It probably is never a simple as this. Just recognizing the process gain is higher when heating than it is when cooling leads one to shift the split point to the left. How much is probably an experimental decision.
One other point: In the heating-cooling application I described, when the controller output is at the split point, both valves are closed, which is their worst controllable point. I would tend to overlap the operating range slightly; say 0% to 55% and 45% to 100% (if I am splitting at 50%), so that within the overlap range, both valves are slightly open. This may require slightly more energy consumption to gain a control improvement. How much to overlap, and whether the control improvement is worth the excess energy cost is a local decision.
Harold L. Wade
Wade Associates, Inc.
A: The “right” answer depends on what is needed and the details of the application. Below are three simple cases. Simple calculations of flows and process needs should aid in making the design decision.
CO is controller output in percentages. V1 is the first valve stem position in percentages. V2 is the second valve position in percentages.
Case 1. Very simple for easy heating and cooling. If these control the flow through something like a reactor jacket, the control dynamics will change greatly, depending on CO, and thus might not be acceptable. If there is a circulating pump on the jacket, then the jacket dynamics change is reduced. Note the serious change in control gain as you approach the 50% point where flow is nominally zero. This calibration is often used for H&V applications. Heating and cooling energy use is minimized when the CO is 50%.
Case 2. Total flow stays more or less constant. This works for applications such as jacket temperature control without a circulating pump. Heating and cooling energy may be lost as part of the cost of good control.
Case 3. The compromise case with overlap in the valve flows to suit the issues. A predetermined minimum flow exists even at CO of 50%
Consider that the relationship between the valve positioner input signal and the inherent valve coefficient may not be linear, and this will also affect the CO versus flow and is part of the control consideration.
PResident, Cullen Langford Inc.
A: Split-range control is often used when there is a need to shift or transition the manipulated variable in order to affect the controlled variable over its expected operating range. For example, in the control of jet velocity out of a paper machine headbox over the entire speed range of the machine, it is often necessary to shift from vacuum to pressurization in the vapor space of the headbox, so the output from a headbox pressure or level controller may be split to two different three-way control valves depending upon whether the operation is in vacuum range (exhausting air/vapor) or pressurization (pulling in air).
Another common example is pH control, where either an acid or base reagent may need to be added, depending upon design pH setpoint and expected pH range. In this case, there may be two valves or two variable-speed or variable-stroke metering pumps.
Here are a few points to consider:
- There is no reason you can't use other splits, such as 0% to 60% or 60% to 100%. In the case of control valves, setting up the positioners or I/Ps appropriately can usually get the stroke and direction desired out of the valve for a given controller output, e.g., 50% to 100% output to 0% to 100% valve stroke. The same is true for most variable-speed drives.
- The physical system should be designed so the split does not occur at a normal operating point, since there is more likelihood of a discontinuity or upset in control at the transition.
- If the gains of the multiple manipulated variables (e.g., vacuum, pressure valve; base, acid pumps) are different, selection of a split somewhere other than right in the middle could be a strategy for helping to linearize the process gain that the controller sees. In this case, assign a larger portion of the controller output range to the manipulated variable with the larger gain, and vice versa.
This is likely not a complete answer, but I hope it is somewhat helpful.
R. H. (Rick) Meeker, Jr., PE
Reliable Power and Controls Corp./
Process Control Solutions, Inc.
A: As commonly happens in control, certain methods of split-range design are holdovers from the past limitations of the control abilities of the hardware. When pneumatics were used, the air tubes splitting the signal were equal diameter, hence, splitting the signal equally.
The 0% to 50%/ 50% to 100% was commonly used in heating/cooling applications using a common medium so an equally split signal may have made sense.
However, it is more likely that the media for heating and cooling are very different.
For example, in plastics extrusion, I've seen heating as an electric heater and cooling as chilled water. Or in reactor control, heating is a fluid heated by a fired heater, and cooling is water chilled with a propane cooler or chilled Syltherm or the like.
The point is that the process dynamics from the heat range is different from the cool range, so the control from a temperature PID is unstable in one region and sluggish in another. Some type of adaptive tuning is needed where the tuning constants are changed, depending upon region of control. Control through the transition then becomes difficult.
One way to control the dynamics across the region is to attempt to linearize the response by translating the temperature you want to control to its basic form, which is actually heat applied. Thus what you are attempting is to linearize across the entire heat range.
If you can describe the medium in terms of BTUs instead of temperature, then maybe you are able to linearize the response through the heating and cooling ranges. Of course, valve dynamics may be incorporated here too. This may then give, as you note, a 0% to 60% / 60% to 100% split instead that would provide for a better control. One application I did was 0% to 18%, 18% to 100%.
Other novel approaches include an overlap or gap in order to control moving through a transition. I've seen 0% to 49%/51% to 100% as a gap control (but here a problem arises if the control wants to live around the 49% to 51% space, which is undesirable). And I've seen 0% to 51%, 49% to 100%, in which both mediums are active (and energy wasteful)
The essence of the solution is linearizing on a parameter common to the range of both outputs you are trying to control. If you can, the split range will probably vary from the standard 0% to 50%/ 50% to 100%. If not, well, adaptive techniques are still in play.
Manager, Systems Technical Solutions Support
Yokogawa Corporation of America
A: There is no good reason at all, would be my reply. If you are trying to maintain constant gain in the control loop, the gain of each valve should be the same. If they are the same size and linear, a 50% split makes sense, but they usually are not. BUT I would also say that I endeavor to avoid designs using split-range outputs.
The better approach is to get two analog outputs from the DCS. You can set these up with separate A/M access and with overlap or underlap between the settings. Much more flexible, and both positioners are calibrated for 0% to 100%, avoiding confusion in maintenance.
Ian H. Gibson
Process, Control and Safety Systems
A: I think the deciding factor is the effect on loop gain, considering the multiplication factor on the gain of the final control device. If you drive a final device over its full range with a reduced input (controller output), you have an increase. I think the reason splits are often set for 50% is simply to equalize this multiplying factor for each control device and give each an equal amount of "working room" in the controller's output range.
Obviously, you could set the split to/for some other percentage, which might be useful in some situations, but the unequal gain factor to the final control elements would have to be considered, and some devices can be harder to calibrate to a greatly reduced input.
Al Pawlowski, PE