Reciprocating compressor speed

There are three types of reciprocating compressor speed 1) low speed (300-500 rpm), long stroke (7 in. and more), 2) medium speed (5DD-800 rpm), medium-stroke (5-7 in.X and 3) high sp e (900-1800 rpm), small-stroke (2-6.5 in.). 

Motor-driven compressors usually operate at constant speed, and other methods of controlhng the capacity are necessary. On reciprocating compressors discharging into receivers, up to about 75 kW (100 np), two types of control are usually available. These are auto-matic-start-and-stop control and constant-speed control. 

Motor-driven reciprocating compressors above about 75 kW (100 hp) in size are usually equipped with a step control. This is in reality a variation of constant-speed control in which unloading is accomplished in a series of steps, varying from full load down to no load. Three-step eontrol (full load, one-half load, and no load) is usually accomplished with inlet-valve unloaders. Five-step eontrol (fuU load, three-fourths load, one-half load, one-fourth load, and no load) is accomphshed by means of clearance pockets (see Fig. 10-91). On some machines, inlet-valve and clearance-control unloading are used in combination. 

Positive-Displacement Compressors Reciprocating compressors are built in different sizes (up to about one megawatt refrigeration capacity per unit). Modern compressors are high-speed, mostly direct-coupled, single-ac ting, from one to mostly eight, and occasion-aUy up to sixteen c inders. 

Mechanical Expanders Reciprocating expanders are very similar in concept and design to reciprocating compressors. Generally these units are used with inlet pressures of 4 to 20 MPa. These machines operate at speeds up to 500 rpm. The thermal efficiencies (actual enthalpy difference/maximum possible enthalpy difference) range from about 75 percent for small units to 85 percent for large machines. 

Vlanv process machines operate at speeds diFFerent From the one oF their drivers. Typical oF cases where the machine rotates. dower than the driver are reciprocating compressors typical examples oF machines rotating/ js / er than the drivers are centriFiigal compressors driven bv electric motors. In either case, ear. are used to match the two speeds, (iears can also be designed to accommodate shaFts that... 

This standard could be adapted to the fuel compressor for the natural gas to be brought up to the injection pressure required for the gas turbine. Covers the minimum requirements for reciprocating compressors and their drivers used in petroleum, chemical, and gas industry services for handling process air or gas with either lubricated or nonlubricated cylinders. Compressors covered by this standard are of moderate-to-low speed and in critical services. The nonlubricated cylinder types of compressors are used for injecting fuel in gas turbines at the high pressure needed. Also covered are related lubricating systems, controls, instrumentation, intercoolers, after-coolers, pulsation suppression devices, and other auxiliary equipment. 

Compressors are usually high-cost items, but easily correlated by brake horsepower vs. S/horsepower. Variations in engine-driven reciprocating compressor prices can be caused by the type of driver, the speed (the slower the speed the more costly, but the more reliable), the total discharge pressure, and the size. 

The basis for evaluation of piston speed varies throughout industry. This indicates that the subject is spiced with as much emotion as technical basics. An attempt to sort out the fundamentals will be made. First, because there are so many configurations and forms of the reciprocating compressor, it would appear logical that there is no one piston speed limit that will apply across the board to all machines. The manufacturer is at odds with the user because he would like to keep the speed up to keep the size of the compressor down, while the user would like to keep the speed down for reliability purposes. As is true for so many other cases, the referee is the economics. An obvious reason to limit the speed is maintenance... 

With the introduction of the new instruments, speed is basically taken for granted. It is a very important parameter for reciprocating compressors, however, because speed is one of the factors in generating displaced volume. For the axial and the centrifugal compressor, speed offers a multiple influence. In the fan laws stated in Chapter 5, speed was the common parameter in both capacity and head. In fact, since head is proportional to speed squared, it becomes quite important that the speed be accurate. 

Many of the compressors require some form of speed matching to the driver. For most drivers to reciprocating compressors, this will be a reducer. For gas turbines to almost all compressors, a gear unit is... 

Several steps can be taken to maximize the run time for the reciprocating compressor. Since wear is a function of rubbing speed, the piston speed can be kept to a minimum. Chapter 3 made recommendations for piston speed. Reliability problems due to valves are reputed to account tor 40% of the maintenance cost of the compressor. Valves are the single largest cause for unplanned shutdowns. Basically, valve life can he increased by keeping the speed of the compressor as low as practical. At 360 rpm, the valves are operated six times a second. At 1,200 rpm, ihc valves operate 20 times a second or 1,728,000 times in a day. It is not difficult to understand why the valves are considered critical. To keep the reliability in mind, valve type, material selection and application considerations such as volume ratio, gas corrosiveness, and gas cleanliness need attention by the experts. One final note is that while lubrication is an asset to the rubbing parts, it is not necessarily good for valve reliability. 

Reciprocating compressors are classified as either high speed or slow speed. Typically, high-speed compressors run at a speed of 900 to 1200 rpm and slow-speed units at speeds of 200 to 600 rpm. 

Figure 10-2. High-speed reciprocating compressor frames and cylinders. [Courtesy of Dresser-Rana Company.)...

The major characteristics of high-speed reciprocating compressors are... 

A recycle valve is needed for surge control as well as for the conditions listed above for reciprocating compressors. At constant speed the head-capacity relationship will vary in accordance with the performance curve. For a constant compressor speed ... 

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. 

As previously discussed in this chapter and in Chapter 10, reciprocating compressors are generally classified as either low-speed (integral) compressors or high-speed (separable) compressors, API has provided a standard and specification for each type of compressor to help the user and the facility engineer provide reliable compressor installations. 

API Specification IIP Specification for Packaged Reciprocating Compressors for Oil and Gas Production Services covers packaged high-speed separable compressors with speeds from 600 to 1,200 rpm. The majority of reciprocating compressors sold in today s market fall into this category. 

Figure 12-5F. Lubricated and nonlubricated balanced opposed process reciprocating compressors, designed to API 618 code. Fixed- and variable-speed drives using gas or diesel engines, steam or gas turbines, or electric motor. Note power drive to connect to right side of cross-head box in center. (Used by permission Bui. PROM 635/115/95-11. Nuovo Pignone S. P. A., Florence, Italy New York Los Angeles and Houston, Texas. All rights reserved.)...

Induction motors are most popular. Synchronous motors are made for speeds as low as 150 rpm and are thus suited for example for low speed reciprocating compressors, but are not made smaller than 50 FIP. A variety of enciosures is available, from weather-proof to explosion-proof. 

Motor-driven reciprocating compressors above about 75 kW (100 hp) in size are usually equipped with a step control. This is in reality a variation of constant-speed control in which unloading is accomplished in a series of steps, varying from full load down to no load. 

The most cost-effective way to produce a high polytropic head is to increase the number of wheels on the rotor. But the longer the rotating assembly, the more difficult it is to properly balance the rotor. Especially for high-speed machines, rotors which become unbalanced are subject to destructive vibration. Therefore, for low-molecular-weight gas services (less than 10), it is not uncommon to use reciprocating compressors. 

Figure 3.24. Control of positive displacement compressors, rotary and reciprocating, (a) Flow control with variable speed drives, (b) Pressure control with bypass to the suction of the compressor, (c) Reciprocating compressor. SC is a servomechanism that opens some of suction valves during discharge, thus permitting stepwise internal bypass. The clearance unloader is controllable similarly. These built-in devices may be supplemented with external bypass to smooth out pressure fluctuations.

Synchronous motors are made in speeds from 1800 (two-pole) to 150 rpm (48-pole). They operate at constant speed without slip, an important characteristic in some applications. Their efficiencies are 1-2.5% higher than that of induction motors, the higher value at the lower speeds. They are the obvious choice to drive large low speed reciprocating compressors requiring speeds below 600 rpm. They are not suitable when severe fluctuations in torque are encountered. Direct current excitation must be provided, and the costs of control equipment are higher than for the induction types. Consequently, synchronous motors are not used under 50 HP or so. 

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