Basics of Centrifugal Compressors: A presentation

Compressors are intended to compress a substance in a gaseous state. Process compressors are used to compress a wide range of gases over a wide range of conditions.

Compressor Applications

  • Refineries
  • Natural Gas Compression & Transportation Services
  • Oil Exploration – Gas Reinjection, Gas lift, etc.
  • Gas Liquefaction
  • Air Compression service
  • Refrigerant compression service
  • Refueling applications

Basic type of compressors

  • Positive Displacement Compressors
  • Reciprocating compressor
  • Screw Compressors
  • Centrifugal compressors
  • Pipeline compressors

Centrifugal Compressors

  • Centrifugal compressor is a “dynamic” machine.
  • Has a continuous flow of fluid that receives energy from an integral shaft impeller.
  • Energy transformed into pressure – partly across the impellers and partly in the stator section called diffusers

Refer to the Cross section & parts (Fig. 1):

  • A. Outer casing
  • B. Stator parts called ‘Diaphragm bundle’
  • C. Rotor
  • D. Impellers
  • E. Balance drum
  • F. Thrust collar
  • G. Hub
  • H. Journal Bearing
  • I. Thrust Bearing
  • J. Labyrinth Seals
  • K. Oil film end seals
Cross section of a typical Centrifugal Compressor
Fig. 1: Cross section of a typical Centrifugal Compressor

Centrifugal Compressor types

Compressors with Horizontal Split casings (Fig. 2):-

Consisting of half casings joined along the horizontal centerline, Employed for operating pressure below 60 bar.

Centrifugal Compressor with Horizontal Split Casing
Fig. 2: Centrifugal Compressor with Horizontal Split Casing

Compressors with Vertical Split casing/Barrel Type (Fig. 3):-

Vertical split casings are formed by cylinder closed by two end covers; hence ‘barrel type’ used to refer these compressors, Employed for high-pressure services up to 685 bar.

Centrifugal Compressor with Vertical Split Casing
Fig. 3: Centrifugal Compressor with Vertical Split Casing

The Horizontal split casings & barrel compressors are further identified based on process stages, i.e.

  • Multistage compressors with one compression stage
  • Multistage compressors with two compression stages (Two compression stages set in same machine/barrel casing. Between the two stages cooling of the fluid is performed in order to increase the efficiency of compression)

Basic Terminologies of Centrifugal Compressor

Surge: Phenomenon of instability which takes place at low flow which involves the entire system including not only the compressor but also the group of components traversed by the fluid upstream & downstream of it. Surge is characterized by intense and rapid flow and pressure fluctuation throughout the system and is generally associated with stall involving one or more compressor stages. This phenomenon generally accompanied by strong noise and violent vibrations which can severely damage the machine involved

Stall: Stall in turbomachinery describes as a situation in which due to low flow values, the stage pressure ratio or head does not vary in a stable manner with the flow rate.

Surge prevention: Surge prevention is effected through experimental tests in which pressure pulsation at a low flow rate is measured on individual stages. On this basis, it is possible to identify the flow values at which the stable operation of the stage is guaranteed.

Capacity Control

Capacity Control is used for the following:

  • Process flow control
  • To optimize fuel/power efficiency
  • Pressure regulation

Capacity can be changed several ways; below are some of them:

  • Speed Regulation
  • Control of Supply gas to the machine
  • Bypassing the discharge flow back to the suction side of the machine

Type of Compressor Drives

Following are the various types of Compressor drives:

  • Electric Motor Drives
  • AC Squirrel cage Induction Motor
  • Synchronous AC Motor
  • Gas Turbines
  • Steam turbines
  • Variable Speed drives
  • Variable Frequency Drive
  • Variable speed (Hydraulic Coupling) drives

Typical Centrifugal Compressor Curve (Fig. 4)

Typical Centrifugal Compressor Curve
Fig. 4: Typical Centrifugal Compressor Curve

Compressor Sealing System

Selection of sealing system is critical for

  • Satisfactory performance
  • Reliability

The type of compressor & method of lubrication used will decide the type of sealing Technology. Sealing System can be divided into two classes based on the type lubrication:

Contacting Sealing System

  • Liquid Lubricated
  • Gas Lubricated

Non – Contacting Sealing System

  • Liquid Lubricated
  • Gas Lubricated

Following are the utilities required for the Compressor:

  • External Fuel gas for seal gas system
  • Instrument Air for the Instruments/Control system/Seal gas system

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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