CHAPTER 11 Reciprocating Compressors * The previous section discussed the various types of compressors, their selection, and process flow. This chapter presents greater detail concern- ing the major components, performance, operational and installation con- siderations, and standard specifications for reciprocating compressors. For normal production facilities, reciprocating compressors far out- number the other types, and it is necessary for the facility engineer to understand the details of reciprocating compressor design. In very large horsepower ranges or booster compressor situations, centrifugal com- pressors are common. These are not discussed in more detail in this book, because these large installations are normally the responsibility of rotating machinery experts. COMPONENTS Figure 11-1 is a cutaway that shows the various components of a recip- rocating compressor. To understand how to specify and maintain a com- pressor properly, it is necessary to have a better understanding of the con- struction of the major components. *Reviewed for the 1999 edition by Lonnie W. Shelton of Paragon Engineering Services, Inc. 286 Reciprocating Compressors 28F Figure 11-1. Cutaway view of typical reciprocating compressor. Frame The compressor frame, shown in Figure 11-2, is a heavy, ragged cast- ing containing all the rotating parts and on which the cylinders and crossheads are mounted. All frames are rated by the compressor rnanu- Figure 11-2. The compressor frame is the rugged casting that contains the rotating parts and on which the cylinders and crossheads are mounted. (Courtesy of Dresser-Rand Company.) 288 Design of GAS-HANDLING Systems and Facilities facturers for a maximum continuous horsepower, speed (rpm), and rod load. The rated horsepower is determined by the maximum horsepower that can be transmitted through the crankshaft to the compressor cylin- ders. The rod load is the force imposed on the piston rod by the pressure differential between the two ends of the piston. 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 of compressor cylinders and for a wide range of applications. Frames are typically classified as separable (balanced-opposed) or integral-type, as shown in Figure 11-3. Separable (balanced-opposed) frames are characterized by an adjacent pair of crank throws 180° out of phase. The frame is separate from the Figure 11-3. Separable (balanced-opposed) compressor (top) and integral-type gas engine compressor (bottom). Reciprocating Compressors 289 driver. Integral-type frames are characterized by having compressor cylinders and power cylinders mounted on the same frame and driven by the same crankshaft. Cylinder A cylinder is a pressure vessel that holds the gas during the compres- sion cycle. There are two basic types: 1. Single-acting cylinders are those where compression occurs only once per crankshaft revolution, 2. Double-acting cylinders are those where compression occurs twice per cranksheet revolution. Figure 11-4 is a cut-away drawing of a compressor with single-acting cylinders. True single-acting cylinders are typical of low horsepower air compressors. Single-acting process compressors are typically double-act- Figure 11-4. Single acting cylinders. (Courtesy of Dresser-Rand.) 290 Design of GAS-HANDLING Systems and Facilities Figure 11-5. Typical double-acting compressor cylinder. (Courtesy of Dresser-Rand Company.} ing cylinders with the outer end suction valves removed. Figure 11-5 is a cut-away of a double-acting cylinder. Cylinders are made of different kinds of materials. Generally, cast iron is used for cylinder operating pressures up to 1,000 to 1,200 psig, nodu- lar iron or cast steel for operating pressure in the 1,000 to 2,500-psig range, and forged steel for pressures greater than 2,500 psig. Like all pressure vessels, the cylinder has a maximum allowable work- ing pressure (MAWP). The maximum allowable working pressure of the cylinder determines the setting of the relief valve that is downstream of the cylinder. The MAWP of the cylinder should be a minimum of 10% or 50 psi greater than its operating pressure. Reciprocating Compressors 291 Figure 11 -6. Cut-away view showing cylinder liner. (Courtesy of Dresser-Rand Company.} A cylinder liner such as that shown in Figure 11-6 may be used to help prolong the life of the cylinder and improve operating flexibility. Any damage caused by the action of the piston or heat generated by compres- sion will affect the cylinder liner, which may be removed and replaced. As the surface of the liner wears, it is much easier and quicker to repair it than to repair the cylinder itself. In addition, liners enable the diameter of the piston to be varied without changing the cylinder and thus provide flexibility to respond to different conditions of pressure and flow rate. The disadvantages of liners are that they increase the clearance (dis- cussed in more detail below) by increasing the distance between the pis- ton and the valve, and they decrease the bore of the cylinder. Therefore, the cylinder will have less capacity and lower efficiency (at high ratios) than if there were no liner. Special Compressor Cylinder Construction Many variations and combinations of cylinder types and arrangements are available from the compressor manufacturers. The compressor manu- facturer will generally make its selection based on the most economical combination it has available. Figure 11 -7. Steeple cylinder. (Courtesy of Dresser-Rand Company.) Reciprocating Compressors 293 Figure 11-7 is a cut-away of a steeple cylinder. This cylinder design is actually two single-acting cylinders coupled together with different-size pistons on the same piston rod. This arrangement allows two stages of compression on the same compressor throw and is usually used in low capacity, low rod load applications. Another variation is the tandem cylinder. The tandem cylinder arrangement again allows two stages of compression on the same com- pressor throw but uses two double-acting cylinders separated by a second distance piece. This arrangement is usually used in low rod load applica- tions where higher capacity is required. Figure 11-8 is a cut-away of the latest innovation in compressor cylin- der design. In this design, the two suction valves and the two discharge valves are installed inside the compressor cylinder bore. The suction valves are stationary and located at each end of the cylinder. The dis- charge valves are connected to the piston rod to form the piston; thus the name valve-in-piston design. This design offers the advantages of lower clearances (thus higher efficiencies), reduced sources of fugitive emis- sions, fewer replacement parts, simpler maintenance procedures, and reduced weight. Distance Pieces 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 fig- ure is a standard distance piece. The piston rod moves back and forth through packing that is contained within the distance piece. The packing keeps the compressed gas from leaking out of the cylinder through the piston rod opening. As the rod passes through the packing it is lubricated. As it goes back and forth, the rod is in contact with the frame lube oil and with the cylinder lube oil and gas. Thus, oil carry-over may occur on the rod from the cylinder to the crankcase. Impurities picked up by the oil from the gas being com- pressed could contaminate crankcase oil. In a single-compartment distance piece, the frame end and the cylinder end contain packing. The space between the cylinder packing and the frame diaphragm and packing is sufficiently long to assure that no part of the rod enters both the cylinder and the frame. This minimizes contami- nation between the gas being compressed and the oil that is used to lubri- cate the crankcase. There are drains and vents off the distance piece and off the packing, so if there is a packing failure, the high-pressure gas has 294 Design of GAS-HANDLING Systems and Facilities Figure 11-8. Valve-in-piston double-acting compressor cylinder, (Courtesy of Dresser-Rand Company.) a place to vent and not build up pressure that could leak through the frame packing into the crankcase. An oil slinger as shown in Figure 11 -9 may be added to further reduce the amount of cylinder lube oil migrating down the rod into the crankcase. 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 conta- minated with the toxic gas. Crosshead, Rods, and Crankshaft The crosshead converts the rotating motion of the connecting rod to a linear, reciprocating motion, which drives the piston as shown in Figure 11-10. The crosshead is provided with top and bottom guide shoes, which ride on lubricated bearing surfaces atached to he compressor frame. In addition, balance weights may be attached to the crosshead to reduce unbalanced forces and moments. The connecting rod connects the crank- shaft to the crosshead. The piston rod connects the crosshead to the piston. The crankshaft rotates about the frame axis, driving the connecting rod, crosshead, piston rod, and piston. Reciprocating Compressors 295 Figure 11-9. API type distance pieces. (Reprinted with permission from API, Std. 618, 3rd Ed., Feb. 1986.) [...]... 11-11 Piston rings and wear bands are made of material that is softer than the cylinder wall with which they are in constant contact, so they must be replaced regularly (Courtesy of Dresser-Rand Company.} Figure 11- 12 Journal bearings allow axial and circumferential oil flow abng the bearing 29 8 Design of GAS-HANDLING Systems and Facilities film between the stationary and rotating parts of the bearing... of Dresser-Rand Company.) 30 6 Design of GAS-HANDLING Systems and Facilities tlon and discharge valves so that the valves can be removed and reinstalled These clearances are called fixed clearances and can be adjusted by: » Removing a small portion of the end of the compressor piston * Shortening the projection of the cylinder heads into the cylinder * Installing spacer rings between cylinder head and. .. Dresser-Rand Company.) Valve Unloaders Inlet valve unloaders are used to deactivate a cylinder end and reduce its capacity to zero Two of the more common types of unloaders are depressor-type unloaders and plug-type unloaders Depressor-type unloaders hold the inlet valve open during both the suction and discharge 30 4 Design of GAS-HANDLING Systems and Facilities Figure 11-18 Cut*away view of plate... with a known flow rate and then calculating an equivalent orifice area that provides the same pressure drop Valves with larger effective flow areas have less pressure drop and better efficiencies, The effects of the seat area, the lift area, and the flow paths are automati- 3 02 Design of GAS-HANDLING Systems and Facilities Figyre 11-16 Cut-away view of poppet valve {Courtesy of 0resser-Roncf Company,).. .29 6 Design of GAS-HANDLING Systems and Facilities Figure 11-10 The crosshead converts the rotating motion of the connecting rod to a linear, reciprocating motion, which drives the piston (Courtesy of Dresser-Rand Company.) Piston The piston is located at the end of the piston rod and acts as a movable barrier in the compressor cylinder It... amount of pressure differential one set of rings can withstand is limited Therefore, several pairs must be installed to handle typical field gas compression applications The basic design of the packing is left up to the manufacturer Lubrication is needed to reduce friction and provide cooling Lubricating oil, which must be finely filtered to prevent grit from entering the 30 0 Design of GAS-HAN DUNG Systems. .. temperature, °R Zs = compressibility at suction conditions or from: where Qg = gas throughput, MMscfd 31 0 Design of GAS-HANDLING Systems and Facilities Compressor Flexibility To enhance compressor flexibility it is desirable to design into the compressor the capability of operating at other than the original design conditions If it is desired to provide flexibility to operate at lower suction pressures:... the piston is at the end of its stroke This is the sum of the volume between the head of the cylinder and the piston, and the volume under the valve seats The total clearance is expressed in percent of the total piston displacement, normally between 4 and 30 % As the piston starts its suction stroke, the gas that remains in the cylinder in the fixed and added clearance areas expands until the pressure... clearance, in3 CCE = crank end clearance, in3 s = stroke length, in Note: CHE and CCE can be obtained from the manufacturer CYLINDER SIZING Typically, in specifying a unit, the suction and discharge pressures, capacity (MMscfd), inlet temperature, and gas properties are given The actual sizing of the cylinders is left to the manufacturer from his specific combinations of standard cylinders, pistons, and liners... sizing and make sure that indeed the compressor will perform Sometimes it is necessary to size a new cylinder for an existing compressor or to verify that an existing compressor will perform in a different service The capacity of the cylinder is a function of piston displacement and volumetric efficiency This is in turn a function of cylinder clearance, compression ratio, and gas properties 30 8 Design of . Company .) 28 8 Design of GAS-HANDLING Systems and Facilities facturers for a maximum continuous horsepower, speed (rpm), and rod load. The rated horsepower is determined by the . has 29 4 Design of GAS-HANDLING Systems and Facilities Figure 11-8. Valve-in-piston double-acting compressor cylinder, (Courtesy of Dresser-Rand Company .) a place to vent and. pressure drop and better efficiencies, The effects of the seat area, the lift area, and the flow paths are automati- 3 02 Design of GAS-HANDLING Systems and Facilities Figyre