LH2 valve leaking at a research centre

This near miss affected a testing installation for liquid hydrogen. A leak developed on a valve installed on a purge line connected with a LH2 storage tank. This shut-off valve served to discharge hydrogen into the exhaust stack of the installation, in case LH2 evaporation from the filling or withdrawal hose was causing excessive pressure. It was also serving a purging purpose, by flushing the filling hose with gaseous hydrogen before filling the cylinder with LH2.
The valve was more than 20 years and was not sealing properly under cryogenic conditions, causing hydrogen gas to escape from the threated connection between the valve stem and the valve case. The valve failed on two internal parts: the metal diaphragm and the O-ring that served as a seal (a sealing with thoric geometry). The metal diaphragm no longer seated properly against the valve seat causing leakage.

tank, piping

A new valve and a new piping system ahd to be installed.

none required

A new model from the same company had to be purchased, because the model of the defective valve was discontinued. The new valve was larger in outer dimensions than the old one. Therefore, the pipe routing around the valve had to be modified. A total of three new welds and an approximately 25 cm long, multiply bent new pipe section were required for installation. The tightness of the welds, the pipe, and the valve against the environment was successfully tested using a calibrated leak detector with a measuring range of from 0 to 1000 ppm H2.