Double-acting cylinders

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. 

After all the previous statements, it would seem very difficult to select a piston. speed. For someone without direct experience, the following guidelines can be used as a starting point. Actual gas compressing experience should be solicited when a new compressor for the same gas is being eonsidered. These values will apply to the industrial process type of compressor with a double-acting cylinder construction. For horizontal compressors with lubricated cylinders, use 700 feet per minute (fpm) and for nonlubricated cylinders use 600 fpin. For vertical compressors with lubricated cylinders, use 800 fpm and for nonlubricated cylinders use 700 fpm. 

To calculate Pd, use Equation 3.1 for single-acting cylinders, Equadofi 3.2 for double-acting cylinders without a tail rod, and Equation 3.3 foi double-acting cylinders with a tail rod. 

A packing is required on double-acting cylinders to provide a barrier to leakage past the rod where it passes through the crank end cylinder closure. The same arrangement is needed at the head end if a tail rod or tan-... 

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. 

Double-acting cylinders are those where compression occurs twice per cranksheet revolution. 

Another variation is the tandem cylinder. The tandem cylindet arrangement again allows two stages of compression on the same compressor throw but uses two double-acting cylinders separated by a second distance piece. This arrangement is usually used in low rod load applications where higher capacity is required. 

Double acting cylinder (sum of head end and crank end displacement)... 

Late in the field life it is desirable to compress the 100 MMscfd for the example field downstream of the separator from 800 psig at 100°F to l.(XX) psig. An engine-driven separable compressor is available from surplus. The engine is rated for 1,600 hp at 900 rpm. Horsepower is proportional to speed. The compressor frame has six 7-in, bore by 6.0-in. stroke double-acting cylinders with a minimum clearance of 17.92%, a rod load limit of 25,000 lb, and rod diameter of 1.75 in. Assume k = 1.26, Z, -0.88, and Zd = 0.85. 

Figure 12-2T. Medium- or high-pressure, double-acting cylinder with flanged liner. The liner is locked in place by a flange between head and cylinder barrel. A step on the liner O.D. permits easy insertion. The cylinder may be made of cast-iron, nodular iron, or cast steel, depending on operating pressure. Note Optional two-compartment distance piece (type D) designed to contain flammable, hazardous, or toxic gases is illustrated. (Used by permission Bui. 33640, June 1985. Dresser-Rand Company.)...

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. 

For double-acting cylinders, % clearance is based on total clearance volume for both the head end and crank end of the cylinder X 100 divided by the total nel piston displacement. The head and crank end % clearance values will be different due to the presence of the piston rod in the crank end of the cylinder. The % clearance values are available from manufacturers for their cylinders. The values range from about 8% for large 36-in. cylinders to 40% for small 3-and 4-in. cylinders. Each cylinder style is different. 

Ap = cross-sectional area of cylinder, in. for double-acting cylinder, use Ap as (2Ap — A,.)... 

Figure 13-14. AV curve for double-acting cylinders. (Used by permission Cooper Cameron Corporation.)...

Pipe Section Rate Factor for Single-Acting and Double-Acting Cylinders... 

Average How Rate of Gas from Cylinder Double-acting cylinders ... 

In a double-acting cylinder, the base frequency will be double that of a single-acting cylinder with its harmonics as additional predominant frequencies. 

Another technique is to use a double-acting cylinder, and when extending the piston, direct the rod end fluid to the head end port to add to the volume from the pump. This is called regeneration and is used to achieve fast close in many applications. 

Figure 2 Four-column press with one double-acting cylinder.

Figure 3 Straight-side press with two double-acting cylinders and a rolling bolster.

A double-acting cylinder is sometimes used in a single-acting application. When applied in this way, one port is vented and the other is pressurized. In this way, the cylinder can be made to either extend or retract under pressure, but not both. 

Figure 4 Gravity close prefill circuit with double-acting cylinder.

The prefill circuit can be used on presses with double-acting cylinders or single-acting cylinders. 

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