Hydrogen compressor explosion and fire at a refinery.

An explosion and fire occurred at a reciprocating recycle hydrogen compressor during commissioning of a new Diesel hydrotreatement Unit. It happened during the reactor pre-sulphidisation step, when the recycle gas contained 9000 ppm of H2S (hydrogen sulphide) and the pressure was at 940 psig (65 bar).
The failure of retaining bolts on head-end suction valve unloader of recycle cylinder on compressor allowed release of process gas.

The compressor was damaged.

The INITIATING CAUSE was the failure of valve bolts on the hydrogen compressor which recycles the H2S-containing hydrogen coming from the hydrotreater.
Investigations revealed that the bolts failed due to material inappropriate to prevent sulphide stress cracking) and inadequate design for the service. The manufacturer did not comply with the company's practice for reciprocating compressors in H2S applications.

Therefore, the ROOT CAUSE was the choice of incorrect material for the components of the valve, as delivered by the manufacturer. This mishap was not noticed/identified by the plant operators.

hydrogen recycling compressor, bolts

DESCRIPTION OF THE PROCESS
A Diesel Hydrotreater Unit removes impurities like sulphur and nitrogen from diesel, in the presence of a catalyst and hydrogen.

Damage to equipment US $100 000 (1993).
Damage to compressor shelter, instrumentation. Estimated 250 man hours spent on investigation.

The ICHEME database (see reference) lists the following lesson learnt:
(1) Standards are essential to the safety of any process plant project. These standards apply to design control, purchasing, construction, and inspection and testing of purchased equipment.
(2) Each group involved in a project, whether contractors, suppliers, designers, procurement, manufacture, construction, etc. plays a key role in assuring the equipment's fitness for use.
(3) All possible process conditions must be detailed in the specification for the purchase of equipment.
(4) HAZOP studies must include all deviations from the design operating conditions such as shutdown, start-up, maintenance, and other activities such as the pre-sulphiding process to check the adequacy of the design.