Allen Bradley 1756-OF8 not driving valve fully open

[shanefaulkner]

Electrical simpleton here, so show me mercy.

Plant previously had a 4-20ma valve that was controlled via pneumatic controller with a P/I. Now they have a ControlLogix 1756-OF8 in 0-20ma mode. Valve will move, but not fully open, and above a certain ma output from the OF8 the channel status shows an open wire. They claim they have tried it with both a 250ohm resistor and 1/4 amp fuse in the circuit with no change. They claim that the valve used to fully travel from 0-100% open when it was controlled via the pneumatic controller.

We will try a different channel on the OF8 when time allows, but I'm wondering what else could be wrong electrically.

 

[Contr_Conn]

Did you change Low Limit on the Limits Tab?

 

[shanefaulkner]

No, they are set to 20 (high) and 0 (low).

 

[doctord]

have you tried a mA simulator to see if the value will move the full distance when you apply 20mA?

 

[drforsythe]

The first thing I would do is measure the actual current that is flowing in the circuit. If the card is not able to source full current, the valve will not fully actuate. I have had this happen in the past when a return wire was inadvertently disconnected and the device was finding another path to complete the circuit. Because it was not properly connected, current was flowing, but not properly and not the way it was intended.

If the actuator has an I/P controller, check to see if anyone tweaked it. We had some old units years ago. Whenever there was an issue, someone would start tweaking the limits- the air pressure, the flow controls, anything with an adjustment screw.

Lastly, the environment our older units were operating in was dusty. The sintered metal filter/mufflers that were installed on the actuator exhaust would collect dust. The dust would mix with oil or moisture in the air (a whole other issue) and get hard. This would reduce exhaust air flow and limit the stroke and speed of the actuators.

Best of luck. Keep us posted on your findings.

 

[rupej]

Check that the valve positioner is adjusted correctly. A lot of times they're not, and the I/P is adjusted to compensate. Now that you've switched to a new reference signal, this may now be revealing itself.

 

[danw]

I, too, suspect the I/P adjustment.


Troubleshooting chart for a Bellofram T1000. Most I/P's operate on the same basis and would have similar recommendations.

I suspect that if you track the valve movement from 0-4ma that you'll see sluggish valve movement.

 

[PLCMentor]

So in summary to a lot of good responses.

1. Check your output - do you have full range 0-20ma actually on the circuit?


2. If output looks good, go to the valve and source with a test device that can supply 0-20ma signal. Do this right at the valve.



With the results of these two tests, you should be able to follow the advice of the other posters here for a solution. If weird results are observed just bring them back here.

 

[shanefaulkner]

It ended up being a wiring issue, they didn't provide many details as to what exactly was found/fixed.

Thanks to everyone for the suggestions.