Rate constraining of control valves

Hi, Can anyone point me to a device for limiting the maximum rate of slew of a control valve? I'm looking for adjustable full scale stroke time
adjustable from about 10 secs to about 1 minute. Something at or close to the valve is required, so that, for example, if the 4-20 signal line breaks the rate limit is respected while the valve moves to the failure position. Valves are to be driven by standard air actuators with I/Ps.
Would prefer something more accurate than would be provided by an air tank and restrictors.
Have looked at a couple of websites for the common manufacturers, haven't come across an obvious fit.TIA
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bruce varley wrote:

Restrictors and dash pots are probably the best you can do passively. When I needed better than that, I drove the device -- valve, door, etc. -- with a motor whose speed I could control. A worm gear is much more reliable day to day than a pinhole.
Maybe there's a new and clever way now, old ways work too.
Jerry
--
Engineering is the art of making what you want from things you can get.

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Bruce, I think this is something I would discuss with my friendly favourite valve/actuator manufacturer. In theory pneumatic actuators have a lag-type response to small changes in position demand; but are inherently rate-limited for larger demand changes by the restriction on the rate at which air can be got into, or out of, the actuator (at least that's the way they are often modelled in simulations) - hence the need for boosters or positioners. It may be difficult to achieve the slew-rate limit you want under only fault conditions without adversely affecting the regulatory performance; but I'd discuss it anyway. It may be possible to do something by modifying a valve positioner.
Kelvin B. Hales Kelvin Hales Associates Limited Consulting Process Control Engineers Web: www.khace.com
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Find the simplest discrete controller that offers RAMP TO SET POINT. and 4 - 20 REMOTE SET POINT input. Don't us the internal set point
Configure it for 4 - 20 PV input and connect the 4 - 20 signal representing VALVE POSITION there.
Use your incoming 4 - 20 valve position command as the REMOTE SET-POINT INPUT.
The valve will move at the RAMP RATE you select on the controller, towards the commanded position
You can configure the PV display to say 0 - 100% valve position and the set point display to 0 - 100%
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