Explosion at the electrolysis unit of a chemical factory

An explosion occurred a the 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. On April 2, a cracking noise was heard in the cell block, which may have indicated minor explosions. 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 report on the accident by the HSE (1976b) estimated that on the basis of the explosion damage, the 1690 l oxygen drum contained a 13.5% hydrogen, 86.5% oxygen mixture, and the explosion produced a shock wave equivalent to 22 kg of TNT. It was calculated that this explosion would have caused an overpressure of 1.03 kPa at 220 m and of 0.21 kPa at 660 m on an open site. The factory was in an urban area with houses 180 m and a school 210 m from the electrolysis plant. Although on an open site some damage might be expected from an explosion of the size described at these distances, none actually occurred. Probably this was due to the fact that the electrolysis plant was housed in a building designed and constructed to direct any blast upward.

INITIATING cause is corrosion, hydrogen entered in an oxygen drum being part of an electrolytic process, causing an explosion.
Root cause is related to lack of continuous gas quality control, which would have indicated presence of H2 on the O2 stream.

alkaline electrolyser cells, oxygen tank (by-pproduct)

DESCRIPTION OF THE PROCESS
Lurgi is a brand name of technology developed by by the speciality gases company Air Liquide 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.

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).

The Health and Safety Commission directed the Health and Safety Executive to perform the investigation.
The investigation report was presented to the Commission on 19 August 1975. HM Factory Inspectorate had by then decided that Laporte Industries Ltd should be prosecuted in the Magistrates Court, and an information alleging a breach of Section 2 of the Health and Safety at work Act 1974 had been laid. It was considered right to delay publication of the report until legal proceedings were completed. The hearing of the case was concluded on 17 March 1976 after several adjournments and the company was fined £300.

The principles of the legislation which regulates the thorough examination of steam boilers, air receivers, etc., should be extended to pressure systems of this type.

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:
(a) cell voltages and temperatures;
(b) gas /electrolyte temperature,
(iii) by the internal examination of the gas ducts by means of a remote viewing instrument