Control.com - Nerds in Control

I have received an enquiry from an end user. Please see their requirements as follows:

They had a butterfly valve c/w double acting actuator mounted with a Reverse action positioner (4mA disc close, 20mA disc open).

They have since changed positoner cam to achieve Direct action (4mA disc open, 20mA disc close). However, they would want the valve to close when there is a signal failure.

I cannot figure out on the right configuration to achieve that requirement.

I NEED ADVICE URGENTLY. (even if it meant to add accessories to achieve it)

Well, assuming they had a really legitimate reason to change from RA to DA, which I can't imagine why... one way to go about this would be to add an alarm trip module in the 4-20mA controller output loop to the positioner so as to deenergize a 4-way solenoid piped between the positioner and the actuator. Set the alarm trip to deenergize the solenoid at ~1-2mA controller output. Pipe the 4-way up to provide air to close the b'fly when the 4-way is deenergized. Of course, with a dbl-acting positioner, you are doomed to fail-last-position on air failure anyway...

Why the heck did they change from RA to DA?

Hello Andrew,

I am not sure if you can configure a control valve to shut off automatically when there is signal failure. To control a control valve you need 2 things, a control signal (4-20ma) and the feedback signal (position). If you don't have these two, chances are you'll end up either with a fully open or a fully closed valve. Which defeats the purpose of the control valve, if you can't even position it to 30% opening, right?

I think the best solution is to have a shut-off valve upstream . You can configure this valve such that any failure to the control valve, it automatically shuts off, thus cutting off the control valve.

ANA

give me more detail on the valve, maybe I can help.

Really not a problem:

Double acting piston actuators will never provide either fail last or fail open-close position. Indeed, if you lose air plant pressure, the valve will close depending on the actual disk position: over a certain opening the valve open, below a certain opening the valve will close and if the valve friction is above the dynamic torque of the valve assembly then the valve will not move.

If fail-closed action is required people generally prefer a spring-and-diaphragm actuator. This option is however not possible if the valve is too large and a piston actuator can be the only solution id large thrust is required.

If you need fail last action you need two pneumatic three way valves on the two pneumatic lines going to the top and the bottom of the actuator. These valves are cheap and robust and are the size of a small regulator. The three-way split valve as a sensing line which is the air plant pressure the two other lines connect the positionner to the actuator. If the sensing line pressure drops below a predetermined value the valve will isolate the actuator casing and the valve will lock in last position.

If a fail-close or open application is required the solution is a little more complex however standard: 4-way valves with a pressurized tank: tha valves sense the air plant pressure; if the pressure falls under a certain treshold value, on valve vents the air out of the actuator and the other valve sends air from the air tank to the other actuator casing.

Again, this is very standard practice and you should call your Fisher representative and he will tell you exactly what is needed for your application (maybe repalcing the actuator to a spring-diaphragm might be the best solution after all!)

Ben Janvier, Principal Process Control Consultant
Emerson Process Management
benoit.janvier@emersonprocess.com