The first thing to look at is temperature and chemical compatibility. Make sure that these fall within the rated capability of the valve. If they do, and the application is a low pressure one, say 50 psi or less, the plastic butterfly valve should do the job.

One thing to remember though is that the soft elastomer liner of the butterfly valve will have a tendency to dry out and harden in compressed air or gas service. This will make it harder for the disc to seal against it over time as well as making the valve more difficult to operate.

It's best to use a butterfly valve with a Viton( liner for compressed gas or air service. This material is less susceptible to drying out than is EPDM. But even with Viton the valve should be factory assembled with a special vacuum lubricant. When applied to all of the Viton( seals, it will help to extend their service life. 

An exception to the use of Viton( is methane gas applications. Butterfly valves with nitrile liners have been used very successfully in this type of service. Vacuum lubricant is still a a good idea on the nitrile liner and seals though.

To sum up, compressed air and gas applications, if the gas is compatible with either a PVC or PPL disc and the liner material, and it's a low pressure application of 50 psi or less, a plastic butterfly valve should be suitable. Just don't forget to use vacuum grease on the liner to prevent it from drying out. And remember all applications are unique and these recommendations will not apply in all cases.

For our purposes we can say a vacuum exists in any enclosed space (like a pipeline) that has been brought below atmospheric pressure by pumping out some of the air or gas in that space.

You've probably heard people talk about "inches of mercury", "bar", or "mm of mercury" when referring to a vacuum. These are unit of measure for a vacuum just like pis is a measure of pressure.

Inches of mercury is probably the most common unit of vacuum measure of industrial applications in the United States. This is the measurement of the difference in height of a column of mercury, in a barometer, at atmospheric pressure, 29.92 inches, and what its height would be at less than atmospheric pressure, or a vacuum.

When some one say's they need a valve for "27 inches of mercury service" they are referring to, in theory, what the height of mercury in a barometer would be if it were measuring the vacuum in the application, not atmospheric pressure. In practice, of course, special gauges are used to measure vacuums.

In most applications Hayward plastic butterfly valves should be suitable for vacuum services down to 26 inches of mercury. The best disc to use is usually PPL because its natural lubricity makes it easier to seat than one of PVC. As in compressed air or gas service the best seal material is usually Viton( and vacuum greased should be used on the liner and seals. All applications are unique and these recommendations won't apply in every case.