Why compressors surge

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. 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 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. 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. 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.

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 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.

It used to be very common to have a dedicated specialized controller performing compressor antisurge control. Visit the SmartProcess Compressor page on Emerson. You can also connect and interact with compressor antisurge experts in the DeltaV forum in the Emerson Exchange community. We invite you to follow us on Facebook, LinkedIn, Twitter and YouTube to keep up to date on all the latest news, events and innovations to help you take on and solve your toughest challenges.

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Share this: Linkedin. Related Posts. Some plants may think of the surge curve as being the point where the surge valve opens. In this case, integral action must be greater than the proportional action.

However, the extra integral action causes a larger overshoot of the surge setpoint, necessitating the setpoint offset to be increased accordingly, which generally corresponds to lower operating efficiency.

Most other plants see the surge setpoint as being the best operating point, when surge valves are open through tuning so that proportional action dominates integral action, preventing overshoot. Using higher controller gain rather than a lower reset time gives a faster correction. For closer operation to the surge curve and to reduce dire consequences from surge, the total must be less than 1 second.

How fast the automation system really needs to be and the required tuning of the surge controller is best determined by running a first-principle dynamic model that includes a momentum balance as well as material and energy balances. Even a fast feedback controller is unable to get a compressor out of severe surge because of the huge jumps in flow. What is needed is an open-loop back up that forces the surge valves to immediately open and holds them open for sufficient time to sustain operating point stability before allowing the feedback controller to start to close the surge valves.

The open-loop backup is triggered by a large predicted overshoot of the surge setpoint to prevent surge, or a precipitous drop in flow indicating an actual surge. An innovation uses a predicted overshoot via a fast future value that is generated by the rate of change of a decreasing flow, with a good signal-to-noise ratio multiplied by the total loop dead time, with updates every controller execution. The open-loop backup simply puts the feedback controller into a remote output mode that is seen by operators.

The remote output is immediately stepped up to a position that typically prevents surge, but is incremented every execution until the future value stabilizes, putting the surge controller bumplessly back in cascade with the surge setpoint computed to sustain an offset from the surge curve.

Many suppliers of standalone compressor controllers have proprietary control strategies providing feedback control, with a backup requiring special expertise and tuning. External-reset feedback ERF , also known as dynamic reset limit, in the surge controller, with a fast readback of actual valve position, enables up and down setpoint rate limits in the analog output blocks to provide fast opening and slow closing of the surge valves without the need to retune the surge controller.

The surge control system principles basically are the same for surge vent valves and surge recycle valves. At least two valves in parallel are used to provide redundancy, particularly because surge valves might not open after sustained operation in closed position, where stiction from seal or seat friction is greatest. For multiple stages, there generally are recycle surge valves and a compressor surge control system for each stage.

Ratio dividing may be used to proportion the different pressure rises for each stage. Figure 4 shows the surge control system with recycle surge valves and two downstream users feeding reactors. Many systems have more reactors. A valve position controller can minimize the pressure setpoint to increase compressor efficiency by pushing a user valve to the maximum effective throttle position e.

The valve position controller has external reset feedback with setpoint rate limits on the compressor pressure controller to provide a gradual, smooth optimization with a fast getaway for a disruption. Not shown in Figure 4 is feedforward action to deal with the fast closing of user valves.

The feedback and feedforward signals must be linearized based on the installed flow characteristics of the surge valves and user valves, respectively. If a high-rangeability, fast and reliable user flow measurement is used for the feedforward, the characterization of the feedforward signal for the feedforward summery is unnecessary.

Much more detail, with a focus on practical essentials, is offered in the book Centrifugal and Axial Compressor Control. In Figure 4, the derivative of the suction flow computed by DY uses a deadtime block to provide immediate updates with good signal-to-noise ratio. This derivative can be multiplied by the deadtime and added to the current suction flow to give a predicted flow one deadtime into the future. This plus reactor feed valve position can be used to provide a feedforward signal to help surge control deal with user i.

The same type of calculation used to give a future value can be used to find and document surge points on the surge curve on the compressor map. Detecting a nearly zero rate of change in pressure for a change in suction flow indicates a surge point. Detecting a severe rate of change of suction flow indicates a surge cycle, with extreme negative rate of change signifying the beginning and an extreme positive rate of change signifying the end of each surge cycle.