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Dec
08

How to Succeed – Part 5

We continue this series with some helpful hints for control system design and implementation. This week and next week we look at control valve selection. There are a lot of misconceptions from sales pitches that lack an understanding of the need for a valve to have minimum backlash and maximum resolution and sensitivity. Most of this stems (pun intended) from using step sizes that are way too large. Today, the smallest step change commonly cited is 0.5%. Maybe I should be happy because the step size was 25% until we had smart positioners that could tell us how much the valve shaft actually moved. What I want are 0.1% steps over the entire throttle range to sort fact from fiction.

Common Misconceptions:
• Rotary valves provide tighter control than sliding stem valves
• Stated rangeability takes into account pressure drop, backlash, and stiction
• “High Performance Valves” (tight shutoff valves) provide high performance
• Piping valves and on-off actuators can be used for control valves
• Piston actuators provide tighter control than diaphragm actuators
• Step tests at 50% open tell the whole story

Rotary valves tend not to have as large a throttle range where the gain (sensitivity) of the installed characteristic is acceptable. Rotary valves that are “High Performance Valves” and piping valves have a markedly reduced throttle range. If you consider the effect of increased backlash and stick-slip of these valves especially near the seat, the actuator shaft used for positioner feedback may not be representative of actual internal flow element (disc, ball, or slotted plug) position due to shaft windup, and a poor inherent flow characteristic, you understand these valves are not really control valves. Similarly, the scotch yoke, rack and pinion, and link arm on-off actuators have excessive backlash or insufficient resolution for throttling service. A diaphragm actuator has the best threshold sensitivity. Higher pressure diaphragm actuators have been developed extending their thrust and torque capability. The next best actuator is a double acting piston. All should have smart positioners with 2-stage or high gain pneumatic relays. High volume spool positioners used on dampers and piping valve posing as control valves have an extremely poor threshold sensitivity requiring step changes of 2% to see a response.

To learn more about what really is important about control valves and variable speed drives read Chapter 7 on Final Element Fundamentals in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industries,  and the articles “Improve Control Loop Performance”, Chemical Processing, Oct, 2007 and “Key Design Components of Final Control Elements”, InTech, March-April, 2011.

Next week I give my checklist for valve selection.