“Ask the Experts” is moderated by noted process control authority Béla Lipták. In this column, he and other experienced process control engineers welcome questions concerning process measurement, control and optimization. If you would like to be on our team of “experts,” please send a resume to email@example.com.
Q: I have a question on the split-range concept. I normally see split-range control designs of 0%-50% and 50%-100% OP (in percentages) from the controller. My question is what determines these ranges? Why not use 0%-60% and 60%-100% ? What is normally the deciding factor?
A: We use multiple valves in applications where automatic switching from cooling to heating (c/h) is involved, and when the rangeability requirement of the process exceeds the range of a single valve, such as pH and many others. The main difference is that in the c/h applications, both valves are closed at the time of switching, and the process gain also abruptly changes, while in the rangeability application, one valve is always open, and the process gain usually does not experience an abrupt change as a result of valve switching.
The goal in all applications is that the loop gain remains constant while switching valves. Assuming, for example, you sequence two linear valves on a 50%/50% split range, and the Cv of the large one is 10 times that of the small one, the loop gain would change by a factor of 10 as you cross the 50% point. This being unacceptable, you could operate the small valve from 0% to 10%, and the large valve from 10% to 100% of controller output, but that is not very convenient when calibrating the positioners.
The sequencing of equal-percentage valves is done as follows: If the small valve has a Cv of 10 and a rangeability of 50:1, its minimum Cv would be 10/50 = 0.2. A line drawn on semi-logarithmic coordinates connecting Cv(Kv) 100 and 0.2 appears in Figure 1. Observe that the Cv of 10 of the small valve falls slightly above the midscale of the controller output (to about 65%), providing a much more favorable span for the calibration of the positioner than did the splitting of the linear valves.
In order to have the two valves act as one without disturbing the smooth equal-percentage characteristics at the points of switching, only one valve must be open at any one time. Therefore the large valve must be prevented from operating at low flows, because in its nearly closed position, its characteristic is not equal-percentage.
As shown in Figure 1, the small valve alone is manipulated until the controller output reaches the value corresponding to its full opening. At this point (if the valves are pneumatic) a pressure switch energizes both three-way solenoid valves, venting the small valve and opening the large valve to the same flow that the small one had been delivering. Switching takes place in one second or less, adequate for all but the fastest control loops.
Figure 1. A method to minimize upsetting the control loop when switching equal percentage values.
When the controller output falls to the point of minimum flow allowed for the larger valve (35%), the solenoids return to their original positions. Thus the switch has a differential gap adjusted to equal the overlap between valve positioners (30%). The range of the positioner for the large valve is found by locating its minimum Cv on Figure 1. A rangeability of 50 would give a minimum Cv of 2.
This same approach can be used to sequence three or more valves. If linear characteristics are required, one should insert a 10:1 multiplier relay in the controller signal to the small valve, so that a 0% to 10% controller output will result in a 0% to 100 % signal to the small valve.
In some applications, including pH, better response can be provided by the use of a “floating” valve position controller, which slowly moves the larger valve while keeping the smaller one near its 50% opening. This way, the small valve provides sensitivity and fast response to the loop within its capacity. The large valve is a sort of automatic bypass, which sets the capacity and has a limited frequency response.
A: The selection of the ranges for the valves in split-range operation should be based on keeping the loop gain constant. The closed loop should behave the same whichever valve is being manipulated by the controller in response to the process load. This will vary with the process and with the size of the valves. In some cases, the gains for the individual operating conditions can be calculated; in other cases, a test may be needed. Ultimately, the loop behavior should be the same for both valves—if it is not, the split point should be adjusted by recalibrating the valve positioners to produce the same behavior.
Process Control Consultant
A: This is a good question—one that should be asked more often.
Consider a typical heating-cooling application. Suppose a fail-open cold water valve is fully open at 0% and closed at 50%. A fail-closed hot water valve is closed at 50% and open at 100%. Then suppose it is discovered that the sensitivity of temperature to changes in the HW valve position is twice as much as the sensitivity of temperature to changes in CW valve position. In other words, the process gain is twice as high on the heating side as on the cooling side. Then a better split would be 0% to 33% cooling, and 33% to 100% heating. That would make the process gain uniform over the entire range of controller output.