Pistons and Rods

The lowly piston, one of the more simple items, has one of the most important functions of the entire compressor. The piston must translate the energy from the crankshaft to the gas in the cylinder. The piston is

Clearance Pockets Compressor
Figure 3-7. Cylinder with clearance pocket. (Courtesy of Dresser-Hand)

equipped with a set of sliding seals referred to as piston rings. Rings are made of a material that must be reasonably compliant for sealing, yet must slide along the cylinder wall with minimum wear. Different rings are used for lubricated or nonlubricated service, with the rings in the nonlubed cylinders needing good dry lubricating qualities. For lubricated service, metallic rings such as cast iron or bronze as well as nonmetallic materials such as filled nylon are used. The nonmetallic materials are becoming more common. For nonlubricated service, the ring material is nonmetallic, ranging from carbon to an assortment of fluorocarbon compounds. Horizontal cylinder pistons feature the addition of a wear band, sometimes referred to as a rider ring (see Figure 3-8).

Pistons may be of segmented construction to permit the use of one-piece wear bands. One-piece wear bands are a requirement in API 618. Pistons have a problem in common with humans—a weight problem. Weight in a piston contributes directly to the compressor shaking forces and must be controlled. For this reason, aluminum pistons are often found in larger low pressure cylinders. Hollow pistons are used but can pose a hazard to maintenance personnel if not properly vented. If trapped, the gas will be released in an unpredictable and dangerous manner when the piston is dismantled.

Pignone Compressor
Figure 3-8. Piston rings and wear band. [Courtesy of Nuovo Pignone)

The piston rod is threaded to the piston and transmits the reciprocating motion from the crosshead to the piston. The piston rod is normally constructed of alloy steel and must have a hardened and polished surface, particularly where it passes through the cylinder packing (double-acting cylinders). Rod loading must be kept within the limits set by the compressor vendor because overloading can cause excess runout of the rod resulting in premature packing wear. This in turn leads to leakage, reduced efficiency, and increased maintenance expense.

In unloaded or part-load operation, rod reversals must be of sufficient magnitude to provide lubrication to the crosshead bearings. The bearings are lubricated by the pumping action of the opening and closing of the bearing clearance area.

Tail rods are dummy rods that protrude from the head end of the cylinder (see Figure 3-9). The purpose of the rod is to pressure-balance a piston or to stabilize a particular piston design. Because of the personnel hazard, a guard must be specified and provided. In a tandem cylinder arrangement, the outboard cylinders are driven with a rod similar to the tail rod.


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

Diagram Dresser Rand Power Piston
Figure 3-9. Diagram of cylinder with piston tail rod. {Courtesy of Dresser-Rand)

valve is used in contrast to the cam-actuated poppet type normally found in piston engines. Reciprocating compressors generally use one of four basic valve configurations:

• rectangular element

• concentric ring

The rectangular element valve, as the name implies, uses rectangular-shaped sealing elements. These valves are the feather valve, channel valve, and the reed valve. These valves are applied to the industrial air machines for the most part. A channel valve is shown in Figure 3-10.

The concentric ring valve uses one or more relatively narrow rings arranged concentrically about the centerline of the valve (see Figure 311). These valves have the advantage of a low stress level due to the lack of stress concentration points. The disadvantage is that it is difficult to maintain uniform flow control with the independent rings. These valves work well with plug type unloaders. Space for the unloader is obtained by eliminating one or more of the innermost rings.

The ported plate valves, as shown in Figure 3-12, are similar to the concentric ring valve except that the rings are joined into a single element. The advantage is that the valve has a single element making flow control somewhat easier. Because of the single element, the number of edges available for impact is reduced. The valve may be mechanically damped, as this design permits the use of damping plates. It has the disadvantage that because of the geometry used, the stress is higher due to the potential of higher stress concentrations. This valve element is probably one of the most commonly used in process reciprocating compressors.

Dresser Rand Reciprocating Compressor






Dresser Rand Cylinder Image

Figure 3-11. Exploded view of a concentric plate valve. (Courtesy of Dresser-Rand)

Figure 3-10. An exploded view of a cushioned channel valve. (Courtesy of Dresser-Rand)

Figure 3-11. Exploded view of a concentric plate valve. (Courtesy of Dresser-Rand)

The poppet valve (see Figure 3-13) consists of multiple, same-size ports and sealing elements. The advantage of the valve is that has a high flow efficiency due to the high lift used and the streamlined shape of the sealing element. The disadvantage is that the valve is not tolerant ot

Dresser Rand Compressors

Figure 3-13. Cutaway of a poppet valve, (Courtesy of Dresser-Rand)

Dresser Rand Compressor
Figure 3-12. Cutaway of a ported plate valve. {Courtesy of Dresser-Rand)

Figure 3-13. Cutaway of a poppet valve, (Courtesy of Dresser-Rand)

uneven flow distribution. The valve is most commonly used in gas transmission service and in low speed, low-to-medium compression ratio compressors. There appears to be an increase in the use of poppet valves in hydrocarbon process service because of the ease of maintenance.

Valve materials must be selected for durable, long-term operation and must also be compatible with the gas being handled. The use of polymer nonmetallic sealing elements is quite common. The valves are symmetrically placed around the outer circumference of the cylinder and can normally be removed and serviced from outside the cylinder without dismantling any other portion. A good design will have the valve and associated parts so arranged that an assembly cannot be installed backwards. The inlet and discharge valves should not be physically interchangeable and should be so constructed as to keep the valve assembly or its parts from entering the cylinder should they become unbolted or break.

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  • Agnese
    How to perform rod runouts on dresser rand recip?
    3 years ago

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