Waste Heat Boiler Failures in Ammonia Plants

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Papers presented at the AIChE’s Ammonia Safety Symposium over the years have described numerous waste heat boiler (WHB) failures. Poor boiler feedwater quality, inadequate design, and insufficient attention to fabrication details were the root causes of these failures. The safety and reliability of both front- and back-end waste heat boilers still need improving. The major failures, reported at the Symposiums, were:

• Jianfeng Chemicals, Fuling, Sichuan, People’s Republic of China. High boiler-water pH damaged a reformed gas WHB. The unit was repaired in-situ in 115 days.

• DSM Fertilizers, The Netherlands. Metal dusting occurred in three waste heat boilers. Low steam-to-carbon ratios and high front-end pressures in modern ammonia plants raise the CO content of the gas leaving the secondary reformer and widen the temperature range in which carbon can form. Preventing carbon formation and carbon adsorption or forming a less stable intermediate carbide can slow metal dusting.

• ICI Chemicals and Polymers, Billingham, England. The ferrules and refractory failed in its primary WHB. The inspection and repair, including ferrule replacement, took 35 days and 4,000 man-hours.

• Rashtriya Chemicals and Fertilisers, India. Rashtriya detected tube leaks in its reformed gas WHB after 600 days onstream. Cracks occurred behind the tube-to-tubesheet weld joint in the boiler’s hot compartment. The boiler’s water had sometimes fallen to a very low level while the plant was operating. About 33 tubes leaked badly and half of the tubes collapsed within one meter of the inlet. Some tubes had pits of two millimeters in diameter and one to two millimeters deep. A whitish phosphate deposit covered the tubes in spite of continuous and intermittent boiler blowdowns. In-situ retubing took 34 days.

• BASF ammonia plant No. 3, Ludwigshafen, Germany. A tube in the plant’s reformed gas WHB ruptured and caused an emergency shutdown. An inspection revealed one hole, several small leaks, and tube erosion. BASF replaced the boiler in a four-week shutdown. Debottlenecking the process air compressor had raised the plant’s throughput and the thermal load on the boiler, which had caused two-phase flow in the tubes.

• Krishak Bharati Cooperative (KRIBHCO), India. Sludge deposits that had been left over from initial chemical cleaning probably caused a reformed gas WHB to fail. KRIBHCO planned to install a new type of blowdown.

• P.T. Pupuk Kaltim, Indonesia. The bottom dished head of Kaltim’s primary WHB ruptured. A refractory failure was the cause. Kaltim replaced the head and cleared debris from the skirt.

• Fertilizers of Trinidad and Tobago. The shell of a primary WHB failed. Extensive cracking on the outer surface of the WHB shell extended downward to the center line of the process gas outlet nozzle. To prevent a recurrence, the level in the water jacket was raised 51 millimeters. Also, the plant considered coating the exposed shell and using an oxygen scavenger.

• Fauji Fertilizer, Pakistan. A double-compartment WHB downstream from the

secondary reformer failed mainly because the water level in the steam drum was lost. The tube-to-tubesheet ligaments were cracked and the tube-to-tubesheet weld, tube holes, ligaments, and welding lips had micro-cracks. A partial retubing was carried out, replacing 101 tubes in the upper portion. This repair lasted for only nine months. Finally, the WHB was replaced with a better design.

• Commercial Solvents Corp., Sterlington, LA. The primary WHB shell failed because the refractory deteriorated and allowed the wall temperatures to exceed design. Having to hammer the shroud slip-joints while installing the tube bundle probably damaged the refractory. Commercial Solvents replaced a section of the shell and installed separate flow meters on each water jacket. The new refractory was superior to the original material.

• Terra Chemicals, Sioux City, Iowa. High temperature caused the pressure shell of a primary WHB to suddenly rupture. The vessel was repaired and a new low-silica (less than 0.1 percent), bubbled alumina, refractory was installed.

• Petrochemical Industries Company, Kuwait. Two horizontal fire-tube WHBs failed. The boilers had long tubes and thick tube sheets, which were sensitive to thermal shocks. Both boilers were replaced with a double-compartment design after repairing and operating the old boilers for a few months.

• KRIBHCO, India. A fire occurred in the pipe between the secondary reformer and primary WHB. The pipe bulged and the water jacket burst open. All electrical and instrument cables in the area were damaged. Hydrogen attack or creep rupture were suspected. Thermal cycling of the transfer line may have damaged the refractory. To avoid hydrogen embrittlement, KRIBHCO changed the shroud material to Inconel-601. It also added a jacket-water level indication to the DCS and installed four thermocouples on each jacket of the pipe leading to the WHB.

• Terra Nitrogen, Courtright, Ontario, Canada. A tubesheet in a fire-tube reformed gas WHB failed catastrophically after nine years in service. A drop in gas temperature at the boiler inlet indicated a leak. Inspection revealed two failures. One was a tube failure just behind the tubesheet-to-tube weld joint. The other was a failure at the tubesheet-to-shell weld joint.

An increase in plant rate earlier in the year had caused the failure. A new boiler front-section was designed for higher plant rates and a better heat load distribution between the two sections. A new riser accommodated the higher steam flow and lessened the formation of steam pockets.

The new boiler design included the installation of larger diameter tubes in the front section to keep the heat flux below the critical value. The heat flux to the second section increased, but Terra was careful to keep it within the critical values. The new design necessitated a bigger diameter shell.

• Canadian Fertilizers (CF), Medicine Hat, Alberta. The shell of a primary WHB failed. Prolonged exposure of the shell to temperatures of 480ºC to 620ºC caused stress rupture (creep). No evidence of hydrogen-related damage or cracking was found. CF removed the stainless steel internal shrouds and refractory from the vessel and baked the entire shell section to remove hydrogen before welding. The incident interrupted production for 15 days. CF believed that 1.25 Cr-0.5 Mo or a higher alloy offered the increased margin of protection necessary to avoid shell failure from overheating.

• Asmidal, Arzew, Algeria. Reformed gas WHBs in two ammonia plants experienced several repeated failures. Local refractory failures had caused overheating. One boiler was repaired; the other replaced.

• KRIBHCO, India. A secondary WHB (102°C) failed in one of KRIBCHO’s ammonia plants. A reduced heat load had produced a lower-than-design circulation, which had allowed sludge to accumulate. Also, the unit had no intermittent blowdown or conventional solid treatment for the initial years. Also, improper tube expansion contributed to the failure.

KRIBHCO inserted sleeves in the weakened tubes to prolong their life and also introduced a new coordinated phosphate treatment and continuous blowdown system. Finally, KRIBHCO installed a new WHB.