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It is also important that each control valve not be oversized and that the master controller not get the slave controller unnecessarily traversing the split range point. For set point changes to the master controller and startup, I favor a feedforward signal of the set point change added to the master controller output and a head start initialization of the slave controller output to preposition the proper valve.
EXCESSIVEcomplexity serves little purpose. Some years ago, I used to look after a bunch of PVC polymerizers, and spent too many sleepless nights trying to tweak the control system for maximum production. A highly exothermic reaction using a water-jacketed reactor and direct injection steam/cooling water. The aim was to bring the batch up to temperature as fast as possible, and then switch to cooling as the exotherm cut in, so that there was minimal (or no) overshoot detectable in the batch temperature.
The best design I devised had P-I-Derivative on PV control on the primary, P-I on the secondary and an override P on the secondary (external feedback) to prevent the jacket temperature exceeding 95C and sending steam to the cooling-water return. Split range on the cooling-water and on the steam using the positioners, with a 5% deadband between them. This provided the balance between the two gains – the batch ran at maximum steam until the jacket temperature reached 95-96C (a bare thermal element in the cooling-water outlet for speed of response), then the derivative response on the primary cut in about 5C below setpoint to drive the secondary to full cooling, and then recover as the batch reached setpoint. With an exothermic reaction, the system doesn’t need heating again until the end of the reaction.
Ian H. Gibson, Process, Controls and Safety Consultant, Melbourne, Australia
OF COURSE, it's not normal practice to put in 2 PID controllers. As such there is freedom to have both heating and cooling occurring at the same time, which is energy inefficient and does not improve control.
The split block implementation is common in most DCS systems. That way, the output of the PID and the positioners can be set separately. The valve setup can set to fail open or fail close and be set to 0-100% as in any other application. Makes it easy for maintenance and reinstallation if necessary. The split allows conversion of the 0-50% of the PID to be converted to 0-100% for the one valve and 50-100% of the PID to 100-0% for the other valve (or vice versa). It also allows to gap or overlap the valve outputs if desired (overlap, again wasteful and gap doesn't often help but insets some additional deadtime. But certainly logic to change tuning constants is needed because of the different process characteristic between the zone..
Alternately, as he as stated, I have done 0-32% of the PID go to one valve and 32-100 to the other. What you are trying to do with that is linearize 2 separate valve characteristics into 1 heating region in order to have a single set of PID tuning constants. That is often tough to do as it is assumed that both cooling and heating are linear in their respective regions and that the valve positioner is also, which is often not the case.
The biggest problem in exothermic reactors is the large amount of heat needed to initiate the exotherm and then to immediately remove the heat when the reaction start to take place. Most PIDs can't keep up with the result of bad temp control.
Bruce Jensen, Yokogawa Corporation of America
I AGREEwith your and Greg answer about the use of a single controller, not dual to prevent fighting. I have used splitting in the DCS because I used I/P converter an pneumatic positioner (hot service, and decoupling in the installation activity of a new power station). This way there are two analogue outputs from the DCS and any tuning is carried out only at the DCS engineering workstation, without involving different people.
The actual solution depends from the hardware available.
Alberto Rohr, Italy
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