Figure 300-45 shows a typical plate valve. This type of valve is actuated by unbalanced pressures on either side of the valve. The valve plates or elements are held against the ports in the valve seat by spring force. The gas pressure overcomes the spring force, the elements lift away from their seats and stop against the guard, opening the valve.
Concentric Ring Valves. The plate valve shown in Figure 300-45 is a concentric ring valve. This type of valve can be fitted with plastic elements, which is an advantage in corrosive services. The concentric ring valve can be used over the widest range of compressor applications and can withstand the most extreme operating conditions. Concentric ring valves have been used for pressures as high as 60,000 psi with differential pressures greater than 10,000 psi and temperatures in excess of 500°F.
Ported Plate Valves. Ported plate valves are very similar to concentric ring valves except that the individual rings are joined to form one or two larger plates. Their chief advantages are ease of manufacture and simpler assembly. Ported plate valves are used primarily for high speed gas field compressors. Ported plate valves are shown in Figure 300-46.
Strip Valves. Strip valves include channel and flexible element valves. They are used primarily for air service, either lube or nonlube. A disadvantage is that they cannot be used with plastic elements and are therefore not very tolerant of dirt or liquids in the gas stream. They have good flow areas and are relatively inexpensive.
The maximum discharge pressure for these valves is about 1,500 psi. The highest recommended operating temperature is 350°F and the maximum pressure differential is approximately 500 psi. Strip valves may be used in compressors with rotating speeds up to 1,800 rpm.
One type of strip valve, called a “feather” by the original manufacturer, employs a single component to serve as the sealing element and the return spring. Application of the feather valve is generally limited to operating pressures less than 1000 psi, differential pressures no greater than 400 psi, and maximum temperatures of 350°F. A feather valve is shown in Figure 300-47.
Another type of strip valve, called a channel valve, is shown in Figure 300-48.
Poppet Valves. Poppet valves have an effective lift area approximately 50% greater than that provided by the same size concentric ring valve. Poppet valves can operate with lifts as great as 1/4 inch and are used extensively in the natural gas transmission industry. They have recently been applied in other process applications such as carbon dioxide injection for tertiary oil recovery. By design, they add built-in clearance, which causes a capacity penalty, especially with high specific heat gases.
The poppet valve utilizes a mushroom-shaped element made from a variety of materials. The sealing element material determines the range of application. Valves with metallic poppets can withstand pressures up to 3000 psi and temperatures to 500°F. However, metallic poppets are seldom used due to inertial effects.
Nonmetallic poppets are limited to 450°F and 800 psi, with compressor speeds up to 1,800 rpm. Typically NYLON, TORLON, and now PEEK are used for the poppet material because of their light weight and conformability to the valve seat. Several OPCOs are using poppet valves with good success. Refer to Figure 300-49.