fire at the vent stack of a hydrogen production plant

A fire at the exit of the vent line occurred when filling a tube trailer. An employee suspected a flame at the outlet of the hydrogen venting line during the filling of a trailer., by sawing the air shimmering around the blowout.
The operator made a plant shutdown and tried to find out the cause. Since the vent line was collecting hydrogen streams from several units at different pressures, the provenience of the vented hydrogen was not easy identifiable. They found a not fully closed hand valve. It was a venting valve from the trailer filling station. After closing it, they discovered no further problem, restarted the plant and continued to fill the trailer.
The inspectors confirmed the occurrence of a flame based on clearly visible tempering colours at the blowout of the vent line. They found also a plate mounted at the blowout, which was not foreseen by the original design. It had been installed to impede bird nesting. It is probable that the increased flow of hydrogen at the vent caused by the left open valve, was ignited by the sharp edge of the plate.

The INITIATING CAUSE of the accidental release was a valve left partially open.
The ROOT CAUSE could be attributed to the difficulty for the operator to check if the valve was fully closed. A contributing factor was the ignition of the vented hydrogen, probably due to the mounting of a metallic plate (not foreseen in the original vent design) at the blowdown location.

vent valve,
vent stack, H2 filling station

DESCRIPTION OF THE FACILITY
This power-to gas plant was coupling an hydro-electric plant with the production of renewable hydrogen. It included a cooler, a compressor, electrolysis with purification and drying unit, a hydrogen storage, a trailer filling station and a research facility.
The electrolyser was an alkaline electrolyser with Nominal power: 1 MW , Production capacity: 200 Nm³/h hydrogen (ca. 450 kg/d), electrolytic fluid: demineralized water with a 30% concentration of potassium hydroxide (KOH).
The electrolyser delivered hydrogen at 20 bar and 80°. After the purified step to a 5.0 purity by removing contaminations and humidity, it was compressed to 300 bar and stored on site.
The storage had a nominal capacity of 1,2 tons hydrogen and had three trailer filling point to fill hydrogen trailer at 200 bar, with a nominal delivery capacity of 1500 kg/d.
The vent line was collecting hydrogen streams from several release points:
(1) Safety valves of the electrolyser (30 bar) , the compressor (30 to 300 bar) and the storage (300 bar)
(1) Regeneration of the drying unit (30 bar)
(2) Venting and safety valves from the trailer filling station (10 to 200 bar)

Nobody was injured, no property loss except some loss of hydrogen.

This is a small incident with negligible consequence. However, some aspects deserve to be highlighted for a return of experience.

(1) The operator delayed to the evening of the incident day the communication of the incident to the regional authority, not using the prescribed communication channel. The authority ordered an immediate shutdown and visited the plant. This caused obviously delays in plant operation and deteriorated the relationship between operator and surveillance authority

(2) The regional authority experts found a perforated metallic plate mounted at the blowout of the collecting vent line. The operator had installed it fearing nesting of small birds. The authority considered this measure not necessary because birds do not use nest places where frequent blowouts take place. More importantly, the operator did not perform a risk assessment able to assess the consequences the plate could have on the normal and emergency operations of the facility. The plate was reduction the vent cross section available for the flow and could cause an increased backpressure at the safety valves. A change in the backpressure will influence the set pressure at which the safety valve will open.

(3) A not properly closed hand valve in the trailer filling station was the cause of the hydrogen release. After connecting the trailer with the connection line of the station, all the trailer lines and connections are flushed backwards from trailer to station with the remaining hydrogen pressure in the trailer. This flushing stream is also released by the vent line to the blowout. This flushing must be done several times and therefore the upper hand valve must be opened and closed every time. After that, the filling can start. For the operator thought the valve was close. However, it was not visible or touchable if the valve is fully closed or not. This impedes an independent check of the execution of the operation.

The authorised expert required several measures, among which the most relevant were:
(1) A temperature measuring must be installed to detect a hydrogen fire, connected to the process control system.
(2) The hand valve must be changed from a valve rod type with a ballpoint to a regulating tip type. The reason is the difficulty to regulate flow by means of a ballpoint valve: already a small opening could lead to a relatively big section allowing high hydrogen flows.