When first asked about pumping hydrogen gas from a vacuum process, I was a bit surprised. Hydrogen as a process gas? But, over the years, it has been asked often enough to show it is not an isolated concern. The questions usually run: Is it difficult to pump? Do the mechanical pumps need Fomblin? And, for processes that need occasional high vacuum, what fluid should we use in the diffusion pump?
Hydrogen at process pressures is not a difficult to pump. It's neither highly reactive nor corrosive and, therefore, doesn't screw up pump oils. So what's the big deal? Well, one fact you probably know about hydrogen (from watching film of the Hindenburg) is that it burns and/or explodes. Another fact you may not know is that hydrogen and oxygen form an explosive mixture over the greatest concentration range of any two materials (any hydrogen concentration between 4.5% to 93.9% in oxygen or between 4% to 74% in air).
Putting those two facts to work, and using the sarcastic Socratic dialogue method to drive home the point, here is an (imaginary) conversation that I've (almost) had many times.
- What is the greatest risk when pumping hydrogen? An explosion.
- And where is that risk greatest? At the mechanical pump's exhaust where the hydrogen (at atmospheric pressure), oxygen (from air), and sparks (from the pump's electric motor or switches) may combine.
- Can I use a hydrocarbon oil in the mechanical pump? Sure, if you're very brave. Until the hydrogen explodes, the oil won't catch fire.
- But what if the hydrogen explodes? Ah! Then you'll be so busy vacating the building (if you're lucky that burning oil will be your least concern).
- But what you've just suggested sounds like a disastrous risk to take. Do you really mean I can use hydrocarbon oil? Of course not! Use Fomblin. I, for one, won't enter your building until you do.
- Is there any other precaution I can take? Yes. Buy a mechanical pump fitted with gas bleed (usually called a chemical or plasma pump) and put the maximum amount of nitrogen, not air, into the bleed to dilute the hydrogen and sweep it from the pump casing. Also duct the exhaust gases (in the best gas-tight manner possible) to the outside world but make sure the vent can't be hit by lightning.
- What about the oil in my diffusion pump oil? Well, there's no oxygen inside the chamber so there's no danger of an explosion unless the system suddenly develops a massive leak
- How could that happen? Oh, when an ion gauge's glass envelope is smashed by an ill-placed crescent wrench.
- Then what happens? Again you'll be too busy vacating the building to care, but if the diffusion pump oil is flammable, its vapor will add a nice high number of joules to the energy of the hydrogen-oxygen explosion triggered by the hot filament in that broken gauge.
- Are you serious? About handling hydrogen? Always!