Reciprocating Compressors – Valves
The compressor valves control the flow of gas into and out of the compressor cylinder. All valves are similar in that the differential pressure across the seat must be greater than the balance spring force before gas may flow through the valve. The lift characteristics, seat area, and flow areas determine the advantages of each design. In older compressor stations, channel valves (Figure 11-15) were commonly used. Channel valves are now considered obsolete and used only in small air compressors In today’s market, there are three common types of valves—poppet valves (Figure 11-16), ring valves (Figure 11-17), and plate valves (Figure 11-18).
Poppet valves are typically used for low compression ratio applications, such as pipeline booster compressors. As the pressure differential increases across each of the individual poppets, they lift and allow gas to pass through the flow openings in the stop plate.
Ring valves are typically used for slow speed, high pressure process compressors. Instead of individual poppets, these valves use concentric rings, which open and close the valve ports.
Plate valves are typically used for high speed separable compressors. Plate valves are similar to ring valves with the rings connected by ribs. Instead of individual elements opening and closing, all valve ports open and close at the same time.
Valve type and size should be specified by the compressor manufacturer. Normally, the manufacturer will quote a valve velocity, which can be calculated from:
At lower velocities the valve has less pressure drop and thus has less maintenance associated with it. Velocities calculated from this equation can be used to compare valve designs.
In addition to valve velocity, the manufacturer can furnish the effective flow area of the valve. This area is determined by measuring the pressure drop across the valve with a known flow rate and then calculating an equivalent orifice area that provides the same pressure drop. Valves with larger effective flow areas have less pressure drop and better efficiencies, The effects of the seat area, the lift area, and the flow paths are automatically included when the effective flow area is used to compare compressor
valves. This in turn provides a better comparison of valve performance than just looking at valve velocity.
In addition to valve efficiency, the following should be considered in valve selection: ease of maintenance, durability, and spare parts required.