Specifying the best valve size can reduce loop variability and enable tighter control. A side benefit can be easier PID tuning and a smaller valve. By understanding key principles, you can become a valuable resource for getting the best automation component that directly affects the process, the control valve.
I became sensitized to control valve sizing early in my career when I found most of the pH reagent valves and many of the Fed-batch reactant valves were riding the seat. This is a bumpy ride. The friction and consequently the limit cycle from stick-slip are greatest near the seat due to the rubbing of the plug against the trim seat for sliding stem (globe) valves. For many rotary valves the cycling is worse. The friction of the ball or disk rubbing against the seal and resulting friction is greater and persists for larger valve positions, causing a greater and more prevalent limit cycle.
We are pretty diligent about making sure the valve can supply the maximum flow. In fact, we can become so diligent we choose a valve size much greater than needed thinking bigger is better in case we ever need more. What we often do not realize is that the process engineer has already built in a factor to make sure there is more than enough flow in the given maximum (e.g., 25% more than needed). Since valve size and valve leakage are prominent requirements on the specification sheet if the materials of construction requirements are clear, we are setup for a bad scenario of buying a larger valve with higher friction.
The valve supplier is happy to sell a larger valve and the piping designer is happier that not much or any of a pipe reducer is needed for valve installation. The process is not happy. The operators are not happy looking at trend charts unless the trend chart time and process variable scales are so large the limit cycle looks like noise. Eventually everyone will be unhappy.
The limit cycle amplitude is large because of greater friction near the seat and the higher valve gain. The amplitude in flow units is the percent resolution (e.g., % stick-slip) multiplied by the valve gain (e.g., delta pph per delta % signal). You get a double whammy from a larger resolution limit and a larger valve gain. If you further decide to reduce the pressure drop allocated to the valve as a fraction of total system pressure drop to less than 0.25, a linear characteristic becomes quick opening greatly increasing the valve gain near the closed position. For a fraction much less than 0.25 and an equal percentage trim you may be literally and figuratively bottoming out for the given R factor that sets the rangeability for the inherent flow characteristic (e.g., R=50).
What can you do to lead the way and become the “Go To” resource for intelligent valve sizing?
You need to compute the installed flow characteristic for various valve and trim sizes as discussed in the Jan 2016 Control Talk Column “Why and how to establish installed valve flow characteristics” You should take advantage of supplier software and your company’s mechanical engineer’s knowledge of the piping system design and details.
You must choose the right inherent flow characteristic. If the pressure drop available to the control valve is relatively constant, then linear trim is best because the installed flow characteristic is then the inherent flow characteristic. The valve pressure drop can be relatively constant due to a variety of reasons most notably pressure control loops or changes in pressure in the rest of the piping system being negligible (fictional losses in system piping negligible). For more on this see the 5/06/2015 Control Talk Blog “Best Control Valve Flow Characteristic Tips”
On the installed flow characteristic you need to make sure the valve gain in percent (% flow per % signal) from minimum to maximum flow does not change by more than a factor of 4 (e.g., 0.5 to 2.0) with the minimum gain greater than 0.25 and the maximum gain less than 4. For sliding stem valves, this valve gain requirement corresponds to minimum and maximum valve positions of 10% and 90%. For many rotary valves, this requirement corresponds to minimum and maximum disk or ball rotations of 20 degrees and 50 degrees. Furthermore, the limit cycle amplitude being the resolution in percent multiplied by the valve gain in flow units (e.g., pph per %) and by the process gain in engineering units (e.g., pH per pph) must be less than the allowable process variability (e.g., pH). The amplitude and conditions for a limit cycle from backlash is a bit more complicated but still computable. For sliding stem valves, you have more flexibility in that you may be able to change out trim sizes as the process requirements change. Plus, sliding stem valves generally have a much better resolution if you have a sensitive diaphragm actuator with plenty of thrust or torque and a smart positioner.
For pH reagent, Fed-batch reactant and fermenter or bioreactor air flow, the rangeability requirement can be extraordinary. If you cannot achieve the rangeability with one valve, you may need to add a small trim valve in parallel with the big valve. This is preferable to split ranged operation because it eliminates the split range discontinuity and adds the ability to make more precise flow adjustments all the time by keeping the small trim valve continually available for direct throttling by the flow or process controller. The large valve is throttled by a valve position controller (VPC) whose controlled variable is current trim valve position and whose setpoint is optimum trim valve position (e.g., 50%). The VPC tuning has traditionally been integral-only with integral action 10 times slower than the flow or process controller integral action to reduce interaction between the loops. Sometimes the VPC action is reduced or eliminated for small deviations of the small valve position from its optimum position (e.g., no VPC action for small valve positions between 40% and 60%). There are now more effective ways of doing VPC by means of external reset feedback, directional move suppression, and an enhanced PID as described in the June 2015 Control feature article “Don’t Overlook the Virtues of PID when Optimizing Processes”
If you take advantage of this knowledge, your supplier will be happy because you are happy. The supplier may also be able to sell you that small trim valve besides the regular valve. For isolation, the supplier can also sell you an on-off valve with low leakage. All of this and more is revealed in the Control May 2016 feature article “How to specify valves and positioners that don’t compromise control”
If you specify the best valve and best size, you might just be famous by Friday. “Thank goodness for Friday.”
For a concise presentation of the concepts and details on the effects of control valves on PID control see my ISA book Good Tuning: A Pocket Guide, 4th Edition. If you are up for a more comprehensive view, see my Momentum Press book Tuning and Control Loop Performance - 4th Edition.
