What is the required pressure (if any) for leak checking newly assembled hydrogen piping systems? It is unclear what pressure is required when constructing a system, prior to initial operation, and the applicable code section seems to suggest exceeding the maximum allowable working pressure or at the very least reaching a pressure that will activate pressure relief. ASME B31.12 seems to be very specific regarding welded pipeline systems; how does it apply to other types of connections from hydrogen fueling stations like double ferrule compression fittings?

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.