There are several resources that can help review designs, such as the Hydrogen Safety Panel and other outside consultants that are members of the Center for Hydrogen Safety.

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.

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.

The Panel recommends performing a pressure test at 110% of design pressure. This requirement should be applied to all systems regardless of construction type since the intent is to ensure pressure integrity and proper installation. All fitting types have modes of failure during installation. For example, there are numerous examples where compression fittings have had ferrules installed incorrectly, tubing improperly inserted, and have been inadequately tightened. In addition, leak checking and pressure testing should always be done in accordance with the locally adopted piping code.  Examples include ASME B31.3 and the Pressure Equipment Directive. 

 
System pressure relief devices will usually need to be removed for the test and temporarily replaced with higher setpoint devices to protect the system during the pressure test. Also, consider a proportional acting relief device.  Piping systems do not require a pop-acting ASME relief valve that are used for pressure vessels since piping system relief devices are more likely to chatter. Chatter can lead to lower than intended flow rate and damage or failure of the valve to operate correctly.   
For high-pressure systems, pneumatic testing is almost exclusively done given the challenge of removing water from a hydro test from the system after the test. Although one might question the wisdom of pneumatically testing at such high pressures, precautions can be taken to ensure a safe test, such as requiring an exclusion zone during the testing.   
 

Explosion testing with hydrogen should be utilized only where there is not an established alternative and then only by personnel experienced in such testing. 
Testing with hydrogen is always a challenge and needs to be approached carefully due to significant differences in properties between hydrogen and propane. Hydrogen can develop significantly higher overpressures and preliminary testing with leaner mixtures and possibly smaller containers should precede full-scale application tests. Documents such as NFPA 2, NFPA 68, and NFPA 60 provide additional guidance on the potential explosion hazards and properties of hydrogen.