Explosion in a hydrogen tank of a food producing plant

Hydrogen needed for the saturation of edible oil was stored at in a spheric tank of 13 m in diameter. The sphere was partitioned into two hemispheres by a neoprene diaphragm attached around the equator. Hydrogen was stored under the diaphragm while the upper hemisphere contained air. An explosion-proof fan was situated on top of the sphere to provide a slightly positive pressure on top of the diaphragm.
When the plant was shut down for the holidays, the fan on top of the hydrogen sphere was also stopped. During plant start up two days later, an explosion occurred in the sphere. The sphere shell was torn into many sections and some of the sections were propelled as far as 370 m. Neighbouring flammable liquid storage tanks and buildings were affected.

[Zalosh and Short, 1978]

The initiating cause was the leaking of the internal tank seal, with diffusion of hydrogen from the bottom half to the upper one containing air.
Ignition of the resulting hydrogen-air mixture was attributed to an electrostatic discharge caused by the motion of the diaphragm when a compressor was started downstream of the sphere. However, another possible ignition source was the explosion-proof fan at the top of the sphere.

A post-incident investigation indicated that hydrogen had leaked past the diaphragm when the fan was shut down. The safety design was inadequate, and also the management of the operation showed shortcoming, in allowing the arrest of a safety system (the fan), designed to create an overpressure the upper hemisphere and impeding hydrogen diffusion into it. Moreover, the fact that the diagraphm was replaced one year before, should have acted as warning.

diaphragm, tank, fan

The storage tank was a sphere of 42 1/2-ft (13 m) diameter, made of a of 3/16-in. (4 .8 mm) thick steel.
The tank fan had been shutdown two days before.

The diaphragm had been installed about a year before the explosion as a replacement for another leaky diaphragm.

Zalosh report mention 'no serious injuries', without quantifying this criterion or specifying if light injuries occurred.
The sections of the spheric tank destroyed by the explosion struck flammable liquid storage tanks and cracked the roofs of adjacent buildings. Most of the windows in the surrounding buildings were broken by the blast wave.
The damage was estimated in the range of 1100,000 and 500,000 US$

The explosion would have been avoided by simple preventing measures, such as avoided the use of an inert gas instead of air in the upper hemisphere, and/or by monitoring the hydrogen concentration there. The fact that the diaphragm had failed previously should have acted as a warning and triggered pro-actively design modification for a better safety. A near-miss is a powerful indication of shortcoming in the safety design of an installation.
it is not known if the designers and the operators of the tank were aware of the values of hydrogen permeability thorugh neoprene. Diffusion of hydrogen across the diaphragm could have been the result of this. Material tests to assess quantiatively their fitt-for-use, before design is a engineering good practice.