Fire from a reactor at a chemical plant

This incident occurred at a reactor at a succinic acid manufacturing plant. The reactor was had just terminated the hydro-generation of maleic anhydride for manufacturing succinic acid. Hydrogen leaked from a mounting joint of a safety valve at the upper part of a reactor. The hydrogen ignited and formed a jet flame.
During inspection at a turnaround shutdown, the safety valve had been previously dismantled and remounted. By this last operation, a smaller gasket than the correct one was used, and on top of that, the force used to tighten the bolts was imperfect. Due to the small size of the gasket and the looseness of bolts, the weight of the piping caused the joint to incline and to develop a gap, which allowed the hydrogen to leak. While an operator went to the site for confirmation, he found a flame jetting from a flange that joined a safety valve to a reactor.

Event sequence:
On May 9th, 1998. During a turnaround shutdown, a safety valve of a reactor was removed for inspection.
On May 16th. The safety valve was mounted again with a fresh gasket.
On May 25th. An airtightness test at the joint was done.
On June 8th. Manufacture of succinic acid started. Raw materials were automatically charged to the reactor, and a reaction started.
two hours later, the feed rate of hydrogen fell, and pressure rose. A low-temperature alarm sounded simultaneously.
5 minutes later, an operator went to the site to open a steam valve. He found a pale flame from the upper part of the reactor. A hose was connected to the water service faucet and the fire was extinguished by spraying water. The operation of the reactor was stopped and nitrogen gas was introduced to the reactor. Occurrence of the fire was reported to the office.

The INITIATING CAUSE was a flange leaking hydrogen.

There was a double mistake during re-assembling after maintenance (wrong gasket and wrong tightening). On top of that, a check had been executed after reassembly and a leak test could not spot the mistakes.

The ROOT CAUSE was shortcoming in management of materials and parts and in inadequate maintenance and inspection procedures.

Gasket, valve

One month before the incident, a maintenance had been performed, and the affected valve dismantled and reassembled. The reaction process, nevertheless, had terminated according to plan.

DESCRIPTION of the CHEMICAL PROCESS
Succinic acid is a dicarboxylic acid with the chemical formula (CH2)2(CO2H)2. One of the common The industrial production route foresees hydrogenation of maleic acid (https://en.wikipedia.org/wiki/Succinic_acid)

none

no evacuation. The area was declared safe after 1/2 hour

The event generated the question on why are the post-assembly tests ineffective in spotting the mistakes.
The usual leak tests after construction consisted in a soap test following piping pressurisation. However, passing such a soap leak test is not a guarantee for tightness on the long-term. This is because the type of gasket and the bolt torque are specified according to physical properties of a flowing liquid, its temperature, long-term use, etc., and cannot be assessed by a short test before operation.
The check of the tightness of bolts is usually conducted by striking a bolt lightly with a small hammer (the so-called “hammering test”).
Since a measurement of torque was not carried out, it was very difficult to find such a defect from a general examination after completion of construction.

The plant improved the control of materials and parts replaced during maintenance.
The bolts shall be tightened equally and according to the requirements.
A new support for distributing the weight of piping was installed.