Explosion at oil storage tank in a refinery

This incident occurred at a plant demonstrating a new technology for the catalytic cracking of vacuum residual oil.
An oil storage tank failed due to high pressure. The overpressure in the tank was the result of a backflow of compressed hydrogen gas. When operators reduced the discharge pressure of the feedstock feed pump, high-pressure hydrogen gas in the reaction system flowed back into a feedstock surge drum and then into the cooling oil receiver.
The pressure of the cooling oil receiver increased, and the oil blew out and the feedstock surge drum ruptured and a fire occurred.

DETAILED ACCIDENTAL SEQUENCE:
in the morning, the unit was SHUTDOWN to replace heavy gas oil piping.
13:00 - A shift leader ordered two operators to stop the feedstock feed pump.
13:08 - One of the operators checked the discharge pressure, which was 19 MPa, and directed the other operator to open the circulation line valve to the feedstock surge drum.
After they informed the control room, they started the operation of opening the valve on the circulation line.
13:15 - As the discharge pressure fell to 1.65 MPa, they began to close the valve at the discharge side of the pump.
13:16 - When the spindle stroke of the discharge valve was 50%, they noticed oil blowing out from the manhole of the cooling oil receiver.
13:17 - They informed to the control room of the oil blowout. When they ran to the site to check what had happened, the feedstock surge drum burned out.

The INITIATING cause of the incident was backflow of compressed hydrogen gas to the oil storage tank, which ruptured due to overpressure. The backflow itself was either caused (i) because of the outlet valve on the pump was not completely closed or (ii) because a combination of a pressure decrease at the outlet of the pump and the non-return value failure as a result of excessive opening of the valve.
According to the analysis of the source, the root cause was a combination of:
(1) The procedure was wrong (the valve operations should have been inverted)
(2) the overall manual character of the operation increases the chance to mistakes,
(3) possibly lack of specific knowledge could be as well a contributing cause.

oil storage tank, oil and hydrogen line, non-return valve

The unit was shutdown, to allow for piping replacement.

Only dmage to the oil tank

The plant was put in emergency shutdown.

Check valves sometimes do not work well when it is necessary.
An operation manual on actions to be taken assuming the check valve does not work is needed.
Basic knowledge on gas dynamics is required: for example, it should not be forgotten that fluid flows from high-pressure to low-pressure.

TECHNICAL
A restriction orifice shall be installed in the circulation line piping.
A check valve shall be replaced.
The back-flow monitoring system shall be changed.
The emergency shutdown valve should be reviewed.

Operation procedures have to be revised.