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Principle of a Centrifugal compressor

Author

Prem Baboo

Sr. Manager (Prod)

National fertilizers Ltd. India,

FIE, Institution of Engineers India,

Technical Advisor & an expert for

www.ureaknowhow.com

1. Basically a simple Dynamic machine.

2. Gas travels essentially radially.

3. Rotor impellers impart Kinetic Energy to gas entering

thru the suction volute.

4. Inlet Guide vanes direct the gas to the Eye of the

impellers.

5. Gas discharged from the periphery of impeller at high

velocity, flow thru the annular diverging passage

called diffusers between inlet wall & diaphragm

converting velocity head into pressure head.

DIAPHRAGMS

These are Separations between stages.

They Form –

1. An open diffuser passage for Gas from Impeller outlet.

2. The return bends.

3. Return Passages to distribute gas to next stage guide

vane & Impeller.

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INLET GUIDE VANES

These direct the flow of the Gas from the return

channel of the diaphragm evenly into the eye of the

next higher stage impeller

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FUNDAMENTAL FORMULAE PV = ZRT

PVk = CONSTN Adiabatic compression PVn = CONST Polytrophic Compression

ϓ= (k-1/k) / (n-1/n) Polytrophic Efficiency Td= {(Pd/Ps)(n-1/n) Ts} = {(Pd/Ps)[(k-1) / (k X ϓ)]Ts}

H = (ZRTs/MW) (n/n-1) {(Pd/Ps)(n-1/n) –1}

EFFECT OF PARAMETERS H = (ZRTs/MW) (n/n-1) {(Pd/Ps)(n-1/n) –1}

lowerTs means Pd/Ps should increase

i.e Pd increase &/or Ps decrease

lower MW meansPd/Ps should decrease

i.e Pd decrease &/or Ps increase

lower Pd means Pd/Ps should decrease

i.e Ps increase

Similarly higher Pd means Pd/Ps should increase i.e Ps increase.

Critical Speed “The speed of the shaft at which frequencies of stationary parts

(casing) and rotating parts (shaft and impeller) become equal, a

sudden heavy vibration is experienced which can cause damage to

both. The speed at which this phenomenon occurs is called critical

speed.”

Surging

“When the head in the system increases beyond the maximum head developed by the compressor it will no longer be able to deliver gases

to the system. At this stage gases will try to flow back into impeller

whereas the impeller tries to blow the gases out. This will results in

unsteady state of system and cause heavy vibration. Sometimes

resulting in damage to compressor.”

The anti-surge controller system consists of a controller which

monitors the relative distance d = (1-S) between the operating point

of compressor and its Surge control line and provide:-

1. Proportional & Integral response to prevent this distance d from

decreasing below the set point.

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2. Increasing the margin of safety when the operating point rapidly

approaches its surge control line & slow decreasing when

operating point moves away from the surge control line.

3. Recycle Trip open loop controller response which prevents

operating point of the compressor from crossing the surge limit

line.

4. Safety On response which adds extra static margin of safety to

surge control line thus reducing the possibility of future cycles

and counts the actual number of surge cycle the compressor has

experienced since the controller was rest.

5. Limiting the discharge pressure of final stage.

Communication with DCS & service computer

It is open loop control response which prevents compressor

operating point crossing surge limit line (SLL ). When ‘RT’ appears

controller jumps to auto & will open 10 % wait for ‘1 sec’ if flow is

established it will stop otherwise it will continue to open the ASV in

steps of 10 % at time interval of 1 sec.

It is open loop control response which adds. Extra safety to surge

control line and hence reducing possibility of future surge cycle and

also counts Actual no. of surge cycles from last manual reset. When

‘SO’ appears one surge cycle is completed and controller jumps into

auto and opens ASV fully to avoid further surge cycles immediately.

d = 1-S

It is relative distance between operating point of compressor and

surge control line (SCL). Negative deviation (DEV) means recycle flow

rate needs to be increased and Positive deviation (DEV) means recycle

flow can be decreased.

Zero deviation (DEV-0) means operating point of compressor is on

surge control line (SCL). Each controller receives four analog inputs

1. Diff. Pressure across flow orifice of 3rd Suction FE – 62.

2. Pd-4- Discharge pressure of 4th stage PT-62

3. Ps-3- Suction pressure of 3rd stage PT-68

4. N -Speed of compressor.

Two inputs are required for indication only

1. Discharge temperature of 4th stage

2. Suction temperature of 3rd stage.

OBJECTIVE OF ANTISURGE CONTROLLER

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To prevent surge induced compressor damage & process upsets

without sacrificing energy efficiency.

To maintain selected process limiting variable within safe range.

To achieve these objective the CCC controller position the recycle

valve in such a way to maintain the volumetric flow above a safe level

METHODS OF PREVENTING SURGE

1. INCREASE PROCESS FLOW TO SUCTION.

2. RECIRCULATE BY OPENING ANTISURGE VALVE.

3. DECREASE PRESSURE RATIO.

4. BY INCREASING SUCTION PRESSURE.

5. OR BY REDUCING DISCHARGE PRESSURE

Chocking

The choke point is the maximum rate of flow that a compressor cans

give at a given speed. At this point the machine can no longer assure a

discharge head. This is not a dangerous phenomenon. This is cause by

the gas exceeding the speed of sound.

Speed of Sound

When the maximum velocity reaches the speed of sound shocks is

produced that limits the flow and in order to increase flow, it is

necessary to increase the speed.

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