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.