Reciprocating Compressors: A Brief Presentation

Reciprocating Compressors: A Brief Presentation

In 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. Fig. 1 shows the classification of Compressors.

Classification of Compressors

Fig. 1: Classification of Compressors

Reciprocating Compressor Types:

  • Single – 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.

Fig. 2 Shows typical configuration of Single and double acting reciprocating compressor.

Single and Double acting Reciprocating Compressor
Fig.2: Single and Double acting Reciprocating Compressor

Construction of Reciprocating Compressors:

Construction of Reciprocating compressors can be divided into two main area:

  • Gas end.
  • Power end.

GAS END:

Parts of the reciprocating compressor (Fig. 3)

  • Cylinder
  • Head
  • Piston & piston rod.
  • Suction valves.
  • Discharge valves.
  • Piston rod Packing
  • Suction and discharge gas jacket
Parts of reciprocating compressor
Fig. 3: Parts of reciprocating compressor

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.

Head:

The 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.

Piston:

  • Piston moves forward and backward to suck and compress the gas. It pushes the gas in discharge pipe during compression stroke.
  • For 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 solid bars. 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.

Clearance in Piston and Cylinder:

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. 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 Rings (Fig. 4):

Piston rings provide a seal that prevents or minimizes leakage through piston and cylinder 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.

Typical configuration of piston rings
Fig. 4: Typical configuration of piston rings

PISTON ROD AND PISTON (Fig. 5):

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.

Typical configuration of Piston Rod and Piston
Fig. 5: Typical configuration of Piston Rod and Piston

Piston rod packing:

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 multiple 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.

Oil Seal:

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.

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.

Valves:

Valves (Suction and Discharge valves) : allow gas to enter into the piston during suction stroke and allow gas to go out into discharge line during compression stroke.

There are normally three types of valves, these are

  • Plate valve.
  • Channel valve.
  • Poppet valve.

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 (Fig. 6):

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.

Typical configuration of Crank Case
Fig. 6: Typical configuration of Crank Case

Crankshaft:

Crankshaft receives the power from Driver and transfer it to piston. The crankshaft is built in a single piece. On the inside of the shaft are holes for passage and distribution of lube oil.

Main Bearings:

The main bearings are built in two halves, made of steel, with inner coating of  antifriction metal.

Connecting Rods (Fig. 7):

Connecting rod connect crank shaft with the piston rod.

The 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 it’s entire length, to allow passage of oil from the big end to the small end bush.

Typical configuration of Connecting Rod
Fig. 7: Typical configuration of Connecting Rod

Crosshead:

Crosshead fastens piston rod to the connecting rod. The sliding surfaces of crosshead are coated with antifriction metal like babbit shoe. 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.

Distance piece:

Distance piece is used to separate the Gas end and Power End of Reciprocating compressor.

API 618 defines 4 types of distance piece which can be used based on criticality of service.

  1. Type A – Short single compartment (Where oil carryover to piston packing is acceptable)
  2. Type B – Long Single Compartment (Where oil carryover to piston packing is not acceptable)
  3. Type C – Long-long two compartment (For critical service like Oxygen and Hydrogen)
  4. Type D – Long-short two compartment (For Process gas services)

Distance piece are provided with drain and vent arrangement and if required continuously purge with buffer gas.

Pulsation Dampeners /Bottles:

Pulsation bottles provided at suction and discharge to the reciprocating compressor, to keep the pulsation within desired limit.

Pulsation study carried out to decide the minimum volume of pulsation bottles.

Lubrication (Fig. 8):

Lubricants reduce friction and therefore wear between moving compressor parts. Lubricant also serves as a coolant.

Typical Lubrication System
Fig. 8: Typical Lubrication System

Generally, two types of systems are uses to lubricate the positive displacement compressors.

  • SPLASH SYSTEM
  • FORCED FEED LUBRICATION

SPLASH SYSTEM:

It is used in older machines. Level 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 LUBRICATION:

A pump is used to feed the oil. Oil is pumped under pressure to the required parts. Following are the main parts of system

Reciprocating Compressor Capacity control Method:

  • By Recirculation
  • By VSD
  • By Valve Un-loader
  • By Volume clearance pocket
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1 thought on “Reciprocating Compressors: A Brief Presentation”

  1. Thankyou sir for wanderfull article with details.
    If possible please also include maintenance requirement/procedure whcich would be very helpful for us as piping layout engineers
    Thanks

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