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The ASME BPV Code, and other Codes by reference, require less than back pressure of 10% of device set pressure from the release flowrate for proper operation of reclosing relief devices such as relief valves. Backpressure from non-reclosing or non-ASME devices may be higher so an analysis is required. It’s not enough to assume the vent system need only be designed for 10% of the set pressure.…

Category: System Design
Keywords: CGA, Vent, Relief valve

Purging can be accomplished in several ways including by using pressure cycles, sweeping, or vacuum. Sweeping is the least reliable but can be effective on simple pipe runs. In most cases, vent systems are open to the atmosphere and the ingress of air from the outlet is likely. However, the vent system should be designed to handle fire or explosion internally. This
generally is not…

Category: Miscellaneous
Keywords: Vent Stack, Purge

It is not possible to define ignition potential by just velocity without more data (i.e. pressure, materials involved, direction of impact). Due to the multiple methods of developing an ignition source (friction, impact, electrical charge) and the low ignition energy, it is assumed that hydrogen in the air will ignite (between 4 -74%), as it does 30-40% of the time with no known ignition…

Category: Miscellaneous

Several programs can predict this such as HyRAM or PHAST. The inputs are critical to a safe
answer.

This is not a simple answer due to the many types of flame lengths and flame orientations due to pressure and direction. NFPA 2 recommends that vent systems should be designed so that if the safety relief valve is relieving at capacity the radiative heat felt by an individual at grade…

Vent stacks and building ventilation systems are different and should be analyzed/designed differently. NFPA 2 has different location requirements for vent stack and ventilation system outlets. There are code requirements for elevation, distances from exposures, and between exposures. 

There are no specific regulatory or code requirements for vent system separation distances. These…

Category: System Design

The vent system should be designed for the temperature at which it operates (ambient for GH2 and Cryogenic for LH2). The outlet of the vent system should be designed for a fire to ensure the mechanical integrity of the vent system. 

The supports should also be designed for these temperatures and the associated expansion and contraction. 

It should be ensured that moisture…

Category: System Design

Several organizations published a paper together on this topic in 2017 (see attached). Based on comparisons with tests and CFD simulations, the following conclusions were drawn:

  1. The gas concentration for vapor cloud explosion blast load calculations for H2 jets can be limited to approximately 10% to 75%. Note that testing for H2-air VCEs in congested environments has been performed…
Category: Explosions

Refer to the white paper completed by the HSP for LH2. The same criteria should be applied to
a vent system. See below.

H2Tools Document Library: White_Paper-Qualified_Individuals_for_Liquid_Hydrogen 

Similar qualifications for vent system design include:

  1. A “qualified” individual for…
Category: System Design

No, but it depends on the application. Nearly all vents less than 4” in size are not purged with N2. This is primarily due to: 1) large flows required to dilute hydrogen below the flammable range, 2) the cost of the nitrogen, 3) the potential blockage of the stack when being inserted a vent header/stack serving a liquid hydrogen system, 4) the potential for backpressure (depending on the…

Category: Miscellaneous
Keywords: Vent, Purge, GH2, LH2

There is no specific requirement not to vent liquid hydrogen from a vent system. Best practice would be to only vent gas from the top of the vessel to relieve pressure. If liquid must be vented, it should be vaporized first. 

Note: It is very unusual to have LH2 flow from a liquid tank out the vent system, as the vent system is connected to the vapor space on the LH2 tanks and there…

Category: Miscellaneous
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