A natural gas processing plant is a facility designed to clean raw natural gas by separating impurities and various non-methane hydrocarbons and fluids to produce dry natural gas.
Processing raw natural gas begins at the inlet gas piping or gas stream of a natural gas processing plant. This piping delivers the raw natural gas to each processing vessel. Two of these processing vessels are an inlet gas-liquid separator and an inlet filter. As the raw natural gas enters the inlet gas-liquid separator the design of the vessel separates most of the liquids from the gases. Separating the liquids from the gases reduces the likelihood of liquid slug or ice and hydrating dams downstream.
As the gas continues downstream of the gas-liquid separator, it enters the first in a series of two inlet filters. After the first inlet filter vessel, additional gas is introduced from two other sources into the gas stream, and then the gas stream enters the second inlet filter vessel. As gas flows through each inlet filter vessel, gas passes through filters that remove solids from the gas and allow liquids to be drained away.
Each inlet filter vessel has a primary pressure vessel with a 54” access hatch at one end to remove filters, a 24” gas inlet and outlet, and a secondary collection vessel underneath to collect liquids. This vessel also has other outlets that allow operators to vent the vessel and verify pressure within the vessel. See the pressure vessel diagram.
During the operation process, trending indicated that pressures were erratic on the inlet piping upstream of the second inlet filter. This erratic pressure represented a
potential blockage in the inlet piping or the inlet filter vessel. So, on February 16, a plan was made to open both inlet filter vessels on the morning of February 20, 2023, to investigate the erratic pressures and replace the filters.
On the morning of February 20, 2023, three operators were tasked with opening the 54” access hatch to remove the filters. These operators first needed to isolate and
depressurize the inlet filter vessel, and a predetermined Lock-Out-Tag-Out (LOTO) procedure would be utilized to ensure a safe opening. However, as the operators
went through the steps of the LOTO they failed to confirm zero pressure inside the vessel with the control room. They also failed to consider the presence of a blockage
inside the inlet filter vessel.
Once the operators reached the last step of the LOTO and removed a bleed plug to verify complete depressurization. They witnessed a normal amount of pressure
released from the vessel that quickly ended. However, this pressure release only came from a section downstream of the blockage. So, believing there was zero
pressure in the vessel, the operators proceeded with loosening the clamp holding the 54” access hatch closed. Once the final bolt holding the access hatch closed was
loose the access hatch unexpectedly opened. Releasing the filters, ice, and hydrate from the vessel striking one operator on the torso and leg. See the failure scenario
diagram and aerial diagram.