What is in this article?:
- Improving Compressed Air System Efficiency: Part 3
- Shutoff and special-duty in-line valves
Piping systems are a critical, but often overlooked, part of the design of a compressed air system.
Shutoff and special-duty in-line valves
Many types of valves are available for compressed air service. The following are the more common types: globe, gate, ball, wafer, butterfly, plug, slow-acting, relieving, wedge, needle, petcocks, and notched ball.
There are a tremendous number of globe and gate valves installed in compressed air service. However, they would be my last choice for shut-off valves in the piping system. They have the highest pressure drop of any of those listed for line service, and they cost two to three times as much. Plug valves have minimum pressure drop for shut-off service, but can also be expensive.
Ball, wafer, and butterfly valves are superior for in-line shut-off service in the compressed air system. I would suggest full-flow ball valves for ½- to 2-in. piping. In larger piping, I suggest either the compression-type wafer or butterfly valves. The butterfly valve has a bolt pattern in the valve body which mates to the adjacent flanges. The wafer valve usually has a grooved ring seat in the valve body for an O-ring, which is compressed between the mating flanges when they are bolted together.
The other six valves are used primarily for point-of-use applications, along with ball valves. Slow-acting valves are a most interesting choice for higher flow applications. They can be spring-loaded, and pneumatically, mechanically, or electrically actuated. When actuated, they can take 10 sec or more to fully open. This not only ramps in the flow (which reduces surge), but also protects downstream components from being slammed as the pressure goes from atmosphere to full line. This can be a thoughtful selection on any large-volume point-of-use installation.
The relieving or auto drain valve is designed to relieve the downstream pressure when in the shut-off position. These are primarily used in subheaders down to the point-of-use and are offered in ball- and plug-type valves. Other types of valves trap pressurized air in the downstream piping, hose, etc. when they are shut off. Another advantage for these types of valves at the point of use is that when partially open they blow air and make substantial noise, allowing for easy troubleshooting. Wedge valves, notched ball valves, and needle valves are used for manual flow control in process or metered recovery applications.
One of the common problems regarding valves and all smaller, frequently used parts in the air system is the large number of stocked sizes for use in the air system. There are 16 sizes available that are 8-in. or smaller. The number of sizes should be minimized to ½, 1, and 2 in. in smaller valves and 3, 4, 6, 8, 10, and 12 in. wherever possible with larger valves. This will limit inventory and reduce pressure drop in the system.
Another issue to keep in mind is the compatibility of valve components with the contaminants and conditions of the air, which might be in a specific part of the system. When acid and caustic gases (common in industrial applications at a low volume — 1 to 5 ppm) combine with water in coolers in the compression process or in dryers, liquid acid will form, which must be dealt with in the piping and drainage systems. Once past the compressor, many of these use water as a carrier. Getting the water out will effectively deal with much of the problem.
Consider the types of contaminants that are present, and select appropriate components. Particular attention should be given to seals and internal valve components. Some lubricants can be very aggressive with rubber components, such as low Nitryl Buna-N. A common material compatible with most compressed air contamination is Viton.
Pipe hangers and support
Hangers in the system should not be fastened tight on the piping. Roller-type hangers work best with air piping sized 4-in. or larger. Larger pipe hangers are also available with spring isolation between the truss contact and the hanger rod. This minimizes the transfer of negative vibration and resonance to the building structural members and downstream equipment. Some longer stroke reciprocating compressors will require inlet and discharge axis support from vibration and pulsation effects on piping for short runs to and from the compressor. You should also consider the use of flexible connectors, particularly on reciprocating type compressors. The focus should be on the direction of the piston travel in the unit. Pulsation or vibration isolation should be perpendicular to the opposing forces.
R. Scot Foss is president of Plant Air Technology, Charlotte, N.C., which specializes in air system auditing and design. This series of articles is based on his book, Compressed Air System Solution Series. To order a copy, click here.