Additional information:  this article is contributed by Mr Peter Lee, Performance Technology, Manager, Emerson Process Management Singapore Pte Ltd, Valve (Fisher) Division

Control Valve - Static vs Dynamic performance

Introduction

Valves have traditionally been specified and selected using mechanical criteria. These criteria focus on the valve’s integrity as a pressure vessel, its flow capacity, its ability to provide the shut-off required, and provide corrosion resistance to the flowing media. Since most valves meet these basic criteria, they are often purchase as a commodity. 

Such mechanical attributes are necessary but insufficient for control valve in throttling service. They focus only on the "Static" performance of the valve. In essence, they are "bench" measurements that say relatively nothing about how the valve will perform under actual operating conditions. A control valve must perform as part of a control system. The control valve is as important a component as the controller and the process sensor. The controller can control the process variable only if the control valve responds quickly and accurately to signal changes. 

Thousands of performance audits conducted by independent consultants and manufacturers have proven that as many as 50% of the valves in service, many of which were selected by conventional considerations, detract from the efforts to optimize control loop performance. Follow-up research shows that the valves’ dynamic characteristics play an important role in reducing process variability, which is part and parcel to process optimization. (Process variability is a measure of how closely the system can maintain the process variable to the set point despite random disturbances). 

Testing Requirements

What is the best environment for conducting comparative control valve dynamic performance tests? The most realistic conditions would be in the user’s process plant. However, the plant environment may make it difficult to acquire detailed measurements, and flow-loading conditions may change significantly from one test to another. Detailed measurements are possible in static bench testing: however, these are likely to be misleading since flow loading is not applied to the valve, and the process variable behaviour is not examined. As a compromise, a laboratory test system may be used to apply consistent, realistic process loading to the control valve and allow collection of detailed open-loop and closed-loop data. 

That is why Fisher has developed the dynamic performance testing loops in which various valves can be compared using a standard input disturbance and systematic tuning procedures. These closed-loop dynamic performance tests under actual loaded conditions provides Fisher with a wealth of information that aids in the design and development of Fisher valves. 

Dynamic vs Static Bench Performance 

1. Valve Friction

A valve tested on the bench may have much lower friction than would occur under normal installed conditions. In some valve body designs, the seal is pushed against the ball by the seal retainer ring when the piping bolts are tightened. Further seal loading occurs in designs which use process pressure against the seal to obtain shutoff rating. The pressure drop across the ball also loads the shaft bearings. 

2. The right variables

In static test, stem position has traditionally been used as the output signal in performing deviation cycles on sliding stem valves. On rotary valves, the shaft position can be easily measured at the positioner/actuator end of the shaft. However, the motion of the ball end of the shaft may vary considerably from what the positioner detects. Two major factors causing this discrepancy are connection design and friction. This discrepancy between the actuator and the ball motion is added to the hysteresis and deadband of the positioner/actuator combination.

Because of these discrepancy, the process variables must be used to deduce changes in ball position while the input signal is varied. 

3. Dynamic Response

For optimum control of many processes, it is important not only that the control valve eventually reaches a certain position, but also how quickly it arrives. The dynamic behaviour of the control valve can significantly contribute to the overall loop dynamics, and therefore, limit the frequency range over which disturbances can be controlled.  

4. Responding to small incremental signal changes

In many process control applications, the control valve is expected to move in increments much smaller than 0.5%. The ability of the control valve to respond to small changes in controller output may limit the use of advanced process control strategies. One factor that could not be determine in static bench test is the unsteadiness of the flow patterns which is controlled by the internal geometry of the valve. When one attempts to make very fine movements of the valve stem, this unsteadiness becomes relatively significant. This finding reinforces the importance of using process variables to get a complete picture of control-valve performance. 

5. Closed-Loop Performance Testing

Not all valves provide the same dynamic performance even though they all theoretically meet static performance purchase specifications and are considered to be equivalent valves.  More and more control valve users are focusing on dynamic performance parameters such as dead band, response times, and installed gain as the means to improve process-loop performance. While it is possible to measure many of these dynamic performance parameters in an open-loop situation, the impact of these parameters really becomes clear when closed-loop performance is measured.  A Control loop is subjected to setpoint changes and to upsets. It establishes new operating conditions to recover from load disturbances or to meet the new setpoint. In all cases, the valve capacity and characteristics has influence on the process dynamic control. Since early 1990s, Fisher has conducted closed-loop process variability performance tests on a wide variety of control valve assemblies and transmitters.

The figure below shows that dramatic differences can exist in the dynamic performance capabilities of various control valve assemblies which supposedly are equal products.

The diagram plots process variability as a percent of the set point variable versus the closed-loop time constant, which is a measure of loop tuning. The horizontal line labeled Manual, shows how much variability is inherent in the loop when no attempt is made to control it (open-loop). The line sloping downward to the left marked Minimum variability represents the calculated dynamic performance of an ideal valve assembly (one with no non-linearities). All real valve assemblies should normally fall somewhere between these two conditions.

 Summary

Control Valves are highly engineered product and should not be treated simply as a commodity. While traditional valve specification certainty play an important role, it is crucial that valve specifications also address real dynamic performance characteristics if true process optimization is to be achieved. The performance of the control loop in the process should be prime consideration when specifying equipment.

It is important to realize that process optimization begins and ends with optimization of the entire loop. A formal loop performance audit should be used to identify and correct problems associated with loop hardware. You cannot simply treat separate parts of the loop individually and expect to achieve coordinated loop performance. Likewise, you cannot hope to evaluate the performance not any part of the loop in isolation. Isolated tests under non-loaded, bench-type conditions will never provide the kind of performance information that is obtained from testing the hardware in actual process conditions.