Clearance pocket

Motor-driven reciprocating compressors above about 75 kW (100 hp) in size are usually equipped with a step control. This is in reality a variation of constant-speed control in which unloading is accomplished in a series of steps, varying from full load down to no load. Three-step eontrol (full load, one-half load, and no load) is usually accomplished with inlet-valve unloaders. Five-step eontrol (fuU load, three-fourths load, one-half load, one-fourth load, and no load) is accomphshed by means of clearance pockets (see Fig. 10-91). On some machines, inlet-valve and clearance-control unloading are used in combination. 

Although such control devices are usually automatically operated, manual operation is satisfactoiy for some services. When manual operation is provided, it often consists of a valve or valves to open and close clearance pockets. In some cases, a movable cyhnder head is provided for variable clearance in the cylinder (see Fig. 10-92). 

FIG. 10-92 Sectional view of a cylinder equipped with a hand-operated valve lifter on one end and a variable-volume clearance pocket at other end. 

One method of volume variations is by speed modulation. Another, more common method, is the use of clearance pockets, with or without valve unloading. With clearance pockets, the cylinder performance is modified. With valve unloading, one or more inlet valves are physically open. Capacity may be regulated in a single- or double-acting cylinder with single or multiple cylinder configuration. 

Larger cylinders normally have enough space for clearance pockets. An additional location is the head casting on the outboard end of the cylinder. Figure 3-7 is an illustration of a cylinder with an unloading pocket in the head. On smaller cylinders, this feature must be provided external to the cylinder. 

Rgure 3-22. A 5-step clearance pocket unloading scheme. Courtesy of Dresser-Rand)... 

An additional capacity control method is the unloader. This method can be used in conjunction with clearance pockets to extend the range of control to zero capacity. On double-acting cylinders, unloading the individual sides one at a time will provide a two-step unloading of the cylinder. On multicylinder arrangements, the cylinders can be unloaded one at a time providing as many steps as cylinders operating in parallel. The unloaders can also be used to totally unload the compressor, as is necessary for electric motor driver startup. 

Clearance pockets and other types of capacity control devices are available to suit any application. 

Figure 11-19. Fixed volume clearance pocket. [Courtesy of Dresser-Rand Company.]...

Variable clearance that can be changed very readily can be built into the cylinder. Figure 11-19 is an example of a fixed volume clearance pocket mounted on the cylinder. This type is separated from the cylinder by a valve that can be opened and closed from the outside. 

More flexibility can be obtained with a variable volume clearance pocket such as that shown in Figure 11-20. This is a plug built into the outer cylinder head. When moved, the clearance volume of the outer end of the cylinder changes. 

The major source of fugitive emissions from a gas compressor cylinder is the piston rod packing. Other sources of fugitive emissions are around the cylinder valve covers, unloader covers, unloader actuator packing, and clearance pocket gasket and actuator packing. 

Figure 12-2E. 250 psig working pressure cylinder used in refrigeration service. Auxiliary stuffing box for added sealing on shutdown. Manual fixed volume clearance pockets for capacity control. (Used by permission Bui. L-679-BIA, 1957. Dresser-Rand Company.)... 

CLEARANCE POCKET shown in the head is the manual fixed-volume type. 

Figure 12-2M. For low compression ratios, designed for 1,000 psi discharge pressure equipped with hand-operated crank and head-end fixed clearance pockets for capacity control. Air-cooled, cast semisteel, double-acting. (Used by permission Dresser-Rand Company.)...

Figure 12-2Q. Cast or nodular iron cylinders for pressures to 1,500 psi. Note double distance-pieces (left, vented or purged, to prevent oil and process gas from leaking past the shaft and right-end fixed clearance pocket). (Used by permission Bui. 85084, 1992. Dresser-Rand Company.)...

The clearance pockets may be of many different shapes and arrangements (see Figures 12-6B, 12-30A, and 12-30B). Fixed volume pockets allow for fixed or set volume changes while the variable volume designs allow for changes to suit a particular operating condition or balance and are of value when the cylinder must be used in several different alternating applications. 

Figure 12-29A. Five-step clearance pocket control for compressor unloading. (Used and adapted by permission ingersoii-Rand Company. Aii rights reserved.)...

Clearance Pocket Open) (Suction Valves Held (Suction Valves Held... 

Note Discharge Valves not Shown not Effective) Clearance Pocket)... 

Constant speed running with inlet valve blocking or intake throttling or external bypass or inlet valve unloading or clearance pocket ... 

Smaller machines may have a valved bypass across the inlet and outlet ports in the cylinder head, or a variable clearance pocket in... 

The second way to unload a reciprocating compressor is with valve disablers. Most of the unloaders you have on your compressors are likely of this inferior type. They typically consist of steel fingers, which are pressed down through the valve-cap assembly. The fingers prevent the valve plate from moving. These valve disablers are far less costly than an adjustable clearance pocket. They may be used on both the crank and head ends of the cylinder. They can reduce the gas flow through a cylinder to zero. 

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