Report on Centrifugal Compressor

8/6/2019 Report on Centrifugal Compressor

http://slidepdf.com/reader/full/report-on-centrifugal-compressor 1/21

CCCCCGYYYYHHHJJUUJJJJJJJJJJJJ ̀

CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE

PREPARED BY: MUHAMMAD YOUSAF

EXECUTIVE PRODUCTION (CO)





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.

Page#1/20





Page#2/20





Page#3/20





Page # 4/20





Page # 5/20





Page # 6/20





Page # 7/20




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.

Page # 8/20




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.

Page # 9/20




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.

Page # 10/20




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

Page # 11 Page #

11





Page #

12

Page

#

12/20




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

Page #

13/20





Page #

14/20





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.

Page #

15/20

y




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

Page #

16/20




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

Page #

17/20




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.

Page #

18/20





Page #

19/20





T

H

E E

Report on Centrifugal Compressor

Documents