operation of compressors

8/4/2019 Operation of Compressors

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4/14/2012 1

Operation of Compressors

Centrifugal


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4/14/2012 Prepared by : M.Ghosh 2

Theory of Compression

Applicable for gases and vapors as theyare compressible.

Compression is a path function.A polytropic process is the real process.

Adiabatic and isothermal are the two

ideal processes.Work is done on gas to achieve adesired degree of compression.


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Volume of gas V1

P1

P2

Isothermal

Adiabatic

Isothermal

PV = Constant

PdV + VdP = 0

P/V = - P/VAdiabatic

PV = Constant

PV-1V + VP = 0

P/V = - P/VPolytropic

P/V = - kP/V

Where k >

Polytropic

Compression Processes


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Energy requirements for compression

Work done = W = PV . With limit conditions of states 1 to 2.Isothermal processWork done = ZnRT Ln( V2/V1)

Adiabatic Process

Work done = ZnRT1[ (P2/P1)-1/ - 1]/(-1)Polytropic ProcessWork done = ZnRT1k[ (P2/P1)

k-1/k - 1]/(k-1)

Where k > Polytropic > Adiabatic > Isothermal


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P

V

1

3

2

12 3

A real process is always polytropic

Compressor calculations are mainlybased on adiabatic process

Isothermal operation is a target

State 3 is by inter stage cooling

Leads to saving of energyApproaching isothermal conditions.

Compressor Intercoolers


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Understanding Characteristic Curves

Volumetric Flow

DevelopedHead

0

Characteristic Curve

System Resistance Curve


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Process

Steam

Compressor

Intercoolers

Turbine

Compressor Schematic

Final Discharge

Compressor

Gear Box


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Centrifugal Compressor Operation

Principle

Conversion of Kinetic energy to Pressure Energy.Application of Bernoullis Theorem.

Volumetric Flow

Increase in rpm/throttle


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Compressor Controls SchematicPerformance Controller

Anti Surge Controller

Flow

Pressure


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Centrifugal Compressor Operation

Anti Surge Control

Sole aim is to ensure minimum flow through the machine

Volumetric Flow

Increase in rpm/throttle


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Factors affecting operation

Suction temperature

Suction humidity

Suction Pressure

Molecular weightCasing and rotor condition

Silencer condition

FrequencySpeed

Discharge destination conditions


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Base

Favourable

Unfavourable

Machine Surge Line

Volumetric Flow

DevelopedH

ead

P

Q

Compressor Performance EstimatesFavourable Unfavourable

Suction Temperature Low High

Suction Pressure High Low

Suction Humidity Low High

Suction pressure drop Low High

Molecular weight High Low

Motor frequency High Low

System resistance Low High


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Compressor Sealing : Labyrinths


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P1 P2

P2 > P1

P1 P2

P2 > P1

P1

Rotor

Impeller

Casing

Balance

Drum

Compressor Operation Dynamics


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P1 P2

P2 > P1

P1

Ft Fr Fb

Ft

Fb

= Fr

Thrust Collar

Thrust Pads



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Ft Fr Fb

Thrust Collar

Thrust Pads


Oil Film

Active or (+)Face

Passive or (-) Face

Lower Temperature zone

Higher Temperature zone


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Compressor SchematicPerformance Controller

Anti Surge Controller

Flow

Pressure


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Vibration monitoring in Compressors

Y = A Sin (t).DisplacementY/t = A Cos (t)..Velocity2Y/2t = - A2 Sin(t) = - A2Y.AccelnYA

Vibration ProbesVelocity or Acceleration probes

Normally two probes will be used

Journal bearings are monitored on Velocity measurements

Thrust bearings are monitored on Displacement measurements

H

V

Vibration measurements give us the following

Shape of the rotor under dynamic conditions

Loading on bearing and portions of the shaft

Used for preventive maintenance of bearing


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Oil system schematic for compressors

LO Hdr

CO Hdr

AccumulatorHead Tank

Main

Tank

Self regulated PCV

Self regulated PCV

N2

LO HX

FiltersPumps

Return

Hdrs


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Oil System in Compressors

Oil could be considered as blood of the machineOil is classified into two main categories

1. Lube oil for lubrication2. Control oil for controls.Lube oil System

1. Provides oil to all bearings for lubrication

2. Has dedicated pumps for the job.

3. Oil temperature , pressures at bearings,quality are all indicativ

symptoms for maintenance

4. Head tank ensures uninterrupted supply during coasting down

in the event of power failure

Control Oil System1. Mainly used for operation of the driver

2. Mainly used for turbines

3. Is equipped with individual arrangements


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Trips /interlocks/AlarmsS

NoTrip/Interlock/Alarm Purpose

1 High discharge temperature Save rotor

2 High vibrations(Alarm and trip) Health of machine

3 Lube oil low pressure Bearings/Rotor

4 High speed (Instrument) Machine Capacity

5 Overspeed (Trip Bolt) Machine capacity

6 Driver interlocks

7 Bearing temperature Alarms

8 Low suction pressure Filtration

9 Low seal fluid flow Prevent leakage

10 Process interlocks


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4/14/2012 22

ReciprocatingCompressors


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Principle of operation

A fixed volume of gas is compressed byapplication of work.

The discharge is not continuous but ispulsative.

The characteristic curve is very steep

Compression ratio per stage is

restricted to 4 for high capacity.

It could go to 8 for low capacities.


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Reciprocating Compressor Schematic

Suction Dampener

Discharge Dampener

Aftercooler

Intercooler

1st Stage2nd Stage

Spillback

Spillback

CW


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Trips /interlocks/Alarms

SNo

Trip/Interlock/Alarm Purpose

1 High discharge temperature(stagewise) Save rotor

2 Low suction pressure Volume goes up

3 Lube oil pressure Bearings/Rotor

4 Process interlocks


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Operational problems

Loss of suction pressure

Poor intercooler performances

High pulsationDemand supply deficit

Safety systems becoming

restriction to operation of themachine.

Passing of NRVs.

operation of compressors

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