DVC6000 Positioner failures 1

[khan101]

Gentlemen,

We are experiencing more than normal failures of these positioners. Usual symptoms are that positioner shuts down when the signal goes below 4 mA and loses configuration. Technician usually recycles power and reconfigures the parameters to make it work again. Last year we sent 2 units back to Fisher for investigation but that did not help either. The only thing they were able to pinpoint was contaminants in the air supply (we have very reliable air supply system). The only thing I can think of is that the unit was cycled on/off an offscale amount of times, which could cause circuit/memory problems but that is just a hunch. Most operators tend to drive the output to minimum when they want to close the valve and the output is driven to -6.9%, which may be below the cut-off value of these units. Just want to know if any of you guys have come across these symptoms as well?

 

[jsummerfield]

I have not experienced such problems. Consider setting minimum output limits to prevent the signal from going below 4 mA. Submit the valve diagnostic data to Fisher to see if that can help identify the problem.

John

 

[JimCasey]

Most digital positioners have nonvolatile memory and should not lose calibration when powered down. After all, they shipped it to you, it was un-powered in the box for a reasonably long time, and it still had the default seetings in it, right?

Still, the Wake-up current is generally about 3.6 ma and you should calibrate the unit to close the valve fully (Output gage goes to zero) at 4 ma. Educate your operators not to drive the output to less than 4 ma to close the valve. Some of the operators may need to be shot to correct this administratively. Password-protected limits in the DCS may also need to be implemented.

 

[CMfgE1]

Sounds like you have a DCS problem and not a positioner problem. Have not experienced this condition ever. I would check the loop power supply. Its possible your voltage drop is causing the power failure. Are you in an XP (explosion proof area)? What DCS sysem are you using?

Theres nothing wrong with your air supply. Your DCS (in most cases) wont recognize a signal below 4ma anyways. and Im guessing your operators are manually shutting the valve down by choosing 0% on the DCS. If not this could be a classic example of operator error. People like to blame intrumentation instead of GMP (good maintenance practice).
So if you could reply with a little more info I would be glad to work on this further.


Good Luck

Quote: "Its not what you know, its who you know" - anybody trying to find a decent job

 

[khan101]

CMfgE1,
Loop power and power supplies have been checked many times and they seem to be in good working condition.
We are using Honeywell TDC as our DCS.
We have faced this problem on more than 1 positioners, both in classified and non-classified area.
Operators are not manually shutting down air supply.

Now other than that, this problem is not very frequent but we usually have once or twice in a month and sometimes in critical loops where it can cause havoc on our process. It has happened when different shifts were on so I can rule out the operator error as well. Our assumption about limiting the power seems to be true because when the output is forced below 0 and hence 4 mA (often 3.8 mA) we understand this causes the device to "reset." The use of barriers compounds the problem as they consume some energy as well. Any further help will be appreciated immensely.

 

[JimCasey]

My sources tell me that this is a known problem and Fisher has put out a memo to the field. If I can get a copy of the memo I will scan it and post the text here.

 

[khan101]

Thanks Jim for confirming my doubts.