Hydrogen leaked from the outlet piping of a hydrogen heating furnace at a fuel oil desulfurization cracking unit during normal refinery operation. The leaking hydrogen caused a localized fire. Dilution water for cleaning polythionic acid collected in the drain nozzle after a turnaround shutdown. The chlorine concentration in this dilution water was high because its concentration in the industrial water was originally high. The chlorine in the industrial water was concentrated by the high temperature, after the plant was restarted, and stress corrosion cracking occurred. Hydrogen leaked and was ignited by static electricity or heat.
A fire occurred at the fuel oil desulfurization cracking unit of a refinery 257 hours after startup of the plant, following a turnaround shutdown. Stress corrosion cracking occurred at the drain valve connection piping located upstream of an emergency shutoff valve at the outlet of a hydrogen heating furnace, resulting in a hydrogen leak and fire. The design operating temperature of the unit was 538 °C.
During the turnaround shutdown, the desulfurization cracking reactor was neutralized and washed with a polythionic acid aqueous solution. The solution remained at the bottom of the check valve after flowing out of the drain valve. During startup, the remaining polythionic acid aqueous solution accumulated at the drain nozzle of the reactor inlet piping. When the high-temperature hydrodesulfurization gas was introduced, the moisture evaporated and the chlorine became concentrated. The temperature in the drain nozzle rose to the temperature at which stress corrosion cracking (SCC) could occur. SCC occurred in a high-stress component, and hydrodesulfurization gas leaked. The cause of ignition was considered to be either static electric sparks or leaking of high-temperature gas into the atmosphere. The main component of the leaking gas was hydrogen, which has a low ignition energy.
Although the drain piping material was austenite stainless steel, which could generate SCC, it is acceptable for normal operation and does not cause any problem if pure water is used for polythionic acid treatment.
Industrial water was used to dilute the polythionic acid. Therefore, the chlorine concentration was high in the polythionic acid aqueous solution that accumulated in the drain valve nozzle. In addition, the chlorine was concentrated by the evaporation of water due to high temperature after starting operation, and SCC occurred. Water management using industrial water is a basic factor, though there was a possibility of SCC due to chlorine.
The stress was generated by thermal expansion at the gusset supporting the drain piping due to a temperature rise after starting operation. There might have been a design error related to the support.
July 3rd, 1999, 22:15. An operator on routine patrol found flames coming out near the emergency shutoff valve at the hydrogen gas heating furnace (F-102) outlet of the fuel oil desulfurization cracking unit. It was reported to the control room immediately.
22:17. Emergency shutdown procedures were carried out and the heating furnace (F-102) was shut down.
22:27. An emergency call was made. The self-defense disaster prevention organization was set up.
22:35. The private fire brigade started fire extinguishing activities until the public fire brigade arrived.
23:56. The fire-extinguishing activities stopped.
July 4th, 00:14. Flames were seen again coming from heat insulation of piping between the check valve and the emergency shutoff valve.
00:18. Fire extinguishing work using steam was started.
01:01. The fire was confirmed to have been extinguished.
- Heating Equipment
- Heat Exchanger
The dilution water for making a polythionic acid aqueous solution was changed from industrial water to pure water to lower the chlorine concentration in the piping.
Water that accumulated in the drain was removed before heating began. Steam tracing was strengthened to prevent condensation of moisture.
The gusset was remodeled into a structure in which thermal expansion is not restricted.
Careful management oversight of startup procedures is necessary to avoid stress corrosion cracking from occurring in hydrogen piping, since it can result in a serious accident. The chlorine concentration can be high in industrial water, and it is desirable to avoid treatment using water with a high chlorine concentration.