How to Size Compressor Anti-surge Valves

by | Jan 16, 2025 | Valves, Actuators & Regulators | 0 comments

Anti-surge valves protect centrifugal compressors from potentially catastrophic damage, but they are typically grossly oversized for common operating conditions. My article in the December 2024 issue of Automation.com, titled “The Art of Compressor Anti-surge Valve Sizing and Selection,” discusses an approach to anti-surge valve sizing that takes into account all operating modes to prevent valve damage and premature failure in a cost-effective manner.

 

Specifying surge

Surge occurs when the compressor is unable to overcome discharge pressure due to a restriction of either the suction intake or discharge outflow. Flow reverses back into the compressor, which collapses the discharge pressure and allows the compressor to push the vapor forward again, until the cycle repeats.

These cycles place enormous stress on compressor internals, potentially inflicting catastrophic damage in a matter of seconds.

 

Surge conditions can create catastrophic damage to a centrifugal compressor in a matter of seconds. Compressor rotor blades (left) and guide vanes (right) can be twisted or snapped off.

Surge conditions can create catastrophic damage to a centrifugal compressor in a matter of seconds. Compressor rotor blades (left) and guide vanes (right) can be twisted or snapped off.

 

Such damage can cost hundreds of thousands to millions of dollars to repair, and it often takes the compressor out of service for weeks, or even months.

 

Surge solutions

The surge condition is avoided using an anti-surge control system consisting of flow and pressure instrumentation, a high-speed controller, and a bypass or anti-surge valve.

Opening the anti-surge valve establishes forward flow, stopping or preventing a surge cycle. These valves are often activated in multiple operating modes under conditions that vary considerably, making valve sizing and selection difficult. Sizing typically prioritizes the large flows that may be required during emergency shutdowns, but these valves will spend much more of their operational time in low-flow modes.

 

Valve damage at low flow

At low flows, oversized valves force vapor through a very narrow opening, creating turbulence that often leads to premature failure of the plug or valve stem.

 

Extended operation at minimum valve opening can create stem or cage damage. Given the size and necessary level of performance of most anti-surge valves, this can result in a very costly repair and a lengthy compressor outage.

Extended operation at minimum valve opening can create stem or cage damage. Given the size and necessary level of performance of most anti-surge valves, this can result in a very costly repair and a lengthy compressor outage.

 

Unfortunately, low flow conditions are rarely considered when the anti-surge valve is specified, with valve damage common in many of these applications.

 

Sizing for all flows

Compressor manufacturers typically recommend sizing anti-surge valves to pass 1.8-2.2 times the maximum flow encountered in surge conditions. This guideline ensures safe handling of emergency shutdowns, but it lacks consideration for more common low-flow operating modes. The lowest flow rates these valves face usually occur during recycle/bypass operation, and during minor surge events.

Typical anti-surge valves may be forced to open at just 2-3% in low flows, leading to premature damage. Comparing valve flow curves can help users select a valve that opens further in minimum flows, while retaining a high maximum flow capacity. In some cases, two separate valves may be recommended to handle high and low flows.

An anti-surge valve sized only for emergency flow will operate at very low openings during continuous bypass (left graph). The right graph shows a valve with the same emergency flow capacity but sized to operate near 10% open at the same low flow condition. This extends valve life dramatically.

An anti-surge valve sized only for emergency flow will operate at very low openings during continuous bypass (left graph). The right graph shows a valve with the same emergency flow capacity but sized to operate near 10% open at the same low flow condition. This extends valve life dramatically.

Final thoughts

Optimal sizing of anti-surge valves requires consideration of both maximum and minimum flows. Additional complicating factors in valve selection may include the need for fast and accurate response, and the ability to handle high pressure drops and elevated noise conditions. Control valve experts can help end users select valves well-suited to their application requirements, yielding long-term savings in reduced maintenance, more efficient operation, and increased uptime.

Visit the Control Valves section on Emerson.com to know how you can meet production schedules, maintain product quality, and improve plant efficiency.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.

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