What is the importance of H2 system cleanliness to safety, and what are recommended cleanliness levels for components and best practices when opening up H2 systems for maintenance/service?

The cleanliness of an H2 piping system is often based on end use requirements since the purity of the system may impact the end use application. Cleanliness required for end use purity is usually much greater than that required to prevent condensation or oxygen content sufficient to create a safety risk. While not required for the same safety reasons as oxygen safety, specifying a system to be “oxygen clean” for systems where a moderate level of cleanliness is required is a typical practice.

 Documents such as CGA 4.1 provide instruction and inspection guidelines for oxygen cleaning. Systems that require very high purity, such as for electronics, will have custom cleaning specifications for that application. One safety-related cleanliness consideration is filtering of particulates since they can cause regulators and safety valves to not operate correctly. This is a frequent concern on recently assembled lines that can have residue from machining, thread cutting, and welding.

Proper purging procedures should be used when opening a system for maintenance to ensure that a flammable mixture of hydrogen can’t be formed when the system is exposed to air. Connections should be covered when open to avoid entry of particles and moisture. Moisture is a particular concern when opening LH2 lines since water vapor can condense on cold surfaces and result in plugging of lines over time.