Are pneumatic valves intrinsically incapable of handling vacuum?

[COWBOYME]

I have designed a basic test stand that requires vacuum to be delivered to two separate test points. I tee-ed directly from the vacuum pump. One line goes to the gauge and one to a pneumatic three-way valve with which I intend to control which test point has vacuum. I am using 1/4" hose and fittings. Unfortunately, I am leaking about 4.5 inHg across the valve (i.e. in either position the other position reads 4.5 inHg). I disconnected and capped the line to the valve and the vacuum returned to full value. There is disunity of thought among my colleagues. Several say that these valves are not designed for Vacuum and as such there is inherent leakage. Others say I merely have a defective valve.

 

[hydtools]

Whether the valves works with a vacuum depends on the type of seals it has. If it has cup seals on the stem, they will allow air to be inhaled by the systen vacuum. These seals act like one-way check valves. They depend in part on system pressure to actuate them, to aid in their sealing.

If the seals are cup seals, you could invert them to work with vacuum. Especially if the seals are spring loaded or self-loading to seal.

If the valve has o-rings for seals, it should work with vacuum.

Ted

 

[budt]

Most manufacturers catslogs offer information about the fluid their valves can be used with. If the catalog is not clear call the manufacturer and talk to a Tech person. I know a lot of ASCO valves specify Air, Oil or Water for their valves.

Since Vacuum is opposite of pressure it can cause wierd problems with some valve designs. A Poppet may stop Pressure from passing as it sets on a Seat with 50 PSI and a spring holding it shut. However, when you chamge to a -13 PSI on the pressurized port it can cause the Poppet to raise off its seat enough to allow atmospheric air into the negative pressure chamber.

Another problem is how the valve is actuated. If it is a Solenoid/Pilot controlled valve they usually require at least 35 PSI to shift position and can't work at the lower Vacuum pressure.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING