ALL ABOUT RECIPROCATING RECIPROCATING COMPRESSOR IN OIL & GAS INDUSTRY. Reciprocating Compressors – Fugitive Emissions Control One of the growing environmental concerns for both new and existing reciprocating compressor installations is fugitive emissions. Fugitive emissions are the l eakage of volatile organic compounds (VOCs) into the atmosphere. The local environmental regulations should be checked at the beginning of the compression project to avoid delays and field modifications. 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. Fugitive emissions can be reduced by supplying improved O-ring seal designs along with piston rod packing cases and actuator stem seal . designs that utilize an inert i nert buffer gas purge. The purge gas and VOCs can then be collected and sent to either a flare or vapor recovery system. The compressor manufacturer must advise the maximum allowable back-pressure on the compressor components. A typical compressor cylinder inert buffer gas arrangement is shown in Figure 11-25.
Reciprocating Compressors – Special Compressor Cylinder Construction
Many variations and combinations of cylinder types and arrangements are available from the compressor manufacturers. The compressor manufacturer will generally make its selection based on the most economical combination it has available. Figure 11-7 is a cut-away of a steeple cylinder. This cylinder design is actually two singleacting 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 compressor throw but uses two double-acting cylinders separated by a second distance piece. This arrangement is usually used in low rod load applications where higher capacity is required. Figure 11-8 is a cut-away of the latest innovation in compressor cylinder 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 discharge 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 emissions, fewer replacement parts, simpler maintenance procedures, and reduced weight.
Reciprocating Compressors – Cylinder A cylinder is a pressure vessel that holds the gas during the compression 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 crankshaft revolution. Figure 11-4 is a cut-away drawing of a compressor with single-acting cylinders. True singleacting cylinders are typical of low horsepower air compressors. Single-acting process compressors are typically double-acting 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, nodular 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 working 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. 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 compression 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 (discussed in more detail below) by increasing the distance between the piston 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.
Reciprocating Compressors – Cylinder Clearance Clearance is the volume remaining in a cylinder end when the piston is at the end of its stroke. This is the sum of the volume between the head of the c ylinder 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 c ylinder in the fixed and added clearance areas expands until the pressure in the c ylinder is equal to the pressure in the line outside of the cylinder. The greater the clearance, the longer it takes for the suction valves to open and the less new gas enters the cylinder. Therefore, less gas will be compressed as cylinder clearance is increased. End clearance is required to keep the piston from striking the compressor head or crank end. Some small clearance is also required under suctlon 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 body or under the valves Variable clearance that can be changed very readily can be built into the cylinder. Figure 1119 is an example of a fixed volume clearance pocket mounted on the cylinder. This type is separated from the cylinder by a valve that can be opened and closed from the outside.
Fixed clearance can also be added to the outer end of the cylinder by adding a fabricated clearance bottle with the desired volume. To change the performance of the cylinder the clearance can be changed by shutting down the compressor, unbolting one bottle, and installing another bottle with a different volume. It is very easy in t hat respect to add clearance and subtract clearance from a cylinder if the cylinder is set up to receive clearance bottles.
More flexibility can be obtained with a variable volume clearance pocket such as that shown in Figure 11 -20. This is a plug built into the outer cylinder head. When moved, the clearance volume of the outer end of the cylinder changes.
Clearance is normally expressed as a percent or fraction of cylinder displacement. It is given by: Single acting cylinder (head end clearance)
Reciprocating Compressors #1 A reciprocating compressor is a positive-displacement machine in which t he compressing and displacing element is a piston moving linearly within a cylinder. Figure 10-1 shows the action of a reciprocating compressor.
In Position 1 the piston is moving away from the cylinder head and the suction valve is open, allowing the cylinder pressure to equal suction pressure and gas to enter the cylinder. The discharge valve is closed. At Position 2 the piston has traveled the full stroke within the cylinder and the cylinder is full of gas at suction pressure. The piston begins to move to the left, closing the suction valve. In moving from Position 2 to Position 3, t he piston moves toward the cylinder head and the volume is reduced. This increases pressure until the cylinder pressure is equal to the discharge pressure and the discharge valve opens. The piston continues to move to the end of the stroke near the cylinder head, discharging gas. Pressure in the cylinder is equal to discharge pressure from Position 3 to Position 4. As the piston reverses its travel the gas remaining within the cylinder expands until it equals suction pressure and the piston is again in Position 1.
Reciprocating compressors are classified as either “high speed” or “slow speed.” Typically, high-speed compressors run at a speed of 900 to J 200 rpm and slow-speed units at speeds of 200 to 600 rpm. Figure 10-2 shows a high-speed compressor frame and cylinders. The upper compressor is called a two throw machine because it has two cylinders attached to the frame and running off the crank shaft. The lower compressor is a four-throw machine because it has four cylinders attached to the frame. The number of “t hrows” refers to the number of pistons.
Reciprocating Compressors #2 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 is compressed, its
temperature increases. Therefore, after passing through the cylinder the gas is usually cooled before being routed to another suction scrubber for another stage of compression. A stage of compression thus consists of a scrubber, cylinder, and after-cooler. (The discharge from the final cylinder may not be routed to an after-cooler.) The number of throws is not the same as the number of stages of compression. It is possible to have a two-stage, four-throw compressor. In this case there would be two sets of two cylinders working in parallel. Each set would have a common suction and discharge. 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. High-speed units are typically engine or electric motor driven, although turbine drivers have also been used. Engines or turbines can be either natural gas or diesel fueled. By far the most common driver for a high-speed compressor is a natural gas driven engine. Figure 10-3 shows a high-speed engine-driven compressor package. The unit typically comes complete on one skid with driver, compressor, suction scrubbers and discharge coolers for each stage of compression and all necessary piping and controls. On large units (> 1,000 hp plus) the cooler may be shipped on a separate skid. The major characteristics of high-speed reciprocating compressors are:
Size • Numerous sizes from 50 hp to 3000 hp. • 2, 4, or 6 compressor cylinders are common. Advantages • Can be skid mounted. • Self -contained for easy installation and easily moved. • Low cost compared to low-speed reciprocating units. • Easily piped for multistage compression. • Size suitable for field gathering offshore and onshore. • Flexible capacity limits. • Low initial cost.
Disadvantages • High-speed engines are not as fuel efficient as integral engines (7,500 to 9,000 Btu/bhp-hr). • Medium range compressor efficiency (higher than centrifugal; lower than low-speed). • Short life compared to low-speed. • Higher maintenance cost than low-speed or centrifugal.
Reciprocating Compressors #3 Low-speed units are typically integral in design as shown in Figure 10-4. “Integral” means that the power cylinders that turn the crank shaft are in the same case (same housing) as the cylinders that do the compressing of the gas. There is one crank shaft. Typically, integrals are considered low-speed units. They tend to operate at 400-600 rpm, although some operate as low as 200 rpm.
Figure 10-5 shows a very large integral compressor. This would be 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.
It should be obvious that one of these large integrals would require a very large and expensive foundation and would have to be field erected. Often, even the compressor cylinders must be shipped separate from the frame due to weight and size limitations. Large integrals are also much more expensive than either high-speeds or centrifugals. For this reason, even though they are the most fuel efficient choice for large horsepower needs, large integrals are not often installed in oil and gas fields. They are more common in plants and pipeline booster service where their fuel efficiency, long life, and steady performance outweigh their much higher cost. There are some low horsepower (140 to 360) integrals that are normally skid mounted as shown in Figure 10-6 and used extensively in small oil fields for flash gas or gas-lif t compressor service. In these units the power cylinders and compressor cylinders are both mounted horizontally and opposed to each other. There may be one or two compressor cylinders and one to four power cylinders. They operate at very slow speed. Their cost and weight are more than similar sized high-speed separable units, but they have lower maintenance cost, greater fuel efficiency, and longer life than the high speeds. The major characteristics of low-speed reciprocating compressors are: Size • Some one and two power cylinder field gas compressors rated for L40 hp to 360 hp. • Numerous sizes from 2,000 hp to 4,000 hp. • Large sizes 2,000 hp increments to 12,000 hp. • 2 to 10 compressor cylinders common. Advantages • High fuel efficiency (6-8,000 Btu/bhp-hr). • High efficiency compression over a wide range of conditions. • Long operating life. • Low operation and maintenance cost when compared to high speeds.
Disadvantages • Usually must be field erected except for very small sizes. • Requires heavy foundation. • High installation cost. • Slow speed requires high degree of vibration and pulsation suppression.
Reciprocating Compressors – Packing Packing provides the dynamic seal between the cylinder and the piston rod. It consists of a series of Teflon rings mounted in a packing case, which is bolted to the cylinder. The piston rod moves in a reciprocating motion through this case. Figure 11-13 shows a typical packing case. The packing case is constructed of a number of pairs of rings, as shown in Figure 11-14.
The gas pressure is higher on one side of each ring. This compresses the rings against the sealing area. Each pair of rings consists of one radial cut ring and one tangential cut ring. The radial cut ring is installed toward the cylinder (pressure) side. Gas flows around the front face of the radial cut ring and then around the outside of both rings. Since the ring outside diameter is greater than the ring inside diameter, a squeezing force is exerted on the rod. This seals the path between the rings and the rod. The
radial cuts are positioned in the ring assembly so that they do not line up with the tangential cuts. Cylinder pressure will force the ring assembly against the packing case lip, thus preventing flow around the rings. The 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 case, is generally injected in the second ring assembly. The pressure differential moves the oil along the shaft. A separate cooling system may be required for high-pressure service (5,000 psi) or where high compression ratios and long packing cases are installed.
Reciprocating Compressors – Piston 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. To reduce friction and improve compression efficiency, the piston will be provided with segmented compression rings as shown in Figure 11 -11. To prevent piston-to-bore contact, the piston may also be provided with removable wear bands that are in continuous contact with the cylinder wall. The compression rings and wear bands are replaced at regular intervals and typically made from soft materials such as brass, Micarta, Teflon, and the newer thermoplastics.
Reciprocating Compressors – 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 figure 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 compressed 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 contamination between the gas being compressed and the oil t hat is used to lubricate 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 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 r od 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 contaminated with the toxic gas.
Reciprocating Compressors – 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 crankshaft 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.
