Principles of Operation
The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.
Control valves may be the most important, but sometimes the most neglected, part of a control loop. The reason is usually the instrument engineer’s unfamiliarity with the many facets, terminologies, and areas of engineering disciplines such as fluid mechanics, metallurgy, noise control, and piping and vessel design that can be involved depending on the severity of service conditions.
Common control valve sizing mistakes
The most common mistake we see with control valves is sizing and usually it is selecting a valve that is too large. An oversized control valve will not only cost you more, but it will be very sensitive and can quickly lead to production headaches.
Small changes in an oversized control valve position can cause large changes in flow. This will make it extremely difficult for the valve to adjust to your desired flow. This can result in “chattering,” where the valve opens and then shuts quickly, causing unnecessary wear on the trim. Any stickiness caused by friction will be amplified by the overly sensitive oversized valve. This will reduce the precision with which you can control flow and may lead to valve positioning errors like stiction and dead band.
You want to size a control valve so it will be no more than 80% open at the maximum required flow rate and no less than less than 20% open at the minimum required flow rate. The idea is to use as much of the valve’s control range as possible while maintaining a reasonable safety factor to allow for surges.
If the valve is too small, it will be obvious immediately, as it will not be able to pass the required flow. In actual practice, under sized valves are fairly uncommon. Commonly, the valve is too large. An oversized control valve will cost more than is necessary, but more importantly, an oversized valve will be very sensitive. Small changes in valve position will cause large changes in flow. This will make it difficult or even impossible for it to adjust exactly to the required flow. Any stickiness caused by friction will be amplified by the overly sensitive oversized valve, reducing the precision to which the flow can be controlled.
Liquid applications must always be evaluated for cavitation. Not only does cavitation cause high noise and vibration levels, it can result in very rapid damage to the valve’s internals and/or the downstream piping. Especially with rotary valves, the prediction of damaging levels of cavitation is more complex than simply calculating the choked flow pressure drop. As a result of flow separation and the formation of eddies within the valve, localized areas of pressure reduction and recovery can cause damaging cavitation at pressure drops well below that which results in fully choked flow. One proven method for predicting cavitation damage in rotary control valves is based on a correlation between calculated sound pressure level and the potential for damage.
In addition to the fact that a noisy valve in liquid service will most likely suffer unacceptable rates of cavitation damage, high noise levels usually cause vibration that can damage piping, instruments and other equipment. Control valves in steam and gas service can generate noise levels well in excess of plant standards, even at moderate pressure drops, especially in sizes above 3 or 4 inches. As a result, the valve sizing and selection process must always include noise calculations.
Installed Flow Characteristic
In nearly all applications, a control valve should have a linear installed flow characteristic (the relationship between controller output and flow in the system). The control valve’s inherent (published) flow characteristic interacts with the system’s flow vs. pressure loss characteristic to yield the installed flow characteristic. If the installed characteristic deviates significantly from linear, it will be difficult or impossible to tune the loop for both accurate and stable control throughout the entire flow range. A computerized analysis of the installed characteristic should be part of the control valve sizing and selection process.
Sizing actuators for on-off service is fairly straight forward, requiring only that an actuator be selected with a torque output slightly higher than the seating and unseating torque of the valve. The situation is more complex with control valves. The torque output of most rotary actuators changes with the angle of opening. At the same time, the valve’s torque requirement depends both on the opening angle and the throttling pressure drop at that particular angle. To ensure adequate spare torque to guarantee smooth, accurate control, a computerized analysis is recommended
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