Heat tracing can be used as a safeguard against freezing. However, it must be understood that: 

• The heat trace system uses a utility supply so may not always be operational.
• The electrical equipment must be properly classified for the area, which can be challenging for the Div 1 classification for vent stack outlets.
• The heat tracing might be damaged by vent stack fires.
• For liquid hydrogen system vents
  • Heat tracing is nearly useless for stopping freezing with a higher flow cold GH2 stream due to the amount of heat needed.
  • The vent stacks can develop liquid air, so the equipment must be robust to survive cryogenic conditions without damage.
• Maintenance on the heat trace equipment might require a system outage for personnel to work on it safely.

Purging is not recommended as a continuous part of vent stack operation. However, maintenance activity is a transient event and it’s prudent and recommended to purge a vent system prior to performing maintenance. It’s always possible that hydrogen could be leaking internally from a valve or other component and therefore create a hazard. Of particular note, care must be taken that proper isolation of the vent system is performed such that the vent system can’t be inadvertently used during maintenance. Since vent systems and stacks rarely have isolation valves to prevent unintended isolation of relief devices, proper maintenance on the vent system may require the entire system or plant to be taken offline.

Design of vent header lines is critical to the safety of the system. From a process perspective, the pipe design must be sufficient to withstand back pressure, thrust forces from the flow, and must be of a sufficient size to not create a restriction that prevents proper flow or activation of the devices. Per ASME BPV Code requirements, backpressure should be limited to no more than 10% of the set pressure.

When more than one source is connected to a single vent, two critical design issues are the pressure rating and flow capacity. The vent header should be of sufficient size to simultaneously meet the required flows from the different sources where it’s possible for them to activate at the same time. This is a particular concern where there may be many, sometimes even dozens, of devices on pressure vessels used for fire protection where all vessels can be exposed to fire at
once.

Pressure rating and set pressures of the devices are also a concern. For example, a 3000 psig set pressure device with the typical 10% allowable back pressure, would allow up to 300 psig in the vent header. If a 300 psig set pressure device were connected to the same header, then it would not activate if required due to that backpressure, leading to possible overpressure of the process system. Best practice would be to use different headers on systems that operate at significant differences in pressure.

Another consideration is to make sure that common vent headers do not create a common mode failure such that redundant devices could be blocked from a common failure. Care must also be taken that incompatible materials (e.g. hydrogen and oxygen) aren’t vented on a common manifold and that contamination (e.g. compressor oil) doesn’t affect other portions of
the system where a source of contamination is present. 

When designing a vent system, the designer must review in a process safety analysis that the hydrogen cannot flow to unexpected locations. It is never a good design to tie a hydrogen vent system into a building ventilation system.

Maintenance is also an issue since vent headers can be an overlooked cross tie between portions of systems that otherwise are properly isolated on the upstream side. For example, if maintenance is being performed on a relief device, and a separate device activates elsewhere on the same header, then backflow could create a hazard while the vent piping is disassembled.

Purging of vent systems is not required and in most instances is not recommended.

A nitrogen purge is generally not needed for a vent system designed in accordance with CGA G-5.5.  However, there are times where this might be considered or required due to the specific design of a system.  Where determined by a risk review to be needed, A continuous purge into a vent system reduces the probability air or oxygen in the piping. Intermittent purging should be evaluated, but if a constant flow of either nitrogen or hydrogen is provided, then the configuration should be adequate. For intermittent purging, an initial full purging with nitrogen or helium is the best safety practice.  If a design and HAZOP condition is that the vent system must be purged with an inert gas for safe operation, then the vent system should be purged prior to putting the system back in service. 

If purging becomes required for a liquid hydrogen vent system, the only acceptable gas per CGA G-5.5 is helium, as this is the only gas that does not solidify at liquid hydrogen temperatures.  

The cleanliness of an H2 piping system is often based on end use requirements since the purity of the system may impact the end use application. Cleanliness required for end use purity is usually much greater than that required to prevent condensation or oxygen content sufficient to create a safety risk. While not required for the same safety reasons as oxygen safety, specifying a system to be “oxygen clean” for systems where a moderate level of cleanliness is required is a typical practice.

 Documents such as CGA 4.1 provide instruction and inspection guidelines for oxygen cleaning. Systems that require very high purity, such as for electronics, will have custom cleaning specifications for that application. One safety-related cleanliness consideration is filtering of particulates since they can cause regulators and safety valves to not operate correctly. This is a frequent concern on recently assembled lines that can have residue from machining, thread cutting, and welding.

Proper purging procedures should be used when opening a system for maintenance to ensure that a flammable mixture of hydrogen can’t be formed when the system is exposed to air. Connections should be covered when open to avoid entry of particles and moisture. Moisture is a particular concern when opening LH2 lines since water vapor can condense on cold surfaces and result in plugging of lines over time.