What safety measures should I put into place if I am using hydrogen for partial pressure or quenching?
Here is some advice gathered from a number of industry experts on what you should watch out for and what precautions are necessary when using a hydrogen atmosphere.
1. The vacuum-furnace leak rate must be 10 microns per hour or less prior any processing with hydrogen.
2. The pressure-relief valve and/or chamber release valve must be vented outdoors via hard pipe on an upward angle to ensure no trapped hydrogen gas. Use of an inert gas added to this venting line for purging capabilities will help ensure no trapped hydrogen gas.
3. If amount of gas venting from furnace is minimal, it is possible to burn the gas at the exit. Be sure to use a check valve.
4. Ensure there are no leaks in the hydrogen supply lines. A vacuum should be pulled on the hydrogen supply lines to check for leaks. Bubble checking of all fittings should be part of routine maintenance.
5. Before opening the furnace door, pump down to less than 100 microns and backfill with inert gas. It would be good to perform this operation one additional time to dilute any hydrogen gas trapped in the pumping exhaust system.
6. Good preventative maintenance schedule, which includes inspecting furnace integrity. O-rings not only if they are dirty but also flat. Take solenoid valves apart and clean the surfaces and components. Oil changes on pumps and condition of the belts are very important to avoid pump failure.
7. Intrinsically safe and redundant control systems to ensure operator mistakes are minimized.
8. Compliance with NFPA 86, especially sections 12.3 and 12.4, is mandatory.
9. Provide an inert gas flow, typically 100–150 cfh, to the mechanical pump plenum at all times when hydrogen is flowing. Install an inert gas line to the pumping exhaust line to purge hydrogen gas out at any time.
10. Have a manual safety shutoff valve added to hydrogen supply line that is easily accessible and prominently labeled.
11. All hydrogen supply lines shall be metal, not flexible plastic.
12. Before addition of hydrogen is enabled, the use of an interlock, which will not allow the addition of hydrogen to the furnace until the furnace has been evacuated to below 100 microns (0.13 mbar).
13. Hydrogen supply lines shall be equipped with high- and low-pressure switches to prevent operation above and below specified pressure ranges.
14. Components that use hydrogen shall be designed and approved for "hydrogen use."
15. Mechanical roughing pumps on all furnaces should have “run monitors” installed and interlocked to ensure that the pump itself (not just the pump motor) is operating.
16. Ballast valves on all mechanical roughing pumps should be plugged to prevent the accidental introduction of air into the pump while hydrogen is being used.
17. Change all vacuum-line air-release valves to normally open style. Pipe those valves to an argon supply equipped with a low-pressure regulator (with pressure gauge) set no higher than 1 PSIG (6,900 Pa).
18. Remove sight glasses from furnace.
19. Always have an operator nearby when operating with a hydrogen atmosphere.
20. Provide five times volume of inert gas standing by at all times ready to purge the furnace of hydrogen in an emergency or a power failure situation.