One of the more heated arguments I had with a client involved his extreme desire to use line size butterfly valves for controlling his process. The entire concept of using control valves for the available pressure drop and for controllability at low flow rates was utterly lost on him. We went round and round until I finally got the client to install spool pieces at each valve location so we could easily replace the butterfly valves with a different valve if we encountered control problems in the future.
A lot of those spool pieces and butterfly valves got replaced.
Concept: Despite what the on/off valve sales person will tell you, an on/off valve with a positioner is NOT a control valve. Such an arrangement can have its uses, but you should understand the limitations of on-off valves and the differences between on-off valves and control valves.
Details: In an attempt to reduce cost, a young engineer may resort to using ball and/or butterfly valves with positioners to control a process. These valves CAN be used in throttling applications, but certain limitations can greatly impact their performance. An automation engineer must understand these limitations and specify this type of valve only where appropriate.
- This arrangement is much cheaper than a standard control valve.
- A ball valve’s flow characteristic can be “characterized” to provide a range of CVs and provide a more linear response if required. (Butterfly valves are not so easily modified.)
- On/off valves tend to have less pressure drop when fully open.
- A butterfly valve’s flow characteristic is essentially closed until about 40 degrees, then flow increases from 20% to 90+% of capacity between 40 and 80 degrees. Such a narrow range of control and poor turndown rarely suits most control valve applications.
- The recovery factor for a butterfly or ball valve is generally poor compared to a control valve. Therefore, these valves tend to have more problems with cavitation and permanent pressure drop when they are throttling flow.
- Precise, repeatable control is difficult to achieve – especially at low flow rates.
One application that IS well suited to a throttling on/off ball valve is a dribble application in batch raw material charging. The typical dribble valve uses a second solenoid to “lock” the valve position in a half-closed position, which slows the charge rate at the end of a charge and helps improve the accuracy of liquid charges. However, the dribble valve solenoid arrangement is prone to maintenance problems and the final charge rate and accuracy tend to drift as the actuator ages. For nearly the same money, a small positioner can be installed on a characterized ball valve and the unit can be continuously throttled at the end to provide very accurate charging. Typical control logic would look something like this:
- If remaining charge amount > 100 lbs, output = 100%
- If remaining charge is <=100 lbs, output = MAX (remaining charge/100, 25)
Such an algorithm charges at maximum speed until the end, then continuously throttles the valve down to 25% as the end of charge approaches. Finally, it trickles in the last few pounds to nail the total charge amount accurately.
Watch-Outs: Beware of using butterfly valves with class VI shutoff in any kind of a throttling application unless the valve is normally at least one-third open. These valves usually torque into the seat very hard and take a great deal of force to crack open from a fully closed position. If the valve is used to throttle in this regime, it will wear quickly and the flow rates will be wildly erratic.
Exceptions: If accurate control at low flow rates and cavitation is not a concern, then ball and butterfly valves can be successfully used in certain applications. However, you must understand the limitations of these valves types and not misapply them.
Insight: Premature valve failure usually occurs due to seat erosion under low flow/high pressure drop conditions or under conditions where the valve is cavitating or flashing. True control valves can be designed to handle and/or mitigate these conditions. Throttling on/off valves are a poor choice for these applications.
Rule of Thumb: Carefully evaluate the application before selecting a valve type. Take time to understand the strengths and weaknesses of the various valve offerings, and choose the valve type that fits the application. Do not skimp on the design to save a few dollars; over the life of the valve, the wrong valve will cost much more than the original savings.