Split-Ranging Multiple Valves

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.

Bruce Jensen
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
Engineering Consultant
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
Almont Engineering