Coke oven gas explosion at a steel plant

[Note of HIAD Validator: this is a preliminary event description, he CSB investigation is still ongoing]

An explosion occurred on an isolation valve on the piping providing coke oven gas to a coke oven. The event took place when workers were performing maintenance activities on the valve. The valve and the piping were in an underground facility, so that the effect of the explosion was considerable. The explosion heavily damaged the structures within the battery, damaging also the above ground components.

DETAILED TIMELINE
On the day of the incident, the company decided to perform a maintenance action on the battery gas isolation valve. [Note of HIAD Validator: the report does not specify it, but is plausible to assume that this action was necessary to be able to full close the isolation valve and isolate the battery].
10:30 a.m. three U.S. Steel employees went to the basement where the valve was located. They were equipped with personal carbon monoxide monitors, and one of them also with a detector of oxygen, carbon monoxide, hydrogen sulphide, and flammable gas.
The operation consisted in the injection of water toward the valve seat through a cleanout port, to clean it from gas residues impeding its full seating.
At the start of the operation, all monitors and detector triggered an alarm. The workers noticed water leaking from the valve’s bonnet flange, heard a “pop” sound, and smelled gas.
The supervisor ordered an evacuation of the basement, both verbally and by radio.
10:47 a.m - The explosion occurred.
10:48 a.m - Workers made the radio evacuation call.

The INITIATING CAUSE was the full rupture of the isolation valve during maintenance.

The reasons for the sudden break are not (yet) reported, because the investigation is not yet concluded. The following fact are known:
(1) The valve was rated for 50 psi.
(2) The maintenance procedure allowed injection of steam at 10 psi to heat the valve before exercising it.
(3) The procedure did not mention the use of water, but the crew started injecting water (at an unknown temperature).
(4) At the time of the incident, at least 22 company’s employees and three contractor’s employees were working on the affected batteries, and an inspection was taking place in the surrounding.

Based on these facts and the evolution of the incidents, the following PRELIMINARY ROOT CAUSES could be identified:
(a) A discrepancy between the operation described in the procedures and the actions performed during the maintenance works.
(b) An under-estimation of the risks related to the planned activities.
(c) Because of (b), failing in preventing measures, such as minimising personnel presence on the location of the activities.

manual gas isolation valve

On 8 of July, more than one month before the incident, a gas leak was discovered from a valve downstream of the isolation valve. The downstream valve was cracked near one of its flanges. A temporary repair was installed, preventing flammable coke oven gas from leaking into the basement. To replace the damaged valve, the coke oven battery had to be isolated from the coke oven gas supply and the piping purged. Additional valves replacements were planned profiting of the partial shutdown.

The gas isolation valve which failed in this event had been produced in 1950 and refurbished by a local vendor in 2013.
Occasionally difficulties were reported in fully seating valves when maintaining them, due to the accumulation of residue over time in the valve seat, coming from the processed coke oven gas.

DESCRIPTION OF THE PROCESS
The plant is a coke manufacturing, converting raw coal into coke, a carbon-rich substance used as fuel in blast furnaces for iron and steel production. This process occurs in coke ovens. A (coke) battery consists in a series of oven operated as one unit. The ovens are kept oxygen-free in order to prevent the coal from burning [12]. This heating process (more than 18 hours at ca. 11000 °C) removes tar, light oils, and other volatile compounds from the coal, which are collectively called “coke oven gas”. The coke oven gas is then processed in a downstream unit and part of it returned to the batteries to be used as fuel to heat the coke oven.

DESCRIPTION OF THE FACILITY
The coke oven gas processing unit supplied processed coke oven gas to the batteries through an underground piping system. The coke oven gas flow splits at a tee to direct it to one of a pair of batteries. After the tee, each battery was equipped with a manually operated isolation valve. It was this valve involved in this incident. Downstream of it, the coke oven gas further split into two parallel streams, each of which flowed down one side of the battery.

The PROCESSED COKE OVEN GAS is a mixture of hydrogen (40 to 60 weight percent), methane (20 to 30 weight percent), nitrogen (3 to 15 weight percent), and carbon monoxide (3 to 6 weight percent), along with smaller amounts of other compounds. It can also contain residue that can accumulate in the bottom of piping and in valve seats.

DESCRIPTION OF THE MAINTENANCE OPERATION:
Valves are subject to a lot of wear and tear, so regular industrial valve maintenance is necessary to keep them functioning correctly. Exercising the valve helps to prevent sticking and corrosion, and can also extend the life of the valve.
‘Exercising’ a gas isolation valve consists in closing and re-opening the valve to ensure it can fully seat and isolate downstream piping. In the company affected by this incident, due to the presence of solid residues in the gas flow which could further impede the full seating of the isolation valve, a practice was used of injecting steam into a valve’s cleanout port to attempt to flush the valve seat during the exercising procedure.

The explosion fatally injured two company employees. Four company employees and one sevice company empolyee were seriously injured. Six additonal workers sustained injuries that did not require immediate hospitalisation.

In this event, the coke oven gas supply piping was in a in a congested area of the basement, underneath a transfer area. Two furnaces and the transfer area were located roughly just 10-20 feet directly above the coke oven gas supply piping, release point for this incident. CSB has issued in December 2025 the following statements:
(1) The buildings could not withstand or protect their occupants from the explosion.
(2) A facility siting safety evaluation had been previously recommended, but the company did not consider it. A siting evaluation and the related hazards mitigation prior to the explosion could have reduced the severity of this incident by preventing the two fatalities and two of the five serious injuries.
(3) The company started efforts to rebuild the facilities and equipment, claiming new measures to avoid or reduce staff presence in those areas. However, without a facility siting evaluation, there are doubts on the strategies aiming at relocating workers.