Reciprocating Compressors – Valve Motion

The motion of valve elements during the compressor cycle are affected by two forces, as shown in Figure 300-50. The first is an aerodynamic force due to flow of gas around the element. The second is the spring force resisting this aerodynamic force.

Forces Acting on a Valve Plate

Plate or strip motion affects compressor performance and reliability. Not only must the valve open and close at the correct time, but it must open completely and seal tightly upon closing. In addition to being able to open fully and close completely, the valve element must not strike the seat or guard with excessive force, or short valve life will result.

Each valve is a damped spring/mass system with natural frequency. If the natural or resonant frequency is forced by some driving frequency, such as the pulsations from the compressor pistons, rapid cycling of the valve element will occur. The element generally bounces between the guard and the seat. This is commonly called “flutter”. Flutter can greatly reduce the life of springs and valve elements.

The manufacturer modifies the valve lift to suit the gas specified. For example, an air compressor might be furnished with a lift of .100 inches. The same compressor valve applied in a low mole weight service such as hydrogen, might use a lift of .032 inches.

The problem with a higher lift valve in the hydrogen service is that hydrogen lacks the damping properties of air. As a result, the valve elements would experience high impact forces and might bounce. Broken valve elements might result.

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