What pressure should the hydrogen vents be designed for? Some guidelines mention 40 barg for deflagration/detonation, but it seems vendors do not consistently adhere to this recommendation.

Vent systems are typically open to the atmosphere, so it’s easy to overlook that they must be designed to withstand significant internal pressure. The two primary sources of pressure within vent systems are: 1) backpressure from the flowing gas, and 2) internal deflagration/detonation.

The large flows of gas exiting relief devices and vents will create backpressure within the vent system. The backpressure often must be limited to 10% to meet code requirements for reclosing relief devices such as relief valves, but can be significantly higher for non-reclosing relief devices such as rupture discs or TPRD’s. For high pressure systems such as fuel stations that might have relief devices set as high as 1000 bar, the back pressure can be substantial. Even 10% would be 100 bar, but there might be situations with rupture discs where backpressure as high as 400-500 bar is possible immediately after activation of the device. The maximum backpressure must be calculated during the design and the vent system appropriately
designed for that pressure.

Vent systems are also subject to internal deflagrations or detonations. The vent system may be full of air prior to a vent from a relief device or manual vent. For a short period of time, a flammable mixture may form until the air is swept out by the hydrogen. Due to the high length/diameter ratio of piping, this can lead to an internal deflagration and even a detonation.

The developed overpressure is a function of a number of variables but CGA S1.3 recommends to design the vent system to withstand 300 psig and comparable EIGA documents recommend 40 bar (~600 psig). These pressure ratings are fairly reasonable for typical vent systems below 6” size, but can be challenging for larger vent headers.