What is the Most Important Thing on Selecting Vacuum Pump?

List the criteria and let the forum suggest the priority.

Among other things, it depends on the vapor pressure of the liquid used to seal the pump. Without looking at detailed data, I suspect your two vendors are correct.

A general search on "vacuum pumps" should be able to turn up the capabilities of various types, thereby guiding you to a suitable selection (if your vendors haven't already suggested some alternatives).

And why would anyone want to get down to that level of vacuum? It seems abstruse.

Bit of an unusual unit for highish vacuum application. Do you mean 0.008 megapascal (abs)? That should be MPaA. If so that's 80 mbara, and shouldn't be a problem for a LR pump, using water at normal temperature. 80 mbara is approx 27.5" Hg vacuum, and a quick look at the Gardner Denver Nash website says "Vacuum pumps to 29+" HgV". If you mean 0.008 mPaA (millipascal) that's a very high vacuum, as others have said, ans not achievable by a LR on its own.

When I was working we bought several pumps rated for 80 mbara.

"Bit of an unusual unit for highish vacuum application"

GA. I agree with above and that follows.

Usual unit for highish vacuum application shall be either Pa, mbar, microns of Hg or thou" of Hg, all in absolute terms.

OP should also give capacity of the pump or volume to be evacuated, the medium pump has to handle and sealing liquid.

Difficult is not impossible.

Try two other vendors.

What are you using it for?

What are you using it in? Is it a chamber, a bag or a room ?

To obtain high vacuum in a chamber you're best using two pumps,a mechanical (Kinny) pump to evacuate down very quickly and a diffusion pump to get the kind of pressures that you are looking at.

Bazzer

But what pressure is that? OP hasn't confirmed yet

I haven't been in the industry for about 20 years now & can't remember the exact pressures we got down to but the scale was microns & we were using the vacuum to do plating on components which meant that it had to be near complete vacuum as possible, As Woodpower said any volatile material will slow the cycle time down considerably as does any porous material, Any way it's all irrelevant unless we know what he's doing & how.

Bazzer

When I worked as a lab chemist, we used the two pump system as noted above by Bazzer. To decide further you need to provide the pump down rate, the leakage or vapor generation rate, whatever needs to be pumped. If there are volatile components of low volatility in the system, you might be able to use a dry ice or liquid nitrogen trap to speed the transfer. How you use it, for what, and your budget are important data.

"FV 0.008 MpaA" was the stated target.

OP has not stated much since.

But is that megapascal (abs), which it ought to be with capital M, or millipascal - see my #4? Factor of 10⁹ is a lot!

I'd say megapascal (abs), as it has the capital M, just lacks the capital P

So would I, but in that case there should be no problem with a LR pump

Pump specification are?

Cap 40 m3/hr; suction pressure FV 0.008 MpaA (Mega Pascal Absolute), this pump will be used on our water transfer system

Apart from volatile & porous material water is a vacuum, or should I say a high vacuum pumps worst enemy.

Bazzer

But 80 mbar is not a high vacuum. And you can't have a liquid-ring pump without liquid, water being by far the most common. For obvious reasons, a different liquid would only be used in special circs where water wouldn't work.

You don't say exactly what it's doing, but depending on the inlet temperature and humidity, and seal water temperature, that duty should be no problem for a LR pump. One from the Vectra SX range by Gardner Denver Nash covers it, can't be more specific as don't know whether you're on 50 or 60 Hz. There are plenty of other suppliers.

So, the specified vacuum will never actually be reached?

Even if all the water was transferred and you were sucking against a dead foot (opposite of head?) the water would then start boiling....

I'm not questioning your spec or application, just tying to get my addled brain around the idea.