Butterfly Valves in Compressed Air & Vacuum Services
Because a butterfly valve uses a disc to control flow, much like a damper in a duct system, some people occasionally want to use it to control the flow of compressed air and other gases, or for vacuum service. Will the valve work in these no-liquid applications? For the most part the answer will be yes, depending on applications specifics.
Compressed Air or Gas Applications
Let's first take a look at compressed air or gas service. Hayward butterfly valves will work in most of these applications; you just have to be aware of a couple of application parameters.
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, about 50 PSI or less, the Hayward 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 the liner, also making the valve more difficult to operate, over time.
It's best to use a butterfly valve with an FPM liner for compressed gas or air service. This material is less susceptible to drying out than EPDM. But even with FPM, the valve should be factory assembled with a special vacuum lubricant. When applied to all of the FPM seals, it will help to extend their service life.
An exception to the use of FPM is methane gas applications. Butterfly valves with Nitrile liners have been used very successfully in this type of service, though vacuum lubricant is still a a good idea on the Nitrile liner and seals.
To sum up: for compressed air and gas applications, if the gas is compatible with either a PVC or PP discs and the liner material, and it's a low pressure application of 50 PSI or less, a Hayward thermoplastic 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.
Before we talk about the application suitability, it might be a good idea to review just what a vacuum is and how it's measured.
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 units of measure for a vacuum just like PSI is a measure of pressure.
Inches of mercury is probably the most common unit of vacuum measurement for 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 someone says 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 butterfly valves should be suitable for vacuum service down to 26 inches of mercury. The best disc to use is usually PP 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 FPM and vacuum grease should be used on the liner and seals. All applications are unique and these recommendations won't apply in every case.