Toxic and flammable gas release from steels production plant

The release of a blast furnace gas occurred at a section of pipework, due to the rupture of valve seal. The gas did not ignite and the fire service attended as a precaution. The gas was both flammable and toxic, containing 21% carbon monoxide and 5% hydrogen.

The onsite emergency plan was activated and the workers sheltered in toxic gas refuges. A concentration of 400 ppm carbon monoxide was measured 200 metres away at the power station, but there were no off-site effects.
The blast furnace was shutdown, with all the gas being discharged through the flare stack.

The investigation identified the sequence of initiating events as follows (see eMARS source):
The plate valve affected by the release was used for isolation of gas main line during shutdowns.
1. During operation it was kept open, but closed due to an electrical short-circuit which caused the hydraulic closure of the valve.
2. The valve did not fully close and the seal was blown off by the gas flow.
3. The leak occurred through the annulus of the valve.
4. The blast furnace shut down but the residual pressure in the gas holder led further release, some gas flared off, but approximately 750,000 cubic metres of gas were lost via the valve.

The INITIATING cause of the release was the wrong automatic closure of a main valve, with consequent rupture of its seal.

The root cause may be the lack of assessment of management system in place to prevent and mitigate potential major accidents (at the time of the accident, the plant had just been classified as COMAH top-tier).

seal, valve, blast furnace

DESCRIPTION OF THE UNIT
The unit affected was the blast furnace system. The blast furnace gas formed during furnace operation is directed from the furnace through the demister into the clean gas system. The failed valve was the demister valve.
It is a plate valve of 3.5m in diameter and is a normally open valve. It is used to isolate the downstream part of the clean gas system when the furnace is off-blast, it is not an emergency shut-off valve.

The major hazard was represented by CO. A concentration of 400 ppm carbon monoxide was measured 200 metres away at the power station, but there were no off-site effects.
This has caused the loss of 130 tonnes of gas from the ruptured valve and a further 77 tonnes of gas from the flare stack. The gas was unburnt because the pilot flame was not alight
(Source: competent authority investigation).

The steel manufacturer remained shut down for over 1 week at a cost of over £1 million.

The affected plant is a COMAH top tier establishment because of the inventory of toxic and flammable gas in the blast furnaces and in storage. The establishment was not covered by the previous CIMAH regulations. This is an ECRA because the loss of 43 tonnes of carbon monoxide is 22% of the top tier threshold inventory.

[COMAH = Control of Major Accident Hazards, a Regulation of 1999 implementing the SEVESO II Directive
ECRA = European Commission Reportable Accidents]

The accidental closure of a valve during operation due to a short-circuit shows that the fault event tree analysis had not been performed with the required accuracy. In particularly, the unit's isolation procedures need to be reviewed to ensure sufficient monitoring as part of safe system.

The was only recently classified as COMAH top-tier site and little previous work has been done to evaluate the management system in place to prevent and mitigate potential major accidents.
As a consequence of the accidents and the related investigation conclusion, the HSE management model was applied and a management system audit proposed.

required to prevent recurrence:
(i) To increase monitoring of isolations when in operation.
(ii) To review and improve flare system operation.
(iii) To review alarm handling.