Digital positioners typically cost thousands of dollars, while solenoid valve costs are in the hundreds of dollars, if that. Positioners typically take either analog signals, which require analog outputs or fieldbus signals from a controller, either of which are usually more expensive per channel than the discrete outputs required by solenoids. Also, for control valves with positioners, we usually include an SOV in the air supply to establish a fail position on loss of power, so you'd probably end up using both a positioner and an SOV for most of your on/off valves. You do the math.

xnuke
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Thanks @xnuke.

That's a good perspective.

Based on your current designs where you include an SOV already with at positioner, do you install the SOV in series with the output port of the positioner?

Reading through the manual of these positioners, apparently when the positioner loses communication to the PLC or loses power it also has a "fail-safe" action that cuts the air supply and vents the compressed air making the SOV unnecessary. Did I read this right?

The reason I can think of the positioner bring value is because of the so called diagnostics built-in.

Will most definitely do the math since I have already prepared a list of valves with SOVs.

Thanks for the input.

I'm used to using non-digital mechanical positioners, where we use a three-port SOV in the air supply with the NC port connected to the supply, the common connected to the positioner/actuator, and the NO port open to atmosphere for venting. When using digital positioners, you may not need the SOV if the positioner can block air and vent the actuator.

I can see the value of diagnostics provided when using on throttling valves, especially those in critical systems, but not on/off valves.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

As stated, a digital positioner cost a lot more than solenoid valve. A solenoid valve is often used in conjunction with a positioner when emergency closure speed is necessary. In general, the air flow through a solenoid valve (that is the Cv) is much higher than a positioner because positioners are used for accurate control. If they have too much air flow, the valve can be unstable and move back and forth trying to get to the set position. Also known as hunting. If money is no object and speed of operation is not important, then by all means you can use a digital positioner for on/off use. Solenoid valves are generally lower cost and more compact as they can often be direct mounted to the actuator.

If it is diagnostics you are seeking, there are several manufacturers of limit switches that include the solenoid valve and output diagnostic data such as number of cycles, speeds of operation, valve torque over time, etc. Generally the same data that digital positioners output. Check out StoneL Axiom, TopWorks, and Westlock for smart limit switches that include the solenoid valve.

Also forgot to mention air quality. Positioners usually require higher cleanliness of the instrument air than solenoids.