Basic Parts Of Reciprocating Compressor

Reciprocating compressors play a major role in the chemical, petrochemical, gas, and general industrial processes. It is the best known and most widely used compressor of the positive displacement type. In this article, we will learn about the reciprocating compressor's basic parts.

Basic Parts Of Reciprocating Compressor 

Cylinders:

• Cylinders of the reciprocating compressor are used in the process industries are separable from the frame. They are attached to the frame by way of an intermediate part known as the distance piece. The cylinder will accommodate both suction and discharge valve plates.

• Normally most of the cylinders used in the process are equipped with replaceable liners. The purpose of the liner is to provide a renewable surface to the wearing portion of the cylinder. This saves the cost of replacing a complete cylinder once the bore has been worn or scored. In the larger, more complex compressors, this feature is standard or readily available as an option. On the smaller frames, particularly the single-stage models, the smaller cylinder size is such that the replaceable liner is not economical and may not be available.

• The cylinders are equipped for cooling to reduce the temperature during the compression cycle. This is usually by means of a water jacket. If water jacketing in the cylinder is not feasible than fins are provided in the cylinder for air cooling. The latter method is limited to either small or special purpose machines. 

• The most common material used in cylinder construction is cast iron for the larger, low-pressure cylinders and steel for the smaller, high-pressure cylinders. In some cases, nodular or ductile iron can be used in lieu of cast iron. For hydrocarbon service, steel is most desirable, although not universally available.
• For capacity control, clearance pocket included in the cylinders. In the case of multi-stage compressors, the clearance pocket is used in the first stage cylinder.

Reciprocating Compressor

Piston:

• The piston is the heart of the reciprocating compressor. The piston translates the energy from the crankcase to the gas in the cylinder. In order to avoid the leakage of compressed gas between the piston and cylinder, the piston is equipped with a set of sliding seals called piston rings. Usually, the rings are made of a material, that having self-lubricating property to reduce the slide friction force between the cylinder and the piston. This to avoid the wear in the cylinder wall and also increase efficiency by reducing the friction force.

• Different types of rings are used for lubricated or non-lubricated service. In the case of non-lubricated service, the rings shall have good self-lubricated property. Usually, this type of piston ring is made up of PEEK and other fluorocarbon compounds.

• For lubricated service, metallic rings such as cast iron or bronze as well as non-metallic materials such as filled nylon are used. Nowadays the compressor manufacturer preferred non-metallic materials because of less weight when compared with metallic rings.

• In the case of horizontal cylinder piston, along with piston rings, an additional ring is used to reduce the avoid the wear between cylinder and piston it is called a wear band or rider ring. The rider ring is one piece configuration, to accommodate the rider ring the piston is segmented construction. As per API 618 one-piece rider rings are required. Rider band life can be extended by rotating a piston 120° to 180° at each overhaul.

• Normally the piston is made up of lesser weight materials such as aluminum and aluminum alloys, this is to reduce the reciprocating compressor shaking forces and Rod load. In the case, piston diameter larger piston diameter hollow pistons are also used to reduce the weight.

Piston Rod:

• 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 run-out of the rod resulting in premature packing wear. This, in turn, leads to leakage, reduced efficiency, and increased maintenance expenses.

Distance Piece:

• The distance piece is a separable housing that connects the cylinder to the frame. The distance piece may be open or closed and may have multiple compartments. It may be furnished as single, double, or extra long.

• The purpose of a long-distance piece is to isolate that part of the piston rod entering the crankcase and receiving lubrication from the part entering the cylinder and contacting the gas. This prevents lubricant from entering the cylinder and contaminating the gas, particularly necessary in non-lubricated cylinders. It can also keep a synthetic lubricant in a cylinder.

• Compartments in the distance piece collect and control packing leakage when the gas is toxic or flammable. The leakage can be directed to a flare or buffer inert gas is used to prevent the toxic gas leakage to the atmosphere.

As per API 618, the distance pieces are classified as four types

1.     Type A

2.     Type B

3.     Type C

4.     Type D

Rod Packing:

• Rod packing is required to prevent the gas leakage along the piston rod where it passes through the crank end cylinder closure.

• If cooling packing is required, the stuffing box may be jacketed for liquid coolant.

The packing rings are the heart of rod packing assembly. The main packing rings types are,

• Tangent Ring
• Radial Ring or Pressure Breaker Ring
• Backup ring

Tangent Ring

• The tangent ring is cut into three segments so that each cut lies on the side of an equilateral triangle. The cuts of this ring maintain sealing contact regardless of variations of the ring’s inside diameter. As wear occurs, the ring segments will close radially to compensate while still maintaining sealing contact at the tangential joints by means of Garter Spring.

Radial Ring or Pressure Breaker Ring

• The radial ring or pressure breaker rings are cut into three segments so that each cut lies on the side of a radial direction of the ring. The cuts of this ring are used to break down the pressure act on the ring. Usually, a radial ring is the first packing ring that faces the cylinder pressure.

Backup Ring

• This type of ring construction is similar to the radial ring. As the name indicate this type of ring is used to backup all the rings when using in the high-pressure application.

CrankShaft:

• Larger compressors, normally above 150 kW(200 hp), have forged steel crankshafts. for compressor less than or equal to 150kW machines ductile iron crankshaft. Crankshafts should have removable balance weights to compensate for rotary unbalance as well as reciprocating unbalance. The crankshaft should be dynamically balanced when above 800 rpm.
• When force lubrication is used, the crankshaft oil passages should be drilled. The Figure shows a drilled crankshaft. In the case of machines above 150kW,

Bearing:

• The main and connecting rod bearings should be split-sleeve, steel-backed, babbitted insert type. Antifriction bearing shall be used for compressor rating less than 150kW. As per API, 618 trapper roller bearings are used as the main bearing for compressor rating less than 150kW.

The cross headpin should have a replaceable bush bearing.

Connecting Rod:

• For reciprocating compressor of above 150 kW(200 hp), have forged steel connecting rod are used and for compressor less than or equal to 150kW machines ductile iron material is used. Like Crankshafts, the connecting rod should have a drilled hole for oil passage. The connection rod is used to connect the crankshaft and the crosshead. The connecting rod converts the rotary motion into reciprocating motion. The connecting rod bolts and nuts shall be securely locked with cotter pins or wire after assembly.

Cross Head:

• All industrial reciprocating compressors are equipped with Crosshead. The crosshead is used to guide the piston in the cylinder bore. The following are the advantage of the crosshead

• The compressor can use a narrow piston, due to the use of narrow piston larger valve area for greater efficiency
• Permits a longer stroke and greater capacity
• Separates crankcase from the cylinder, allowing control of oil carryover into the cylinder.
• Gives greater stability to the piston, eliminating piston “slap” and reducing ring wear
• Permits stronger piston design and higher operating pressures.

Valve:

• The reciprocating compressor cylinder valves are of the spring-loaded, gas-actuated type. Reciprocating compressors generally use one of three types of valve configurations:

• Ring Type valve
• Poppet Type valve
• Plate Type valve

Poppet Type Valve

• The poppet valve (see Figure) 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 of uneven flow distribution. The valve is most commonly used in the medium pressure range. Nowadays these types of valves are used in hydrocarbon service because of easy maintenance.

• Poppet valves are recommended up to 15 Mpa differential pressure or 30 Mpa discharge pressure and 600rpm.

Plate Type Valve

• The plate valves, as shown in Figure, 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. These valves are mostly used in the industrial process compressor.

• Plate valves are recommended up to 20 Mpa differential or 40 Mpa discharge pressure and 1800 rpm.

Ring Type Valve

• As the name implies the ring-type valve consisting of concentric ring valve uses one or more relatively narrow rings arranged concentrically about the centerline of the valve (see Fig ). 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.

• Ring valves are recommended up to 30 Mpa differential or 60 Mpa discharge pressure and 600 rpm.

Lubrication System:

• Force type lubrication is used for the medium and heavy-duty compressors. The main component of the force type lubrication is the main oil pump, dual oil filter, lube oil cooler, pressure regulating valve, lube oil tank, and its associated accessories.

• Splash type lubrication is used for a small compressor of 100kW. This type of system lubricating oil by the splashing of crank-throw moving through the lubricant in the sump.