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Old November 9th, 2015, 06:16 PM   #1
ddeshi2
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Troubleshooting 4-20 mA loop

Hello everyone,

We have a backpressure valve with digital positioner (two-wire) connected to an analog output card ( Automation Direct). This valve has to control the pressure by opening or closing depending on the pressure transducer value connected to analog input module. Everything worked fine until last week when all of a sudden the valve started behaving erratic. We opened the valve and checked for any mechanical faults, but everything looked fine. We checked the digital positioner and changed it to a new one because we found some moisture on the old positioner circuit board. After changing the positioner the valve has stopped responding( neither opening/closing) to the PLC signal. The positioner has an option to set either in auto/manual mode and I'm able to open and close the valve by placing the positioner in manual mode but not from HMI. When the positioner was placed in manual mode I checked the mA signal by connecting multimeter in series and found that at 0% it was 4 mA. I slowly increased the valve opening but the multimeter value didn't change ( showing 4 mA at 100%). What does this mean? Does it mean the positioner needs 4-20 mA calibration? or is it something else?I'm new to this field and appreciate your help in figuring out the problem .

Thanks,

Andy
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Old November 9th, 2015, 06:29 PM   #2
Phil Buchanan
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Opening or closing the valve is not going to change your 4-20 signal from your description of the process. The only thing that will change the 4-20 to the valve is the PLC analog output card and it won't change until the PSI transducer connected to the analog input changes again depending on if your description is correct.

4-20 ma is not how much current the vale is using its a signal going to the valve and it won't change just because you do a manual position change to the valve.
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Old November 9th, 2015, 08:05 PM   #3
danw
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There are I/P's (current-to-pneumatic transducers) that convert 4-20mA to a pressure signal, like 3-15 psi, or there are digital positioners, which have a mechanical linkage connection to the valve stem which supplies feedback to the positioner so that the positioner can control the position of the valve stem ensuring that the valve stem position matches the demand signal from the controller.

I suspect you have a positioner because you describe it as 'digital', but it isn't clear. There are dumb electropneumatic positioners and smart positioners.

If it is a smart positioner, then it needs to go through a set up routine to find the stem movement limits and check performance. Taking a smart positioner out of the box, bolting it in position and connecting an air supply is not enough to get it running properly.

You need to find the manual for a smart positioner and go through whatever configuration and tuning procedure is needed to get it running. Proper mechanical linkage connection to the valve stem is very important.

I/Ps generally work out-of-the-box.

PBuchanan is correct, using manual mode on the positioner does not affect the 4-20mA signal from the controller. Your controller probably has a manual mode that forces the output to a given state, like 4.00 mA or 12.00 mA.
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Old November 9th, 2015, 11:32 PM   #4
JRB
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When the positioner was placed in manual mode I checked the mA signal by connecting multimeter in series and found that at 0% it was 4 mA. I slowly increased the valve opening but the multimeter value didn't change ( showing 4 mA at 100%).

What does this mean?



The positioner doesn't produce the 4-20mA signal, the plc analog output does.

So, you would have to place the valve's HMI analog plc output in manual mode at the HMI, and then enter 0-100%, to observe the current
signal (4-20mA) changing on your series connected ammeter at the valve, (if your HMI control has manual capability). The manual
control on the positioner disregards the analog input (from the plc analog output) when in manual. If the positioner is/was normally
in auto, you would leave it in auto to test its position manually, and test current, using the manual analog plc output from the HMI.

If the valve is driven manually to 100% (20mA) from the HMI and 20mA is not present at the positioner, there is a problem with the plc analog output.

Most likley, the positioner should be in auto at all times, where it responds to the 4-20mA input signal from the plc analog output.

Use the plc auto/man to run/test the valve operation, and measure changing mA.

Only put the positioner in manual when you want to drive the valve to some manual position irregardless of the 4-20mA input signal.

If 20mA is present from the plc manual AO and the valve doesn't open fully, the "digital positioner" must need to be configured properly.
Check the digital positioner configuration parameters for auto valve position value at 100% (20mA).

Perhaps the original erratic output operation was somehow attributable to the "pressure transducer connected to analog input module",
instead of the valve or positioner.
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Old November 10th, 2015, 01:34 AM   #5
willxfmr
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Here is how I would attack this issue:

1) Start simple and check to see if the new unit was wired correctly. A wire landed in the wrong spot, or incorrectly would explain the sudden failure. When a new device is installed, an "oops" is often the cause of problems.

2) Does the new unit have dip switches or some other means of setting it for a 4-20 signal rather than say a 0-10v signal? If so, have you checked to make sure it is setup correctly? Again, the new install oops will often bite you in the backside.

3) While you have the unit open, check to make sure you don't have a broken wire. Lift both wires from both ends, twist them together at one end and Ring them out at the other end. If the wires are good, make sure they get landed correctly after testing. You do not want to find out what kind of headaches can be created by causing new issues while troubleshooting the first problem.

4)With the simple stuff out of the way, move on to the slightly more complex. Check to see if the new unit is giving the correct signal to the PLC input. If you are not getting a good input, the output will not do much. I can't be much more specific than that since I don't have the information on what kind of unit we are dealing with.

5) If none of the above have helped, then it's time to get serious. Remove the PLC from the equation and try to drive the valve by sending a 20mA signal to it with the calibration source of your choice. Did the valve go full open? If not the issue is most likely at the valve, and you need to repeat steps 1-3.

6) If it still doesn't work, replace it with another new unit, or contact the manufacture. DOA parts are not unheard of, so don't assume that the new unit you installed works unless you can prove it does. If the valve does go full open with your 20mA signal, then work from the other direction. Make sure the input card is set for a source signal, and using the same signal generator, send a signal to the PLC input. Check and make sure the PLC sees the incoming signal. If not you may have a bad input card. Check to see that the right output command is being sent to the output card. If the output card is being told to output say, 15mA, check to see if it is actually doing that. The issue may be with the output card. If giving the correct input signal to the PLC delivers the correct output signal, repeat steps 1-4.

If none of this resolves the problem, curse loudly and blame the Chinese for making junk products. Don't worry, about where the parts actually came from. At that point it doesn't really matter. Good luck, and I hope this helps.
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Old November 10th, 2015, 08:24 AM   #6
JohnCalderwood
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What is the manufacturer/part number of the positioner?

At least then one of us could identify what is required in the setup and commissioning. Some positioners will be factory set, other require to learn the limits and do a self-check.
I recently tried to recommission a PMV positioner that had lain in the stores for a while, played around with it, then eventually had to strip it and found that a spring had come loose and it was not reacting like it should. Sorted that and it then worked....
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Old November 10th, 2015, 08:56 AM   #7
osmanmom
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i will refer to engineering/process documentation manual if available,
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Old November 10th, 2015, 03:00 PM   #8
Bit_Bucket_07
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Quote:
Originally Posted by ddeshi2 View Post
Hello everyone,

We have a backpressure valve with digital positioner (two-wire) connected to an analog output card ( Automation Direct). This valve has to control the pressure by opening or closing depending on the pressure transducer value connected to analog input module. Everything worked fine until last week when all of a sudden the valve started behaving erratic. We opened the valve and checked for any mechanical faults, but everything looked fine. We checked the digital positioner and changed it to a new one because we found some moisture on the old positioner circuit board. After changing the positioner the valve has stopped responding( neither opening/closing) to the PLC signal. The positioner has an option to set either in auto/manual mode and I'm able to open and close the valve by placing the positioner in manual mode but not from HMI. When the positioner was placed in manual mode I checked the mA signal by connecting multimeter in series and found that at 0% it was 4 mA. I slowly increased the valve opening but the multimeter value didn't change ( showing 4 mA at 100%). What does this mean? Does it mean the positioner needs 4-20 mA calibration? or is it something else?I'm new to this field and appreciate your help in figuring out the problem .

Thanks,

Andy

Without knowing the details regarding the equipment in question, FWIW, I witnessed a similar situation in which moisture in the instrument air supply was soaking the control board inside the valve leading to failure. Upon replacing the control board, it was necessary to run through a setup/calibration procedure before the valve would respond to a 4-20 mA signal. After paying for repairs twice, the client finally installed addition dryer and drainage equipment to keep moisture from entering the positioner via the air supply. No failures have occurred since this was done.
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Old November 11th, 2015, 10:02 AM   #9
ddeshi2
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Thank you all for the suggestions! The wiring is fine without any short or open. I have ordered a loop calibrator and waiting for it to arrive. Will keep you posted.

Thanks,

Andy
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Old November 11th, 2015, 05:35 PM   #10
JRB
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You know, it's November, and here in the east it's been pretty warm. That's why I had my window open the other day when
I heard my kitty cat outside screaming bloody murder. I was crafting a detailed response to this post about the 4-20mA
thing, but I grabbed my eight foot bullwhip, and proceeded to the source of my whailing friend instead, only to find him
thirty foot up a giant pine tree with a coyote snarling at him from the bottom. When I cracked the whip, all four of the
coyote's feet left the ground, and he yelped out of sight. I was certain we wouldn't see him again. Next thing you know,
I'm hanging from a schwabisch at thirty feet in the air coaxing my little buddy off of a two inch tree branch with a
handfull of Kit & Kaboodle (TM), when the coyote came back. As a baby, my kitty used to leap onto my CarHart (TM), and
traverse me like a tree branch. But now, agitated from the return of the coyote, and his grip about to slip on the tree
branch, he jumped onto my head, and when he couldn't achieve the grip he's used to on my T shirt, guess what happened to
my face? And so, during my heroic attempt to save him, I wound up hurling my best friend to the forest floor, and the
coyote, from thirty feet in the air to get him off my head, where he proceeded to bounce off the leaf litter and climb
up another tree.

Bleeding about the face, I was able to decend, and using a different noise producing tool, exactly one of those creatures
is no longer with us, while the life of the other, is pending. Its been raining for days and he's still out there, on a
branch, mulling it over. Anyway, after the trauma, I was able to finish my reply, and had I simply refreshed the page before
posting a bunch of malarchy that was pretty much already stated by professionals in other replies, you wouldn't have had
to endure this monumental story of my selfless dedication to a complete stranger's post, even if my face was still intact.
It just shows how things can change in an instant. But hey, there's less puss in my facial expression today, so things are
really lookin up!!

Keep it out of the trees! ....and Thank You in advance for all your prayers.
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