The Art of Compressor Anti-surge Valve Sizing and Selection

Summary

Centrifugal compressors depend upon an anti-surge valve to keep the equipment out of dangerous operating zones and avoid catastrophic damage. To ensure the valve can quickly move the compressor out of surge when an emergency shutdown occurs, the valve is typically grossly oversized. This works well for the relatively rare case where the compressor is running at full capacity and is suddenly blocked in, but the same valve will work very poorly under much more common operating conditions, such as continuous recycle or slow approaches to surge conditions.

This article discusses a more holistic approach to anti-surge valve sizing, where all the operating modes are considered in the valve design. This technique has been successfully utilized to resolve a number of situations where an incomplete and/or incorrect anti-surge valve specification resulted in severe valve damage and premature failure due to faulty operation at low flow conditions.
 

The scourge of surge

Surge occurs when either the suction intake or discharge outflow of a compressor is blocked or restricted. For example, consider a centrifugal compressor running at rated capacity with a valve on the discharge closing. As the restriction on the discharge flow increases, the discharge pressure of the compressor rises to compensate, and the resulting flow through the compressor falls.

If the valve continues to close, the pressure will continue rising until the compressor reaches its maximum pressure limit and flow stalls. As the flow stops and the fluid momentum is lost, the compressor can no longer contain the pressure at its outlet, and the pressurized discharge relieves backwards through the compressor. As soon as the discharge pressure collapses, the still spinning compressor pushes the vapor forward again, building pressure and setting up the next surge cycle.

The rate of surge cycles varies from one every a few seconds, to multiple cycles per second in a high-speed compressor. With each cycle, the thrust bearings and compressor blades are subjected to enormous stress. It only takes a few surge cycles to inflict damage, and continued surge conditions can destroy the compressor internals (Figure 1).

Anti-surge valve solution

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 (Figure 2).
 

• Startup: The compressor starts from a dead stop. Under these conditions, the anti-surge valve is normally fully open to ensure flow remains high until the compressor reaches operating conditions and can be brought into operation. At that point, the anti-surge valve is closed.
• Normal shutdown: The compressor is being taken offline. If the compressor is operating in parallel, the anti-surge valve is gradually opened to take the unit’s flow out of the header, and it is then fully opened to maintain flow as the compressor is shut down.
• Emergency shutdown: In many designs, this is the only condition considered for anti-surge valve sizing, leading to issues as described below. The compressor is running at full capacity and suddenly blocked in. The anti-surge valve must handle the full compressor flow, plus provide additional capacity to immediately move the compressor out of surge.

Low flow

• Continuous recycle or bypass mode: This condition is the one most likely to occur, and it requires the anti-surge valve to open slightly to maintain some amount of minimum flow. This condition often occurs when the actual flow requirements are less than the minimum flow of the compressor, so the difference must be recycled. This condition can be caused by downstream users or equipment outages, either of which force the compressor to operate at a point it is not designed to handle.
• Normal surge events: In this case, a smaller downstream user may drop off-line, causing the compressor to approach surge before it can slow down. Ideally, the anti-surge valve will only open slightly to move the compressor away from surge, and then close once the compressor speed has been throttled.

As mentioned above, the emergency compressor shutdown requires the largest flow, and this condition is typically used to size the valve. However, that condition is the least likely to occur. Low flow modes are much more common and require the valve to operate at these conditions for an extended length of time.
 

Low flow anti-surge valve damage

When an oversized valve is forced to operate at very low flows, problems arise. This is particularly true for anti-surge valves because they typically must handle very high pressure drops. Most valves control poorly when the valve is barely open since the flow paths are not yet fully established and turbulence through the cracked seat is significant. This turbulence can cause the valve plug to vibrate and ring like a bell, with the vibration potentially causing high frequency bending moments in the valve stem connection to the plug (Figure 3). Extended operation with the valve at minimum opening will often result in premature valve failure due to cracking of the plug, and/or total failure of the valve stem connection to the plug.
 

Anti-surge valve sizing

Historically, compressor manufacturers have solely focused on the emergency shutdown condition when sizing the anti-surge valve. The typical specification suggests the user should consult the minimum and maximum surge condition flows and size the valve to pass 1.8-2.2 times that flow, provided the valve cannot pass more than the maximum stonewall compressor flow. Stonewall is a maximum flow condition where vapor reaches sonic velocity, choking the compressor and allowing no more flow to pass.

Key takeaways

When sizing and specifying an anti-surge control valve, it is important to look beyond the typical maximum flow surge conditions. While these flows are certainly important and critical for surge avoidance during an emergency shutdown, it is just as important that the valve be able to operate at very low flows, so design parameters for minimum recycle flow should be considered when specifying anti-surge control valves. For normal operating upsets where surge prevention may be required, the valve should be designed to handle the difference in flow between the surge control line and surge limit line, which is typically 10% of compressor capacity. All this information must be fully understood before the proper anti-surge control valve can be specified and selected.

Although knowing the required flows and process conditions is required, it is only part of the challenge. Complicating factors, such as extremely fast and accurate response, critical pressure drops, and high noise conditions make valve sizing and material selection even more difficult. Often an anti-surge valve will require a combination of high-capacity and high-diagnostic positioners, characterized flow trims, oversized actuators, pneumatic boosters, and the proper materials of construction to handle the broad range of flow and performance parameters.

Once the parameters are understood, end users are encouraged to consult a control valve expert to select the best combination of actuator, valve style, trim, and positioner accessories to suit the application. The extra effort to evaluate the full range of requirements can yield enormous savings in reduced maintenance, more efficient operation, and increased uptime.

_{Figures all courtesy of Emerson}