Compressed Air System

Shut off and special duty valves

Compressed air valve selection is application-specific.

By Scott Foss, President, Plant Air Technology

There are many types of valves available for compressed air service. The following is a list of the more common types for a variety of applications seen in the field:

  • globe valves
  • gate valves
  • ball valves
  • wafer valves
  • butterfly valves
  • plug valves
  • slow acting valves
  • relieving valves
  • wedge valves
  • needle valves
  • petcocks
  • notched ball valves

Starting from the beginning

There are a tremendous number of globe and gate valves installed in compressed air services and they should be the last choice for shut-off valves in the piping system. There are a number of drawbacks to globe and gate valves. They have the highest pressure drop of any valve for line service. In addition, they cost two to three times as much as the other valve types. They are heavy and difficult to install. They are remarkably difficult to remove and service and tend to leak more often than other available valves for the same service.

Plug valves have minimum pressure drop for shut off service, but usually cost more than ball, butterfly, or wafer valves. They are not as easy to work with and cost more than ball, wafer, and butterfly valves.

Ball type, wafer, and butterfly valves are superior for inline shut off service in compressed air piping systems. Use full flow ball valves for 1/2-inch to 2-inch piping. In larger piping, use either a compression-type wafer valve or a butterfly valve. The butterfly valve bolt pattern mates to the adjacent flanges. The wafer valve usually has a grooved ring seat in the valve body for an O-ring that is compressed to form a seal between the mating flanges when bolted together. Remember that valves in compressed air service are either fully opened or fully closed. There is usually no service in between. It is difficult to tell if stem type valves are fully open even if they have rising stems. This can add to an already high pressure drop in service.

Minimize pressure drop

One facility thought they needed to replace their 1-inch square drive impact wrenches with larger #5 spline drive wrenches for maintenance. Examination of the system revealed a non-rising stem valve in the upstream piping that was restricting the air flow. When the valve was opened, the 1-inch tool worked fine. From that point on, the facility only installed quarter turn valves that could easily be identified as open or closed.

The other six valves, along with ball valves, are for point-of-use applications. The slow acting valve is a most interesting valve for higher flow applications. They can be spring loaded or actuated pneumatically, mechanically, or electrically. When actuated, they take 10 seconds or more to fully open. This not only ramps the flow into the users to reduce the need for more on-board supply energy (reduces surge) but also protects downstream components from being "slam" actuated from atmosphere to full line pressure. This can be a thoughtful selection on any large volume point-of-use installation.

Auto drain valve

The relieving or auto drain valve dumps the downstream pressure when it is in the off position. These valves are used primarily in subheaders down to the point-of-use. I feel strongly about the safety this valve offers the user. Auto drain valves are offered in ball and plug type styles. When shut off, other types of valves trap pressurized air in the downstream piping. Then, before you can do any work, you must relieve the air pressure by cracking a hose fitting.  The only reason for shutting off the valve is to limit waste when not in service, to alter the downstream arrangement, or to perform maintenance. In addition to the safety angle, you can trap parts or wreck tooling if you cannot relive the pressure. Another advantage for these types of valves at the point-of-use is that they blow air and make substantial noise when partially open. Other types of valves restrict flow and cause a pressure drop. You may not be able to detect this problem with other types of valves such as a non-rising stem valve.

Wedge valves, notched ball valves, and needle valves are for manual flow control in process or metered recovery applications. The needle and notched ball valve provide accurate control. The self-cleaning capability of wedge valves makes them well suited for particularly dirty applications.

One of the common problems with valves and smaller, frequently used parts in the air system is the number of sizes commonly stocked. There are 16 sizes available at 8-inch and under. It is easy to install improperly sized parts in the distribution system.

It is a good practice to identify components, including valves, for air service. Hold the number of sizes to 1/2-, 1-, 2-inch in smaller valves and 3-, 4-, 6-, 8-, 10-, and 12-inch whenever possible with larger valves. This limits inventory and pressure drop in the system.

Contamination

Another important issue is the compatibility of valve components with the contaminants and conditions of the air in a specific part if the system. Every compressed air system has contaminants present until they are removed either by intent or at the point-of-use. Each contaminant, except those in a vapor form, can be removed with properly designed clean up. Much of the vapor condenses, wets out, or entrains in other contaminants upstream in the system. Some might get downstream. Most lubricated compressors wash out airborne contaminants such as dirt, acid and caustic gas, solvents, stack emission, and other industrial airborne contaminants. Acid and caustic gases (common in industrial applications at a low volume--one to five ppm) form acids when they combine with the water in coolers or in dryers.

Much of this will break down the lubricants. In some cases, when controlled, this is a better alternative to the downstream damage that can be done to system's components or the cost of parts that are compatible with these aggressive contaminants. Once they get past the compressor, many of these use water as a carrier. Removing the water effectively deals with much of this problem. In the particular case of the valves, carefully select upstream valves and the clean-up equipment.

Consider the types of contaminants present and select appropriate components. Pay particular attention to stem valve packing, seals, and internal valve components. Some lubricants can be very aggressive with rubber components such as low nitile Buna N. Viton is a common material compatible with most compressed air contaminants.

Thread compounds, sealants, and gasketing

 

There are many opinions regarding sealant, thread compounds, and gasketing materials. The most important criterion is compatibility with the materials present in the air in the specific part of the system where the material is used. This also applies to gasketing material. Downstream, where screwed fittings predominate, thread sealant compounds must be used. Most mechanics have little training in assembling threaded fittings for air service. Air piping resonates and is highly subject to leaks. Teflon-type tapes and sealant are the most popular sealant for air piping, but with little justification. In many other applications such as water, Teflon is an acceptable sealant that is easy to work with.

However, Teflon tapes and sealants have been found to breakdown with air piping. There is also strong evidence that particles from this type of material frequently get into downstream equipment and controls. This is particularly true when dissimilar metals mate using sealed threads. Common materials which are threaded near the point-of-use in an air system include brass, bronze, black iron, carbon steel, aluminum, magnesium, copper, and zinc. Advanced pipe sealants that set up without the presence of oxygen seem to work more effectively and support the resonance present in compressed air piping. One thing is absolutely true, regardless of the material selected, if you do not clean the threads prior to applying sealant, the sealant will not do its job. Apply the sealant two to three threads back from the tip to avoid migration of sealant into the system. The application of the sealant is as important as the selection of material.

Pipe hangers and support

Hangers in the system should not be tightly fastened to the pipe. Roller-type hangers work best with air piping 4 inches in size or larger. Larger pipe hangers are also available with spring isolation between the truss contact and the hanger rod. This minimizes the transfer of vibration and resonance to the building structural members and downstream equipment. Some larger stroke reciprocating compressors require inlet and discharge axis support from vibration and pulsation effects on piping for short runs to and from the compressor.

Also consider the use of flexible connectors, particularly on reciprocating compressors. Focus on the direction of the piston travel in the unit. Pulsation or vibration isolation should be perpendicular to the opposing forces. Contact the manufacturer of both the flex connector and the compressor for best advice regarding these issues.  Aside from critical lengths for the inlet and discharge piping on reciprocating compressors, it seems to be difficult to get much assistance in specific installation technology. Do not be disappointed if the equipment manufacturer is reluctant to provide installation advice beyond the boundaries of their equipment.

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