Reciprocating compressors are the best known and most widely used compressors in positive displacement type. Reciprocating compressors play a major role in the chemical, petrochemical, Oil & Gas, and general industrial application.
In this article, we will learn about the basic component of the reciprocating compressor.
Cylinders
The cylinder is a pressure-retaining part of the reciprocating compressor. The cylinder accommodates both suction and discharge valves. For smaller capacity reciprocating compressor (up to 100kW), the cylinder is directly connected to compressor frame. For larger capacity reciprocating compressor (above 100kW), the cylinder is connected to compressor frame through distance piece.
Normally most of the cylinders used in the process industries 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 out or metal scoring.
For the larger capacity compressor, cylinders are provided with replaceable linear. In smaller capacity compressors, particularly for single stage models the replaceable liner is not economical and may not be available.
The cylinders are equipped for the cooling arrangement to reduce the temperature rise 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 multistage compressors, the clearance pocket is used in the first stage cylinder.
Piston
The piston is the heart of the reciprocating compressor. The piston transfers 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 piston rings are made of a material, that having self-lubricating property to reduce the slide friction force between the cylinder and the piston.
To avoid the wear in the cylinder wall and an increase in 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. Piston rings are 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 manufacturers preferred non-metallic materials because of less weight and lower frictional force when compared with metallic rings.
For a horizontally mounted cylinder, along with piston rings, an additional ring called wear band or rider ring is used to lower the wear between cylinder and piston. 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 a required. Rider band life can be extended by rotating a piston 120° to 180° at each overhaul.
Normally the piston is made up of less weight materials such as aluminium and aluminium alloys, this is to reduce the reciprocating compressor Shaking forces and Rod load. For larger diameter pistons, hollow type pistons are 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. MOC of piston rod is alloy steel and must have a hardened and polished surface particularly where it passes through the Cylinder packing (double-acting cylinders).
(To read about mechanical properties for Materials, Click Here)
Rod loading must be kept within the limits set by the compressor manufacturer because overloading can cause excess runout of the rod resulting in premature packing wear. This in turn leads to leakage, reduced efficiency, and increased maintenance expense.
Distance Piece
The distance piece is a separable housing that connects the cylinder to the frame. The distance piece may be open or closed type and may have multiple compartments. Distance piece may be furnished as single, double, or extra-long type.
The purpose of longer distance piece is to prevent the lubricant entering from crankcase to cylinder. Thereby preventing the gas contamination due to lubrication entering in the cylinder side.
In the case of lubricated cylinders, the distance piece prevents also keep a synthetic lubricant in a cylinder from being corrupted by the crankcase lubricant.
Compartments in the distance piece collect and control packing leakage when the gas is toxic or flammable. The leakage can be direct 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 three types
- Type A
- Type B
- Type C
- Type D
(In order to shorten this article, the distance piece types are not elaborated in this article. To read more click here).
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 rod packing cooling needed, the stuffing box is jacketed for liquid coolant.
The packing rings are the important element of rod packing assembly. The below-listed packing rings are widely used in a reciprocating compressor.
- 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. If wear occurs in the tangent ring, the garter spring will tighten the ring segments to compensate for the wear.
Radial Ring or Pressure Breaker Ring
The radial ring or pressure breaker ring is cut into three segments so that each cut lies on the side of a radial direction of the ring. The cuts in the ring are used to break down the pressure act on the ring. The radial ring is the first ring of road packing to face the cylinder pressure.
Backup Ring
The construction of the backup ring 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.
Crank Shaft
Larger compressors, normally above 150 kW(200 hp), the crankshaft is made up of forged steel. Compressor less than or equal to 150kW, the crankshaft is made up of ductile iron.
The crankshaft is connected to drive, to transfer the rotary motion.
When force lubrication is used, drilled oil passage is provided in the crankshaft. The Figure shows a drilled crankshaft.
Crankshafts should have removable balance weights to compensate the rotary unbalance as well as reciprocating unbalances. The crankshaft should be dynamically balanced when above 800 rpm.
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 crosshead pin should have a replaceable bush bearing.
Connecting Rod
Purpose of the connecting rod is 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.
Like Crankshafts, the connecting rod has drilled hole for oil passage.
The connecting rod is made up of forged steel for 150 kW (200 hp) reciprocating compressor. For compressor less than or equal to 150kW, ductile iron material is used.
Cross Head
All industrial 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 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
Reciprocating compressors generally use one of three types of valve configurations:
- Poppet Type Valve
- Plate Type Valve
- Ring Type Valve
Poppet Valve
Poppet Valves are not tolerant of uneven flow distribution. The poppet valve is used in a medium pressure range compressor. 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 type valve contains concentric rings of valve plates. The concentric ring valve plates are a single element. Since the valve plate is made of a single element, flow control is much easier when compared to other types of valves.
The stress act on the valve plate is much higher than the Poppet valve. These type of valves are widely used compressor handling of Oil-free gases.
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 centre line of the valve (see Fig ). These valves have the advantage of a low-stress level due to the lack of stress concentration points. It is difficult to maintain uniform flow control across the ring-type valve.
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 compressor. 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 the small compressor(up to 100kW). This type of system lubricating oil by the splashing of crank-throw moving through the lubricant in the sump.
I hope this article is much useful to understand the Reciprocating Compressor Basics Components.
Happy Reading….Cheers…!!!