Can a rotary vane pump handle water vapour?

Yes — but only with the gas ballast valve open, and only within the pump's stated water-vapour tolerance. Run it without gas ballast and the vapour condenses inside the pump, turning the oil milky and destroying both its sealing and its lubricating function within hours.

The mechanism is worth understanding because it tells you when you are safe. A rotary vane pump compresses gas before pushing it out of the exhaust valve. If the gas contains water vapour, compression raises its partial pressure — and if that exceeds the saturation pressure at the pump's operating temperature, the water condenses right there in the compression space and mixes into the oil.

Gas ballast defeats this by admitting a controlled amount of dry air into the compression space part-way through the cycle. The extra gas means the exhaust valve opens sooner, at a lower compression ratio, so the water's partial pressure never reaches saturation and it leaves the pump as vapour. The trade-off is direct and unavoidable: gas ballast raises ultimate pressure, typically by about a decade, and increases oil consumption through the exhaust.

The number on the data sheet is water-vapour capacity, usually in g/h — how much water the pump can pass per hour with gas ballast open at its rated operating temperature. Exceed it and you condense regardless of ballast. Note the temperature condition: a pump that has not warmed up has a much lower real capacity, which is why the standing advice is to run the pump 20–30 minutes with gas ballast open before and after a wet process.

If the oil has already gone milky, changing it is not enough on its own — run the pump warm with gas ballast open and the inlet blanked for half an hour to drive off dissolved water, then change the oil. Doing it in the other order just contaminates the fresh charge.

When the vapour load is continuous and large, a rotary vane pump is the wrong tool no matter how you run it. Dry screw, claw or liquid ring pumps handle sustained condensable loads without an oil charge to protect — tell us what you are pumping and at what rate and we will tell you honestly which family fits.