Surge is characterized by fast flow reversals through a compressor and is caused by a large-scale breakdown of flow patterns within the compressor. Surge happens at low flow rates, often when the downstream demand decreases. When flow decreases below a certain minimum point, flow patterns in the compressor become unstable and fluid can move back through the compressor from the high-pressure side to the low-pressure side.
Because surge is a fast, high energy phenomenon, it can introduce excessive dynamic loads on internal components, such as thrust bearings, seals and blades as well as introduce unwanted pipeline vibrations. 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.
Anti-surge control valves have been used on compressors for decades. Most anti-surge valves recycle flow from the outlet of the compressor back to the inlet to keep a minimum amount of flow going through the compressor.
Combining digital valve controllers with anti-surge valves can help meet the challenging requirements of anti-surge applications where speed and accuracy are important - speed requirements are typically less than 2 seconds and overshoot criteria is in the 5% or less range. Further, online tuning/diagnostics are required and the valves must respond to small step changes without overshoot or undershoot.
The goal of any compressor controller is to operate turbomachinery within a safe operating envelope. Aside from surge limits, compressor controllers must handle a number of physical limitations including choke limits, power limits, process limits and rotational speed limits. The task of operating a compressor within this region becomes particularly complex on multistage compressors or when multiple compressors are operated in parallel. While many compressors today are controlled by sophisticated and integrated control systems, the majority of anti-surge valves are still specified only by stroking speed.
Valves that are designed only for stroking speed rarely control well in throttling applications. Because of this, the performance of the anti-surge system suffers. Not only does the performance of the anti-surge system suffer, plant availability and throughput hinges on the performance of these valves.
The optimized anti-surge valve with digital valve controller is designed with both open- and closed loop performance in mind. 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 anti-surge 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. For new units, this means that a smaller compressor could be used with an optimized anti-surge valve compared to one that uses a valve selected by traditional means.
Requirements for Anti-surge 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. Figure 1 shows the performance of an 18” anti-surge valve designed for only fast stroking speed when subjected to a series of step tests. As the step size becomes larger, the valve ‘hunts’ for position - a tell-tale sign of a poor performing anti-surge valve.
Because stable valve performance is critical in this application, an unstable servo loop requires that the valve be run in manual control making startup and shutdown difficult. This also equates to reduced compressor throughput and efficiency since the system must be de-tuned because of the valve’s poor performance.
However, by designing with both open- and closed-loop performance in mind, adequate protection of the compressor is ensured along with accurate throttling control to maximize compressor output and efficiency. By designing the system for fast acting, accurate closed-loop control, the compressor anti-surge system can perform at its peak potential by allowing for higher system gains.
Figure 2 shows the performance of the same NPS 18” anti-surge valve designed with the enhancements of the optimized anti-surge valve.
Stable performance throughout the entire valve travel ensures that the valve will respond quickly and accurately to any changes initiated by the anti-surge controller. The symmetric performance in the opening and closing directions can improve tuning and controllability.
In addition to other benefits, digital valve controllers can provide online tuning and live feedback so that adjustments can be done remotely at faster speed, all components can be tuned together remotely and non-intrusive diagnostics including performance diagnostics, triggered diagnostic, on seat diagnostic can be done online. This allows the user to identify any potential issues without any shutdown and actively preplan for any necessary improvements or upgrades during next shutdown. It also allows for partial stroke testing to ensure movement of the valve at desired stroking speed during demand, as these valves operate infrequently.
Anti-surge valves can be found in nearly any production or process facility, but are most suited for challenging and daunting performance needs of the olefin and LNG industries. Olefins and LNG facilities commonly integrate capacity and anti-surge control into the anti-surge controller since these two systems can fight one another if left independent. This integration requires excellent performance from the anti-surge valve in order to operate efficiently.
When anti-surge valve applications use a digital valve controller, and anti-surge-specific control algorithms are built in, with the proper valve, actuator and accessories, compressor throughput and efficiency can be achieved, increasing plant uptime.