Centrifugal and axial compressors can experience a potentially destructive condition known as surge. Emerson’s Mikhail Ilchenko describes it for me.
Surge is a condition that occurs in compressors when the amount of gas they are trying to compress is insufficient for the size of the compressor and the blades lose their ability to transfer energy from the shaft to the fluid, causing a reverse flow of the gas. 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 compressors are designed such that these valves are only open on startup or under reduced rates.
Given the possible harm to the equipment, a special surge prevention algorithm is required to efficiently protect the compressor. The controller response time also must be higher than for majority of other control loops. The Emerson DeltaV PK controller is a good example of fast-acting controller for compressor antisurge control.
This antisurge control loop also requires fast acting antisurge valves and differential pressure transmitters, because the overall control response is a function of the response time of the antisurge valve, measurement devices and control algorithm relative to the dynamics of the compressor parameters.
The system speed-of-response becomes more critical the closer the compressor operates to its 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 flow as low as possible, thus operating closer to the surge line.
Mikhail explained to me that antisurge control is only needed when the gas flow is low, so most compressors are designed to have their antisurge valves closed for most of the time.
A second consideration is that any antisurge system is providing closed-loop regulatory control only before compressor goes into surge. When a compressor is surging, the system only needs to detect this condition and keep opening the antisurge valve in steps until the compressor stops surging.
It’s a discrete, open-loop response that is called Surge Detection and the speed requirements for that function is covered by API Standard 670, 5th edition. It sets a requirement that electronic logic solver shall have a total program execution time of 100 msec or less and the overall compressor surge detection response time shall be less than 500 msec.
When it comes to preventing surge before it happens, the overall response time required depends on the worst-case scenario for the potential flow drop event. One would need to determine what can potentially happen upstream or downstream of the compressor to cause the most rapid flow drop.
Generally, the bigger the compressor, the faster pressure changes upstream and downstream in the case of process disturbance. For example, if there is a Fail-Close control valve downstream and it suddenly loses air. On the other hand, the bigger the volume between compressor and that potential flow restriction, the slower pressure there will be changing, and dynamics will be smoother.
It used to be very common to have a dedicated specialized controller performing compressor antisurge control. With modern advanced technology, controllers have become faster and systems like the DeltaV control system can address this function safely and efficiently without the need for a specialized, “black box” controller.
Visit the SmartProcess Compressor page on Emerson.com for more on how these control algorithms combined with smart field devices help improve compressor operational safety, stability and efficiency. You can also connect and interact with compressor antisurge experts in the DeltaV forum in the Emerson Exchange 365 community.