1. Can a positive displacement compressor self-regulate it’s capacity?
No, a reciprocating compressor cannot self-regulate it’s capacity against a given
discharge pressure. As it’s characteristic is constant volume, it will continue to
displace gas until it receives a signal not to do so.
2. What are the functions of compressor control and protection systems in
dynamic compressors?
 Provide stable control and operation at all the operating conditions in
datasheet
 Provide protection against operation in surge area of the performance curve
3. What are the most commonly used head-capacity control methods for
centrifugal compressor?
 Speed Control
 Adjustable Inlet Guide Vanes
 Suction throttling
 Discharge throttling
 Blow-off
 Recycle
4. Centrifugal Compressor Curve for speed control
If the compressor is operating at the constant pressure point, Y (flow rate Qy),
and if the process requires a higher gas flow, the discharge pressure immediately
falls and the operating point moves to the right and downward along the
characteristic curve for the given speed. The pressure transmitter will sense the
lowering pressure, and the pressure controller will send a control signal to the
turbine governor. The governor will increase the speed of the compressor
(through an increase in turbine speed), which results in an increase in the system
pressure back to the pressure setpoint. The new operating point would be located
on the desired pressure line but further to the right.
If the process required less gas flow, the discharge pressure would begin to
increase and the control system would decrease the speed of the compressor
(through a decrease in turbine speed) until the pressure setpoint is restored. The
flow could be reduced until point X was reached.
Point X is set at the minimum operating point before the surge line (surge control
line). Antisurge controls, which are discussed later in this module, will prevent
the operating point from moving to the left of point X on each speed curve. If the
process required a flow rate of only point Z, the volume of gas (Qx - Qz) would
have to be blown off or recycled. The operating control would have to be shifted
from variable-speed control to blow-off control, which is the only control that is
available when the process requires flows that are below the stable operating
range.
5. Centrifugal Compressor Curve for inlet guide vane control
Adjustable guide vanes are used for the control of axial and single-stage
centrifugal compressors. Single-stage compressors frequently incorporate an
axial inlet, and they do not require fixed guide vanes.
Pre-rotation adjustable guide vanes pre-whirl the gas that enters the compressor
stage in the direction of rotation, which develops less head than at design.
Between full open and maximum pre-whirl position, adjustable guide vanes
provide some degree of reduced horsepower over the suction throttling valve.
Counter-rotation adjustable guide vanes are used to extend the useful operating
range of any dynamic compressor. The range of operation is extended through a
change of the angle of attack and the inlet gas velocity to the impeller blade. For
the high flow region, the angle of attack is increased to eliminate flow separation
and to effect an increase in the produced head of the impeller or blade. The
elimination of flow separation and an increase in the produced head will increase
the capacity range of the impeller.
Adjustable inlet guide vanes are expensive, limited in effectiveness, and present
many maintenance and operational problems. At Saudi Aramco, centrifugal
compressor adjustable inlet guide vanes have proven to be mechanically
unreliable in general services; therefore, prior to control selection, the economics
of inlet guide vanes must be considered because of their higher initial cost,
complex mechanism, maintenance, and requirement for frequent adjustment.
Adjustable inlet guide vanes should not be used on process centrifugal
compressors, and they should never be used in any sour gas service. Saudi
Aramco primarily uses adjustable inlet guide vanes for axial and single-stage
centrifugal air compressors.
6. When is suction throttling control used in centrifugal compressors?
When the compressor does not have inlet guide vanes and is driven by a constant
speed drive.