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Control pressure at discharge

Q: I have five pumps running parallel, transferring water. Due to pressure fluctuation at discharge, which depends on the flow requirements of the user, I am planning to install a pressure control valve at the pump discharge to keep the pump running at an optimum condition. My candidate is a globe valve, but because the line size is quite big (14 in.), the globe valve is not adequate. An engineering consultant suggested a butterfly type. According to past experience, using this kind of valve is not effective for pressure control. What kind of valve is best for a 14-in discharge?

A: Mark Coughran, senior control engineering consultant, Emerson Process Management, provided this answer.

I have seen problems in the past with butterfly valves on large water lines. See, for example, page 10.54 of Process/Industrial Instruments and Control Handbook, 5th edition, McGraw-Hill. However, things have improved since then in the valves, actuators, positioners and application software.

The problems were typically caused by a combination of factors:

1. wrong size of valve; valve diameter = pipe diameter, giving high process gain at low travels and low process gain at high travels;
2. butterfly disk shape contributing to a poor installed flow characteristic;
3. backlash in the disk-to-shaft and shaft-to-actuator connections and inside the actuator;
4. poor performance from the valve positioner;
5. installed torque not properly accounted in selection of the actuator

Today, control valve vendors offer the following:

1. valve sizing software that predicts the installed flow characteristic so that you can see your controller will be able to handle the flow vs. output curve;
2. butterfly valves with disk shapes giving more favorable flow and torque characteristics;
3. zero-backlash connections inside the valve, the actuator and the connection of the valve to the actuator;
4. valve positioners that respond to 0.1% signal changes in less than one second;
5. actuator sizing software that predicts the installed torque.

Globe valves typically are too expensive for large process water lines. Your control valve vendor should be able to provide test data on modern butterfly control valves (valve + actuator + positioner) in similar applications. If not, consider a segmented ball valve whose size will probably be smaller than the pipe diameter.  On a 14-in. line, this should give a good flow characteristic, reasonable actuator size and dead band less than 0.3% — for much lower cost than a globe valve.

Next, you should think about the control strategy.  At a minimum, use a controller tuning method that does not oscillate but responds at your required speed, such as Lambda tuning. If you need to control the five lines separately, there will be interaction and balancing concerns. The options range from individual PID controllers to a multivariable controller. All the options are easy to configure and tune in a modern DCS.

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