Unloaders and clearance devices are used to:
• Unload the compressor for startup;
• Maintain fixed suction or discharge pressure in a process;
• Control capacity (flow rate);
• Maintain optimum loading on driver under varying compressor operating conditions; and
• Maintain operating pressures, temperatures, or piston rod loads within allowable limits under varying compressor operating conditions.
Unloaders are applied to the suction side of a compressor cylinder to deactivate one or both ends of the cylinder. Unloaders are installed on suction valves only, so the gas circulating through the valve will be cooler than if the unloaders were installed on discharge valves. Metal fatigue and attendant valve failure occurs more quickly at elevated temperatures. The two types of unloaders, plug and finger, are shown in Figure 300-24.
There are two kinds of plug-type unloaders (as shown in Figure 300-24, Items A and B). Item A is fitted on a suction valve. Item B is applied to an individual gas port on the suction side of the cylinder. With either kind, when the unloader is actuated, an opening is maintained during both the suction and discharge strokes. With this opening, the gas is no longer trapped, but is free to flow back to the suction side during the compression stroke. Normally, only one plug-type unloader is required on a cylinder end. Either kind can be operated manually or automatically with air or gas as the motive fluid.
The finger type suction valve unloader (Item C of Figure 300-24) is applied to all suction valves on a cylinder end. This type is sometimes called a “suction valve depressor”. When actuated, the fingers push down on the suction valve plate(s), and hold them open. Again, the gas flows back to suction during the compression stroke.
The plug-type tends to be more reliable than the finger-type. Valve plate failure can be caused by the concentrated loads where the fingers push on the plate. The disadvantage of using plug-type unloaders is that there is less overall available valve area in the cylinder, resulting in higher valve losses.
One European compressor valve manufacturer markets a “stepless” controller for their valve unloaders. In this scheme, the finger type unloader holds the suction valve plate open for a specific length of time corresponding to the desired capacity. Since the suction valves are not allowed to close at the normal time (at or near bottom dead center), in effect, the displacement of that end of the cylinder is reduced. Therefore, it is theoretically possible to provide an infinite number of capacity steps depending on valve timing.
The stepless system has been applied to quite a few installations in Europe, but it has not gained much favor in the domestic petroleum applications due to its complexity. One USA manufacturer offers a similar stepless control system, but it is typically applied only to storage (pump-in, pump-out) compressors having continually varying suction and discharge pressures.
There are two kinds of clearance devices: clearance pockets and clearance spacers. Pockets can be fixed or variable volume. (Figure 300-25 shows examples of fixed and variable volume pockets.) Fixed volume pockets can be either manually or pneumatically (air or gas) operated.
Variable volume pockets are normally only manually operated. Hydraulic actuation has been tried, but the results have generally been unfavorable. Variable volume pockets are applied only to the head end of a cylinder, because it is physically impossible to locate them on the crank end. Occasionally, variable-volume pockets may be capable of more adjustment than required for the load step. Opening the pocket too far can cause overheating problems because the volumetric efficiency becomes too low. External mechanical stops can be installed to limit handwheel travel to the length required in the capacity control design.
Fixed volume pockets are normally applied to the head end for the same reason, although fixed pockets can be applied to suction valve caps on either the head or crank ends. Fixed-clearance pockets often look like an ell-shaped piece of pipe, pointed upwards to prevent liquid accumulation. This type of pocket can be installed by the manufacturer, or as a field retrofit.
Valve cap pockets require a special suction valve with an opening at the center of the seat so that the pocket volume is in direct communication with the cylinder’s interior when the pocket is opened. Installation of valve cap pockets on discharge valves is not recommended because of the possibility of collection of oil or liquids in the pockets.
Clearance spacers are sometimes used to increase the clearance volume. A spacer can be installed between the cylinder head and cylinder body. Also, a compressor valve can be designed with a two-piece yoke (hold-down for valve). By using one piece of the yoke as a spacer, the valve seat and guard can be moved farther away from the cylinder bore, thus increasing the clearance. A shutdown is required to change clearance with spacers.