Fire in a LH2 Storage Vessel

The event occurred at a liquid hydrogen spheric storage consisting of two large spheric vessels.
A working team began a procedure to sample the LH2 storage vessels and the associated transfer pipe. This procedure was being performed to determine the conditions within the system.
The procedure consisted in the following operations:
(1) activation of the flare stack for the LH2 storage area.
(2) Pressurisation of storage tank number 1 with GN2 was initiated, with a target pressure of 10 psig, at which point samples were planned to be taken.
At 5 psig, a loud noise was heard in the upper area of tank number 2. Smoke was seen exiting the flare stack and from the insulation on vent lines for both tanks. At this time, tank number 1 was vented and the pressurisation system was secured.

Damage resulted to both storage vessels and to some of their piping. Especially vessels number 2 experienced extensive internal damage. There was further extensive damage to three bellows on a pressurisation line, vent line, and instrumentation port due to excessive axial movement. There was also evidence of heat buildup in both vent lines and charring of insulation.

The INITIATING CAUSE was the formation of an air-hydrogen explosive mixture in the liquid hydrogen storage and the related piping system.

The LH2 vessels and vent system were not properly maintained after the last use in 2003, nevertheless, the were left with hydrogen in the system.
A fault tree process was used to determine that air entered the system via two possible mechanisms:
(1) The vent line was disconnected at several places for installation of new bellows, allowing air to enter the vent line system; and
(2) the purge panel was deactivated allowing air to enter the system via the flare stack.
During the sampling procedure, the hydrogen/air mixture reached a level sufficient to allow combustion, and the flame from the flare stack pilot burners provided ignition. The result was a fire/deflagration that travelled back through the vent line and into the LH2 tanks, causing further fire/deflagration/detonation. Check valves and the vent valve in the system could not to arrest a hydrogen flame as it travelled in the vent system towards the tanks.
ROOT CAUSE was lack of (adequate) inspection and maintenance procedures specific to idling systems. Management failed in planning actions guaranteeing their safety.

vessels, stacks, piping, valves

Since 4 years, The LH2 vessels and vent system were not properly maintained.

DESCRIPTION OF THE CRYOGENIC STORAGE SYSTEM
The LH2 storage system consisted in 2 spherical pressure vessels with a volume of 225,000 US liquid gallons (850 m3, approximately 60 tons of liquid hydrogen), with a maximum working pressure of 50 psig (approximately 3.5 bar). An eight-inch transfer piping connected them to the usage point.

The mishap resulted in physical damage to both storage tanks, as well as to some of the piping for both tanks. External to the storage tanks, there was extensive damage to three bellows on a pressurization line, vent line, and instrumentation port due to excessive axial movement. There was also evidence of heat buildup in both vent lines. Significant charring was noted on the tank number 2 external vent line insulation. Internally, extensive damage to the number 2 storage tank was noted.
The internal bellows on the manway was distorted and ruptured. The pressurization ring was found lying in the bottom of the tank, where it had fallen after becoming dislodged from its holding brackets. The internal tank personnel ladder was also dislodged and lying against the side of the tank. The three anti-vortex baffles were displaced and the screens were destroyed. There was damage to the inactive level sensor system including cable damage. Tank wall discoloration as well as aluminum and metal slag found in the bottom of the tank and charring of the external vent line insulation indicated a high temperature mishap.

the investigation report issued the following recommendations:
Recommendation 1: The organisation should define necessary activities in order to place hydrogen systems in long term periods of inactivity. The defined activities should address requirements for rendering inert, isolation (i.e., physical disconnect, double block and bleed, etc.) and periodic monitoring.
Recommendation 2: The organisation should develop a process to periodically monitor hazardous systems for proper configuration (i.e., a daily/weekly/monthly check sheet to verify critical purges are active).

Corrective action included repair of the damaged hardware by a qualified contractor. The recommendations of the investigation report were adopted, by documenting organisational policy and procedures for establishing standby and mothball conditions for facilities and equipment.