The post, Biggest Valve Sizing Mistake, appeared first on ControlGlobal.com's Control Talk blog.
There is a common mistake made in the sizing of most control valves. The intentions that lead to this mistake may be good but the results are insidiously bad. While you would think that the proliferation of improvements in technology and communications would lead to better awareness, the problem appears to getting worse because of pervasive persistent misconceptions fostered by missing fields on valve specification forms.
Presently, valve specification forms have fields for maximum flow, available pressure drop and leakage. Most people filling out the form would think that a valve that can easily handle a greater flow with a lower pressure drop and less leakage would be better. This leads often to rotary valves with tight shutoff seals. These valves are cheaper than sliding stem valves and the actuators often included are designed for a rotary stem and can handle greater shutoff pressures. The resulting ball and butterfly valves have piston actuators designed more for on-off action. These valves are usually already in the piping specification extensively for automated sequential actions and shutdown. While rangeability may not be on the valve specification, it is thought to be extraordinarily great for these valves due to a prevalent definition of rangeability being a maximum flow divided by a minimum flow whose Cv is within the specified inherent flow characteristic. Gosh, you don’t even need to be concerned with piping reducers. What seals the deal is the very attractive price. Not understood is that these on-off valves posing as throttling valves are a disaster. A low valve pressure drop to system pressure drop ratio causes distortion of the installed flow characteristic making the characteristic much more nonlinear. The backlash of the actuator linkages and the keylock actuator shaft to stem to ball or disk connections is excessive, the stiction from the ball or disk seal and shaft packing is terrible, and the resolution of the piston actuator poor. The result is limit cycles and a real rangeability that is lousy to the point of being a disaster for any loop where control better than within 5% of setpoint is desired. Often the oscillations are blamed on other sources due to lack of understanding. The real rangeability is drastically reduced to perhaps 10% of the stated rangeability due to distortion of the nonlinear installed flow characteristic and the backlash and stiction that gets worse near the closed position. Operating valve positions are much less than expected due to conservative factors built into pump sizing and maximum flow specified. Most valve suppliers will not do response testing and if requested, the testing will not be done below 10% valve position because of the deterioration in response. The user is setup for a terrible scenario of limit cycling.
So what can we do? Please, add in a backlash and resolution (e.g., 0.5%) and response time (e.g., 2 sec) and installed flow characteristic valve gain (e.g., 0.5 to 2.0 % flow per % signal) requirement at 10%, 50% and 90% positions for step changes of 0.25%, 0.5%, 1%, and 2% for all valves plus 50% step change for surge valves and gas pressure valves. To achieve these specification requirements, use splined shaft connections, integrally cast stem and ball or disk, v-notch balls and contoured disks, and low friction seals for rotary valves and a valve pressure drop that is at least 25% of the system pressure drop for maximum flow, low friction packing (e.g., ultra low friction (ULF) packing), sensitive diaphragm actuators (now available for much higher actuator pressures) and digital positioners tuned with maximum gain and no integral action for all valves. The installed flow characteristic should be plotted with the help of process and piping engineers for the worst case. When asked why the valve cost is higher, tell them the cheaper valve will cause sloppy control putting the plant safety seriously at risk.
Remember bigger is not better and cheaper is not really cheaper in the long run.
For much more than you have ever wanted on valve response checkout the Control article “How to specify valves and positioners that don’t compromise control" and the associated white paper “Valve Response - Truth or Consequences”.
For much more on valve specification see the ISA Mentor Program Q&A post “Basic Guidelines for Control Valve Selection and Sizing”
If you want more information on opportunities to learn what is really important, please join the ISA Mentor Program and ask the questions whose answers can be shared via Mentor Q&A Posts.
You can also get a comprehensive resource focused on what you really need to know for a successful automaton project including nearly a thousand best practices in the 98% new 2019 Process/Industrial Instruments and Controls Handbook Sixth Edition capturing the expertise of 50 leaders in industry.
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