Calculate with Confidence: Properly Size Pressure Relief Devices for Compressed Gas Portable Containers Using the Newly Updated CGA S-1.2
August 8, 2019
On June 28, 2019, CGA released the 10th edition of S-1.2 – Pressure Relief Device Standards-Part 2-Portable Containers for Compressed Gases. S-1.2 is part of a series CGA publishes about sizing pressure relief devices (PRDs):
- CGA S-1.1 – Pressure Relief Device Standards-Part 1-Cylinders for Compressed Gases
- CGA S-1.2 – Pressure Relief Device Standards-Part 2-Portable Containers for Compressed Gases
- CGA S-1.3 – Pressure Relief Device Standards-Part 3-Stationary Storage Containers for Compressed Gases
As their titles indicate, each of the publications above focuses on determining the sizing of PRDs for a specific type of storage container for compressed gases. The term “compressed gases” is used in a broad sense here, indicating gases that are in either gas phase or liquid (cryogenic) phase.
In this article, we’ll provide an overview of the changes made, while exploring reasons to obtain this latest version of the publication.
Finding Guidance on Using the Right PRDs
But first, you may be wondering what exactly the Compressed Gas Association’s S-1.X series of pressure relief device standards covers. After all, doesn’t the Boiler and Pressure Vessel (BPV) code produced by the American Society of Mechanical Engineers (ASME) mandate the requirements for pressure vessels?
Although it’s true that the BPV code contains a wealth of information about designing and building pressure vessels, including the requirement for PRDs, it doesn’t apply to all pressurized containers.
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. These regulations also include requirements for PRDs on cylinders and portable containers. However, neither the BPV code nor DOT or TC regulations include details for sizing a PRD. This is what the S-1.X series of pressure relief device standards does.
A PRD is necessary to maintain pressure in a pressurized container below a level that could lead to its rupture – a catastrophic event that can harm people and damage equipment.
PRD sizing depends on the container’s contents 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.
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 reach 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 portable liquid cylinder 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 tube trailer with a MAWP of 2200 psi has a PRD set at 1000 psi. You won’t be able to fill the tube trailer to a pressure higher than 1000 psi. Once you reach that set pressure, the PRD will open and prevent filling the tube trailer to a higher pressure.
By losing the ability to store gas at a higher pressure, you’ll need to fill the tube trailer 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.
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.
The Key to Optimum PRD Set Points and Flow Capacities
So where do you find the optimum PRD set points and flow capacities to meet your specific needs? You can use one of the S-1.X series of standards that applies to your equipment.
For portable containers, you’ll want to use S-1.2.
PRD set points and flow capacities depend on the following factors (for all the S-1.X standards):
- 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 S-1.2. We’ve provided a list of these updates below. In general, the new S-1.2 contains more detailed methods for sizing PRDs. Because the methods are more detailed, the 10th edition of S-1.2 includes sample calculations to make it easier for you to start using the updated equations.
The most recent edition of S-1.2 includes a new easy-to-follow table, Table 1, that makes it much easier to calculate PRD settings as a function of equipment type. All subsequent tables were renumbered.
Table 1 covers the following portable containers:
- DOT-4L / TC-4LM
- UN portable tanks / ISO containers
- MC / TC 338 cargo tanks
- CGA / TC 341 cargo tanks
- MC / TC 331 cargo tanks
For each type of container, Table 1 lists the following information:
- Test pressure
- Primary set pressure
- Primary flow rating pressure
- Secondary set pressure
- Second flow rating pressure
- Optional secondary set pressure
- Required secondary flow rating pressure
In previous editions, the information was scattered throughout the publication. Having all the information in a single place saves you time, while promoting accuracy and consistency. You no longer need to wonder whether you’re using the right numbers.
These changes and the others listed below have made S-1.2 easier to use and provide more accurate results. Now the results are even more precise than those from previous editions. When you use the new edition, you will be able to specify the most optimally-sized PRDs.
CGA will be petitioning U.S. DOT to reference the 2019 edition of S-1.2 in its regulations in place of the previous edition. These regulations cover the transportation of hazardous materials. The regulation reference is 49 CFR Part 171.7.
Detailed List of Changes in this Edition
Here is the list of changes to the 2019 S-1.2:
- Added DOT-4L/TC-4LM cylinders to the scope of CGA S-1.2 and defined them as liquid cylinders for more specific terminology.
- Added buckling pin devices to the list of acceptable non-reclosing pressure relief devices.
- Added definition of overall length of a pressure vessel to provide consistent measurements.
- Defined portable tanks as UN portable tanks and ISO containers for more specific terminology.
- Defined service pressure for DOT-4L/TC-4LM cylinders for more specific wording.
- Provided more options for acceptable PRDs for all containers, compared to the previous requirement for at least one spring-loaded PRD.
- Removed requirement that piping and hoses must carry a marking of a UV or UD stamp because ASME B31.3 has no such mandate.
- Provided more specific requirements for overfill protection of vessels in flammable or oxygen service.
- Developed a new Table 1 to show set point and flow rating pressure requirements as a function of equipment type and size.
- Added specific requirements for inlet and outlet pressure drop on spring-loaded relief valves.
- Provided more specific requirements for chlorine and LG gas container PRDs.
- Added requirements for liquefied compressed gases, refrigerated fluids, and cryogenic fluids.
- Added extensive rewrite of the section to determine the relief capacity of liquefied compressed gases, refrigerated fluids, and cryogenic fluids, including the effects of inlet and outlet piping.
- Rewrote section for PRD flow capacity requirements for conditions excluding fire.
- Added methodology to calculate flow capacity requirements for thermal expansion relief valves.
- Rewrote section for PRD flow capacity requirements for fire conditions.
- Rewrote section on PRD flow capacity requirements for liquefied compressed gases, refrigerated fluids, and cryogenic fluids in insulated containers.
- Added PRD requirements for DOT-4L and TC-4LM insulated cylinders.
- Added inspection requirements for thermal relief valves.
- Added appendix A with a sample calculation of PRD capacity in U.S. units.
- Added appendix B with methodology to calculate Gi and Gu in U.S. and metric units.