What are typical cleaning requirements for hydrogen systems?

The end use of hydrogen usually drives the cleaning for initial construction. 

1. When the hydrogen is used for combustion and many industrial applications, normal care during construction and leak testing is usually adequate. 
2. When the hydrogen needs to be very pure, as is the case for PEM fuel cell applications, more attention needs to be paid to installing clean system components, keeping them clean during construction, and cleaning prior to introduction of hydrogen into the system. Design and construction firms often lean heavily on standards developed for oxygen service for guidance for these applications. While "oxygen clean" might not be necessary for the same safety reasons as for oxygen systems, "oxygen clean" is well recognized and there are several standards which provide guidance and good practices for different levels of cleanliness. Examples include: 
   1. ASTM G93, Standard Practice for Cleaning Methods and Cleanliness Levels for Material and Equipment Used in Oxygen-Enriched Environments; 
   2. CGA G-4.1, Cleaning Equipment for Oxygen Service; 
   3. SAE ARP1176A, Oxygen System and Component Cleaning and Packaging; (d) MSS SP-138, Quality Standard Practice for Oxygen Cleaning of Valves & Fittings. 

For liquid hydrogen systems, the additional concern is with removing all substances that will be solid at liquid hydrogen temperatures. That includes all gases except helium, so it is generally necessary to purge air out of the system using hydrogen (when it can be done safely, e.g. for small volumes) or helium.