CGA’s S-1.3 Publication Guides Optimal PRD Sizing for Compressed Gas Stationary Containers

Finding Guidance on How to Correctly Size PRDs

If you’re looking for help with how to correctly size a pressure relief device, you’ll want to reference one of the safety publications in the CGA S-1.X series.

It’s true that the Boiler and Pressure Vessel (BPV) code produced by the American Society of Mechanical Engineers (ASME) mandates requirements for designing and building pressure vessels, including for PRDs. It’s also true that some pressurized containers – such as cylinders and portable containers – are built to specifications included in U.S. Department of Transportation (DOT) or Transport Canada (TC) regulations, and these regulations include requirements for PRDs on cylinders and portable containers.

However, neither the BPV code nor the DOT or TC regulations include details for sizing a PRD. That information is contained in the S-1.X series of pressure relief device standards from CGA.

PRD sizing depends on the container’s contents (called “lading” in the CGA S-1.X publications) and operation.

To protect a container, you must select a PRD with the proper set pressure and correct flow capacity. The PRD must open at the right setting to relieve overpressure. At the same time, the PRD must have enough flow capacity to relieve pressure faster than pressure can build up.

Both set pressure and flow capacity play critical roles in ensuring a container’s safe and efficient operation.

Set Pressure

A PRD with a set pressure that’s too high – above the maximum allowable working pressure (MAWP) of the container – will not protect the container. The pressure can exceed the container’s MAWP limit before the PRD can open and relieve the pressure. This over-pressurization can cause the container to rupture.

For example, a bulk cryogenic storage tank with a MAWP of 250 psi must not operate above 250 psi. If the PRD is set at 400 psi, the vessel can reach pressures up to 400 psi before the PRD relieves the pressure. Granted, containers’ MAWPs have safety factors built in, but it is still unsafe to operate a vessel above its MAWP.

Alternatively, a PRD with a set pressure that is too low – well below the MAWP – will not allow you to use the container to its full capacity. While not a safety issue, operating a container below its full capacity can lead to costly inefficiencies.

For example, let’s say that a gas receiver with a MAWP of 2200 psi has a PRD set at 1000 psi. You won’t be able to fill the receiver to a pressure higher than 1000 psi. Once you reach that set pressure, the PRD will open and prevent filling the receiver to a higher pressure.

By losing the ability to store gas at a higher pressure, you’ll need to fill the receiver more often. Your operating costs will increase because you must make more fills. With a properly-set PRD (2200 psi), you can pressurize the vessel at a higher value, allowing for greater storage and fewer fills.

Flow Capacity

A PRD with a flow capacity lower than the maximum flow rate of an overpressure condition cannot maintain the container pressure below the MAWP. Pressure builds up faster than the PRD can relieve the pressure. Eventually the container pressure can rise high enough to cause a rupture.

A PRD with a flow capacity much higher than is needed will keep the vessel pressure below the MAWP, so there is no safety issue. However, the larger PRD will cost more than the optimum-sized (smaller) PRD. This creates inefficiencies from having larger and more expensive components than needed, or could cause the PRD to “chatter,” opening and closing quickly, which can lead to premature failure of the PRD.

The Key to Optimum PRD Set Points and Flow Capacities

To find the optimum PRD set points and flow capacities to meet your specific needs, reference the CGA S-1.X  standard that applies to your equipment. For compressed gas stationary storage containers, you’ll want to use CGA S-1.3.

PRD set points and flow capacities depend on the following factors (this applies to all the publications in the CGA S-1.X series):

• Type of vessel: gas cylinder, liquid container, gas receiver, tube trailer, liquid storage tank, liquid delivery vehicle, and others
• Type of vessel design: liquid or gas service, vacuum-jacketed or not, type of insulation
• Fluid stored in the vessel: flammable or inert, oxygen or not, liquid or gas
• Types of upset conditions: fire, loss of insulation

A CGA task force of subject matter experts developed the revisions to this safety publication. We’ve provided a list of these updates below. In general, this version of CGA S-1.3 contains more detailed methods for sizing PRDs. Because the methods are more detailed, the 9th edition includes sample calculations to make it easier for you to start using the updated equations.

CGA S-1.3 has a large footprint by virtue of being referenced in many model codes, including those published by the National Fire Protection Association (NFPA) and the International Code Council (ICC). The following model codes reference CGA S-1.3:

• NFPA 1, Fire Code
• NFPA 2, Hydrogen Technologies Code
• NFPA 52, Vehicular Natural Gas Fuel Systems Code
• NFPA 55, Compressed Gases and Cryogenic Fluids Code
• NFPA 58, LP-Gas Code
• NFPA 59A, Standard for the Production, Storage, and Handling of Liquefied Natural Gas (LNG)
• NFPA 400, Hazardous Materials Code
• International Fire Code (IFC)
• International Fuel Gas Code (IFGC)

What’s New in this Edition

Major changes to the 2020 edition of CGA S-1.3 include:

• Added new figure 1 with nomenclature for containers of liquefied compressed gases, refrigerated fluids, and cryogenic fluids.
• Added appendix A with methods and examples to calculate G_i and G_u for determining pressure relief device capacity.
• Added appendix B with flow diagram figures, dimensional definitions, capacity calculation equations, and sample calculations for a vacuum-insulated container and its relief system, including:
  • Calculation of required relief capacity for loss of vacuum under ambient conditions
  • Calculation of required relief device capacity for loss of vacuum under fire conditions
  • Calculation of relief system capacity for loss of vacuum under ambient conditions for the primary reclosing pressure relief valve
  • Calculation of relief system capacity for loss of vacuum under fire conditions for the primary nonreclosing rupture disk
• Updated table with thermal conductivity values of refrigerated fluids at the mean temperature between saturation and 1200 degrees Fahrenheit (1660 degrees Rankine). Added values for the thermal conductivity of air at that mean temperature for the cryogenic fluids listed in the table. Added explanatory material for the mean temperature numbers.
• Added “minimum net flow area (MNFA)” to appropriate equations.
• Provided two sets of pressure relief device calculations for containers holding liquefied compressed gases, refrigerated fluids, or cryogenic fluids: one for insulated containers and one for containers that are either uninsulated or have lost insulation.

For more details about this safety standard, the full Table of Contents may be downloaded for free from the CGA S-1.3 publication details page on the CGA portal.