Some of the most consistently popular posts here on the blog involve compressor anti-surge control. It’s a condition in centrifugal and axial compressors where a flow reversal condition can occur. Wikipedia defines centrifugal compressor surge as:
…the point at which the compressor cannot add enough energy to overcome the system resistance or backpressure. This causes a rapid flow reversal (i.e., surge). As a result, high vibration, temperature increases, and rapid changes in axial thrust can occur. These occurrences can damage the rotor seals, rotor bearings, the compressor driver and cycle operation. Most turbomachines are designed to easily withstand occasional surging. However, if the machine is forced to surge repeatedly for a long period of time, or if it is poorly designed, repeated surges can result in a catastrophic failure.
In a new whitepaper, Increased Plant Availability through Proper Anti-Surge Valve Selection, Emerson’s Riyaz Ali highlights the importance in valve selection in not only avoiding surge conditions, but also operating the compressor more reliably.
The cost of replacing compressor seals alone is on the order of $20K to $50K. Over time, surge can introduce fatigue failures that can damage the entire compressor.
He explains the traditional solution to avoid surge conditions:
Antisurge control valves have been used on compressors for decades. Most antisurge valves recycle flow from the outlet of the compressor back to the inlet to keep a minimum amount of flow going through the compressor.
The stroking speed of these valves has historically been a key selection criterion:
Valves that are designed only for stroking speed rarely control well in throttling applications. Because of this, the performance of the antisurge system suffers. Not only does the performance of the antisurge system suffer, plant availability and throughput hinges on the performance of these valves.
Systems that have been designed only for stroking speed end up with a lack of robustness in the control servo loop. This means that the system likely will be unstable when operated outside of open-loop requirements. This can cause excessive overshoot and instability when the valve responds to a set point change.
Optimizing control valve response with digital valve controllers can help:
The open-loop response is tied directly to stroking speed while the closed-loop response is directly related to the actual control function of the antisurge controller. Better control yields improved system gains, which equates to faster action and tighter control. This tighter compressor control allows the compressor to operate more efficiently while increasing compressor throughput.
Read the whitepaper to see before and after performance by applying digital valve controller technology to anti-surge valves. Riyaz concludes:
With the advent of digital technology, the controllers are now expected to provide more than just antisurge control. They must be designed to handle physical limitations of the compressor as well as to maximize the throughput and efficiency of the compressor. In many cases, this has been accomplished by combining the throughput and capacity control requirements into the antisurge controllers.
This was possible due to specific control and tuning algorithms in digital valve controller, which allows a user to tune the valve from the control room and adjust the gains and damping functions immediately to see the results in control room.
When antisurge valve applications use a digital valve controller with antisurge-specific control algorithms built in, and when coupled with the proper valve, actuator and accessories, increased compressor throughput, reliability and efficiency can be achieved, increasing plant uptime.