Explosion at the electrolysis unit of a chemical factory

An explosion occurred at a Lurgi electrolyser of a chemical factory. The plant suffered extensive damage and one man died. The investigation concluded that the explosion probably occurred on the oxygen separating drum into which hydrogen had leaked. After the accident the company carried out a mass balance on the hydrogen flows just prior to the explosion and found that no less than 50% was unaccounted for.

The ingress of hydrogen into the oxygen drum was apparently due to corrosion/erosion in the electrolysis cells. The internal breakdown of the cells had probably been initiated some time before the accident. Three days before, a cracking noise was heard in the cell block, which may have indicated minor explosions. Based on explosion damage, the HSE report calculated that the oxygen drum contained a 13.5% hydrogen, 86.5% oxygen mixture, and the explosion produced a shock wave equivalent to 22 kg of TNT.

The INITIATING CAUSE was the degradation of the electrolytic cells, with hydrogen entering the oxygen section of the electrolyser, causing an explosion.

A continuous gas quality control would have indicated presence of H2 on the O2 stream. Indeed, there was a system of monitoring the purity of the hydrogen and oxygen streams by hourly gas analyses performed by the process operator. The evidence suggested that these analyses were not always carried out and that assumed values were entered in the process log. One operator stated that he only did the analyses two or three times in every 12 h.

The failure to execute a critical safety procedure indicates shortcoming in management of the operations (ensuring execution of procedures) and a lack of safety culture. This statement is reinforced by the fact that 3 days before a precursor of the incident occurred and was noticed, but operators failed to take actions.

alkaline electrolyser cells, oxygen tank (by-pproduct)

DESCRIPTION OF THE PROCESS
Lurgi is a brand name of technology developed by by one company for the production of chemicals. "Lurgi electrolysis" is an electrolysis system based on the alkaline elctrolysis principle, able to produce high-purity hydrogen at high pressure (30 bar) without a compression step.
It uses a potassium hydroxide eletrolyte. The cells have a diamter of 1.5 m and a thickness of 25 mm.

The lightweight roof was almost completely blown off the building (fig 1) but adjacent and other buildings were hardly affected. There was no damage outside the factory, but extensive damage to the electrolyser and the death of the plant operator from injuries (caustic burns).

Based on calculations, the HSE reort concluded that this explosion should have caused an overpressure of 1.03 kPa at 220 m and of 0.21 kPa at 660 m on an open site and was expected to produce damage to buildings up to a distance of hundredths of metres. On the contrary, no damaged occurred. Probably this was because the electrolysis plant was housed in a building designed and constructed to direct any blast upward.

The event was the object of a court case. The company was fined £300.

The HSE report concluded that the principles of the legislation which regulated the inspection of steam boilers, air receivers, etc., should be applied as well to pressure systems of this type.

Moreover, the following precautions should be taken in the operation of this type of plant:

1. Both hydrogen and oxygen quality should be monitored by intrinsically safe continuous analysers linked to indicator /recorder/controllers. These should actuate visual and audible alarms when oxygen purity falls to 98.8% or hydrogen purity to 99.7% and shut down the plant when oxygen purity falls to 98% or hydrogen purity to 99.5%. The continuous monitoring instruments should be regularly checked, serviced and calibrated.

2. The continuous monitoring should be backed up by hourly manual gas analyses carried out by the operators, and these in turn should be checked by a similar analysis by laboratory (or other skilled) personnel once every 24 hours. Persons making such tests should be under clear instructions to report any variations in gas purity and to close the plant down instantly in the event that purity falls below 98% for oxygen or 99.5% for hydrogen.

3. The internal condition of the plant should be systematically monitored :
(i) By the measurement and study of sludge formation.
(ii) By the studying of the pattern of cell voltages and temperatures and gas /electrolyte temperature.
(iii) By the internal examination of the gas ducts by means of a remote viewing instrument.