Hydrogen release from a desulphurisation unit in a refinery

The charge pump on a hydro-desulphurissation unit tripped. The charge flow control valve then opened wide on low flow. The high pressure hydrogen supplied to the hydrodesulphurisation unit flowed backwards through a faulty non-return valve to the charge tank.

The tank vents were not designed for this high flow of gas and so the weak roof to the wall seam failed. Fortunately, the gas did not find a source of ignition and so there was no fire. The pump could have failed due to reverse rotation, but did not.

The INITIATING cause was a mechanical equipment failure

The root cause could be perhaps found in a design or operation cause (failure of proper maintenance or inspection). The safety design did not take into account this type of event sequence. Backflow due to pump or compressor tripping is a well-known mishaps.

desulphurisation unit, control valve, pump tank

Tripping of the rector unit

According to the ICHEME database, the following measures were requested:

1. To identify all critical non-return valves which could cause a serious incident when failing. These valve have to be repaired to “as new condition” at scheduled shutdowns.

2. To identify the case where reverse flow should not be relied on as the sole means of preventing a serious incident. In those cases, to modify operations in such a way that the flow control valve close any time that the pump tripped.