same problems,including cracks in the welds. We had to replace thisboiler and selected another tubesheet design. In thisdesign the tubes are inserted again in the tubesheet,which has an Incoloy overlay to make the boiler morerepair-friendly in the future.Ahmed: In our opinion, the problem was partial dry-out of the boiler which produces mechanical stressesand material damage. We have taken measures toavoid the total power failure and boiler feed watersupply interruption for the future. We were quite satis-fied with the original design, so no changes were madein this.Huurdeman: The cause of your problem was lack ofboiler feed water. With an interrupted water supply,you were still sending a lot of steam through the pri-

mary and secondary reformer. For what reason are youusing such a large amount of steam? This amount ofsteam and the resulting heat input into the boiler willfurther decrease the boiler water level. We have usedthese large amounts of steam in the past. To our expe-rience, the amount of steam can be decreased substan-tially when you lose feed and fuel on the primaryreformer. Once the process air lines and the primaryreformer tubes are purged, the amount of steam can bereduced and doing so will decrease steam productionin the boiler.Ahmed: My colleague Zaheer Anwar will answer thisquestion.Anwar: I tend to agree with you. In case of boiler feedwater interruption, we now reduce the amount of cool-ing steam to primary reformer and secondary reformer.


Documents

1996: Failure, Repair and Replacement of Waste Heat Boiler...The waste heat boiler (WHB) (E-208A/B) installed downstream of the secondary reformer is a double compartment design (namely,