explosion on a testing unit of rockets

While preparations were being made for a LH2 turbopump test, an explosion occurred above the test cell. Major damage was caused throughout the test facility. A short duration fire caused minor damage.

At the time of explosion, a check was being made of the run and catch tank valves. The run tank valve was opened allowing LH2 to enter the run header. The run and return headers were at ambient temperature and the vent valves were closed. LH2 was also being transferred to the gas generator tank which required pressurisation of the LH2 run tank. Based on the estimated damage, with blast pressures of 2-3 psig, 10 pounds TNT equivalent was deto¬nated initially and followed by a second detonation equivalent to about 20 pounds TNT.

[Ordin, NASA (1974)]

The explosion was due to over-pressurisation of the bellows assembly in suction duct , followed by activation of a burst disk and subsequent release of about 60 pounds of hydrogen.
The IGNITION SOURCE was probably a static charge produced by torn edges of burst disk. Forcing the liquid hydrogen from the run tank into the warm header caused flashing of the liquid and buildup of high pressures sufficient to rupture bellows. The relief valves were not large enough to handle the gases evolved.
The bellows failure produced two separate clouds of hydrogen gas joined by a flammable cloud zone which allowed common ignition. A large amount of H2 was considered in the second cloud as being released from the run header.

bellow, burst disks, valves and header