On November 29, 2020, at approximately 6:42 p.m., a major fire occurred at a Chemical Company in USA, after sections of piping supplying water to a coal gasification unit failed, releasing flammable chemicals that ignited (autoignition).The company estimated the property damage to be $1.1 million.
At the time of the incident, the company was feeding oxygen and a slurry of coal and water to one of its coal gasifiers to generate product gases, primarily hydrogen and carbon monoxide. The gasifier operated at a pressure of approximately 1,000 pounds per square inch. Pressurized water was injected into the gasifier to cool the hot product gases
Figure 1. Simplified diagram of the water supply system involved in the incident. (Credit: CSB)
At approximately 6:41 p.m., a pipe support failed near the water supply control valve at location #1, shown in Figure 1. This pipe support was welded to the water supply piping, and its failure opened a large hole in the piping, releasing most of the available water supply. Roughly one minute later, two of the four water supply pipes to the gasifier ruptured at location #2, shown in Figure 1, releasing hot, flammable product gases that autoignited, resulting in a fire around the gasifier. The company operators shut down the gasifier, which stopped the release of product gases, extinguishing the fire.
The company's investigation found that fatigue cracking at the pipe support weld caused the initial water supply piping failure. The piping support had severely corroded over time and was no longer supporting the pipe, as it had lost contact with the structure. The company’s last visual inspection of this piping system, which occurred in 2018, did not identify any problems with the pipe support.
With Water Supply 2 being released through the breached piping, Water Supply 1 should have been able to provide sufficient water flow to the gasifier. However, the check valve in the water supply piping had been stuck in the open position for an extended period prior to the incident and had not been inspected or otherwise tested. As a result, Water Supply 1 preferentially flowed through the open hole in the piping at location #1 (Figure 1).
Without a steady water supply to the gasifier, hot product gases exited into the water piping, creating localized high-pressure and high-temperature conditions that ruptured two of the four water supply piping connections to the gasifier. Hot, flammable product gases were released from these two ruptured pipe locations, resulting in the fire.
Before the incident, the company's process hazard analysis (PHA) had not accurately assessed the potential for loss of water flow to the gasifier. The company’s technical staff had mistakenly concluded that a loss of water flow would trigger a shutdown of the gasifier due to a low liquid level.
Probable Cause
Based on the company's investigation, the CSB determined that the probable cause of the incident was the rupture of piping connected to the gasifier. The failure of the piping released hot, flammable material from the gasifier that ignited, resulting in the fire. The gasifier piping failure was caused by the loss of water flow through the piping, which occurred when a large hole developed in a different section of piping in the water system after a welded pipe support broke, and a check valve in the piping system did not close because it was stuck in the open position. The company's process safety management systems contributed to the incident. The company did not maintain the integrity of the piping support, which was severely corroded. Additionally, it was not testing the integrity of the check valve, which had been inoperable for an extended period before the incident. Furthermore, not accurately assessing the loss of water flow in its PHA contributed to a lack of effective safeguards to protect the gasifier’s piping.
Source:CSB.gov
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