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
Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.
