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8/6/2019 Report on Centrifugal Compressor
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE
PREPARED BY: MUHAMMAD YOUSAF
EXECUTIVE PRODUCTION (CO)
8/6/2019 Report on Centrifugal Compressor
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE
A five days course on “Centrifugal Compressor Operation and Maintenance” was conducted by General Electric (GE) Oil and Gas Company at “Florence Learning Center”, Italy (October 18 to 22, 2010). I was one of the participants among many professionals from different countries of the world having wide range of experience in the field of Centrifugal compressors. This course has given me the opportunity to enhance my knowledge & capability to understand compressor’s operational dynamics & maintenance related aspects. I therefore recommend this particular course to other individuals & especially with GE (Nuovo Pignone) which has vast experience in Centrifugal compressors manufacturing & maintenance. Some of the “Salient Points” have been covered in this report with pictorial display to serve better understanding & training purpose.
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE Surging Compressor surge is an unsatable phenomenon where the gasflows forward and backward across the machine alternatively.This phenomina occurs at certain conditions of flow, pressureratio and rotational speed. As a consequence some damagescan occur e.g.
• Shaft vibration. • Instability in axial thrust • Bearings seizure. • Seals damge.
Surging features Flow reverses in 20 to 50 milliseconds. Surge cycles at a rate of 0.3 second to 3 seconds per cycle.
Compressor vibrates. Temperature rise. “Whooshing” noise or “clanking noise”. Conventional instruments and human operators may fail to recognize surge.
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE Factors leading to surge
During normal operation;
• High discharge pressure (increase of plant
resistance).
• Reduction of suction pressure.
• Increase of suction temperature.
• Gas composition changes (decrease
molecular weight)
Start up. Shut down.
Anti surge system
Anti surge system avoids that the machine
operation falls on the surge line. Substantially the
protection consists in the opening of control valve
that joins the compressor discharge to suction. As a
consequence a part of flow outgoing from the
machine is re‐circulated.
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE This causes an increase in the suction flow and so theoperating point moves away from the critical condition.
What is Chocking?When the flow rate increases beyond a certain valueit may happen that in some duct is achieved thespeed of sound. This phenomenon determines thepressure drop due to friction and high incidence of the flow on the fix and mobile ducts. As a
consequence there is an enormous reduction inefficiency.
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE Dry gas seal systemThe aim of this system is to minimize the escape of the gas from acentrifugal compressor. It has replaced the traditional oil film sealwhich has many operational problems. Dry gas seal system shows high stability operation. It is self regulated in the way to keepconstant the gas leakage
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE What is a vibration?
Vibration is simply the motion of a machine or machine part back and forth from its position of rest. The cause of vibration must be a force which is changing in either its direction or its amount. It is the force which causes vibration, and the resulting characteristics will be determined by the manner in which the forces are generated.
Some definations
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE
The amplitude and frequency of the rotor vibrations are very important for a centrifugal compressor. They are the measure of the mechanical behavior of a compressor.
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE The rotor vibration amplitude must not cause
Contact between rotor and stator. Oil seals and dry gas seals overloading. Fatigue in the bearings.
Vibration classification
The typical vibrations of the centrifugal compressors can be generally classified with refrence to the nature of the vibration, cause and the frequency. According to the nature of vibration cause, the vibrations may be classified in:
Free Forced Self excited
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE Free vibration
Free vibrations are excited by an impulse or step force applied to
the rotor. These impulses may be due to the following causes;
Internal rubs. Loose rotor system components.
Surge. Slug of liquid.
The vibration can occur with one or several system natural
frequencies.
Forced vibration
o Forced vibrations take place when an external time dependent
force transfers energy to the rotor‐bearings system that reacts
vibrating at the exciting frequency. The most common sources
of excitation are
o rotor bow
o Unbalance in the rotor system
o Coupling misalignment
o Aerodynamic cross coupling forces
When the excitation frequency coincides with one of the rotor natural
frequency, resonance occurs.
Self excited vibration
Self excited vibrations are typical of a rotor‐bearing system
when the applied actions are related to rotor displacement and
velocity. Resulting forces have components which are
prependicular to shaft motion. Such forces are consequences
of the circumferential pressure variations in the annulus
chambers, variations determined by an eccentric shaft rotation
in the presence of fluids
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CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE Compressor parts where these phenomina may take place are
Journal bearings. Oil
seals
rings.
Gas labyrinth seals.
Problems of vibrationThe main problem that we have to consider when a rotor bearingsystem is subject to periodic forces is the phenomenon of resonance What is resonance? A system is in resonance when vibration displacement becomes
maximum. This occurs when the frequency of a periodic force(excitation frequency) applied to a rotor bearing support systemcorresponds to a natural frequency of that system How we can avoid this? A complex system like a centrifugal compressor rotor has many
natural frequencies that we call critical speeds the most important
are the first and second. It is necessary to determine the fieldoperation speeds that avoid the resonance. Generally a centrifugal compressor works between first and second critical speed.
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