Fire in a refinery

The incident occurred at a hydrocracking unit, which had been shut down for maintenance. A fire occurred at a flange during the pressure tests before start-up. The fire resulted from ignition of high - pressure hydrogen-rich gas escaping from two shell-to-shell nozzle flanges on the hydrocracker reactor charge - effluent heat exchangers. Four workers had been sent to tighten flanges, which had been found leaking and had caused a partial depressurisation of the system. By the time they had assumed position, the pressure had dropped from 1600 psig to 1050 psig (respectively 110 bar and 72 bar).
At the instant the hammer wrench was struck for the second time the ignition of the leaking gas occurred followed at both flanges, followed by a flash fire and a continuous flame.
The unit was then depressurised via the relief system and the fire was extinguished after burning for approximately 45 minutes. Unit damage was confirmed to the insulation.

Probable causes of ignition were:
1. Spark caused by striking the hammer wrench
2. Static build-up from the escaping hydrogen
3. Auto ignition of hydrogen due to expansion heating
4. Auto ignition of iron sulphide scale.

The workers should have never bee sent at the palce of the leak without preventing search for a possible explosive gas (mixture) by gas detecting devices and strict safe procedures.

hydrocracker, flange

The hydrocracking unit had been shutdown for a scheduled regeneration of the preheater and hydrocracker reactors and minor maintenance. After completion of these actions, preparations were made to bring the unit on stream.
(2) The unit was nitrogen purged and evacuated several times and nitrogen was then fed into the pre-treating an hydrocracking sections, checking for leaks at 100 PSIG.
(2) The nitrogen was then vented to the relief system and the system evacuated Hydrogen-rich gas was introduced and again the preheater and hydrocracker were checked for leaks up to 450 PSIG.
(3) The make-up hydrogen compressor was placed in service and the pre-treatment unit system was checked for leaks at 1250 PSIG. A moderate leak was successfully stopped.
(4) The hydrocracker section was being raised to a final test pressure of 1600 PSIG when two shell-to-shell nozzle flange leaks developed in the inner connections of the hydrocracker reactor charge-effluent exchangers. These severe leaks developed from flanges that had not been disturbed during shutdown regeneration and start-up.
(5) The make-up hydrogen compressor was shut down at about the same time due to vibration. The loss of this and the leaks caused the system pressure to fail.

Damage to equipment (insulation) and 4 injuries (first and second degree burn).

Flange leaks are not unusual where hydrogen is circulated during start-up. It is common practice to fit known leak prone flanges with stream rings to prevent flash-fires and to heat flanges that are leaking as quickly as possible to normal process temperatures. Leaking flanges not equipped with permanent steam rings are to be protected with steam lances until the leak stops.

Unit operating and current maintenance practices include now the following corrective measures:
1. Continued use of nitrogen for evacuating and testing equipment prior to start-up; nitrogen to be used through the full range of test pressures.
2. Should major leaks occur during or after start-up with hydrogen in the system, the system pressure will be reduced to minimise leak. If a leak should continue at low pressure nitrogen will be readmitted before work is initiated.
3. Employees will wear protective clothing and equipment as required while stopping leaks occurring during and after start-up.
4. Exchanger flanges that have been opened and have leaked in the past will be tightened using hydraulic torque wrench equipment prior to pressure testing.
This equipment will also be used in stopping process leaks that are found during unit start-up.