Centrifugal Compressor fundamentals

1. 1
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.

2. 2
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

3. 3

4. 4
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.

5. 5
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

6. 6
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.
*********************************************