Fire in secondary reformer outlet line to waste heat boiler

Fire in Secondary Reformer Outlet Line to Waste Heat Boiler Jagmohan Singh, I? Basu, and B.M. Rao Krishak Bharati Cooperative Limited (KRIBHCO), PO. Box Kribhconagar, Surat-3945 15, Gujarat, India

Tnis paper describes the failure of the pressure shell of an interconnecting pipe between the secondary reformer andpri- m a y waste beat boiler that led to afire in Ammonia Unit I ajer 13-and-a-halfyears of operation. The fire incident result- ed at KRIBHCO in aplant shutdown of 15 days. It also describes analysis, the repair work carried out to put the plant back in service in the least possible time, and the remedial measures taken.

INTRODUCTION Krishak Bharati Cooperative Limited (KRIBHCO)

located at Hazira, Surat, in the State of Gujarat, India, operates a nitrogenous fertilizer complex comprised of two streams, each with a capacity of 1,350 MTPD of ammonia and four streams of 1,100 MTPD of urea. Ammonia plants, such as Units I and 11, are based on steam reforming of natural gas. They were designed by M.W. Kellogg of the U.S., and were commissioned in September and November 1985, respectively. In order to reduce the plants' energy consumption, a sin- gle purge gas recovery unit, common to both plants, was installed and commissioned in August 1988. After 10 years of operation, reformer tubes in both the units were replaced with thinner microalloy tubes, and the ammonia synthesis converters were modified to radial flow. Both plants have operated consistently at a level of about 115% of design capacity for the last 15 years.

The process gas from the secondary reformer (equipment 103-D) contains a mixture of hydrogen, nitrogen, carbon monoxide, carbon dioxide, residual methane, and steam. The process gas at 34 kg/cm2 and 980" C is cooled to 400" C in two vertical bayo- net-type primary waste heat boilers (101-CA and 101- CB), operating in parallel, and then further cooled to 350" C in a secondary waste heat boiler (102-0. Fig- ure 1 shows the arrangement of the secondary reformer and waste heat boilers. The secondary reformer, primary waste heat boilers, and interconnecting piping are of standard Kellogg design, with an

external water jacket to give a uniform temperature to the pressure shell. The pressure shell of the interconnecting piping between the secondary reformer and primary waste heat boiler is made of 30 mm thick, 915 mm dia, ASTM A-516 Gr 70 carbon steel material. The inner side has a 200 mm thick bubble alumina refractory (Greencast 97 L) lining. A liner (shroud) of 6 mm thick SS 310, with expansion joints, is provided to protect the refractory from erosion and collapse. Con- ical gas shields are provided between the pressure shell and the refractory to prevent gas bypassing. The shroud is welded to a gas distributor located at the bottom of the primary waste heat boiler. The water jacket is fabricated of 6 mm thick ASTM A285 Gr 8 plate. Figure 2 shows a cross-section of the interconnecting pipe.

The water jacket of the interconnecting pipe pressure shell, as shown in Figure 3, is integrated with each primary waste heat boiler. Demineralized water and/or return condensate from the surface condenser is provided as a cooling medium through a level con- trol valve. The level in the jacket is set in such a way that an overflow is always maintained through the jacket, ensuring they remain water-filled at all times. Low-level alarms are provided to warn the operator in case of any abnormality. Water flow to the jacket is also measured and its indication is available in the field. Loss of water from the jackets could cause undue stress in the transfer line piping. It is recom- mended by the process licensor that the reformer be taken out of service in the event of loss of the jacket water.

THE INCIDENT Ammonia Unit I was undergoing start-up after its

annual turnaround. The primary reformer was lit up on April 17, 1999. The start-up of the CO, removal section took longer, due to leakage of thread joints of instrument tappings of the semi-lean solution flash tank, and vanadation of the CO, removal section. At

Process Safety Progress (V01.21, No.3) September 2002 205

1 PROCESS AIR

PROCESS PRIMARY REFORMER

BFW

I STEAM 43 GAS TO.-I'

1 0 2 4

BFW

I -GAS TO

102-c

PRIMARY WASTE HEAT SECONDARY PRIMARY WASTE HEAT BOILER I01 -CA REFOMER BOILER 101 -CA

Figure 1. Arrangement of secondary reformer and primary waste heat boilers.

0140 hrs on April 24, 1999, while the ammonia synthesis converter was heating up, operators heard an explosive sound from the secondary reformer area, followed by a big gas cloud on fire. The gas cloud had engulfed the area up to the electrical substation on the south side, and to the Benfield section on the east side. Feed and fuel gas to the primary reformer, and process air to the secondary reformer, were immediately cut off, initiating a crash shutdown of the plant. The fire was confined to the secondary reformer area and controlled within 10 minutes. The fire damage was minimal and, more importantly, no one was injured. The plant operators heard a sound like leaking gas a few seconds ahead of the explo- sion. They also saw a mist of steam with splinter sparks near the thermowell region of the interconnecting pipe, and the jacket area near the boiler had turned red.

OBSERVATIONS AND DAMAGE The failure had taken place on the topside of the

interconnecting pipe, very close to the conical gas shield towards the primary waste heat boiler (101-

CB). This line had a large gaping hole and the pressure shell bulged around the opening. The water jacket had burst open very close to the thermowell region after ballooning (See Figures 4 and 5). The pressure shell wall of the failed region had drastically thinned down.

The conical gas shield protruded through the opening and the refractory portion around it had blown out completely (See Figure 6). The water jacket cover at the top of boiler 101-CB was found slightly lifted from its position due to gas blowing through the jacket overflow opening, and its shell had bulged near the inlet nozzle. The inner diameter of this nozzle had increased by about 40 mm, due to bulging. The liner had opened up at the longitudinal weld joint, slightly away from the burst opening and towards the secondary reformer side of the interconnecting pipe (See Figure 6). All electrical and instrument cables in that area were damaged, as were a few transmitters, level switches, and thermocouples.

An investigation of the DCS alarm summary the following day revealed that the jacket water low-level alarm had appeared three minutes before the incident.

206 September 2002 Process Safety Progress (V01.21, No.3)

Table 1. Mechanical properties of samples from the pressure shell ASTM A-516 Gr 70.

574.48 _ _ _ ~ _ 532.91 __ _ _ _ _ - _ _ - U.T.S N/mm2_ 32.8 % Elensation 27.6

V notch charpy impact strength 16.33 13.0 _ _ - ___ -

Figure 2. Cross-section of interconnecting pipe between secondary reformer and primary waste heat boilers.

REPAIR In order to establish the soundness of the shell

material around the inlet nozzle area of the boiler, the following checks were carried out.

A fluorescent magnetic particle test was carried out and no surface cracks were noticed. Hardness measurements were taken and values ranged between 150-200 VHN. In situ metallography was performed at three dif- ferent locations to determine if there was any high- temperature hydrogen attack. N o grain boundary fissures were noticed in the microstructure. Based on these results, it was decided to replace

the damaged portion of interconnecting pipe only. The boiler shell with the distorted inlet nozzle would be used so that the plant could be brought back to production within the shortest possible time.

The damaged portion of water jacket, pressure shell, refractory, and liner were removed, as was the tube bundle and the gas distributor at the bottom of the primary waste heat boiler to facilitate the repair work. The replacement pipe of 1.2 m out of 2.5 m length was fabricated by rolling 32 mm thick SA 516 Gr 70 plate in a conical shape to match the diameter at both ends. A plate of higher thickness (32 mm) than the original 30 mm was selected to minimize the mismatch at the boiler shell nozzle joint due to bulging. The pipe was cut in two halves along the longitudinal seam to facilitate installation of the shroud, conical gas shield, and refractory (See Figure 7).

The bottom half of the new 32 mm thick pipe was installed and welded to the pressure pipe at the secondary reformer end, and to the boiler shell nozzle at the other end. A preheat temperature of 100-150° C


t Over flow to sump

L

Water Jacket-

7 Thermowell - - -

Pressure shell

Liner Inconel601

Conical gas shield

Condensate

Figure 3. Integrated water jacket system of interconnecting pipe and primary waste heat boiler.

208 September 2002 Process Safety Progress (V01.21, No.3)

was maintained during welding. These joints were also welded from inside after back gouging (See Fig- ure 8). A new shroud and conical gas shield, made from Inconel-601 plate, were installed. The shroud was wrapped with 6 mm thick cardboard to form the required expansion. Refractory pouring in the bottom half of the interconnecting pipe was completed up to boiler 101-CI3. Then, the top half of the interconnec- tion pipe was welded.

The welding joint to the boiler nozzle end was also carried out from the inside of the boiler shell to fill up the gap and to match the pipe and the distorted shell nozzle. The longitudinal joints and the top portion of the circuinferencial joint at the secondary reformer side could not be welded from inside, due to inacces- sibility. The top half of the refractory work up to the conical gas shield was completed pouring through the pouring nozzle. The refractory work from the conical gas shield to the boiler shell was carried out from within the boiler. All the joints were DP tested and stress relieved. The shroud and gas distributor inside the boiler were installed. The tube bundle was boxed up and the water jacket restored. Due care was taken for welding, such as the selection of electrodes, devel- oping the correct welding procedure, welders' qualifi- cation, and proper NDT checks. N o special refractory

dry out procedure was used. Dry out was done online with a slow start-up. The plant was back in production after 15 days.

ANALYSIS The reasons for the failure of the pressure shell

could be hydrogen attack on the pressure shell of ASTM A-516 Gr 70 material, o r metal creep. Failure of the refractory between the pressure shell and shroud has to occur first so that the pressure shell is exposed to high temperatures in excess of 343.4" C, which is the design metal temperature. The refractory failure may be due to thermal cycling of the transfer line over a period of time, or mechanical shocks caused to the refractory during repairs at the bottom of the waste heat boiler during turnaround.

Figure 9 shows the fractured edge along the thickness of the tube. It is evident that the wall thickness of the pressure shell at this region was drastically reduced. Severe bulging in the ruptured area indicates that it was locally overheated. Figure 10 shows the general microstructure of the sample taken from the pressure shell from a region far away from the rupture. The structure shows ferrite (white grains) and perlite (darker grains) arranged in alternate bands.


The area surrounding the rupture contained creep fissures (See Figures 11 and 12) and the microstructure shows a sphereoidized structure in the perlite region due to high temperature (See Figure 13). There was no evidence of hydrogen damage in the samples examined. Mechanical properties of samples taken from the pressure shell ASTM A-516 Gr 70, close to and far from the burst-open region are shown in Table 1. They also discount the possibility of hydrogen attack, otherwise, the mechanical properties would have been severely reduced.

During boroscopic inspection from the bottom of the 101-CB boiler, at the time of annual turnaround, it was found that the gas distributor was buckled. The boiler tube bundle was lifted for further inspection. It was observed that the shroud around the inlet nozzle, along with the gas distributor, and part of the refractory in that zone were also damaged. Surface repair of the refractory was carried out and the shroud, along with the gas distributor, was replaced with Inconel- 601. A similar job was also carried out at the 101-CB boiler bottom in February 1995. Deterioration of the shroud at the bottom of the boiler, due to high temperature and carburization, in the other primary waste heat boilers, was also observed. The microstructure of the shroud material SS 310 (See Figure 14) shows severe carbide precipitation at the grain boundaries and intermetallic precipitates in the matrix of the

grains. The phase transformation occurred in the material at 980" C, over a long period.

It can be concluded that the failure of the pressure shell was a result of localized refractory damage and short-term high-temperature stress rupture. Due to the rise in temperature and internal gas pressure, hot ten- sile rupture had occurred. The reasons for the refractory failure might be due to repair work done during the turnaround at the boiler inlet nozzle area, or voids and cracks in the refractory in that region that might have gone unnoticed.

MEASURES TAKEN To avoid hydrogen embrittlement, the shroud material was changed to Inconel-601 instead of SS 310. Jacket water level and flow indications were brought on to DCS for close monitoring. Four thermocouples on each jacket pipe of the interconnecting pipe to the waste heat boilers were provided for benchmark and fiiture reference. It was decided to cut the water jacket for NDT inspection of critical areas of the shell during turn- arounds. Action for procuring a new boiler shell was initiat- ed as the fire-affected shell had bulged around the inlet nozzle area.

210 September 2002 Process Safety Progress (Vo1.21, No.3)

Monitoring of jacket vents for presence of explosive gases had been started. However, it was stopped as water vapor in the sample was damag- ing the explosimeter.

7bis paper (3d) was originally presented at the 46th Annual Safety in Ammonia Plants and Related Facili- ties Symposium held in Montreal, Quebec, Canada, from Janua y 14-1 7, 2002.

Figure 14. Microstructure of shroud SS 310 showing carbide precipitation(400x).


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Fire in secondary reformer outlet line to waste heat boiler