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.

API 520, Sizing, Selection, and Installation of Pressure-relieving Devices Part I - Sizing and Selection, was written for use in the process industries in gas and liquid service.

CGA S1.3, Pressure Relief Device Standards-Part 3-Stationary Storage Containers for Compressed Gases was written for fixed equipment in gas service.

Which standard to use depends on the choice by the owner/designer considering regulations that may apply. In the USA, the API standard is used most often in process plants where local regulations for relief valve sizing usually do not apply. For other industrial facilities and for commercial and residential facilities, regulations frequently apply. The CGA standard is more often referenced in regulations.

Equation 6.3.1.1 in CGA S1.3 is based on modeling to API methods described in: Heitner, T. Trautmauis, and M. Morrissey, “Relieving Requirements for Gas Filled Vessels Exposed to Fire,” 1983 Proceedings-Refining Department, Volume 62, American Petroleum Institute, Washington, D.C., pp. 112-122.

This method considers the transient nature of the vessel warming in combination with the venting of the vessel. It also includes the inherent delay in the gas temperature increase over time. This approach is used as the basis for CGA S1.3 and has proven valid for industrial gas and related vessels.

An annual inspection of safety devices is recommended. Testing requirements will be based on the type of device and a quantified risk analysis. Typical replacement or function testing of relief valves is between 5 and 10 years depending on the application within the industrial gas industry. Rupture discs are not tested but are frequently replaced on an interval based on manufacturer recommendations and a mechanical integrity program established by the operator. For transportation applications, rupture discs are usually replaced at vessel requalification at either 5 or 10 years depending on test method.

Regarding cylinders, it is not necessary to capture the fuse-backed devices which are on the cylinder itself. However, all other relief devices and vent valves must exhaust from a vent system designed in accordance with CGA G-5.5. Also, note that NFPA 2-7.1.17 requires compliance with CGA G-5.5 regardless of storage quantity when the vent system is servicing pressure relief devices. Cylinders in storage that are capped and within an approved cylinder storage area do not require a vent system.