Cylinders compressors

FIG. 10-86 Two -stage double-acting compressor cylinders with intercooler. 

Lubrication of compressor cylinders can be tailored to the application. The cylinders may be designed for normal hydrocarbon lubricants or can be modified for synthetic lubricants. The cylinder may also be designed for self lubrication, generally referred to as nonlubed. A compromise lubrication method that uses the nonlubed design but requires a small amount of lubricant is referred to as the mini-lube system. 

A more rare form of driver is the steam cylinder. Most arrangements combine the steam driver and compressor on the same frame with the steam cylinder opposite the compressor cylinder. Each cylinder s connecting rod is connected to a common throw on the crankshaft. A flywheel is used to provide inertia. For air service, the units are built as single- and two-stage units, with other combinations available for process service. 

The compressor cylinder valves are of the spring-loaded, gas-actuated type in all but a limited number of portable compressors. This kind of... 

Hartwick, W Power Requirement and Associated Effects of Reciprocating Compressor Cylinder Ends, Deactivated by Internal By-Pas.sing, ASMS 75-DGP-9, New York, NY American Society of Mechanical tine neens, 1975. 

The large, heavy-duty, integral engine-compressor has long been the workhorse of the gas compression industry. The in-line or V-shaped power cylinder, horizontal compressor cylinder configuration is lamitiar. 

As pointed out in Volume 1, Chapter 3, a compressor may have any number of stages. Each stage normally contains a suction scrubber to separate any liquids that carry over or condense in the gas line prior to the compressor cylinder (or case for centrifugal compressors). When gas... 

High-speed units are normally separable. That is, the compressor frame and driver are separated by a coupling or gear box. This is opposed to an integral unit where power cylinders are mounted on the same frame as the compressor cylinders, and the power pistons are attached to the same drive shaft as the compressor cylinders. 

Compressor cylinders [dry or water-cooled) of cast iron, nodular iron, or forged steel are engineered to suit required pressures and capacities. 

Figure 10-5 shows a very large integral compressor. Tbi.s would he typical of compressors in the 2.000 hp to 13,000 hp size. The size of this unit can be estimated by the height of the handrails above the compressor cylinder on the walkway that provides access to the power cylinders. This particular unit has sixteen power cylinders (eight on each side) and four compressor cylinders. 

There are some low horsepower (140 to 360) integrals that are nonnaf ly skid mounted as shown in Figure 10-6 and used extensively in small oil fields for flash gas or gas-lift compressor service. In these units the power cylinders and compressor cylinders are both mounted horizontally... 

Rotary compressors are positive-displacement machines. Figure iO-7 shows a typical vane compressor. The operation is similar to that of a vane pump shown schematically in Figure 10-10 of Volume 1, 2nd Edition (Figure 10-9 in 1st Edition). A number of vanes, typically from 8 to 20, fit into slots in a rotating shaft. The vanes slide into and out of the slots as the shaft rotates and the volume contained between two adjacent vanes and the wall of the compressor cylinder decreases. Vanes can be cloth impregnated with a phenolic resin, bronze, or aluminum. The more vanes the compressor has, the smaller the pressure differential across the vanes. Thus, high-ratio vane compressors tend to have more vanes than low-ratio compressors. 

At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure. 

A suction pressure throttling valve can also be installed to protect the compressor from too high a suction pressure. This is typically a butterfly valve that is placed in the suction piping. As flow rate to the compressor increases, the valve will close slightly and maintain a constant suction pressure. This will automatically limit the flow rate to exactly that rate where the actual volume of gas equals that required by the cylinder at tlie chosen suction pressure setting. It will not allow the suction pressure to increase and the compressor cylinder to thus handle more flow rate. 

Suction scrubbers are required on the unit suction to catch any liquid carry-overs from the upstream equipment and any condensation caused by cooling in the lines leading to the compressor. They are also required on all the other stages to remove any condensation after cooling. On each suction scrubber a high level shut-down is required so that if any liquids do accumulate in the suction scrubber, the compressor will automatically shut down before liquids carry-over to the compressor cylinders. 

Each frame is designed for a maximum number of cylinders. The frame itself does not indicate the number of stages or the duty of the compressor. An individual frame can be used for many different sizes o compressor cylinders and for a wide range of applications. Frames are typically classified as separable (balanced-opposed) or integral-type, as shown m Figure 11-3. 

Figure 11 -5. Typical double-acKng compressor cylinder. (Courtesy of Dresser-Rand Company ...

A distance piece provides the separation of the compressor cylinder from the compressor frame as shown in Figure 11-9. At the top of the figure is a standard distance piece. 

A two-compartment distance piece may be used for toxic gases, but it is not very common. In this configuration, no part of the rod enters both the crankcase and the compartment adjacent to the compressor cylinder. That is, even if there were one failure, the crankcase oil cannot be contaminated with the toxic gas. 

The piston is located at the end of the piston rod and acts as a movable barrier in the compressor cylinder. It is generally made from materials such as aluminum or cast iron and has a hollow center. Small-diameter high-pressure cylinders may be provided with a combined piston and rod machined from a single piece of bar stock. 

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. 

Traditional compressor cylinder designs require cooling water jackets to promote uniform distribution of heat created by gas compression and friction. Some of the perceived advantages of water-cooled cylinders are reduced suction gas preheat, better cylinder lubrication, prolonged parts life, and reduced maintenance. 

Many manufacturers, users, and compressor applications still require that compressor cylinders be supplied with liquid-cooled cylinders. Figure 11-21 includes schematics of several types of liquid coolant systems. 

Forced coolant systems using a mixture of glycol and water are the most common for natural gas compressors. Normally, the compressor cylinder cooling system and compressor frame lube oil cooling system is combined. A single pump is used to circulate the coolant through the cylinders and the lube oil heat exchanger and then to an aerial cooler where the heat is dissipated. 

The two basic types of cylinder lubrication systems are the pump-to point system and the divider-block system. The pump-to-point system provides each lubrication point with its own lubricator pump. Thus, if the compressor cylinders and packing require six lubrication points, the lubricator box would be supplied with six cam driven pumps. The divider-block system uses one or more lubricator pumps to supply a divider block, which then distributes the flow to each of the lubrication points. The two systems are sometimes combined such that each stage of compression is provided with its own pump and a divider block to distribute the flow between the cylinders and packing of that particular stage. 

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. 

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