Monday, June 6, 2011

Process Automation - Split range control

Process Automation - Split range control

WE HAVE a tempered water system supplying cooling and heating to an exothermic reactor.
We are using a Cascade/Split range control method. The Master controller is for the Reactor
Temperature, the Slave is for the Tempered Water Temperature. The slave outputs goes to a
splitter, which then controls the steam valve and water valve for the tempered water.
The diagram below is a typical split range control for example: a tempered water system. The
water temperature is the PV for the PID. A typical splitter would be 0-49% for cooling, and 51-
100% for heating (with steam). In some applications, another option is to shift the split point if
the heating process gain is higher than that of cooling (different ‘strengths’ in the valves). For
example a 0-32% for cooling, 34-100% for heating split.

My question is in regards to the difference in ‘strength’ between the heating valve and cooling
valve (different process gains). Would the diagram below work? It seems that because there is
a difference between the heating and cooling gains that we could have each valve have it’s
own PID controller. That way we can tune each valve for it’s own gain. The Add and
Subtraction of 5 degrees is to make sure each PID is not seeing the same setpoint (to give
them a ‘deadband’) and are not constantly fighting each other.

Process control authority Bela Liptak brings in specialists from his
cadre of co-authors to answer a reader's question. Find out which
books and online courses they recommend for continued
education. Find out what the difference is in strength between a
heating valve and cooling valve for split range control


5/29/2009 this an acceptable method of control? What is wrong with it?

David Rolfe

THE NORMAL configuration is to use an =%, fail closed steam valve with a positioner that
operates it between 50-100% of the output signal from the reverse acting slave PID set at 10-
20% proportional band and a little integral. The water valve is also =%, it fails open and it
operates between 0-50% of the output signal from the reverse acting slave PID. In order to
prevent reset windup in the master, we also provide the master TIC with external reset from
the slave transmitter output.

In your existing configuration, I do not see the need for the splitter, as the positioners fulfill that
function. In the proposed new configuration, I don’t see the need for inserting dead band at the
PID (particularly not such large one as +- 5 degrees F), because the dead band can be
provided by setting the positioner ranges 0-49% and 51-100%.

Because the gain, time constant and deadtime of the process is different during cooling from
that which exist during heating, it is reasonable to modify the tuning constants when switching
from cooling to heating, but 1) most of the integral and derivative changes have to be done in
the Master and 2) I would not adjust the gain in two separate slave PIDs, but in the same one.
This is because if you have integral in the positional algorithm of the slave controller, the
internal reference would be lost at the time of switching.
I will also ask some other colleagues about your question.

Béla Lipták
I AGREE with your assessment of the proposed system. The dead zone inserted between the
two slave controllers will cause cycling in the primary loop. Additionally, the bias introduced
into the set points will cause offset if external reset feedback is applied.
There is usually no conflict in a batch reactor control system between PID settings for heating
and cooling, because heating only brings the primary temperature up to set point, where
cooling takes over. Therefore the cascade system should be tuned for cooling, and this usually
gives acceptable results for heating.

Where this is not the case, separate PID settings can be scheduled into the slave controller,
depending on which valve is open. This is easy to do with digital controllers, and is a standard
feature in some controllers (e.g., Foxboro).

Greg Shinskey

I HAVE seen dual PID controllers used for pH control where there was an acid and base
reagent. Unfortunately the difference between the set points needed to keep the controllers
from fighting and insuring both valve are not open at the same time is highly dependent upon
the dynamics and tuning of each PID. So I prefer a single split ranged controller with its tuning
scheduled per the valve throttled as you mentioned. So far as the split range point, operators
expect 50% but a different point may help compensate for the difference in process and valve
gains if the controller gain cannot be scheduled. Since the process dead time and time
constant is also different for heating and cooling, the integral time will also be different but if
mostly proportional action is used for the slave controller, it may not need to be scheduled.

I favor the use of the splitter in the DCS configuration instead of split ranged analog positioners
because it eliminates special positioner calibrations. This was particularly important in the days
of pneumatic positioners. However, if the positioner is digital, smart, and has its calibration
accessible from the control room, the accuracy and maintainability of split ranging in the field is
no longer as much an issue. It is especially important that the valves do not have a deadband
or stick-slip as the trim goes into and out of the seat that is greater than half of the split range
gap. The deadband and stick-slip cited for many valves is at the ideal throttle position of 50%
and does not show the effect of the extra friction from seats and seals at shutoff.

1 comment:

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