Compressors clearance unloaders

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... 

Reciprocating compressor capacity may easily be adjusted by changing compressor speed, changing compressor cylinder clearance, unloading compressor cylinder inlet valves, recycling gas from unit discharge to unit suction, or a combination of these methods. All these methods may be accomplished either manually by the operator or automatically by the control panel. 

Figure 3.24. Control of positive displacement compressors, rotary and reciprocating, (a) Flow control with variable speed drives, (b) Pressure control with bypass to the suction of the compressor, (c) Reciprocating compressor. SC is a servomechanism that opens some of suction valves during discharge, thus permitting stepwise internal bypass. The clearance unloader is controllable similarly. These built-in devices may be supplemented with external bypass to smooth out pressure fluctuations.

For a constant speed driver such as motor, capacity of a reciprocating compressor can be controlled by Unloading inlet valve and/or unloaditig pockets or reservoirs at the cylinder. Lift suction valve (suction valve unloading) will prevent compression and open pocket to cylinder (clearance unloading) will increase cylinder clearance volume. Either action will reduce compressor capacity. If gas demand is intermittent, on-ofT control of motor can be used to control capacity. 

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. 

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. 

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

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. 

Cylinders may be single- or double-acting, and both unloaders and clearance pockets are used for capacity control (Section 9.1.6.3B). If the compressor takes less than the full cell output of hydrogen, the excess must be vented. This can be by way of the low-pressure seal pots or by controlled venting somewhere in the compression train. This can present problems in process control. Section 11.4.2.5A describes some of the systems that have been used. 

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. 

The best way to unload a reciprocating compressor is to increase the cylinder clearance. When the piston completes the end of its stroke, the space between the piston face and the cylinder head is called the clearance. Increasing the clearance reduces the scfd flow of gas to a compressor running at a constant speed and a fixed suction pressure. The effect of adjusting the cylinder clearance is proportional to the suction stroke volumetric efficiency (Evs). Evs is calculated as follows ... 

All compressors that use in-line impellers must be monitored closely for axial thrusting. If the compressor is subjected to frequent or constant unloading, the axial clearance will increase due to this thrusting cycle. Ultimately, this frequent thrust loading will lead to catastrophic failure of the compressor. 

---