Compressor Surge Control

Any process manufacturer that has centrifugal compressors knows about a condition known as surge. Emerson's Pete Sharpe described it to me:

Surge is a condition that occurs on compressors when the amount of gas they are trying to compress is insufficient for the speed of the compressor and the turbine blades lose their forward thrust, causing a reverse movement in the shaft. This condition can have catastrophic effects on the machine, so compressor manufacturers include anti-surge valves that recycle gas from the discharge to the suction when a low flow is detected. Usually these valves are designed to be only open on startup or under reduced rates.

This surge condition is analogous to an aircraft "stall" condition, or pump cavitation.

Given the possible harm to the equipment, specialized anti-surge controllers exist that have very high-speed execution and special surge detection algorithms. It also requires fast acting antisurge valves and differential pressure transmitters. The required control response is a function of the response time of the surge valve, measurement devices and control algorithm relative to the surge dynamics of the compressor. In an earlier surge control post, ModelingAndControl.com's Greg McMillan noted that surge valves typically have response times from 1 to 5 seconds.

The system speed-of-response becomes more critical the closer you operate to the surge limit. Since recycling gas from the discharge back to the suction of a compressor wastes energy, the most economic operation is to keep the recycle flows as low as possible, thus operating close to the surge line.

This analog may not be a perfect fit, but it reminds me of the Grand Canyon. The closer you are to the edge the better the view down, but it is also increases your risk.

Pete was recently asked about his thoughts on doing anti-surge compressor controls within an automation system, in this case, the DeltaV system. His first point is that surge control is only needed when the gas flow is low, so most compressors are designed to have their surge valves closed for the majority of the time.

If you are trying to operate right on the surge line, Pete agrees that these specialized, dedicated anti-surge controllers may be necessary. However, if you choose to set the surge flow controller setpoints more conservatively to include a safety margin, anti-surge control functions can be safely implemented within the DeltaV system, or in a Foundation fieldbus-based arrangement within the fieldbus devices.

These more conservative settings mean that the compressor would go into recycle or blow-off condition earlier than is absolutely necessary. Energy is wasted only when the recycle valve opens. So, to evaluate whether an expensive, dedicated surge control system is required, plant engineers need to answer the following questions:

• How often is the surge valve open?
• Under what conditions does it need to open?
• How quickly do changes in gas rates, composition, and pressure occur?
• How much energy is wasted by having surge controllers set 5% higher than the surge point?

If the answers to these questions provide for this more conservative operation, Pete notes that he has worked with process manufacturers to execute 1-second anti-surge control within a DeltaV controller. These strategies can run at up to 100msec, which is faster than the valve response time. In the ModelingAndControl post referenced earlier, Greg suggested a control strategy.

A recent DeltaV whitepaper, Fast Control Response Requirement describes 300msec control by having the control algorithms run in the Foundation fieldbus devices--a practice known as control in the field. In this case, the digital valve controller on the surge control valve would run the analog output block and the PID control block.

The advances in technology allow the operating target to safely move closer to the surge curve. The advantages of incorporating this control into your automation system is an integrated operator interface, advanced measurement diagnostics, alarming, and reporting information on the compressor. All good stuff to keep your process operating effectively.

Surge-Control-Considerations-in-Centrifugal-Compressors.mp3 | iTunes

Before the holidays, ModelingAndControl.com's Greg McMillan had another set of eBooks, Centrifugal and Axial Compressor Control Student Text and Instructor's Guide. These books were originally published in 1983 and the copyrights were returned to Greg.

As described in the "About the Book" section, the text is geared for instrumentation and process control design engineers. The text describes:

...the surge phenomena and illustrates the effect of the operating conditions on surge. Covers various methods of throughput and surge feedback control. It explains the need for preprogrammed open-loop surge control and describes newly developed methods for implementation and coordination with feedback control.

Greg was recently asked by a colleague how surge control might be applied in automation systems like the DeltaV system. He responded:

DeltaV provides an excellent solution for nearly any control system that prevents surge by the manipulation of a control valve if you make the execution time of the module 0.1 to 0.2 seconds so that it is much faster than valve. Surge control valves have a response time of 1 to 5 seconds. Also, you should configure an open loop back in the DeltaV module because once a compressor gets into surge a feedback PI loop can't get it out of surge no matter how fast it is due to the flow reversals every couple of seconds. The open loop back up is simply a CALC block in DeltaV that puts the PI in ROUT and increments the surge valve position when activated. When deactivated it returns the PI to its preferred mode. The trigger for activation is a crossing of a line between the actual surge curve and the surge controller set point allowing for some error in the knowledge of the surge curve. The clear point for deactivation is a point well to the right of the surge controller set point for at least a couple of seconds.

This set of eBooks joins his other freely available eBooks:

• A Funny Thing Happened On The Way To The Control Room
• Biochemical Measurement and Control
• Continuous Control Techniques for Distributed Control Systems

If your process includes centrifugal or axial compressors, you may want to take a look these latest additions to the eBook library.