Vacuum pumps are used to maintain insulating vacuum in cryogenic vessels and piping and to purge hydrogen and associated purge gases from systems. Hydrogen is more difficult than heavier gases to pump because of its high thermal velocity (almost four times the average velocity of air molecules) and its small molecular size. Also, hydrogen will diffuse rapidly in an evacuated space. Therefore, careful consideration should be given to the specifications for a vacuum pump intended for use in hydrogen service.
Several pump technologies are used, depending on the vacuum requirement, and these include positive displacement, momentum transfer, and entrapment pumps. Vacuum pump capacity should be based on the total calculated peak demand of the system, and the pump should meet this peak demand when continuously operating. Regardless of the technology, vacuum pumps used for pumping hydrogen should be rated for hydrogen service. Several aspects of vacuum technology pertaining to hydrogen best practices are noted below.
While in general a vacuum insulating space on a vessel or pipe should not contain hydrogen, leakage from the primary enclosure or piping can release hydrogen into the space, so the vacuum pump must be designed for safe use for gas mixtures including hydrogen.
Positive Displacement Systems
Condensation on the interior walls of rotary piston and rotary vane vacuum pumps can occur when large quantities of water or other condensable vapors are being pumped. Gas (usually room air) is admitted through a ballast valve to avoid condensation problems. The valve is positioned to allow the gas (which could be as much as 10% of the pump displacement) to enter the chamber during the compression stage. This could result in the creation of a combustible mixture within the pump when a system containing hydrogen is being evacuated.
The exhaust from rotary mechanical pumps should be vented outside because of entrained oil vapors. The vent line should not run vertically from the exhaust connection because water or other vapors that condense on the vent line could drain into the pump and contaminate the fluid. Alternatively, a sump could be added at the exhaust connection to collect the vapors before they can flow into the pump.
Momentum Transfer Systems
Turbo-molecular pumps may require a special blade design to optimize performance for pumping hydrogen.
Cryogenic pumps use cold surfaces and adsorbents to condense and accumulate vapor deposits. The accumulations are subsequently isolated by valves for disposal. The cryogenic cooled material, if trapped and not allowed to freely exhaust, can create high pressures when warmed. In addition, care must be taken with the exhaust process because these materials may have the potential to react with one another, or with the atmosphere, when warmed.
Getters are materials that absorb gases in vacuum systems. In some instances, getters containing hydrogen, when exposed to high concentrations of oxygen as might occur with an air leak, can cause heating sufficient to act as an ignition source.
Elastomeric materials used in hoses (such as rubber and thermoplastic) will absorb and permeate hydrogen readily, so the length of such hoses should be minimized.
Any metals exposed to hydrogen, including those used in instrument diaphragms, should be evaluated for hydrogen embrittlement. For information on material selection, please see Material Compatibility.