There are many designs of storage systems where multiple vessels might be needed to obtain the required storage quantity. Regulations differ between vessels and modules which are intended for stationary or transportation purposes. Similarly, there are differences in codes globally. The issues of requiring shutoff valves on individual vessels and requiring TPRD’s are linked since generally vessels need to be in communication with the pressure relief system. A valve on each vessel will require one or more TPRD on each vessel.

There are trade-offs between the risk of having a large number of valves and TPRD’s, each of which is a potential leak/release source, and having a large number of vessels manifolded together with fewer valves/TPRD’s but then larger banks that contain larger quantity of hydrogen that will be released in a single event. The main disadvantage of multiple vessels being connected to a single TPRD is that a much larger release is possible since each vessel can’t be isolated. This may also require a much larger device to be able to vent the multiple vessels which then means a higher release rate with the associated larger vapor cloud, risk of explosion, and radiation profile. The decision is dependent upon the risk assessment and might be
different depending on the location and application. When a tube has its own, individual TPRD and isolation valve (which must be closed during transport), a smaller release of hydrogen would occur.

When it is necessary or desirable to work on a part of a hydrogen system while another part of the system remains in operation or in standby condition. An example is a system with two compressors, where one is normally operating and the other acts as an installed spare. The two block valves are closed, and the bleed (vent) valve is open. This arrangement assures that any hydrogen leaking through the valve on the pressurized side will be vented to atmosphere rather than entering the part of the system being maintained.

The need for DBB is generally identified during the hazard analysis. Important variables include liquid or gas service, pressures, and procedures for operations and maintenance. Some organizations have a matrix for isolation required based on risk, frequency of maintenance activity, and work permit requirements.  High severity or frequent maintenance will usually be DBB. Low frequency, low hazard or where additional protective measures are present will not.   

Safety codes globally have a requirement to provide a positive means to isolate energy sources and hazardous substances prior to performing maintenance. For gaseous hydrogen systems, methods such as a blind flange, a double block valve arrangement or a double block and bleed valve arrangement can provide that positive isolation.

Installing a blind flange requires breaking the supply line and inserting a solid insert that blocks the flow. The disadvantage of this approach is that it is more laborious than the other options and a method of isolation is needed to safely install the blind flange. The components involved are a lower cost than the other options but that cost is offset by the additional labor and system down-time required. For these reasons, they typically are only used for long term isolation.

A double valve arrangement is an effective approach that can be implemented quickly. A disadvantage of the double valve is that hydrogen may leak through the first valve and allow pressure to build between the valves without any indication. Aside from leading to a false sense of security, the pressure may also push its way through the second valve into the downstream plumbing and work area. While it may seem unlikely for two valves to leak, there sometimes is a common mode failure where both valves are damaged at the same time.

A double-block-and-bleed valve arrangement has a third valve to act as a means to vent, or "bleed" pressure between the two block valves. . In this configuration, leak through of the first valve cannot pressurize the second blocking valve, thereby eliminating the leak-through failure mode of the double block valve arrangement. For hydrogen systems, the outlet of the bleed valve should be routed to a safe venting location. A double block and bleed system can also be automated. In that situation the block valves are designed to fail closed and the bleed valve to fail open. Double block and bleed valves can also be used to safety depressurize and vent the downstream section prior to the isolation.

It is difficult to provide trustworthy answers to these questions without understanding the design and configuration of the specific installation. It may be best to consult with a pressure systems expert to evaluate the specific installation and uses. The gas provider may also be a good resource for specifics on gas equipment use. Other beneficial resources include the HSP Best Practices online resource and the DOE Hydrogen Safety Training for Researchers, the AIChE Laboratory Safety Course.

NFPA 2, Hydrogen Technologies Code, and NFPA 45, Standard on Fire Protection for Laboratories Using Chemicals, may also be helpful in identifying requirements and best practices. In addition, the project designed should consult with the local authority having jurisdiction for safety guidance that is applicable to locally adopted codes.