On January 31, 2023, at 6:40 a.m., an accidental release of high-pressure carbon dioxide (CO2) occurred when a vessel overpressured and catastrophically ruptured at a CO2 gas plant in Texas. Vessel fragments propelled up to 1,200 feet away and heavily damaged a control room (Figure 1). Kinder Morgan estimated the property damage to be over $14 million.
The incident occurred in the site’s CO2 compression system. Upstream of each compressor was a vessel called a scrubber that removed liquid from the CO2 feed stream. Removing liquid from compressor feed streams is critical to preventing compressor damage.
On one of the high-pressure compressor system scrubbers, a liquid level control valve (called a “dump valve”) became stuck in the open position, which was common at the site during cold weather. The outside temperature at the time was 22 degrees Fahrenheit (°F). The liquid drained out of the high-pressure scrubber through the stuck-open valve, and then high-pressure CO2 gas also started flowing out through the stuck-open valve. The high-pressure gas was released into the low-pressure drain system, and as it did, the CO2 became cold—as low as negative 30 °F (due to the Joule-Thompson effect).
The cold CO2 caused ice or hydrate to form in a drain system vessel’s outlet piping, including the piping to its emergency pressure relief valve. When this happened, the high-pressure CO2 could not vent through the pressure relief valve. The pressure in the drain system vessel continued to build until it reached about 550–700 pounds per square inch gauge (psig) and catastrophically ruptured. The vessel fractured in a brittle mode, producing fragments that flew up to 1,200 feet away.
The ruptured drain system vessel, which was made of carbon steel and measured 15 feet long by 6 feet wide, was rated to a maximum pressure of 125 psig at 650 °F. The blocked pressure relief valve had a set pressure of 15 psig. The scrubbers that drained liquid to the drain system operated at pressures ranging from 450-1,000 psig.
The failed vessel did not have instrumentation installed to allow operators to monitor conditions such as pressure, temperature, or level, and the scrubber liquid level and dump valve position were not recorded. The system’s lack of instrumentation prevented operations personnel from identifying the abnormal operating condition and taking corrective action.
In two previous process hazard analysis (PHA) reviews, the company had identified the potential for CO2 gas to blow through a scrubber dump valve, but this scenario was identified as an operational problem, not a safety problem. The PHAs did not identify the potential for ice or hydrate formation in the drain system due to a stuck-open dump valve. The PHAs also did not include a full review of the downstream closed drain system, as the closed drain system was viewed as a low-pressure utility system with low safety risk.
The company's investigation concluded that the incident resulted from the thermodynamic properties and system pressures not being sufficiently accounted for, resulting in inadequate vessel and piping design, inadequate overpressure protection, and inadequate instrumentation. The investigation recommended, among other corrective action items, that the site create an engineering design standard that addresses hazards associated with introducing high-pressure fluids into drain systems.
The company reported that 5.6 million pounds of carbon dioxide were released. In addition, the company reported that natural gas (111,000 pounds), hydrogen sulfide (2,000 pounds), carbon monoxide (1,400 pounds), sulfur dioxide (1,200 pounds), and nitrogen oxides (NOx) (260 pounds) were also released.
Probable Cause
Based on the company investigation, the CSB determined that the probable cause of the incident was the overpressure of a drain system vessel after a scrubber dump valve became stuck in the open position. Ice or hydrate formed in the vessel outlet piping, blocking the relief path. Inadequate vessel and drain system design and the lack of needed equipment instrumentation contributed to the incident.
Source:CSB.gov
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