Surge occurs when the pressure behind a centrifugal compressor becomes higher than the outlet pressure, causing fluid to reverse flow into the compressor. This unstable phenomenon repeats in cycles around 1-2 Hz. To prevent surge, anti-surge control systems detect when the operating point approaches the surge line and open a recycle valve to increase inlet flow and move the operating point away from the surge line. Proportional-integral and proportional-integral-derivative algorithms are commonly used to control the recycle valve based on differences between the process variable and set point.

1. Surge is defined as the operating point at which centrifugal compressor peak head capability
and minimum flow limits are reached. Actually, the working principle of a centrifugal
compressor is increasing the kinetic energy of the fluid with a rotating impeller. The fluid is then
slowed down in a volume called the plenum, where the kinetic energy is converted into
potential energy in form of a pressure rise.
When the plenum pressure behind the compressor is higher than the compressor outlet
pressure, the fluid tends to reverse or even flow back in the compressor. As a consequence, the
plenum pressure will decrease, inlet pressure will increase and the flow reverses again. This
phenomenon, called surge, repeats and occurs in cycles with frequencies varying from 1 to 2
Hz. So, the compressor loses the ability to maintain the peak head when surge occurs and the
entire system becomes unstable. A collection of surge points during varying compressor speed
or varying inlet gas angle is fitted as surge line. In normal conditions, the compressor operates
in the right side of the surge line. However, during startup/emergency shutdown, the operating
point will move towards the surge line because flow is reduced. If conditions are such that the
operating point approaches the surge line, flow recirculation occurs in the impeller and diffuser
(Figure 1). The flow separation will eventually cause a decrease in the discharge pressure, and
flow from suction to discharge will resume. Surging can cause the compressor to overheat to
the point at which the maximum allowable temperature of the unit is exceeded. Also, surging

2. can cause damage to the thrust bearing due to the rotor shifting back and forth from the active
to the inactive side. This is defined as the surge cycle of the compressor.
Anti-Surge Control Systems
These systems detect when a process compression stage is approaching to surge and
subsequently take action to reverse the movement of the operating point towards the surge
line (SL). This decreases the plenum pressure and increases the flow through the compressor,
resulting in stable working conditions. It is normally achieved by opening a control valve in a
recycle line (Anti-Surge Control Valve or ASCV), returning the discharge gas to the inlet of the
compressor via a suction cooler. The resulting increase in compressor inlet volume flow moves
the operating point away from surge.
Due to inaccuracies in measurements and response times of transmitters and valves, Anti-surge
control achieves a surge control line (SCL) parallel to the surge limit line. The control line is
offset to the right of the surge line by a margin; typically equal to 3- 10% of inlet volume flow at
surge (Figure 2). However, a lower margin is also desirable because higher efficiency could be
obtained by closing the recycle valve.
Figure 2: Surge Protection Definitions.
In real operation, compressor performance curves in the coordinate system are unique for
constant given suction conditions. This means that the variation of inlet conditions leads to
changing compressor performance curves. On the other hand, since inlet temperature
decreases, inlet molecular weight increases or inlet pressure increases, the differential pressure
across measuring device would go up for the same inlet capacity. So for the purposes of
control, new coordinate system is used which must be invariant (or independent) to changes in
inlet conditions. Therefore, the effects of inlet temperature, gas molecular weight and
compressibility factor are not required to be considered in the controlling system. In this
regard, several invariant coordinates are obtained by compressor manufacturer. They use
dimensional analysis for the generation of these invariant systems (or compressor map). The
two most important systems are:

3. 1) Compressor polytropic head (or differential pressure) versus square of flow rate in suction
and
2) Pressure ratio versus square of flow rate in suction.
Flow through the compressor suction is equivalent to the pressure drop in orifice or venturi
installed at the inlet or outlet of compressor. Thus, pressure loss in orifice or venturi can be
calibrated as a function of compressor flow rate.
A compressor map is illustrated by superimposing both performance and system resistance
curves independent of rotational speed (Figure 3). For compressors with inlet guide vanes,
compressor map is represented by a new family of curves that do not depend on suction
conditions either. This additional coordinate could be a function of either guide vane position or
equivalent rotational speed.
Figure 3: Compressor map.
Anti-Surge Controller System And Algorithms
Proportional–integral (PI) and proportional–integral–derivative (PID) are two major control
algorithms which are used to control imperfectly known compression systems. The basic
procedure of these algorithms is that the controller output should be a function of the
difference (Error, e) between two values which should be controlled (process variable, PV) and
its set point (SP) (Figure 4).
When operating in the stable region at the right-hand side of the SP-line, where the error, e, is
positive, the controller output is forced to be zero and integrators should be reset to avoid
wind-up. As the flow decreases due to a disturbance, PV decreases as well and — at a certain
point — where PV< SP, the error, e, becomes negative. Here, the controller comes into action
opening the anti-surge valve. This action pushes the PV back to the stable region at the right-
hand side of the SP-line. Moreover, small disturbances should not lead to big reactions. But a
fast and resolute opening of the valve is required when the control line is exceeded in the
direction of the surge limit. Therefore, the controller has a nonlinear gain behavior when the

4. controller deviation (PV-SP) is negative.
Figure 4: Compressor controller schematic. Earlier matter about nonlinear gain controller leads
to considering derivation term in logic control of system. Actually, effect of the derivative (D-
action) term is that it often allows the control response to be accelerated without increasing
the risk of instability, because it is a measure how fast the system is responding and action will
tend to counter the oscillatory action. However, it will also make the system more sensitive to
signal noise. Thus, the simpler PI algorithm is sometimes more useful than full PID control. But
it (PI) is limited in speed of its response and is unable to take the machine out of surge in the
event that the operating point crosses the surge line. In other words, the D-action should be
incorporated by changing the set point due to a change in flow (dF/dt). When the flow
decreases rapidly (dF/dt << 0), the danger of going to surge exists in compressor. Therefore, the
original set point (SP) is increased with a higher margin. As a consequence, the PV-line passes
the SP-line earlier. The controller will react earlier and have a higher output, resulting in an
increased control effort. It is should be noted that for the smaller PV/SP, D-action is more
important because, it will cause a higher value of the weighing factor of this margin. So, the
effect of the D-action due to a decrease in flow is the increase in set point, resulting in a
negative error earlier than without the D-action. Logic controller without derivation and with
just the proportional plus-integral control system is adequate for many changes in plant
operating conditions. For activation of this method, second control line is located between the
proportional plus-integral control and surge lines. When this control line is crossed, there is a
step increase in the output from the controller that causes the recycle valve to open. This kind

5. of valve opening is called valve jumping (Figure
5).
Figure 5: PI algorithm with step line. The controller output decays exponentially with time to a
point where the proportional plus-integral control system resumes control. Moreover, the
momentary valve position calculated by the PI algorithm is overridden by an adjustable additive
component. Anti-Surge Input Requirements
As earlier mentioned, flow rate is the main data obtained from suction or discharge. Moreover,
pressure and temperature in suction and discharge are needed to establish the operating point
on the compressor performance curve. Recommendations for the minimum input requirements
for a given system arrangement can be found in Figure 6.
Figure 6: Compressor Surge Control System Layout.
Additionally, it should be noted that a molecular weight change of 50% or a variation of 10 or
more is considered a significant change in the mixture gas composition. In such conditions,
compressor map and relevant surge line is individually generated for each gas component and
the magnitude and direction of each is compared with the other. Obviously, the finalized surge
line shall cover and include all unstable regions, obtained by the performance maps.

6. The response time of a transmitter for the measurement of a process variable used by the
surge control algorithm should be less than 100 milliseconds. Moreover, the emergency
shutdown situation will require the fastest response time from the control system. The
shutdown operation does not affect the surge control algorithm because the sole function of
the controller under the shutdown operation is to fully open the control valve as quickly as
possible. In such condition, the recycle control valves should be able to move from fully closed
to fully open in less than 2.0 seconds.
Anti-Surge System layout
Figure 6 is an illustration of the Anti-Surge System layout. On centrifugal discharge, a check
valve shall be installed to prevent reverse flow and minimize surge. The check valve shall be
located as close as possible downstream of the compressor to minimize mass inventory. In
other words, a check valve at the compressor outlet will limit the downstream volume and
increase the required system response time of the surge control system. In parallel operations,
compressor units should have check valves installed to assure effective surge control of each
compressor.
In order to protect the compressor adequately from surge, the discharged flow must be
restored very quickly. For this reason the anti-surge recycle connection and anti-surge control
valve shall be located upstream of the check valve as close as practical to the compressor
discharge connection. Moreover, a cooler in the recycle line may have to be considered, to
suitably control the suction temperature and thereby prevent the compressor from going into
surge. The line-up and tie-in point of the recycle line on the suction side of the compressor shall
be at the upstream of the suction scrubber in order to prevent liquid from entering the
compressor.
If the compressor bypass control line is taken downstream of the discharge cooler (called “cold
bypass line”), an additional hot gas bypass line upstream of the discharge check valve is
required to protect the compressor against surge during start-up, trip and normal operation
(Figure 7).
In “hot bypass lines,” the suction cooler is installed after the recycle line, which is taken directly
from the discharge and recycled to the upstream of the suction cooler.
Figure 7: Hot and Cold Recycle Valves.
Recycle lines and their components shall be sized to handle compressor flow rates under rated
conditions and all other specified operating and start / stop conditions.

7. Anti-Surge Valve Specification
The anti-surge valve (ASV) is a fail open solenoid valve. This means that it needs a high signal of
20 mA to close the valve, and a low signal of 4 mA to open the valve. When a failure occurs, the
valve will usually receive a low signal and it will open, which is the safe position.
In startup and normal stop, the compressor will initially operate with the valve at 100% open.
The valve characteristic for 100% open should be sufficiently removed from the compressor
operational region, during compressor startup. ASV should be sized for 1.8 to 2.2 times of
maximum surge point flow rate. Oversized valves will not provide the sensitivity required for
particular operational settings and will restrict the performance of the compressor; actually, the
valve characteristic lines will become too far from the right of the operation.
Operational flexibility should be considered so that one compressor unit could be isolated from
the other units at a station, while two or more compressors are running as parallel or series.
Moreover, this may require independent bypass loops and quick closing of valves at the suction
side of each compressor. Quick closing of valves will prevent overloading the other units if one
compressor is shutdown.
Therefore, to provide maximum protection and operating flexibility, it is recommended that
surge protection systems with their own dedicated recycle loops should be provided for each
process compression stage.
Authors:
Ali Ghanbariannaeeni is a rotating equipment specialist with NARGAN Engineers and
Constructors, Tehran, Iran. Ph: 9821-8891-4926-30. Email: A.Ghanbarian@ nargan.com, www.
nargan.com.
Ghazalehsadat Ghazanfarihashemi is a rotating equipment specialist with Sazeh Consultants
Engineers, Tehran, Iran. Ph: 9821-8873-9924. Email: Gh.Ghazanfari@ sazeh.Co.ir, www.
sazeh.com.