Reciprocating Compressors

A reciprocating compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure.

From: The Engineer's Guide to Plant Layout and Piping Design for the Oil and Gas Industries, 2018

Reciprocating Compressors

Justin Hollingsworth, ... Franzisko Maywald, in Compression Machinery for Oil and Gas, 2019

Service Types and Operating Conditions

Reciprocating compressors have been widely used for over 200 years ever since gases needed to be compressed. In the last 50 years however their dominance as the compressor type of choice has been eroded as other compressor types have been developed. Reciprocating compressors can be used in almost any compression application, but other compressor types are generally preferred for certain applications. Centrifugal compressors tend to be preferred when the power is above 2 MW, the mole weight (MW) is greater than 10, and the discharge pressure below 100 MPa. Screw compressors are preferred when the power is in the range 10–500 kW and the discharge pressure is below 30 bar. Roots blowers are used for discharge pressures below 0.1 MPa gage. Power levels below 100 kW tend to be the province of diaphragms, vanes, etc.

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Reciprocating Compressors

Heinz P. Bloch, in Petrochemical Machinery Insights, 2017

Introduction

A top-rated manufacturer of reciprocating compressors keeps its customers informed of relevant updates. Service bulletins are offered to you by compressor manufacturers because they want to promote an ongoing sales and service relationship with a user's organization. However, the operation of your plant involves factors not within the manufacturer's knowledge, and operation of the plant is within your control and responsibility. The responsibility for the plant’s reliability, operability, and profitability rests with you. It should not surprise us that an equipment manufacturer will tend to disclaim responsibilities beyond parts, and even that responsibility ends as warranties expire. The manufacturer's disclaimers will always include, but not be limited to, direct, consequential, or special damages. The vendor/manufacturer will use legal language making this position abundantly clear. Moreover, the manufacturer considers his service bulletin confidential and proprietary, places it under copyright and/or other intellectual property protections, and provides these bulletins for authorized use only. So be sure to obtain all kinds of written consent before you involve others.

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Storage

In Lees' Loss Prevention in the Process Industries (Third Edition), 2005

22.9.6 Ancillary facilities

Ancillary facilities for refrigerated LPG tanks include the refrigeration system, the pumping facilities and vaporization equipment.

Reciprocating compressors are used for the direct re-liquefaction of vapour boiled off due to heat inleak. Use is also made of reciprocating compressors on refrigeration sets. Malfunction of a refrigeration set can cause the liquid refrigerated to be cooled below its normal temperature.

Reciprocating compressors can be a source of leakage of flammable vapour. They are subject to crankcase explosions and should normally be protected against this by an explosion relief valve.

Pumps should not normally be located inside a bund, where they introduce an additional hazard and are themselves at risk. It is desirable to locate them outside the bund. This also allows them to be used to pump down the storage tank during a fire.

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Accidents involving compressors, hoses, and pumps

Roy E. Sanders, in Chemical Process Safety (Fourth Edition), 2015

Reciprocating compressors

Reciprocating compressors are widely used in the chemical, oil, and gas industries for moving compressible fluids reliably. Reciprocating compressors have the advantage over other types of compressors (e.g., centrifugal and rotary) when it comes to handling wide-capacity swings and generating a high-discharge pressure [1].

When properly designed, installed, and operated, these compressors serve well. However, there are stories about problems with liquids from condensation of saturated vapor streams entering the suction side of the compressor. The typical reciprocating compressor design is not very tolerant of an accumulation of liquids in the suction. There is the possibility of blowing the compressor heads off if the compressors are subjected to incompressible fluids.

Just like any other piece of important operating equipment, the operator must understand the details of the equipment and its limitations. Trevor Kletz offers many excellent short stories in his latest book, Still Going Wrong! He provides cameo descriptions of process-plant errors with a message [2].

Kletz provides a story about a packaged unit containing a reciprocating compressor. The compressor was started in error. The discharge valve was in the closed position and the pressure increased until the packing around the cylinder rod blew out. This compressor was equipped with a relief valve that was merely a sentinel to warn the operator to take action. The sentinel was incapable of relieving the full output of the equipment. It is not an acceptable design standard today, but this example supports the need for available process-safety information on all equipment [2].

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Pressure System Design

In Lees' Loss Prevention in the Process Industries (Fourth Edition), 2012

12.10.2 Reciprocating Compressors

Reciprocating compressors are utilized for high compressions. They have long been the machines used for compression to the high pressures required in ammonia plants and on offshore platforms. Accounts of reciprocating compressors are given by Whittaker (1975), Barnes (1976), Messer (1979), and Woollat (1993). Relevant standards are BS 7322: 1990 Specification for the Design and Construction of Reciprocating Type Compressors for the Process Industry and API Std 618: 1986 Reciprocating Compressors for General Refinery Services.

Reciprocating compressors can be provided with capacity control, or turndown, by the use of volume pockets and by drive speed control. Some principal malfunctions on reciprocating compressors are: (1) valve leakage, (2) cylinder/piston scoring, (3) piston ring leakage, (4) gasket failure, (5) tail rod failure, and (6) vibration, as well as the general compressor failures such as those caused by liquid slugs or loss of lubricating oil or cooling water.

Leakage of the suction or discharge valves is one of the commonest failures. There are a number of symptoms of valve malfunction. The valve may become unusually hot, the cylinder capacity may fall, the discharge temperature may rise, and the interstage pressures may be abnormal. The suction pressure may rise and the discharge pressure fall unless automatically controlled.

On some machines, there is nothing to prevent a suction valve being fitted on the discharge, or vice versa, by mistake. If this happens, it is possible to create very high pressures in the cylinder, particularly the high pressure cylinder, and so cause failure of the cylinder, the piston, or the drive system. Preferably, this feature should be designed out. Where this is not the case, there should be procedures to minimize the probability of error.

The piston rod in high pressure cylinders is sometimes balanced by a tail rod on the other side of the piston. There have been some serious accidents in which the tail rod has broken off and flown out like a projectile. There should be a ‘catcher’ of sufficient strength to prevent escape of the tail rod if it does break. Tail rods are frequently surface hardened and it is important for them to be free of surface cracks. They should be regularly inspected. A tail rod failure allows the escape of high pressure gas.

The reciprocating movement of the piston inevitably causes some degree of vibration. This vibration may be transmitted to, and cause failures in, the process pipework. Small auxiliary piping on the machine tends to be particularly vulnerable to fracture from this vibration. It should be anchored to reduce vibration and inspected regularly.

Changes in the discharge temperature are often a sign of malfunction on a reciprocating compressor. High discharge temperature may be associated with valve failure, piston ring leakage, increased compression ratio, gas composition change, or loss of cooling water.

On air compressors lubricated with oil, a high discharge temperature can result in an explosion. Such air compressor explosions, and the discharge temperature limits necessary to avoid them, are discussed in Chapter 17. Some compressors are required to produce compressed air which is free of oil. It is very desirable, for example, that instrument air be oil-free. Carbon ring compressors are often used for this purpose.

The reciprocating compressors were one of the potential sources of the gas leak investigated in the Piper Alpha Inquiry (Cullen, 1990). The questions of the tolerance of such machines to ingestion of liquid slugs and of the bolt tightening practices used were considered in evidence by Bett (1989).

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The Compressor

Antony Barber C.Eng., M.Sc., F.I.Mech.E., M.R.Ae.S., in Pneumatic Handbook (Eighth Edition), 1997

Reciprocating compressors

Reciprocating compressors are suitable for operating over a very wide range of speeds, with a practical maximum of about 10 bar delivery pressure from a single-stage unit, and up to 70 bar for a two-stage machine. Multi-stage reciprocating compressors may be built for special purposes capable of supplying delivery pressures up to and in excess of 700 bar. By carefully selecting the number of stages, the designer can also produce a machine which approaches the ideal or isothermal compression curve more closely than with any other type, with the possible exception of very large volume axial flow compressors (see the chapter on Compressor Performance). In the double-acting compressor the space on the other side of the cylinder is enclosed and so both sides are used for compression, giving two compression strokes for each revolution of the crank shaft. Alternatively, one side of the piston may be used for one stage and the other for the second stage in a two stage compressor. Individual cylinders may be also used for multi-stage compression, disposed in a number of arrangements.

Most reciprocating compressors are single-stage or two-stage, ranging from fractional horsepower units to very large machines with input requirements of the order of 2250 kW. The smaller compressors are usually single stage of a single or V-twin layout with air cooled cylinders and are powered by electric motors.

Intermediate sizes comprise a variety of different configurations. Vertical compressors may comprise one or more cylinders in line, ‘V’, ‘W’ and ‘H’ arrangements for multi-cylinder units, and also ‘L’ configuration which has both vertical and horizontal cylinders disposed about a common crankshaft. The angled arrangement of cylinders offers certain advantages, notably reduced bulk and weight and superior machine balance (since with careful design the primary forces can be accurately balanced). The ‘L’ configuration with a vertical low pressure cylinder and a horizontal high pressure cylinder is advantageous for larger machines, facilitating installation, assembly, maintenance and dismantling. Larger reciprocating compressors are commonly horizontal double acting, tandem or duplex. Basic two-stage units are shown in Figure 2. Variations include transferring the side thrust by means of a crosshead. When the space under the piston is used, it is essential to use a crosshead to convert the rotary action of the crankshaft into reciprocating motion in order to obtain a satisfactory seal of the piston rod where it passes through the compression chamber. The crosshead ensures that all the side thrust component from the crank shaft is taken by the crosshead guide and not transferred to the piston and cylinder. Single-acting reciprocating compressors are normally of the trunk piston type, whilst differential-piston double-acting compressors can be of either type – see Figure 3.

Differential or stepped pistons may also be used as shown in Figure 4. Here one stage of compression takes place in the annular space between the shoulder of the piston and the corresponding shoulder in the cylinders.

Figures 2 to 4 also show designs where a compressor piston is connected to the same crank as the reciprocating engine which drives it. A common arrangement uses three cylinders of a four-cylinder block to form the engine, whilst the fourth cylinder forms the compressor cylinder. This is a convenient way of balancing the forces with a single cylinder compressor.

Figure 6 shows the pattern of reciprocating air compressors in industrial service, according to a recent survey. This represents most of the industrial units in service. For special purposes, as mentioned above, compressors are found well outside the range indicated. The versatility of reciprocating compressors means that they are the most common of all types. The disadvantages of reciprocating compressors are:

FIGURE 6. –Lubricated air compressors in industrial service – all piston types. (CompAir Broom Wade)

They require special foundations to cater for the unbalanced inertial forces of the reciprocating pistons and connecting rods.

The maintenance they need has to be done by skilled personnel.

The inlet and delivery valves are prone to failure.

The discontinuous flow of the compressed medium can cause vibrational resonance in the delivery passages and the distribution system.

In mobile compressors and in the medium range of stationary compressors, reciprocating compressors tend to have been superseded in recent years by screw and sliding vane designs in oil flooded and dry versions. They still maintain their dominance for large stationary (factory) applications

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TROUBLESHOOTING PNEUMATIC CIRCUITS

R. Keith Mobley, in Fluid Power Dynamics, 2000

Reciprocating Positive-Displacement Compressors

Reciprocating compressors have a history of chronic failures that include valves, lubrication system, pulsation, and imbalance. Table 21-3 identifies common failure modes and causes for this type of compressor.

Table 21-3. Common Failure Modes of Reciprocating Compressor

Like all reciprocating machines, reciprocating compressors normally generate higher levels of vibration than centrifugal machines. In part, the increased level of vibration is due to the impact as each piston reaches top dead center and bottom dead center of its stroke. The energy levels also are influenced by the unbalanced forces generated by non-opposed pistons and looseness in the piston rods, wrist pins, and journals of the compressor. In most cases, the dominant vibration frequency is the second harmonic (2×) of the main crankshaft's rotating speed. Again, this results from the impact that occurs when each piston changes directions (i.e., two impacts occur during one complete crankshaft rotation).

Valves. Valve failure is the dominant failure mode for reciprocating compressors. Because of their high cyclic rate, which exceeds 80 million cycles per year, inlet and discharge valves tend to work-harden and crack.

Lubrication systems. Poor maintenance of lubrication-system components, such as filters and strainers, typically causes premature failure. Such maintenance is crucial to reciprocating compressors because they rely on the lubrication system to provide a uniform oil film between closely fitting parts (e.g., piston rings and the cylinder wall). Partial or complete failure of the lube system results in catastrophic failure of the compressor.

Pulsation. Reciprocating compressors generate pulses of compressed air or gas that are discharged into the piping that transports the air or gas to its point(s) of use. This pulsation often generates resonance in the piping system, and pulse impact (i.e., standing waves) can severely damage other machinery connected to the compressed-air system. Although this behavior does not cause the compressor to fail, it must be prevented to protect other plant equipment. Note, however, that most compressed-air systems do not use pulsation dampers.

Each time the compressor discharges compressed air, the air tends to act like a compression spring. Because it rapidly expands to fill the discharge piping's available volume, the pulse of high-pressure air can cause serious damage. The pulsation wavelength, λ, from a compressor having a double-acting piston design can be determined by

λ=60a2n=34,050n

where λ = Wavelength, feet

a = Speed of sound = 1,135 feet/second

n = Compressor speed, revolutions/minute

For a double-acting piston design, a compressor running at 1,200 rpm will generate a standing wave of 28.4 feet. In other words, a shock load equivalent to the discharge pressure will be transmitted to any piping or machine connected to the discharge piping and located within twenty-eight feet of the compressor. Note that, for a single-acting cylinder, the wavelength will be twice as long.

Imbalance. Compressor inertial forces may have two effects on the operating dynamics of a reciprocating compressor, affecting its balance characteristics. The first is a force in the direction of the piston movement, which is displayed as impacts in a vibration profile as the piston reaches top and bottom dead center of its stroke. The second effect is a couple, or moment, caused by an offset between the axes of two or more pistons on a common crankshaft. The interrelationship and magnitude of these two effects depend upon such factors as (1) number of cranks; (2) longitudinal and angular arrangement; (3) cylinder arrangement; and (4) amount of counterbalancing possible. Two significant vibration periods result, the primary at the compressor's rotation speed (×) and the secondary at 2×.

Although the forces developed are sinusoidal, only the maximum (i.e., the amplitude) is considered in the analysis. Figure 21-1 shows relative values of the inertial forces for various compressor arrangements.

Figure 21-1. Unbalanced inertial forces and couples for various reciprocating compressors.

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Maximizing Machinery Uptime

In Practical Machinery Management for Process Plants, 2006

Locking or Holding Mechanism Failure

Reciprocating compressors utilize bolted joints to unite the various component assemblies into one structure. Therefore, there is a high dependence upon bolted joint integrity to maintain reliable operation and any loosening of these joints makes a mechanical failure inevitable. Estimates of compressor failures due to this failure mode alone vary from 10 to 20% of all recorded failures. To improve bolted joint reliability, and achieve a significant reduction in failure incidence, the design of all joints must be analyzed carefully and a maximum incorporation of reliable design details should be done.

Designing Out Looseness.

By applying the integral design approach, a design study of the machine should be made to minimize the number of bolted joints and change to one-piece components. An example of this is to specify one-piece valve cage-cover designs as opposed to the standard two-piece construction (Fig. 13-16).

Figure 13-16. One piece valve cage cover (Cook Manley).

Resisting Loosening Failures.

The vast majority of failures in reciprocating compressor joints are due to a loss of bolt/stud tension arising from joint relaxation and vibration loosening. These factors are caused by the following:

A low design elasticity of the bolt/stud connection.

Embedment of joint surfaces and nut surface.

Insufficient thread engagement.

Oversized holes.

Temperature cycling of the joint.

Excessive joint surfaces.

Vibration overcoming the friction forces between the nut and stud threads, leading to nut rotation.

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COMPRESSORS

R. Keith Mobley, in Fluid Power Dynamics, 2000

Pulsation

Reciprocating compressors generate pulses of compressed air or gas that is discharged into the piping used to transport it to its points of use. These pulsations can, and often do, generate resonance in the piping system. In addition, these pulses can cause severe damage to other machinery that may be connected to the compressed air system.

Since most compressed air systems do not use pulsation dampers to minimize the pulsations generated by reciprocating compressors, resonance or the impact of these pulses, called standing waves, generated by these compressors can result in severe damage to other machine trains included in the compressed air system. Although this is not a failure mode of the compressor, it must be prevented to protect other critical plant equipment. Each time the compressor discharges a volume of compressed air, the air tends to act like a compression spring. It will rapidly expand to fill the available volume of discharge piping, the pulse of high pressure air can result in serious damage.

The full wavelength of the pulsation generated by a double-acting piston design can be obtained by:

λ=60a2n

or when a is 1,135:

λ=34,050n

where λ = wavelength, feet

a = speed of sound, feet/second

n = speed, revolutions/minute

Note: For a single-acting cylinder, the wavelength will be twice as long.

In the example, the compressor running at 1,200 rpm will generate a standing wave of 28.375 feet. In other words, a shock load equivalent to the discharge pressure will be transmitted to any piping or machine connected to the discharge piping that is located within 28 feet of the compressor.

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