Synloop Superheater Issues...Local PWHT 2m Outlet Nozzle (2.25Cr-1Mo) Inlet Nozzle (2.25Cr-1Mo)...

Synloop Superheater Issues

PT. Kaltim Pasifik Amoniak (KPA) operates a 2000 MTPD ammonia plant in Bontang, East

Kalimantan, Indonesia. At the time of its commissioning in early 2000 it was the world’s largest

capacity ammonia plant. Between May 2002 and August 2004 two significant problems were

experienced with the Synthesis Gas (SG) Steam Superheater in the ammonia synthesis loop. The first

problem was a through-wall leak at the weld of the dished bottom head of the vessel. A crack

approximately one third of the way around the circumference was discovered in May 2002. This was

the closing weld of the vessel. Repairs were made and the condition of the seam was monitored

closely for the next two years. Repeat indications of problems at the weld seam were found and other

repairs were necessary with eventual replacement of the entire exchanger

In September 2003, leakages at 60 tube-to-tube sheet welding joints were detected by pressure test.

These 60 welds were repaired.

Unfortunately, this leakage allowed water to enter the process side during start up and shutdown

situations, which were subsequently blown into a hot downstream S-50 converter eventually

destroying its internals and necessitating complete replacement of the internals and the catalyst,

charge. However, further leakage at tube-to-tubesheet welds was found six months later and again in

the next three and a half months after repairing. Finally, the SG Steam Superheater was completely

replaced with the new one in 2005.

This paper discusses the damage seen; the root causes, and suggested design changes, which were

incorporated into a replacement vessel to avoid the re-occurrence of similar problems. Also outlined

are the results of a recent inspection that showed the new exchanger to be free of problems after two

years of steady operation

Hotdo M. Pasaribu

PT. Kaltim Pasifik Amoniak,

Bontang, East Kalimantan, Indonesia

Introduction

T. Kaltim Pasifik Amoniak is a joint

venture company owned by Mitsui and

Co. Ltd and Toyota Tsusho. Its ammonia

plant is situated in Bontang, East Kalimantan,

Indonesia, with a nominal production capacity

of 2,000 MTPD. It is an export-oriented plant

located in a remote area with an objective to

fulfill ammonia market requirements in the

world. The plant was designed and

commissioned in early 2000 by Haldor Topsoe

A/S as the process licenser and Mitsubishi

Heavy Industries, Ltd as the main EPC

contractor. Ammonia production started at the

end of March 2000.

At the time of commissioning in early 2000 it

was the world’s largest single-train ammonia

plant.

The plant operated consistently at a level higher

than its design capacity for two years before the

initial failure occurred in the synloop

superheater, E-0500.

P

111 AMMONIA TECHNICAL MANUAL2008

Brief Description of SG Steam Superheater

(E-0500)

The synthesis section of the plant uses Topsoe’s

S-250 technology. This features their S-200 and

S-50 converters in series separated by a vertical

U tube high-pressure steam superheater, which

removes reaction heat from the S-200 converter

outlet stream and adjusts the temperature for

optimum conversion in the downstream S-50

converter.

It superheats the saturated steam leaving the

main Steam Drum (V-0201). This saturated

steam is superheated to 515 °C and is used in

high-pressure steam turbines.

The steam leaving the unit has the following

specifications:

Conductivity @ 25°C <0.2 mS/cm

SiO2 <0.02 ppm

Total Iron (Fe) <0.02 ppm

This design of the superheater and it’s location

between the two synthesis converters was, to the

best of our knowledge, the first of its kind and

KPA’s experience illustrates the caution

required in operating a synloop with such

equipment located between two synthesis

converters.

Figure-1. shows the process flow of the

ammonia synthesis section of the KPA plant.

An outline of the channel section of the E-0500

superheater is shown in Figure 2 below

Main operating conditions are set out in the

process datasheet as follows

No Description Tube Side:

Synthesis Gas

Shell Side:

Steam

1 Inlet Temperature, oC 439 324

2 Outlet Temperature, oC 376 380

3 Inlet Pressure, kgf/cm2G 138 120

4 Outlet Pressure, kgf/cm2G 138 115

(Mol %) a b

H2 55.18 52.52

N2 18.39 17.51

1st Converter

R-0501

115K/G 380 oC

120K/G 324oC

SG Steam

Superheater

E-0500

138K/G 439 oC

a

138K/G 376 oC

137K/G 418oC

2nd Converter R-0502

b

324 oC

120K/G 324oC

WHB

E-0501

136K/G 340oC

304 oC

270oC

BFW Preheater E-0502

136K/G 281oC

112AMMONIA TECHNICAL MANUAL 2008

Figure-1: Process flow of ammonia synthesis loop

,

Figure-2: Outline of E-0500 channel

Materials and dimension of main components are as follows:

No Description Material Specification Dimension

1 Tube 321H SS 400 U-tubes, 38mm OD, 4.5mm t

2 Tube sheet 2.25Cr-1Mo steel 515mm t

3 Weld Overlay Inconel 600 6mm t

4 Fillet Weld Inconel 600

5 Shell 2.25Cr-1Mo steel 2086mm OD, 106mm t

6 Head 2.25Cr-1Mo steel 60mm t

Tube material was selected as 321H SS to

prevent the nitriding because the inlet

temperature of the synthesis gas is higher than

nitriding temperature of low alloy steel.

Inconel 600 overlay on the 2.25Cr-1Mo

tubesheet is also provided to avoid nitriding on

low alloy steel. The pass partition plate is made

of Incoloy 800 and prevents the shell and head

from contacting the hot inlet gas. Therefore,

shell and head material was selected by taking

into consideration the risk of nitriding and

hydrogen embrittlement/hydrogen attack

according to latest edition of Nelson curves as

referred to in API 941 standard.

The Incident and Immediate Action Taken

On May 05, 2002, during routine plant

checking, the field operator heard an abnormal

noise & smelled ammonia in the area around E-

PG

Steam 324

� 380

�

376�

439�

11m

Manhole (2.25Cr-1Mo)

Tubresheet (2.25Cr-1Mo, 515mmt)

Head (2.25Cr-1Mo, 60mmt)

Shell (2.25Cr-1Mo, 106mmt)

(2) Shell to Head

Local PWHT

2m

Outlet Nozzle (2.25Cr-1Mo)

Inlet Nozzle (2.25Cr-1Mo)

Internal (Incoloy800)

Tube (SUS321H, 38mmOD×4.5mmt)

(1) Tube to Tubesheet

Pipe (SUS347)


0500 SG Steam Superheater. A high explosive

level was found within a distance of about 10-15

cm from the unit. Finally, it was determined that

the abnormal noise came from a crack at the

outer surface of the bottom hemispherical head,

located just above the manhole. The plant was

immediately shutdown.

Figure-3: Crack location

Inspection revealed that a 110 mm through-wall

crack had formed in a circumferential seam

weld while the inside weld surface showed a

crack running around approximately 1/3rd

of the

circumference of the vessel as shown in Figure-

5

As recommended by the vessel manufacturer,

the weld (identified as weld seam No. 34) was

repaired by cutting off the bottom head and

completely re-welding it back in place using the

same procedures as applied during the original

manufacturing process including re-post weld

heat treating (see Fig. 4)

On completion, the May 2002 repair was judged

a success and this equipment was re-

commissioned.

Later on, during plant normal operation the

pressure drop across the second ammonia

converter slowly started to increase. The cause

at the time was unknown. Eventually, the plant

was unable to manufacture 2000 tones/day. In

September 2003 the pressure drop across the S-

50 second converter suddenly increased to the

extent that the plant could not be operated at all

and had to be shutdown. Internal inspection of

the downstream second converter showed that

the catalyst had flowed down into the inlet pipe

(see Fig. 13).

Fortunately it was possible to run a bypass

around the S-50 ammonia converter and the

plant could be operated at 90% capacity for

some 9 months until reconstruction of the

converter was completed in July 2004. As part

of the reconstruction, the catalyst and the

internals of the converter were completely

removed and renewed. At the suggestion of the

process licensor the gas inlet piping was also

reconfigured to minimize the possibilities of

pressure drop build up during operation

Regretfully, though thorough inspection found

no indications of defects following the repair of

weld seam 34 in 2002, new cracks were found

in September 2003 and at one point a crack

penetrated the vessel wall by up to 13

millimeters. Extensive repairs to the seam were

thus made for the second time and as a

consequence, post weld heat treatment was

repeated.

During this same September shut down, it was

also decided to pressure test the tube bundle of

E-0500 (HP steam side) and the pressure test

identified 60 tube-to-tube sheet leaks (see Fig.

10). These leaks were repaired; the exchanger

Steam Outlet Steam Inlet

Syngas Inlet Syngas Outlet

Weld seam no. 34

(circumferentially crack

location)

Manhole


was re-tested to confirm the repairs prior to re-

streaming.

Again, in March 2004 as a result of another

inspection (see Fig. 7), defect indications at

seam# 34 were found on which the 3rd

repair

was then carried out. However, this time the

repair method differed significantly from

previous repairs as the welding and post weld

heat treatments conditions were improved.

Smaller weld passes were applied using a

temper bead process followed by post weld heat

treatment designed to comply as closely as

possible to BS5500 requirements with

additional restrictions to endeavor to minimize

residual stresses introduced by the treatment.

The vessel was inspected again in July 2004,

after three and a half months of operation and

no significant cracking was detected. However,

in view of the fact that several post weld heat

treatment to the vessel wall had been carried

out, there was a major concern that the

hardness/strength of the base material had

dropped. Consequently, the vessel designers and

manufactures have recommended that the vessel

should be down rated to operate at a lower

pressure than the original design.

Furthermore, several preventive measures were

carried out to detect gas leakage promptly and

prevent cracks on the tube-to-tubesheet welds as

long as possible prior to complete replacement

with the new vessel in 2005.

A summary of the history of the vessel is given

in table-1.

Failure Investigation and analysis

A failure investigation was carried out to verify

the status of metallographic structures & crack

related structures to service performance and to

interpret and determine the type, cause & mode

of failure on which corrective action could be

initiated to prevent the recurrence of future

similar failures.

Those activities were conducted by the KPA

team with cooperation and support from our

metallurgical consultants, the main EPC

contractor, the vessel manufacturer, as well as

Haldor Topsoe as the process licenser.

The following damage processes were identified

1. Cracking at the heat affected zone of closing

weld 34.

2. Cracking of tube-to-tube sheet welds leading

to leakage.

3. Delamination of the cladding on the tube

sheet.

4. Nitriding of the 321 stainless steel tubes.

Following are the possibilities for the root

causes of the above failure:

Cracking at Weld 34

Characteristics of the damage

� Cracking initiated in the HAZ of weld 34 on

the inside of the vessel in both the head and

the shell side of the weld.

� Cracks ran perpendicular to the surface of

the vessel and did not follow the

microstructure. These cracks initiated in the

HAZ and then ran into the weld.

� Cracks ran circumferentially.

� Cracks were purely intergranular.

� Cracks propagated all the way through the

vessel wall and caused leakage of synthesis

gas to atmosphere.

� The vessel has contained a very significant

through wall defect and yet leaked rather

than ruptured.

� Crack propagation rate had increased.

� Cracks grew from being undetectable to near

to the maximum allowable defect size in 5

months of operation.

� Weld 34 is a closing weld that had been

locally post weld heat-treated.


Possible causes

As a result of investigation, the cause of

cracking was probably due to inadequate post

weld heat treatment of a closing weld which left

the vessel with low residual stresses and then

cracking occurred as a result of hydrogen and

nitriding attack. Residual stresses can be

introduced by post weld heat treatment if high

thermal gradients are present. One major cause

could be if the temperature on the inside of the

vessel is kept hotter than the outside of the

vessel at the PWHT temperature or on cooling.

Thick nozzles were present next to weld 34 and

the pattern of cracking seen had obviously been

influenced by these nozzles. These acted as

huge heat sinks and PWHT’s given prior to

2004 had not been conducted in such a way as

to apply heat evenly to these regions. As a result

it is considered impossible for weld 34 to have

been evenly heated and cooled during the earlier

PWHTs given.

Leaking Tube-to-Tube Sheet Welds


� Cracks initiated at the root of the weld and

secondary cracks were present in this area.

� Cracks ran in the nickel-based weld.

� There was typically a major crack, which

was open and relatively straight.

� Secondary cracking present was multi-

branched and typical of stress corrosion

cracking.

� Small secondary cracks were filled with

corrosion product (rich in Chromium, iron

and oxygen).

� Corrosion product /deposited on the surface

of large cracks. These deposits were iron

oxide and the chromium content was very

low.

� There were deposits present at the bottom of

the crevice between the tube and the tube

sheet next to the weld that was similar to the

deposit in the major crack.

� Significant numbers of tubes leaked in E-

0500. Leaks were present in both the inlet

and outlet side of the inverted U-tube

bundle.

Possible causes

The environment required to cause cracking in

Inconel 600 type alloys is not extremely severe

and does not necessarily require any specific

contamination. The problem is often referred to

as primary water stress corrosion cracking

(PWSCC). When condensed water is formed

which then evaporates during operation a

concentration of chemicals can occur in the

bottom of the crevices formed. Therefore, the

primary cause of cracking is considered to be

stress corrosion cracking due to a significant

stress, the presence of condensed water during

shut downs, and the choice of a material which

is prone to cracking.

However, one major influential factor is also the

presence of hydrogen in the water. In the case of

E-0500, it is probable that the tubes had not

been expanded to achieve a pressure seal within

the tubesheet area so that cracks allowed

passage of synthesis gas into the steam system

when the vessel was on line and boiler feed

water into the bottom channel when the vessel

was off-line.

The cracking seen probably occurred by the

formation of a single crack, which grows

through wall. Once this had leaked through the

wall, ammonia and steam leaked through the

crack at different times. The secondary

cracking and corrosion probably then occurred.

Delamination of cladding on the tube sheet


� The cracks propagated along the interface

between the two materials.

� The cracks initiated at the holes in the tube

sheet.


� A layer of precipitates is formed at the

interface.

� The cracks had not propagated through the

cladding

Possible causes

Delamination is only a major problem when the

cracks run through the cladding material. Where

fillet welds are applied at tubes in a tube sheet

this can be a major problem. However, if the

cladding is not stressed by surface attachments

such as fillet welds there is less driving force to

cause delamination and there is little or no

reason for cracks to run through the cladding. In

this case the cladding continues to protect the

underlying material.

In the case of E-0500 it is possible that the

delamination had been beneficial as it reduced

the stresses in the adjacent fillet welds and

probably assisted in arresting the PWSCC

cracks in them.

Therefore, the cause of delamination on the

tubesheet overlay could have been cracking due

to nitriding by synthesis gas from the tube side.

That does not occur without through wall

cracking on fillet weld

Nitriding of the 321 stainless steel tubes


� Nitrided layer formed on inlet tubes to a

depth of 0.2 mm (1/25th

of the wall

thickness) in 3.7 years of operation.

� Nitrided layer cracking on the ends of the

tube

Possible causes

Damage expected from the choice of materials

and the operating environment

Final Action Taken

It was finally decided to replace the existing E-

0500, as it was not safe to continue to operate

with the damaged vessel in the long term. KPA

requested the vessel manufacturer and the main

EPC contractor to review the design of the

exchanger. It was also suggested to install a

drain on the bottom channel as a preventive

measure against cracks on the tube-to-tube sheet

welds which then resulted in water

accumulation in the bottom of the process side

of the vessel. Since it could not be drained, the

wreckage of the internals of the S 50 converter

was attributed to the flushing of the

accumulated water trapped in the bottom head

of the vessel into the hot converter on a short

outage.

Based on these suggestions the re-design of the

SG Steam Superheater was carried out and

submitted to KPA who gave final their approval.

Finally, the new superheater was installed in

July 2005. This was manufactured without a

closing weld, with a fully weld overlayed

channel, with 321stainless steel tubes and with

321 welded J preparation tube-to-tube sheet

welds. The operating procedure was also revised

to ensure that no water could build up on the

process side and be blown into the S 50

converter as discussed below.

Summary of Design Improvements for E-

0500

As a result of the known issues at KPA in the E-

0500 vessel a number of options for a redesign

of the vessel were assessed. Final design

improvements are as follow,

1. Stainless fillet weld (SS347) for tube-to-

tube sheet weld and stainless overlay

(SS309L+SS347) on tube sheet was

employed instead of Inconel 600. (see Fig.

12.1)

2. Two-step PWHT was carried out for tube

sheet overlay to reduce the hardness of the

bond area.


3. J groove was made for the tube-to-tube

sheet weld and the welding procedure for

the same was confirmed with a mock-up

test.

4. The bottom channel was heat treated in the

furnace as one piece after finishing all

welding.

5. A flat cover was employed instead of

spherical head for the bottom channel (see

Fig. 12.2). A drain was installed on the

bottom head so that any tube-to-tube sheet

weld leakage could be detected and drained

before establishing loop circulation.

6. Inconel overlay was carried out for the

whole inside surface of the bottom channel

including nozzles.

7. The simplified radius was used for the

corner between tube sheet and the channel

shell to avoid the stress concentration on

the Inconel 600 overlay. (see Fig.12.3)

8. Cl contamination was strictly watched

during the fabrication. Wipe test after the

washing of the drilled holes was made (the

Cl content shall be less than 0.15g/m2)

9. A sensitization test was carried out on test

coupons to simulate the stainless steel

cladding (under layer of SS309L and a top

layer SS347) on the tube sheet.

Start-up/shut-down Operation of E-0500

As a result of the lessons learned from this

experience operating procedures during

synthesis section start up and shutdown were

modified to the following. In summary

synthesis loop pressure should be always kept

higher than steam side pressures, and it must be

avoided to introduce water/steam to the

synthesis loop, during plant start-up and shut-

down

In addition, from the viewpoint of water quality,

the following should also be carried out:

1) Start-Up Operation

- Heating up condition of steam piping

and shell side of E-0500 shall be

carefully confirmed.

- Shell side of E-0500 shall be brought

on-line after the outlet temperature of

Ammonia Converter; R-0501 becomes

more than 340°C (more than dew point

of HP steam).

2) Shut-Down Operation

- In case that the synthesis section is

stopped, the inlet and outlet manual

block valves at steam line shall be closed

as soon as possible.

- Pressure of E-0500 shell side shall be

reduced to atmospheric pressure (0.2

kg/cm2G) by opening drain valve at

steam side.

A N2 purge on E-0500 shell side is

introduced on shutdown to prevent

condensation of steam and thus

eliminate the possibility of SCC due to

impurities in water.

The performance

In July 2005, the new SG Steam Superheater

was installed. In order to ensure its service

performance after running for one year, an

observation/inspection of tube-to-tube sheet

welds and weld joint #15 & #34 was carried out

during the plant shutdown in

September/October 2006. The results showed

that the vessel was in a good condition.

An external and internal inspection was also

done during the 2007 turnaround; the inspection

results by UT indicated that the new vessel had

the maximum tube expansion within 1 mm of

the back of the tube sheet so that the risk due to

the ingress of oxygen, condensation and the

concentration of contaminants during startup

that could create corrosion in the crevice can be

reduced.

Therefore, it was concluded that such equipment

was still in a good condition after two years

operation and safe for operation.


Conclusions

(1) The cause of shell to head welding joint

failure was cracking by nitrogen, hydrogen

and high stress or Alkaline SCC.

High residual stress from the local PWHT

was the main cause of damage. The less

than required PWHT was probably the

result of the design of the bottom of the

vessel where large inlet, outlet, and

manway nozzles were located too close to

the subject weld. Therefore, it would be our

recommendation that no closing weld shall

be located at the bottom of the vessel (i.e.

no localized PWHT in this area).

(2) The cause of cracks on the tube-to-

tubesheet weld joint was primary water

stress corrosion cracking (PWSCC).

PWSCC normally requires condensed

water to be present. This indicates that the

cracking occurs when water is present in

the bottom of crevices around the tubes,

and any traces of chlorides could have

penetrated the tube-to-tube sheet crevices

where they would concentrate to possibly

cause the problems. This can only occur

during a start-up or a shutdown. The

presence of open major cracks and the high

rate of crack growth during shutdowns

suggests that a significant stress was

present in the fillet welds

(3) It is mandatory to ensure that crevices are

not formed at the back of the tube sheet

during fabrication with a similar design or

at least the maximum tube expansion can

be produced within 1 mm of the back of the

tube sheet to reduce the risk due to ingress

of oxygen, condensation and the

concentration of contaminants during

startup that will create corrosion in the

crevice. This resulted in intergranular

corrosion and subsequently initiation and

propagation of cracks by fatigue.

(4) It must be ensured that the heat input is

sufficient to bring the thermal mass of the

tubesheet to above the dew point of the

startup steam before steam is allowed to

enter the shell side. Prior to this steam

entry the shell side nitrogen blanking

should be maintained.

321 stainless steel will slowly nitride and

the tubes will have a limited life if there is

any significant applied or residual stress

present Austenitic stainless steels are

highly susceptible to SCC with even small

amounts of chloride and thin films of

condensed liquid

References

1. Borsig, "Failure Analysis of the Crack in

Seam 34 of the SG Steam Superheater",

dated 31 May 2002. (Confidential, not

published)

2. Firth, D., Materials Performance

Technologies Report No. 33502.02,

"Determination of maximum allowable

defect sizes and crack growth rates for weld

34 in superheater E0500", issued October

2003. (Confidential, not published)


Technologies Report No 33502.06 “MPT

Examination of E-0500 Crack Samples

Taken in March 2004”, issued April 2004.

(Confidential, not published)


Technologies Report No 33052.07

“Summary of the damage seen to

superheater E-0500, the possible root

causes and recommended improvements for

a new vessel”, issued April 2004.

(Confidential, not published)

5. Mitsubishi Heavy Industries Ltd, “Analysis

of Cracking in Tube to Tubesheet Welding

joint and Shell to Head Welding Joint for

KPA E-0500 Steam Superheater”, issued

May 2004. (Confidential, not published)

6. Mitsubishi Heavy Industries Ltd,

“Summary of Preventive Measures against

cracks on E-0500 SG Steam Superheater”,


issued July 2004. (Confidential, not

published)

7. API Practice 941 Steels for hydrogen

service at elevated temperatures & pressure

refineries and petrochemical plants 1998.

Table-1: History of KPA vessel E -0500


Figure-4: Crack location and repair work in 2002

Figure-5: Distribution of cracks in the closing welds in May 2002


Figure-6: Distribution of cracks in the closing welds in September 2003

Figure-7: Inside cracks indication by MT in the closing welds in 2004


Figure-8: Typical crack

��

Inlet side (Manhole side)

�� Outlet side (Manhole side)

TubNo.:Row-Column

(VIEW FROM BOTTOM SIDE)

Figure-9: Penetrant Test results of E0500 tube-to-tubesheet welding joint

90�

180�

270�

0�

��

��

��

Weld Seam Tube Tube inside

indication length

��L: Linear

D:

Rounded

��


Leak at the Inlet tube No 3-1 (by soap test)

Leak at the outlet tube No 13-9 (by PT)

Figure-10: Example of E-0500 tube-to-tubesheet joint leak test and PT Indications


Figure-11: Distribution of leaking tube-to-tubesheet at E-0500

Figure-12.1: Material configuration on tube to tubesheet welding joint

Figure-12.2: Outline of Flat cover structure with Inconel600 overlay

Figure-12.3: Configuration of corner R on tubesheet to shell welding joint with overlay

Local PWHT

Flat cover

Furnace PWHT

Weld Overlay

(Inconel600,

5mmt)

2.25Cr-1Mo steel

106mmt Inconel 600

309L SS / 347 SS

Simplified Radius

Overlay after welding tubesheet and shell

Weld Overlay 309L SS/ 347 SS

6mmt

Fillet weld 347SS

Leg 6.6mm

515mm

50mm 309LSS

347SS Fillet weld Inconel600 Leg 4mm

Weld Overlay Inconel600

6mmt

515mm

50mm


Figure-13: A big pile of Second Ammonia Converter catalyst found at inlet pipe after cutting


Synloop Superheater Issues...Local PWHT 2m Outlet Nozzle (2.25Cr-1Mo) Inlet Nozzle (2.25Cr-1Mo)...

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