Vacuum Isolation Valve

[TheMak]

Hey everyone!

New member here. I joined eng-tips because I frequently have engineering questions that none of my real life friends seem to be able to answer and I haven't yet figured out how to get google to show me highly specific information regarding machine design and component names.

That brings me to the question I have for today. I'm sorry if this sounds like a noob question or if I should already know this but here goes: I need to know the name (not brand name) of a particular type of valve. Basically, I know what I want and I need to know what it's called in the market. I'm looking for a valve that is used on reciprocating vacuum pumps. This valve is one directional, i.e. it allows the air to flow through in one direction but stops air from flowing in the reverse direction. I'm basically designing a small reciprocating vacuum pump and I need to know where I can find the valves I need and what valves I need to search for to ensure that they can keep the air out of the chamber once it is evacuated. I'm not familiar with the industry lexicon and would greatly appreciate some help.

If my question is in any way incomplete or you need more information to answer it, I will be happy to provide such information.

Best,
Mak.

 

[berkshire]

What you are looking for is called a check valve. On a vacuum pump You would have two, an intake, and an exhaust valve.
These valves take several forms, depending on the manufacturer of the pump. In one pump it can be a flat carbon disc, rising and falling against a machined metal sealing face. In another make of machine it can be a spring reed, pushed aside by the incoming air then snapping back into position to prevent the escape of that air. On yet another machine the valve can be a spring loaded hardened steel ball sometimes this is referred to as a poppet valve.
These valves are usually incorporated into the cylinder heads of the vacuum pumps.
B.E.

You are judged not by what you know, but by what you can do.

 

[Compositepro]

Berkshire is correct but there are some additional subtleties involved with vacuum pumps as opposed to compressors (vacuum pump are a type of compressor used to compress gases from below ambient to ambient pressure). Most check valves have cracking pressures that are a significant compared to the vacuum pressure. The valves in a vacuum pump are designed for the purpose in such a way that they do not restrict gas flow. Unless you are concerned about maintaining only a crude vacuum, standard check valve are unsuitable and are not used in vacuum systems. You may need an actuated valve that closes upon loss of power or reduction in vacuum.

 

[TheMak]

Thanks for the responses guys! (Or girls, lol).

I'm designing a vacuum pump that is going to be connected to an evacuation chamber with a volume of 1 cu.ft. From what I gather from the answers, it looks like there are only 2 of these check valves used in this application.

For some reason that I still can't fathom, I was under the impression that there would be 3, of which 2 would be in the cylinder head and one would be somewhere between the inlet to the cylinder and the evacuation chamber.

The vacuum I'm going for is 10-20 millibars. Is it possible to find check valves that are made for such a purpose?

Berkshire, that's exactly what I was thinking of, especially the valve with the hardened steel ball because that's the one I have worked with, only in the application I saw them used, they were carefully weighed and this weight was what kept them closed to maintain a certain pressure as opposed to a spring.

Compositepro, can you please clarify a little bit? You mentioned that depending on the vacuum I'm going for I would need an actuated check valve. Is 10-20 millibars around the level where normal check valves are ineffective? I didn't think this was a very high vacuum. And when you say that I need an actuated valve that closes upon the reduction in vacuum, do you mean that the valve closes when the pressure rises above 20 mbars or when it falls below 10 mbars? This is assuming that those are my limits.

Thanks again for the replies!

 

[berkshire]

The Mak,
The intake and exhaust valves on a vacuum pump. Once the pump stops running will always allow a tiny bit of leakage. You are better off using a Manually operated valve, or a solenoid operated valve rather than just a check valve. You mention that your chamber will only be 1cuft. Will this cycle or do you intend to hold the vacuum for a period of time? You have to understand that we have no idea what you are doing with this. I am used to pulling a vacuum on a large part, and holding that vacuum for 6 to 8 hours as part of a resin curing process. I also do air conditioning work where I am used to pulling a higher vacuum for one to two hours.
B.E.

You are judged not by what you know, but by what you can do.

 

[Compositepro]

You say you are designing a vacuum pump. This sounds like a hobbyist question. Reciprocating pumps are not used to get 10 mbar because of issues with unswept volume in the cylinders between the intake and exhaust valves. Generally they are capable of 27-28 inches of mercury. Achieving 10 mbar requires a compression ratio of over 100.

Buy a proper vacuum pump. You can get a new one from Harbor Freight for $100 on sale. Used pumps are also readily available. The best lab pumps are oil-sealed rotary vane designs that can achieve 0.001 mm of mercury. The small ones will take several minutes to pump down your chamber.

"I haven't yet figured out how to get google to show me highly specific information". I refrained earlier from commenting on this statement, but it did catch my attention right away. You cannot bypass the requirement for a basic, and time consuming, education by Googling for information only when you need it. Try searching for information on vacuum pump design and spend a few hours or days absorbing some of it. What you learn will be applicable to many other machines as well. That is how it works.