See H2Tools, Best Practices: Purging, for a description of different purging approaches for hydrogen systems.
There are several resources that can help review designs, such as the Hydrogen Safety Panel and other outside consultants that are members of the Center for Hydrogen Safety.
Welded joints are always best, but they cannot always be used as a connection to tanks and tubes, as mechanical joints are needed for maintenance. Supports for the reaction forces can help ensure the mechanical joints in the piping does not pull apart.
If large diameter or thick-walled tube is installed with compression fittings, the use of hydraulic swaging is recommended.
Regardless of the piping method, reaction forces should be reviewed and supports designed for the reaction forces.
Pressure testing of the vent system is recommended to ensure the vent system will withstand relief device activation.
This decision would depend on the system design, system operation, and a hazard assessment. Likely it would be better to run all hydrogen vents to a common vent or flare system, but this might also restrict the ability to isolate smaller sections for maintenance.
CGA G-5.5 provides several options for vent stack outlets but not all options, nor does it tell when one outlet type is better than another design.
Figure 7, is one design, but in my opinion, not the best design. For instance, for warm gas, typical no-flow, normal scenarios (like a rupture disc on a vent system), Figure 5, a capped vent pipe is the simplest.
My preference depending on the application is a design not included yet in CGA G-5.5 or Figures 6 or 8 depending on if the flow must exit vertically or not.
Water, snow, or ice mustn’t plug the vent system, which may occur with Figure 7 if the weep holes become blocked.
The potential of an explosive atmosphere is inherent with any vent system and must be addressed through adequate design. Purging for most vent stacks is impractical due to availability or cost. In addition, and particularly for LH2 systems, the purge gas can cause potential safety issues. The primary way that explosive atmospheres are addressed is through ensuring that the design of the vent system can withstand an internal deflagration or detonation. This is not that difficult for smaller systems (less than 6”) but can be challenging when vent systems are larger and/or operate more as ducting than pipe. Where the vent system can’t be built strong enough for the potential internal overpressure, purging can be a necessary and prudent safeguard.
Additionally, the amount of O2 in the vent stacks is typically small (i.e. 1.22 scf /.1 lbs. in a 3” dia/25 ft tall vent stack). As hydrogen flows into the stack the time that there is a flammable (between 4 and 74%) region within the vent stack is also small.
For a detonation there must be the correct amount of hydrogen and oxygen. In a 3” vent stack, 25 ft tall there is ~ 1.25 cu ft of oxygen at atmospheric pressure. (=.1 lbs/.0032 lbmoles). The flammable range of H2 is 4-74% H2. At the stochiometric ratio, there is ~.0064 lbMoles of H2 that can react with the O2 in the vent stack. This represents ~.013 lb of h2 that can react. This is quite small amount energy release.
Calculations
Radius – 1.5”
Piping Volume = (1.5/12)^2*3.14*25 ft = 1.22 scf
Weight – 1.22 scf/12.08 scf/lb =.1 lb
Moles - .1 lb/32 lb/lbmole =.0032 lbmoles
H2 + ½ O2 = H20
.0064+.0032 = .0064
.0064 lb moles H2 X 2lb/lb mole = .0128 lb H2
Pressure relief systems may use reclosing devices like relief valves, non-reclosing devices like rupture discs, or a combination of both in parallel. Some systems may also be equipped with emergency blowdown systems that are operated by control systems. Selection of the proper devices is dependent on the system design and relative hazards. Variables that affect the selection include the type and size of vessel(s), location, pressure, and inventory.
The compressed gas industry is sensitive to the consequences of a premature activation of non-reclosing relief devices and the associated risk. More early activations have occurred than activations in real fire events. CGA S1.3, Pressure Relief Device Standards-Part 3-Stationary Storage, Containers for Compressed Gases allows for non-reclosing devices, but also recommends having a reclosing device as primary.
API 520, Sizing, Selection, and Installation of Pressure-relieving Devices Part I - Sizing and Selection, provides guidance on relief device selection and installation aimed at process plants. What might make sense in a process plant that has the potential for flammable liquid pool fires that might expose a gas storage vessel to an external fire for an extended period may not apply to other facilities.
Specific considerations not necessarily discussed in either CSA or API standards include:
· A prolonged fire exposure to a vessel may heat the vessel to a level where it is too weak to withstand the relief device set point. For this scenario, a reclosing device would not protect the vessel from reputing whereas a non-reclosing device might.
· Rapid depressurization of a vessel containing high pressure hydrogen can lead to cold temperatures at the nozzle of the vessel and to a lesser extent to the entire vessel. In an external fire case, the cold temperature would likely be mitigated. However, cold temperatures could develop in non-fire venting cases. For metal vessels, the strength of the vessel increases as the vessel cools, thereby reducing susceptibility to failure. But if the vessel is made from carbon or low alloy steel, the vessel may become vulnerable to brittle fracture.
· A depressurization with a non-reclosing device may form a large vapor cloud. Non-reclosing devices are typically larger and depressurize the vessels at a faster rate. There is a high probability that a vapor cloud will form and find an ignition source, resulting in a deflagration. The resultant fireball and overpressure can cause damage and injure people.
It depends on the facility and risk assessment, but generally multiple pressure and temperatures to one vent stack is not the best practice unless all are similar in pressure and temperature, and the streams have compatible composition and flow rate. Care must also be taken to prevent reverse flow and misdirected flow between portions of the system. Additionally, one vent stack can become a single mode of failure for an entire process or facility. Specific considerations for vent systems include the following:
Several codes and standards address vent systems, but not all topics are fully covered in each. Here is a list of codes and standards that address hydrogen vents:
ANSI/AIAA G-095A, Guide to Safety of Hydrogen and Hydrogen Systems (formerly NASA Hydrogen Safety Standard).
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.
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