Mark: I have seen a lot of loops where the filter time was too small or too large. If the filter time is too small, noise causes the valve to move, which self-inflicts a disturbance and wears out the packing. If the filter time is too large, the loop may oscillate, since integrating process performance and tuning is particularly sensitive to the additional lag.
Stan: What do you recommend projects do for these important loops?
Mark: I recommend that users request the supplier to provide the dead band, resolution, response time and installed characteristic for control valves. These lost-motion and response parameters should be the results of small steps in valve signal (e.g. 0.5%) per the current test methods established by the ISA-75.25.01-2000 (R2006). The user should plot the integrating process gain, which is the product of the control valve gain, process variable gain and measurement span gain, versus controller output. The control valve gain is the slope of the flow versus stroke plot of an installed characteristic curve. The process variable gain is the slope of a plot of the ramp rate of the process variable versus valve flow provided by the process engineer for various setpoints, and the measurement span gain is 100% divided by the process variable span used by the controller. The resulting integrating process gain must have dimensions of % PV per sec per % flow (units of 1/sec). The user should use tuning rules for integrating processes to estimate tuning settings, and decide if gain scheduling is needed for changes in the gain with operating point. Once the loop is commissioned, auto tuners and adaptive controllers should be used to identify the actual integrating process gain and dead time for the expected range of setpoints and load disturbances.
Greg: The installed characteristic and lost motion near the seat determine the real rangeability of the control valve as estimated by the equations on slide 21 of Deminar #2 posted April 22, 2010 on http://modelingandcontrol.com/. Deminar #2 and its posted review also provide a more in-depth discussion of dead band (backlash), resolution (stiction) and the resulting limit cycles.
Top Ten Things You Don't Want to Hear in a Project Definition Meeting
10. I don't want any smart instrumentation talking back to me.
9. Let's study each loop to see if the valve really needs a positioner.
8. Let's slap an actuator on our piping valves and use them for control valves.
7. We just need to make sure the control valve spec requires the tightest shutoff.
6. What is the big deal about process control? We just have to set the flow per the PFD.
5 Cascade control seems awfully complex.
4. The operators can tune the loops.
3. Let's do the project for half the money in half the time.
2. Let's go with packaged equipment and let the equipment supplier select and design the automation system.
1. Let's go out for bids and have the purchasing agent pick the best deal.