SYED ASGHAR ABBAS NAQVI
Reciprocating CompressorIn a reciprocating compressor, a volume of gas is drawn into a cylinder, it is trapped, and compressed by piston and then discharged into the discharge line. The cylinder valves control the flow of gas through the cylinder; these valves act as check valves. There are two types of reciprocating compressor.
Reciprocating CompressorTypesSingle Acting compressor
It is a compressor that has one discharge per revolution of crankshaft. Double Acting Compressor
It is a compressor that completes two discharge strokes per revolutions of crankshaft. Most heavy-duty compressors are double acting..
Reciprocating CompressorDifferent parts of double acting compressor are listed below.
Suction valve. Suction gas jacket. Piston. Cylinder.Discharge valve.Discharge gas jacket
Construction of Reciprocating Compressors Reciprocating compressors can be divided into two main groups.
Gas end.Power end.
Gas End GAS END
Parts of the reciprocating compressor that handle process gas are grouped in this category.
Cylinder & Liner Piston reciprocates inside a cylinder. To provide for reduced reconditioning cost, the cylinder may be fitted with a liner or sleeve. A cylinder or liner usually wears at the points where the piston rings rub against it. Because of the weight of the piston, wear is usually greater at the bottom of a horizontal cylinder. A cylinder liner is usually counter bored near the ends of the outer ring travel i.e. counter bores are made just ahead of the points where the end piston rings stop and reverse direction. Shoulders may form in the liner where the rings travel stops unless counter bores are provided.
PistonFor low speed compressors (upto 330 rpm) and medium speed compressors (330-600 rpm), pistons are usually made of cast iron.Upto 7 diameter cast iron pistons are made of solids. Those of more than 7 diameters are usually hollow (to reduce cost). Carbon pistons are sometimes used for compressing oxygen and other gases that must be kept free of lubricant.
As the compressor reaches operating temperature, the piston and rod expand more than liner/cylinder does. In order to prevent seizure adequate clearance should be provided, at the same time clearance must be close enough to permit adequate support of piston rings. Similarly end clearance is also important.A cold piston is usually installed with one third of its end clearance on the crank end and two third of its end clearance on the head end.
PISTON ROD AND PISTONGenerally, the piston rod is fastened to the piston by means of special nut that is prevented from unscrewing. The surface of the rod has suitable degree of finish designed to minimize wear on the sealing areas as much as possible. The piston is provided with grooves for piston rings and rider rings.
PISTON ROD AND PISTON
Piston rod packing ensures sealing of the compressed gas. The piston rod packing consists of series of cups each containing several seal rings side by side. The rings are built of three sectors, held together by a spring installed in the groove running around the outside of the ring. The entire set of cups is held in place by stud bolts. Inside channels are there for cooling, gas recovery and lubrication of the piston rod packing.
Piston rod packing
Piston rod packing
Piston rod packing
Piston rod packing
Piston rod packing
Oil SealAn arrangement of scraper rings serves to keep the oil, entrained by piston rod, from leaking out of the crankcase. The oil scraped is returned to the crankcase reservoir.
An arrangement of scraper rings serves to keep the oil, entrained by piston rod, from leaking out of the crankcase. The oil scraped is returned to the crankcase reservoir.
Piston RingsPiston rings provide a seal that prevents or minimizes leakage through piston and liner. Metal piston rings are made either in one piece, with a gap or in several segments. Gaps in the rings allow them to move out or expand as the compressor reaches operating temperature. Rings of heavy piston are sometimes given bronze, Babbitt or Teflon expanders or riders. Lubrication is a must for metallic rings. Teflon rings with Teflon rider bands are sometimes used to support the piston when the gas do not permit use of a lubricant.
HeadThe ends of cylinder are equipped with removable heads, these heads may contain water/liquid jacket for cooling. One end is called head-end head and other crank-end head. The crank-end contains packing (a set of metallic packing rings) to prevent gas leakage around piston rod.ValvesThere are normally three types of valves, these are
Plate valve.Channel valve.Poppet valve.
Plate ValveHeavy-duty compressors use a form of plate valve. The part that closes against a valve seat is a flat metal plate. The plates are held tightly against the seat by a set of springs.
Channel ValveThese valves use channel shaped plates instead of flat plates. Above each channel is a bowed, steel tension spring. Spring pushes from the stop plate and channels cover the slots in the valve seat.
Poppet Valve These are shaped like the valves in an automobile engine. These are separate, round poppets to seat against holes in the valve seats. Poppets are made of low friction material (e.g. Bakelite), they provide a low-pressure drop and are often used when ratios of compression are low and also for high flow rates.
ValvesThe valves are of automatic type. They open due to the effect of the differential pressure between cylinder and the suction or discharge chambers, and close due to the force of springs acting on the plates plus the differential pressure across the valves.
Major Components of ValvesSeat
The main body of valve that withstands the differential pressure when the valve is closed
Major Components of Valves contd.Counter seat
The valve component that limits the lift and contains the springsRings
The element that withstands the differential pressure, ensuring gas sealSpring
The element that acts on the rings, causing valve closingShim
The element that determines the valve lift.Stud bolt, nut & pin
These elements fasten all the components together.
Power End Parts of reciprocating compressor that assist in transferring power and converting rotary motion into reciprocating motion are grouped in this category.
Crank Case Crank case supports the crankshaft. All bearing supports are bored under setup condition to ensure perfect alignment. Crankcase is provided with easy removable covers on the top for inspection and maintenance. The bottom of the crankcase serves as the oil reservoir. A main pump for lubrication of the crank mechanism is placed on the shield mounted on the side opposite the coupling and is driven by compressor.
Main BearingsThe main bearings are built in two halves, made of steel, with inner coating of antifriction metal.
The crankshaft is built in a single piece. On the inside of the shaft are holes for passage and distribution of lube oil. Crankshaft
Connecting RodsThe connecting rod has two bearings. The big end bearing is built in two halves. It is made of metal with inner coating of antifriction metal. The connecting rod small end bearing is build of steel, with inner coating of antifriction metal. A hole runs through the connecting rod for its entire length, to allow passage of oil from the big end to the small end bush.
CrossheadCrosshead fastens piston rod to the connecting rod. The sliding surfaces of crosshead are coated with antifriction metal i.e. babbited shoes. These are of interchangeable type. That permits it to slide back and forth within the crosshead guides. The shoes have channels for the distribution of lube oil. The lubrication is obtained under pressure; it comes out from the two guides of the crosshead slide body. Connection between connecting rod and crosshead is realized by means of a gudgeon pin. The piston rod is connected to the crosshead by nut arrangement, tightened by means of hydraulic device.
LubricationLubricants reduce friction and therefore wear between moving compressor parts. Lubricant also serves as a coolant.
Lubrication SystemsGenerally, two types of systems are uses to lubricate the positive displacement compressors. SPLASH SYSTEMIt is used in older machines. A supply is maintained in the crankcase. Oil is splashed up by the rotation of the crank and the counter weight into the collecting ring. Centrifugal force throws the oil outward through an oil passage to the crank pin.
Forced Feed SystemOil is pumped under pressure to the required parts. Following are the main parts of system:STRAINER
Oil from the crankcase first passes through a coarse strainer. The strainer is removable so that chips or broken off pieces can be cleaned out of the strainer.
Forced Feed System
Bypass Relief ValveThe compressor will be completely damaged if the oil flow is appreciably reduced or stopped by a plugged filter. In order to prevent this damage, forced feed system is fitted with a bypass relief valve, a low-pressure alarm and bypass shutdow