It depends on the service. Variables include disk design, corrosion resistance, number of cycles, and how to close the operating pressure is to the rupture disc set pressure. One approach is to ask the manufacturer for their recommendation. Typical practice for U.S. Department of Transportation vessels and discs is to replace them at the tube requalification (i.e., every 5 to 10 years in the U.S. depending on the retest method used). Unless there are specific reasons for frequent replacement, it's usually best not to replace discs too often since even replacing the discs potentially introduces risk. Reasons to replace the disks more frequently could include corrosion on the disk and fatigue from repeated pressure cycles. (The closer to the setpoint these are used—greater than 70-80%—the more fatigue and likelihood for premature failure).

The answer could be no devices at all, just a TPRD, just a PRD, or both. It depends on the potential overpressure scenarios identified during a hazard assessment. TPRDs typically are not used on ASME pressure vessels since they are not ASME compliant devices and since  system siting provides protection from engulfing fires. However, TPRDs are frequently used in portable applications for both hazardous materials transportation and as vehicle fuel tanks.

HSP members generally agree that rupture discs are problematic due to spurious activations and large releases. They should be avoided in most applications, since the entire vessel contents will empty and the tank will require inerting before replacing discs and be put back in service. TPRD’s can provide additional protection against spurious activation, but have also experienced this issue, plus might not meet code requirements for stationary service.  Spring loaded or pilot-operated relief devices may also prematurely activate but usually will reclose to limit the size of release. However, The Panel recommends complying with the adopted edition of ASME Boiler and Pressure Vessel Code (which provides direction for stationary vessels), NFPA 2 - 2023 (para 7.1.5.5), and CGA S1.3 (which is referenced by NFPA). NFPA 2, Section 7.1.5.5.1, requires the use of a PRD device to prevent the maximum design pressure of the vessel from being exceeded. The use of CGA S1.3 is also required. CGA S1.3 provides guidance on when protection needs to be provided, the types of devices to be used, and the sizing of those devices. For most overpressure scenarios, the vessel usually must be protected at MAWP and within the rules of ASME Code and CGA S1.3. 

This usually means a spring-activated pressure relief valve since rupture discs and TPRD’s rarely can be set as precisely as needed. If there are thermal exposure scenarios, methods to eliminate the thermal exposure and/or adding supplemental  thermal activated pressure relief devices may be appropriate. Spring-operated relief valves are often used to meet thermal exposure scenarios but have limited effectiveness and may not be optimal. The number of either pressure or temperature protective devices depends on the demands present and the device sizing. The need for redundancy of devices depends on the results of a risk assessment. Means to service the devices should be considered in the design. The location of device discharges must also be considered, particularly when devices are used for thermal protection. The installation of devices, inlet piping, and vent piping must be designed per ASME requirements.  

A few additional topics to consider: Ambient temperature effects should be considered when designing relief systems. While selection and sizing are detailed in the standards, performance may be affected by pressure effects that are not considered under the standards. For example, a code-compliant pressure relief valve could open if ambient temperature or process conditions are not sufficiently addressed. Flow capacity and setpoint are important issues to discuss for relief devices. Setpoints are usually based on MAWP or design pressure, as well as the type of overpressure exposure. For example, overpressure due to fire will often have a higher allowable pressure accumulation. Flow capacity is normally determined by the flowrate into the vessel from a compressor or pump, failure of upstream control valves, and/or heat flux into the tubes. Heat flux can be considered separate or in addition to the compressor or pump max flow capacity depending on risk assessment results. 

Although relief devices are usually required by code or the hazard assessment, they also are subject to spurious or erroneous activation. Considering hydrogen’s likelihood of ignition (when mixed with air), the use of devices should be balanced with other means of shutdown and pressure protection. Where equipped, s, the PRD should normally be the means of overpressure protection with the highest pressure setpoint. To ensure that the above issues are properly addressed, the HSP recommends that the design of high-pressure hydrogen systems be done by engineers with experience in this application. Regarding specific recommendations, the Panel does not endorse specific equipment manufacturers, models, or brands.
 

The HSP recommends against the use of glycols for pressure tests due to the difficulty of adequately removing all glycol that might be left in a system after a hydrotest. The HSP recommends a pneumatic test at 110% of the system maximum allowable working pressure (MAWP), which is acceptable by code. Due to an increased danger with pneumatics vs hydrotesting, establish a pressure test zone for personnel in the area. 

Several concerns with glycols are: 1) Freezing in a liquid hydrogen   system that could lead to safety issues such as blockage of lines and instrumentation. There could be dead legs or low spots that cannot be emptied or cleaned easily leaving enough to freeze important components such as small gauge lines, vent stacks, and relief devices. 2) Freezing at cold ambient temperatures or within dispenser piping that chills the dispensed gas can also lead to blockage of lines and instrumentation. 3) Off-spec hydrogen if not well cleaned from the lines, vessels, instrumentation, and dead legs of a system. Glycol can interact with materials such as aluminum and lead to pitting and corrosion.

Another concern for glycol solutions is that they are flammable. A leak generated during pressure testing could generate a spray that is readily ignitable by nearby ignition sources. For automatic sprinkler antifreeze solutions, the requirement is to use UL-listed solutions that have undergone flammability testing. with the pressures used in applicable systems. These pressures are at least an order-of-magnitude lower than what is typically used for hydrogen piping. This safety concern can be mitigated by preventing personnel and ignition sources from being near the piping when tested.

As a general rule, pneumatic testing is preferred over hydrostatic, especially for systems with complex piping geometry, as long as proper precautions are taken for the Pressure-Volume (PV) energy. The HSP recommends using a pneumatic test at 1.1 times the design pressure using a clean, dry, inert gas such as nitrogen. This is a best practice for field testing where cleaning afterward is even more difficult. Helpful resources that describe precautions to take are: 1) UK HSE G4 Safety in pressure testing, says to (a) perform a hazard analysis considering stored energy, blast effects, and missile formation; (b) develop a written procedure; and (c) examine system prior to test. 2) ASME PCC-2, Repair of Pressure Equipment and Piping, says to limit stored energy in any one test loop to 271,000,000 J (200,000,000 ft-lb) (0.07 tons TNT). If the stored energy requirement can’t be met, barriers must be erected or separation distances in excess of 60 m (200 ft) must be used. The HSE document offers broad guidance, while the ASME document provides more specific information about precautions as a function of the stored energy.

H2-air flammability limits vary with temperature  . The H2-air lower flammability limit is virtually the same as the H2-O2 lower limit. However, the H2-O2 upper flammability limit increases substantially to about 95% at room temperature and gets even higher at elevated temperatures.