Unloading pockets

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. 

The proper way to reduce the volumetric efficiency is to increase the starting volumetric clearance. This is done with an adjustable unloading pocket, as shown in Fig. 29.4. This device, also called the head-end unloader, works by increasing the starting volumetric clearance. This is known as the volume of gas trapped between the cylinder head and the piston, when the piston position is at bottom dead center, as explained in Fig. 29.2. 

Figure 29.4 An unloading pocket reduces engine load and volumetric capacity.

Many reciprocating compressors come equipped with unloading pockets. This is an open area at the end of the cylinder whose length is adjustable. The compressor piston cannot compress the gas in the unloading pockets. The purpose of the pockets is to allow the operator to decrease compressor capacity without wasting horsepower. If you are looking for maximum capacity, make sure these adjustable pockets are closed. 

Next, check the unloader pockets. An unloader is a device used to reduce the capacity of a cylinder, without reducing the compressor s efficiency. Figure 25-5 illustrates the function of an unloading pocket. Increasing the clearance between the piston and the cylinder head will reduce the volume of the gas compressed per stroke. 

Close the suspect unloader pocket and note the effect on the engine s fuel gas manifold pressure. 

If the fuel gas manifold pressure does not increase, the unloader pocket did not really close, and it is probably broken. 

Constant-speed control should be used when gas demand is fairly constant. With this type of control, the compressor runs continuously but compresses only when gas is needed. Tiiree methods of unloading the compressor with this type of control are in common use (1) closed suction unloaders, (2) open inlet-valve unloaders, and (3) clearance unloaders. The closed suc tion unloader consists of a pressure-actuated valve which shuts off the compressor intake. Open inlet-valve unloaders (see Fig. 10-89) operate to nold the compressor inlet valves open and thereby prevent compression. Clearance unloaders (see Fig. 10-90) consist of pockets or small reservoirs which are opened when unloading is desired. The gas is compressed into... 

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. 

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. 

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. 

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. 

For a discussion of this important performance control topic, refer to Cylinder Unloading, later in this chapter and Figure 12-27. A reasonable average range is 7-22% with fixed valve pockets. ... 

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

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

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. 

Clearance unloaders are small pockets into which the gas is forced on the compression stroke and expands into the cylinder on the return stroke, thus preventing compression of additional gas. 

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