Head-end unloaders

By examining the curve for the initial compression with no unloaders, it shows that the horsepower requirement crosses the +3% overload line about one-third of the way through the suction pressure range. Figure 12-32 shows the effect of adding first one unloader and then a second one. The simplest way to handle this is a head-end unloader on each of the two parallel cylinders. 

Using a head-end unloader on each cylinder, die head-end horsepower should he reduced ... 

Now determine the operating line for the condition of hoth head end unloaders open, at two suction conditions ... 

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. 

The head-end unloader has a great advantage in that it unloads the driver without increasing the amount of work required, per mole of gas... 

In practice, the head-end unloader can reduce compression work required per cylinder only by roughly 30 percent. 

For double-acting cylinders, the clearance at the head end should be calculated separately from that of the crank end, because for small cylinders, the volume occupied by the piston rod is significant when cylinder unloading is considered. 

If each head-end is unloaded to the point of requiring only the 187 hp, the unit will not he overloaded at the maximum point. The head-end E would have to he... 

The head end % clearance will he 22.1% because this is the condition with the unloader open on each cylinder. Reading curve ... 

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. 

The two-stage compressor shown in Figure 25-10 is restricted, due to its mechanical configuration, to adjustment of the head-end (i.e., suction) cylinder only. Therefore, if it is necessary to unload a compressor by disabling one stage, it is best to disable the crank end. If the head end is removed... 

Table 25-1 shows that for the compressors discussed, the temperature rise for the individual cylinders varied from 28°F for the No. 1 cylinder crank end to 42°F for the No. 2 cylinder crank end. The key point of this table is that compression efficiency varies inversely with temperature rise. As both the suction and discharge pressures were the same for all cylinder ends, the reason for the variable temperature rise was different efficiencies of compression. Because the work performed by the piston at each cylinder end was about the same (except for No. 2 cylinder head end which had a bad unloader), the observed temperature increases were inversely proportional to the gas flows. This means that if the No. 1 cylinder crank end was moving 30 MMscfd of gas, then the No. 2 cylinder crank end was moving only 20 MMscfd and the No. 1 cylinder head end was moving 23 MMscfd. 

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. 

NOISE IN CYLINDER 1. Loose piston. 2. Piston hitting outer head or frame end of cylinder. 3. Loose crosshead lock nut. 4. Broken or leaking valve(s). 5. Worn or broken piston rings or expanders. 6. Valve improperly seated/damaged seat gasket. 7. Free air unloader plunger chattering. 

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 ... 

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