NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Pressure Relief Devices for 

High-Pressure Gaseous Storage 

Systems: Applicability to 

Hydrogen Technology

A. Kostival, C. Rivkin, W. Buttner, and 

R. Burgess

National Renewable Energy Laboratory

Technical Report

NREL/TP-5400-60175

November 2013
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Pressure Relief Devices for 

High-Pressure Gaseous 

Storage Systems: Applicability 

to Hydrogen Technology 

A. Kostival, C. Rivkin, W. Buttner, and 

R. Burgess

National Renewable Energy Laboratory

Prepared under Task No. HT12.7210

Technical Report

NREL/TP-5400-60175

November 2013
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/help/ordermethods.aspx

Cover Photos: (left to right) photo by Pat Corkery, NREL 16416, photo from SunEdison, NREL 17423, photo by Pat Corkery, NREL 

16560, photo by Dennis Schroeder, NREL 17613, photo by Dean Armstrong, NREL 17436, photo by Pat Corkery, NREL 17721.

 Printed on paper containing at least 50% wastepaper, including 10% post consumer waste. 
iii

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Table of Contents

1 Introduction........................................................................................................................................... 1

2 Pressure Relief Devices (PRDs).......................................................................................................... 1

2.1 PRD Background........................................................................................................................... 1

2.2 Common Types of PRDs............................................................................................................... 2

2.2.1 Advantages and Disadvantages........................................................................................ 2

2.3 Technical Classifications for PRDs............................................................................................... 3

3 Overview of PRD Failure Incidents..................................................................................................... 5

3.1 PRD Failure Incidents................................................................................................................... 5

3.2 Analysis of Failure Incidents......................................................................................................... 7

4 PRD Codes and Standards.................................................................................................................. 8

4.1 Industry Position on PRDs for High-Pressure Gas Storage .......................................................... 9

5 PRD Applications for Hydrogen........................................................................................................ 10

5.1 Issues with PRDs Used for Hydrogen Service............................................................................ 10

5.1.1 Hydrogen Embrittlement................................................................................................ 10

5.1.2 Flammability .................................................................................................................. 10

5.2 Acceptable PRDs for Hydrogen Service ..................................................................................... 11

6 Conclusion .......................................................................................................................................... 12

References................................................................................................................................................. 13

Appendix.................................................................................................................................................... 14
iv

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

List of Figures

Figure A1. Pressure relief valve activation characteristics......................................................................... 14

Figure A2. Frangible burst disk device cross-section ................................................................................. 15

Figure A3. Fusible metal plug device ......................................................................................................... 16

Figure A4. Frangible burst disk and fusible metal plug device cross-section............................................. 17

Figure A5. Spring-loaded pressure relief valve device cross-section ......................................................... 18

Figure A6. Pressure relief valve operation chart......................................................................................... 19

List of Tables

Table 1. Pressure Relief Device Category Definitions [3]............................................................................ 2

Table 2. Comparison of Advantages and Disadvantages for Different PRD Types..................................... 3

Table 3. Pressure Relief Device Codes and Standards ................................................................................. 8

Table 4. PRDs for Hydrogen Applications................................................................................................. 11
1 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

1 Introduction

Pressure relief devices (PRDs) are viewed as essential safety measures for high-pressure gas 

storage and distribution systems. These devices are used to prevent the over-pressurization of gas 

storage vessels and distribution equipment, except in the application of certain toxic gases. PRDs 

play a critical role in the implementation of most high-pressure gas storage systems and anyone 

working with these devices should understand their function so they can be designed, installed, 

and maintained properly to prevent any potentially dangerous or fatal incidents. As such, the 

intention of this report is to introduce the reader to the function of the common types of PRDs 

currently used in industry. Since high-pressure hydrogen gas storage systems are being 

developed to support the growing hydrogen energy infrastructure, several recent failure 

incidents, specifically involving hydrogen, will be examined to demonstrate the results and 

possible mechanisms of a device failure. The applicable codes and standards, developed to 

minimize the risk of failure for PRDs, will also be reviewed. Finally, because PRDs are a critical 

component for the development of a successful hydrogen energy infrastructure, important 

considerations for pressure relief devices applied in a hydrogen gas environment will be 

explored. 

2 Pressure Relief Devices (PRDs)

2.1 PRD Background

Although there are several different PRD design platforms developed for gaseous applications, 

they all have the same basic function. PRDs are installed on high-pressure gas storage containers 

and related distribution lines in the vicinity of key components for the system as part of the 

safety system; this is often required by code. In the event of an over-pressure situation induced 

by, for example, overfilling, exposure to excessive heat, or a liquid to gas phase change, the PRD 

will activate and allow the gas to escape to a safe location. The PRD should be set such that this 

venting will relieve excess pressure before the internal pressure exceeds the maximum allowable 

working pressure (MAWP) for the system, which could result in an explosion or other 

catastrophic failure [1]. Most PRDs are calibrated to relieve pressure at the MAWP. However, 

since there is a factor of safety included in the MAWP, PRDs for certain applications can be 

safely set above MAWP. These exceptions include PRDs used to control a process fluid and 

supplemental valves to protect the system in the event of fire exposure (see Figure A1 in the 

Appendix). Another function of PRDs is to prevent damage to any sensitive system elements that 

might result from exposure to excessively high pressures for which they are not rated. 

The PRD requirement is often waived for circumstances where a release may be a higher risk 

than the impact of an unlikely over-pressurization. For certain toxic gases, pressure relief devices 

are not permitted because exposure to the gas is considered to be a greater hazard than potential 

failure of the storage container [2]. 

It is important to note that pressure relief devices are an engineering control to prevent overpressure and should not be relied on as a primary system control. 
2 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

2.2 Common Types of PRDs 

There are hundreds of different PRD models commercially available from various companies; 

however, the vast majority can be categorized by one of the four generic platform types, as listed 

in Table 1. Each type of PRD performs the same basic function of relieving excess pressure 

buildup; however, it is important to consult the codes and standards governing pressure relief 

device selection to ensure that the selected PRD design is appropriate for the intended 

application. 

Table 1. Pressure Relief Device Category Definitions [3]

Type Description 

Frangible burst 

disk

A non-reclosing metal disk designed to burst when the static pressure at the inlet 

exceeds the specified set pressure. The bursting of the disk allows the 

pressurized gas to pass through the outlet vent and relieve the internal pressure 

of the system. This device empties the entire contents of the system (see Figure 

A2 in the Appendix). 

Fusible metal plug 

device

A non-reclosing device designed with a metal alloy that has a low melting 

temperature (either 165°F or 212°F nominal) that yields when exposed to excess 

temperature and opens the outlet to allow the pressurized gas to escape and 

relieve the internal pressure of the system. This device empties the entire 

contents of the system (see Figure A3 in the Appendix)

Frangible burst 

disk backed by a 

fusible metal plug

A non-reclosing device designed with a frangible burst disk backed by a fusible 

metal plug. This device will only operate in the event of both extreme temperature 

that causes the metal plug to yield and excessive pressure that causes the disk to 

burst such as in the event of a fire. This device empties the entire contents of the 

system (see Figure A4 in the Appendix). 

Spring-loaded 

pressure relief 

valve

A reclosing device in which a spring holds the valve closed until the static 

pressure increases above the set pressure of the spring. When the valve is 

exposed to pressure at or above the set pressure the disk unseats, opening the 

valve and allowing the gas to vent, thus relieving the excess pressure (see Figure 

A5 in the Appendix). The device will relieve over-pressure up to the maximum 

relieving pressure, at which point the valve is completely opened. Due to a slight 

amount of hysteresis, this device will typically relieve pressure back below the set 

pressure in a stage known as blowdown (see Figure A6 in the Appendix). This 

device does not empty the entire contents of the system. 

Although there are four PRD platform types, there are only two main activation mechanisms—

one resulting from over-pressure and the other initiated by elevated temperature. Both activation 

mechanisms allow the PRD to perform the basic function of relieving excess pressure. It is 

important to consult the codes and standards governing pressure relief device selection to ensure 

that the PRD has the appropriate activation mechanism for the intended application. 

Additionally, it is pertinent that the selected PRD is sized appropriately for the application. If any 

system changes are made, the PRD sizing and selection should be reevaluated and the PRD 

should be replaced if necessary. 

2.2.1 Advantages and Disadvantages

In order to make sure the appropriate PRD is selected for each application, it is important to 

understand the advantages and disadvantages of each type of device. A basic breakdown of the 

benefits and drawbacks of each type of PRD is presented in Table 2. 
3 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Table 2. Comparison of Advantages and Disadvantages for Different PRD Types

Type Advantages Disadvantages

Frangible burst disk • Reliable operation

• Low maintenance

• Minimal leakage

• Relieves entire tank contents

• Very loud noise when burst

• Will not necessarily operate in a 

fire

• Not reusable

• Relief set point is not very 

precise

Fusible metal plug 

device

• Relieves pressure when 

system is exposed to high 

temperature (e.g., a fire) to 

prevent over pressurization

• Low maintenance

• Does not protect against over 

pressure not caused by fire 

(e.g., filling/compressor error)

• Relieves entire tank contents

• Not reusable

• Must be in position exposed to 

elevated temperature to 

operate

Frangible burst disk

backed by a fusible 

metal plug

• Protects cylinder from over 

pressurization caused by fire 

• Low maintenance

• Only operates if tank becomes 

over pressurized in a fire 

• May not relieve pressure if 

vessel walls are weakened/fail 

due to fire

• Relieves entire tank contents 

• Not reusable

Spring-loaded pressure 

relief valve

• Easily adjusted set pressure 

• Does not relieve entire tank 

contents when operated

• Reusable

• Does not reduce tank pressure

in the event of a fire

• Less reliable than other PRD 

options if not maintained and 

inspected regularly

• Regular maintenance schedule 

required

Since each installation has different requirements and constraints, there is no single “best” 

pressure relief device type. Each type is suited for different applications and has different 

operation requirements to prevent different over-pressurization scenarios. 

2.3 Technical Classifications for PRDs

As outlined in the CGA S-1.1, “Pressure Relief Device Standards Part 1—Cylinders for 

Compressed Gases,” the four major categories of PRD designs can be further partitioned into 11 

pressure relief device designations. These classifications, which are applicable to PRDs for all 

high-pressure gas storage containers, are listed below. 
4 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Type CG-1 

Rupture disk device that bursts at a predetermined set pressure. 

Type CG-2 

A fusible plug device using a fusible alloy with a nominal yield temperature of 165°F (73.9°C). 

This device is only to be used for service not exceeding 500 psig (3,540 kPa) at 68°F (20°C). 

Type CG-3 

A fusible plug device using a fusible alloy with a nominal yield temperature of 212°F (100°C). 

This device is only to be used for service not exceeding 500 psig (3,540 kPa) at 68°F (20°C). 

Type CG-4 

A combination device using a frangible burst disk backed by a fusible alloy with a nominal yield 

temperature of 165°F (73.9°C). 

Type CG-5 

A combination device using a frangible burst disk backed by a fusible alloy with a nominal yield 

temperature of 212°F (100°C). 

Type CG-7 

A pressure relief valve that opens to relieve excess system pressure and then closes and reseals to 

prevent further fluid flow once the pressure is below the set relief pressure of the device. These 

devices are not be used for service exceeding 500 psig (3,540 kPa). 

Type CG-8 

A rupture disk followed in series by a pressure relief valve. 

Type CG-9 

A fusible plug device for service pressures between 500 psig (3,540 kPa) and 6,000 psig (41,400 

kPa) using a fusible alloy with a nominal yield temperature of 217°F (102.7°C). 

Type CG-10

A fusible trigger device that uses a fusible metal alloy to trigger the pressure relief mechanism on 

the device. These devices should activate within 90 seconds and should not be used for service 

pressures exceeding 13,100 psig (90,300 kPa). 

Type CG-11

A pressure relief valve that opens to relieve excess system pressure and then closes and reseals to 

prevent further fluid flow once the pressure is below the set relief pressure of the device. These 

devices are capable of activating and resealing multiple times. 
5 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Type CG-12

A combination device incorporating a fusible trigger device and a pressure relief valve that can 

be activated by either temperature or pressure. These devices are not to be used for service that 

exceeds 13,100 psi (90,300 kPa). 

Through careful examination, it can be observed that each of these Compressed Gas Association 

(CGA) pressure relief device classifications utilize features found in one or more of the four 

main categories of PRD, although each have different specifications for safe usage. The CGA 

has rated these PRDs for acceptable use on DOT (Department of Transportation) specification 

cylinders for more than 200 gases and cryogenic liquids. Table 3 in the CGA S-1.1 standard [3]

should be consulted for the specific requirements regarding the type(s) of PRD that are required 

for each gas, as well as specific details about the PRD location requirements for compressed gas 

cylinders. For application-specific sizing and location requirements related to PRD usage for 

stationary storage vessels, the CGA S-1.3 standard should be consulted. 

3 Overview of PRD Failure Incidents

Although PRDs are an important safety measure in the storage of high-pressure gases, they also 

present their own risks and challenges. To bring perspective to the consequences of these 

incidents, several failure events specifically involving hydrogen have been analyzed. There have 

been several recent incidents within the hydrogen industry involving the premature failure of a 

pressure relief device. The repercussions from these failures vary from no damage or injury to 

death of personnel and extensive structural damage. These device failures can also have political 

consequences; hydrogen gas is still viewed by some as an unsafe energy source. Unreliable 

technology can significantly hinder progress toward public acceptance of hydrogen as a viable 

energy alternative. 

Before analyzing failure incidents, it is necessary to distinguish between the failure modes for a 

PRD. Two main failure modes are feasible: 

• Type 1 Failure: A PRD should activate but does not [4]

• Type 2 Failure: A PRD activates when it should not [4]

A Type 1 failure has far more hazardous potential than a Type 2 failure because it results in a 

situation where the internal pressure within a vessel can exceed its MAWP and potentially burst. 

Fortunately, Type 1 failures are very uncommon. Type 2 failures occur more frequently and 

typically result from improperly maintained equipment, improper selection or sizing, or 

compromised structural properties. To understand what can cause a PRD to fail, as well as to 

describe the resulting aftermath of a device failure, a few sample incidents involving the failure 

of a device used for hydrogen service are described below. A full incident report for each of 

these events can be found on the H2Incidents database at http://www.h2incidents.org/. 

3.1 PRD Failure Incidents

Tube Trailer Leak Through Over-Pressure-Protection Rupture Disk 

A failed rupture disk on a hydrogen delivery tube trailer was detected by an employee at the 

filling facility. The leak was reported and a response team safely contained the leak and vented 
6 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

the hydrogen gas through an exhaust tube. The burst disk had been rated for 4,000 psi and had 

failed around 2,100 psi. Analysis of the failed burst disk, as well as other devices on the trailer, 

showed the disks were made of a high strength steel alloy that was susceptible to hydrogen 

embrittlement. There was no equipment damage or personal injury in this event [5]. 

Unexpected Failure of Rupture Disk on Liquid Hydrogen Tank

A burst disk PRD ruptured prematurely when a cryogenic liquid hydrogen storage tank became 

slightly over-pressurized due to heating from ambient temperature. Investigation showed that the 

hydrogen piping was creating back pressure on the disk during manual venting, which caused the 

PRD to rupture below the set pressure. There was no equipment damage or personal injury in 

this event [6]. 

High-Pressure Burst Disk Failure

A burst disk PRD ruptured prematurely at 5,200 psig during a filling operation to 6,000 psig. The 

failure resulted in a violent release of a vent line that was improperly secured, which caused 

significant damage to nearby system components. Fortunately, there were no personnel in the 

area when the disk failed. This device was rated to 10,000 psig; metallurgical analysis showed 

the rupture disk was composed of pure nickel, a material that is not suitable for hydrogen service. 

Hydrogen embrittlement in the disk led to stress cracks and eventual failure [7]. 

Hydrogen Tube Trailer Burst Disk Ruptures Prematurely While Filling

A burst disk on a hydrogen tube trailer ruptured prematurely during the filling process. This 

resulted in hydrogen gas venting through the exhaust tube as designed. The gas ignited exiting 

the tube, which caused a flame to shoot out of the tube until the gas leak was contained. The 

burst disk failed about 1,000 psig below set pressure as a result of improper requalification 

following an inspection. The percussion from the failed burst disk startled a mechanic working 

under the trailer, who subsequently sustained minor injury during his hurried exit. No property 

damage resulted from the incident [8]. 

Pressure Relief Device Fails at Fueling Station

A pressure relief valve failed at a high-pressure hydrogen storage station, causing the release of 

hydrogen gas through the vent stack where it ignited and burned for approximately 2.5 hours. 

Approximately 300 kg of hydrogen was released. The venting equipment functioned as designed; 

only a canopy, which had been placed over the equipment after the initial installation, was 

damaged by the flame. The relief valve had failed due to hydrogen embrittlement of the high 

strength steel used for the nozzle. The event caused the evacuation of local businesses and 

elementary schools and generated significant attention by local media. No injuries resulted from 

the failure; the incident resulted in less than $300 of structural damage, but the station was out of 

service for several months during the following investigation [9]. 

Hydrogen Explosion at Coal-Fired Power Plant

The premature failure of a rupture disk device during a routine delivery, and subsequent failure 

of vent piping, caused the buildup of hydrogen gas and a resulting explosion at a coal-fired 

power plant. The PRD, which had been recently serviced, did not contain the fusible metal 

backing that was specified. In addition to the significant structural damage caused by the blast, 

the delivery driver was killed in the explosion, and 10 others were injured [10].
7 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

3.2 Analysis of Failure Incidents

As shown by this brief list of events, there is a wide range of severity that can result from a PRD 

failure. Often, the incident results in normal operation of the venting equipment and 

subsequently there is minimal damage or personal injury. However, in cases like the coal-fired 

power plant, there can be significantly greater consequences from a device failure. It is worth 

noting that all of the reported PRD failure incidents are of Type 2 failure classification; the PRD 

fails below set pressure. This implies that all of these cases of PRD failure are a matter of 

reliability with the PRD designs. These cases were all a result of improper selection, design, or 

improper servicing. Since issues with hydrogen service are still being discovered, there are still 

design improvements that need to be made to assure the safe and reliable use of hydrogen as a 

commercially viable energy alternative. However, as a general statement, pressure relief devices 

are a reliable last line of protection against an over-pressurization event. 

A few major takeaways from this discussion of various hydrogen-related failure incidents 

include the following: 

• Planning and properly executing regular maintenance on PRDs is important to prevent 

premature activation below the set pressure. 

• Proper training is required for anyone who works on or near PRD systems to understand 

the basic function of PRDs as well as what to expect when a PRD activates. 

Subsequently, response procedures should be developed in the case of a premature 

activation. 

• It is important to ensure that any device used for hydrogen is composed of a material 

suitable for hydrogen service (see section 5.1.1 Hydrogen Embrittlement). 

Understanding the design and function of these devices will help anyone working with PRDs 

respond appropriately in the event of a failure. Additionally, by understanding how these devices 

fail, the systems can be designed to prevent such incidents, resulting in overall safer PRD 

technology. 
8 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

4 PRD Codes and Standards

To guide the PRD selection process for each application, there are several codes and standards 

that should be consulted. Table 3 below provides a useful reference guide for the applicable 

codes and standards and what relevant information can be found in each. 

Table 3. Pressure Relief Device Codes and Standards

Title Relevant Information

CGA S-1.1 – 2007

Pressure Relief Device Standards 

Part 1 – Cylinders for Compressed 

Gases

Thirteenth Edition

General

• PRD Technical Type Definitions

• Gas Specific PRD Assignments

Compressed Gas Cylinder Specific

• PRD Type Specific Application Requirements

• PRD Design / Construction Requirements

• PRD Testing Requirements

• PRD Identification/Marking Requirements

• PRD Maintenance Requirements

• PRD Service Life/Replacement Period Requirements

CGA S-1.2 – 2005

Pressure Relief Device Standards 

Part 2 – Cargo and Portable Tanks 

for Compressed Gases

Eighth Edition

Portable Compressed Gas Tanks and Cargo Specific

• PRD Type Specific Application Requirements

• PRD Design / Construction Requirements

• PRD Testing Requirements

• PRD Identification/Marking Requirements

• PRD Maintenance Requirements

CGA S-1.3 – 2005

Pressure Relief Device Standards 

Part 3 – Stationary Storage 

Containers for Compressed Gases

Seventh Edition

Stationary Storage Container Specific

• PRD Type Specific Application Requirements

• PRD Design / Construction Requirements

• PRD Testing Requirements

• PRD Identification/Marking Requirements

• PRD Maintenance Requirements
9 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

4.1 Industry Position on PRDs for High-Pressure Gas Storage

High-pressure gas storage vessels generally fall within one of two categories: mobile storage 

vessels and stationary storage vessels. Mobile vessels and their relief systems must meet the 

minimum DOT requirements as specified within 49 CFR. Stationary vessels are usually built to 

ASME requirements but can also be DOT vessels (e.g., a parked tube trailer that is supplying a 

process). As a generally accepted practice within industry, high-pressure gaseous storage 

systems are designed with a PRD in direct pneumatic connection to the pressure vessel that meet 

the requirements of either DOT or ASME code, or as required by the governing CGA standards. 

These PRDs are seen as providing an additional level of safety to prevent catastrophic failure of 

a storage vessel. In light of recent incidents involving PRD failure, some stakeholders have 

asserted the position that PRDs can, in specific scenarios, actually introduce additional and 

unnecessary failure risk to the system. Their position states that when properly sited to avoid the 

possibility of engulfing fire, the only credible over-pressure event is the risk of a pressure source 

(e.g., compressor, pump, regulator) malfunctioning thereby overfilling the vessel and increasing 

its pressure above the MAWP. ASME code requires the installation of a PRD for scenarios 

deemed credible by the designer, and such PRDs must be in a position to maintain the pressure in 

the vessel below MAWP. For an external source of pressure, the PRD must be located either on 

the vessel or between the pressure source and the vessel, and it must be set at or below the 

MAWP. 

These stakeholders have asserted that for non-liquefied compressed gases, the only credible 

internal event that would cause an over-pressure situation would be an engulfing fire. ASME 

codes state that if storage containers are sited in compliance with NFPA codes, such that the 

possibility of an engulfing flame is minimized, PRDs installed directly in contact with the 

storage vessel are not required. Therefore, these firms hold the position that PRDs are not 

necessary on high-pressure gaseous storage vessels that meet NFPA location standards as well as 

ASME PRD standards for protection from external pressure sources. 

Again, this is the position of certain stakeholders within industry; this stance has not yet been 

universally accepted by the industry as a whole. If enough evidence can be provided to support 

this opinion, there is potential for the governing codes and standards to be revisited and updated 

as necessary. 

It is important to note that this position pertains only to high-pressure stationary gas 

storage containers and explicitly excludes cryogenic fluids or other condensed fluids. 
10 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

5 PRD Applications for Hydrogen

As the world’s energy demand continues to grow exponentially, new sources of energy are 

constantly being explored. Hydrogen could prove to be a very effective alternative fuel to power 

transportation, backup power, and various other energy needs. While working toward a 

commercially viable hydrogen infrastructure, a major challenge is the development of safe, 

effective methods for handling the highly flammable hydrogen gas. As such, it is important to 

understand the issues with PRD application in a hydrogen environment, as well as the types of 

PRDs approved for hydrogen service. 

5.1 Issues with PRDs Used for Hydrogen Service

Due to its small molecular size and low ignition energy, hydrogen can be challenging to store 

safely at high pressures. PRDs are implemented as a final safety measure for hydrogen storage 

containers to prevent over-pressurization incidents. Some issues with PRDs used for hydrogen 

service include leakage and hydrogen embrittlement, which can lead to device failure. 

Unfortunately, due to the frequency of Type 2 failures as shown by the failure event summary 

(see section 3.1 PRD Failure Incidents), PRDs may not be reliable enough to be applied on a 

broad commercial scale necessary for the development of hydrogen energy infrastructure.

Studies are underway to understand the failure modes of these devices so that they can be 

designed to improve reliability. As a result of the frequency of PRD failure there has been recent 

discussion within industry about the necessity of PRDs on hydrogen storage containers for 

certain over-pressure scenarios. However, until the codes are updated to reflect a change of 

policy that might result from these discussions, PRDs should be installed to meet the applicable 

standards for the particular application, be it mobile or stationary. 

5.1.1 Hydrogen Embrittlement

A major consideration when designing components for hydrogen service is the prevention of 

hydrogen-assisted fracture, which occurs through a process known as hydrogen embrittlement. 

Hydrogen has a very small molecular size and can easily permeate into most metals. Once 

diffused, the hydrogen forms areas of embrittlement, such that it weakens or “embrittles” the 

metal by reducing ductility and promoting cracking. Three main factors that can affect the 

magnitude of hydrogen embrittlement are material, environmental conditions, and mechanical 

loads or stresses [11]. Higher-strength steel alloys, which are typically used in high-pressure 

applications for other gases, are known to experience a greater loss in ductility when used in 

hydrogen service than lower strength steels do. Minimizing stresses in the element will prevent 

failure resulting from hydrogen embrittlement, as the metal will still retain some level of 

ductility. Materials recommended for hydrogen service include 300-series stainless steels, 

copper, and brass, as they are the least susceptible to hydrogen embrittlement when exposed to 

the gas for prolonged periods [11]. 

5.1.2 Flammability

Because hydrogen has a low ignition energy in air (20 μJ), it is important to design relief systems 

to vent away from personnel and equipment to prevent injury or explosions. Often hydrogen will 

spontaneously ignite (due to static, particle impingement, or the shock wave from a high pressure 

release), which can result in the false perception of a serious incident even in the event of a 

functional pressure relief event. Additionally, hydrogen burns with a pale blue flame that can be 
11 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

nearly invisible in daylight [12]. As such, PRDs should always be vented up away from areas 

where hydrogen gas could potentially congregate and create an explosive condition, or where 

personnel could haphazardly come in contact with an inconspicuous hydrogen flame. 

5.2 Acceptable PRDs for Hydrogen Service

PRDs that are acceptable for hydrogen service on DOT compressed gas cylinders are listed in 

Table 4 (taken from Table 3 of the CGA S-1.1 standard [3]). 

Table 4. PRDs for Hydrogen Applications

Device Type Device Code

CG-1 N 

CG-4 J 

CG-5 J 

CG-7 N 

CG-10 J 

CG-11 N 

The accompanying letter codes J and N are defined as follows:

• J—“For cylinders, this device is required in only one end. For tubes, this device is 

required at both ends and each device shall be arranged to discharge upwards and 

unobstructed to the open air in such a manner as to prevent any impingement of escaping 

gas upon the containers or personnel.” [3] 

• N—“This device is required in only one end of tubes. The device shall be arranged to 

discharge upwards and unobstructed to the open air in such a manner as to prevent any 

impingement of escaping gas upon the containers or personnel.” [3]

For stationary storage vessels, the PRD application requirements are based on relief flow 

capacity calculations; there are not specific PRD configuration assignments for usage in a 

hydrogen environment. To calculate application-specific sizing and configuration requirements 

for PRDs on a stationary storage vessel, sections 5 and 6 of the CGA S-1.3 should be consulted. 
12 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

6 Conclusion

PRDs are pertinent safety components for pressurized gas storage systems. The four major types 

of PRDs can be categorized by the 11 CGA classifications that describe most of the PRDs 

currently commercially available. Although they are implemented as a safety measure, there is 

some small additional risk involved with the use of a PRD, specifically when used for hydrogen 

service. However, as the failure modes for these devices are analyzed, the reliability will 

improve until the risk of device failure is minimized. The indicated CGA standards provide the 

minimum requirements for safely designing and implementing PRDs into various compressed 

gas storage systems. These standards should be consulted when selecting a PRD for a specific 

design application. PRDs are applied in most types of high-pressure gas storage, but their 

application in hydrogen is specifically pertinent as the world moves toward a cleaner, more 

sustainable energy future. As such, PRD design and implementation requirements for hydrogen 

are still being reviewed and augmented as necessary. 
13 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

References 

1. “Safetygram-15: Cylinder Pressure Relief Devices.” Air Products , 1999. Accessed July 1, 

2013: http://www.airproducts.com/~/media/Files/PDF/company/safetygram-15.pdf. 

2. “Safe Handling of Compressed Gases in the Laboratory and Plant.” Matheson Gas, 2011. 

Accessed June 28, 2013: http://www.mathesongas.com/pdfs/products/guide-to-safe-handling-ofcompressed-gases-publ-03.pdf.

3. Pressure Relief Standards Part 1—Cylinders for Compressed Gases. Thirteenth Edition. CGA 

S-1.1. Chantilly, VA: Compressed Gas Association, 2007. 

4. Sunderland, P.B. “Pressure Relief Devices for Hydrogen Vehicles.” Presented at the Third 

European Summer School on Hydrogen Safety, July 21–30, 2008. Accessed June 28, 

2013: http://www.hysafe.org/science/eAcademy/docs/3rdesshs/presentations/ESSHS2008Sunder

landP2.pdf. 

5. “Tube Trailer Leak through Over-Pressure-Protection Rupture Disk.” H2Incidents, 2010. 

Accessed July 1, 2013: http://www.h2incidents.org/incident.asp?inc=267&cat=3&val=12. 

6. “Unexpected Failure of Rupture Disk on Liquid Hydrogen Tank at Manufacturing Plant.” 

H2Incidents, 2010. Accessed July 1, 

2013: http://www.h2incidents.org/incident.asp?inc=259&cat=3&val=12. 

7. “High-Pressure Burst Disk Failure.” H2Incidents, 2010. Accessed July 1, 

2013: http://www.h2incidents.org/incident.asp?inc=255&cat=3&val=12. 

8. “Hydrogen Tube Trailer Burst Disc Ruptures Prematurely While Filling.” H2Incidents, 2009. 

Accessed July 1, 2013: http://www.h2incidents.org/incident.asp?inc=252&cat=3&val=12. 

9. “Pressure Relief Device Fails at Fueling Station.” H2Incidents, 2013. Accessed July 1, 

2013: http://www.h2incidents.org/incident.asp?inc=311&cat=3&val=12. 

10. “Hydrogen Explosion at Coal-Fired Power Plant.” H2Incidents, 2008. Accessed July 1, 

2013: http://www.h2incidents.org/incident.asp?inc=192&cat=3&val=12. 

11. Harris, A.; San Marchi, C. Investigation of the Hydrogen Release Incident at the AC Transit 

Emeryville Facility (Revised). SAND2012-8642. Livermore, CA: Sandia National Laboratories, 

2012. Accessed June 20, 2013: http://prod.sandia.gov/techlib/accesscontrol.cgi/2012/128642.pdf. 

12. “Hydrogen Gas Safety.” Los Alamos National Laboratory, 2000. Accessed July 2, 

2013: http://www.usmra.com/repository/category/mine_gases/hydrogengassafety.pdf. 
14 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Appendix

Figure A1. Pressure relief valve activation characteristics 

Illustration by Arlen Kostival, NREL 500041
15 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Figure A2. Frangible burst disk device cross-section 

Illustration by Arlen Kostival, NREL 500038
16 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Figure A3. Fusible metal plug device 

Illustration by Arlen Kostival, NREL 500040
17 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

 

Figure A4. Frangible burst disk and fusible metal plug device cross-section 

Illustration by Arlen Kostival, NREL 500039
18 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Figure A5. Spring-loaded pressure relief valve device cross-section 

Illustration by Arlen Kostival, NREL 500043
19 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Figure A6. Pressure relief valve operation chart 

Illustration by Arlen Kostival, NREL 500042