Hydrogen Tech World December 2022 | Page 36

[ safety ]
spray water on the vent stack outlet . Only helium should be used for LH2 systems if a purge gas is required , since it is the only gas that will not solidify at LH2 temperatures .
3 . Incorrect supports The design of supports for vent stacks should consider reaction forces ( especially from pressure safety relief valve activating ), local wind , and seismic forces , and ensure that the supports are designed for vent system expansion / contraction due to temperature changes . The end of the vent stack must be supported such that the reaction forces sum to zero . This is accomplished by vertical flow upward or a tee stack that equalizes the flow through both opposing legs of the tee . ‘ L ’ type or single-exit horizontal flow direction vent stacks are discouraged due to the large directional reaction forces . Even with tee stacks , harmonic motion can occur if the top vent stack outlet supports are more than 0.5 – 0.6 m ( 1.5 – 2 ft ) below the top of the vent stack .
4 . Incorrect piping expansion / contraction Consider pipe and fitting expansion / contraction due to temperature changes . Make sure piping connected to the vent piping ( such as conduit or propane for flaring ) expands or contracts at the same rate as the hydrogen piping or the attached conduit / piping is designed to move independently of the hydrogen vent stack . Installation errors have caused vent stack failures from changes in flow direction ( forces on elbows ) and the use of compression fittings on thick walls and / or large-diameter tubing being incorrectly installed . Changes in flow direction create reaction forces within the piping that can result in failures . Typically , vent systems are not pressure tested after installation ; thus , incorrectly connected joints may fail during the operation of the vent system . An example of this is swaged tubing on thick-walled or large-diameter piping . This type of connection requires hydraulic machinery to ensure a correct connection with good mechanical integrity .
5 . Incorrect temperature rating for joints or vent system Ensure the melting point for the joints and vent system components is above the hydrogen flame temperature during a release . This is especially a risk for plastic piping and copper-brazed joints . Welded 304 or 316-type stainless steel is the recommended material for the vent system . Plastic , cast iron , ductile or malleable iron , and high silicone should be avoided . Materials with low melting points ( i . e ., aluminum , copper , brass , and bronze ) are not recommended because of their reduced strength at high temperatures . Carbon steel and many polymers are not suitable materials for LH2 hydrogen systems with operating temperatures below -29 ° C ( -20 ° F ).
6 . Air ingress during a release Ensure there are no openings in the vent system that can pull air into the vent stack during a flow event ( i . e ., from a venturi effect ), which may result in a fire or deflagration in the vent stack .
7 . Undersizing the vent system Vent systems should be designed for the maximum peak flow at the operating pressure of every stream that can flow simultaneously . The design should address the distance and pressure drop of each flow stream .
8 . Interaction between flow streams Ensure any interaction between the flow streams is considered for all the design parameters . For example , combining multiple hydrogen streams into one stack is common . Designers should ask whether a single vent stack is acceptable or if multiple vent stacks are needed . Operating conditions should be evaluated in this instance . In the case of two streams – e . g ., one with a pressure of 700 barg and the other at 12 barg – two stacks are recommended . Another scenario may be combining GH2 at cold temperature ( from an LH2 source ) with a stream of GH2 at ambient
36 Hydrogen Tech World | Issue 7 | December 2022