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Loop checking
Continuous process industries, DCS questions. topic
Posted by a on 25 October, 2010 - 3:58 pm
Hello all,

I started the loop checking with ma source & multimeter since I don't have HART communicator and pressure calibrator. I connected the PT, ma source and multimeter in series to DCS. I adjusted the current from ma source but I got the reading erratic reading in DCS. Shall I connect the TX in series with ma source to DCS for loop checking. What will happen if I connect ma source parallel to TX. In either case I am giving external supply. Please guide me.


Posted by Sam on 26 October, 2010 - 6:33 am
Hi there,

Ok let's do some loop checks without a HART communicator.

You can use a mA source to do the following.

1)
Test the loop for continuity from the transmitter to the DCS. This means disconnecting the two wires from the transmitter and connect the mA source directly to these two wires and simulate 4 to 20 mA to the DCS. You then contact the CCR and ask them if they receive what you simulate. If it is pressure they should receive whatever the calibration of the transmitter is on the pressure faceplate in the CCR. You have two settings on the mA source,"Sink and Source". I can also sometimes forget on what the setting should be so try "sink" first then switch over to "source" if CCR said they cannot see any change. Your mA source should have a display on it but you can also connect a multimeter in series to confirm what the mA is that you are sending to the CCR.

2)
The next step after step "1" should be:
To re-connect the two wires to the transmitter and use a pressure pump to pump the transmitter up to its full calibrated pressure. In this case connect a multimeter in series on the negative output wire to see what the mA is that you send to the CCR. Again contact the CCR to confirm they see what ever pressure you have pumped to on the CCR faceplate. The mA source is not used in this test. Once this test is successful the loop check on the transmitter is complete.

3)
The next thing you can do with a mA source:
You can simulate the 4 to 20mA to a control valve. You can do this at the control valve itself by again disconnecting the two wire to the positioner and connect the mA source where the two wires was connected. Again you can connect a multimeter in series as well to confirm the mA you send to the positioner. Stroke check and calibrate the valve and the postioner in this manner in the field.

4)
If you have a feedback positioner on the valve you can again disconnect the two wires to the feedback positioner and connect the mA source directly to the two wires and simulate the loop by sending the 4 to 20 mA to the CCR and ask them to confirm the values. Again the next step is to reconnect the wires and ask the CCR to do a stroke check from the CCR on the control valve and to see if they receive the correct feedback. If they manually send a command of 30% valve opening they must receive a 30% valve position feedback. If not reconnect you mA source and re-calibrate the positioner or feedback positioner and test again from the CCR. Once the test from the CCR is successful the loop check on the valve is complete.

This basic method of loop checking applies to pressure,level, temp, flow and density and all control valves. Loop checking a on/off valve is as simple as asking the CCR to open and close it from the CCR. If it does not work check the solenoid valves. Obviously working with the HART is much easier but like you said you don't have a HART so doing it this way is fine and the way we use to do it before HART and SMART.

If you have two or four wire instruments you might have to switch between sink or source to get it working, can't remember which but either one of the two setting will work.

NB!!
Don't forget to switch over the red lead of the multimeter when you measure mA.


Posted by Ashwini on 26 October, 2010 - 9:07 am
Sam,

I was looking through your loop check details; couple of things here, pl correct me if im wrong:

- item #3 loop checking a control valve should not require a mA source..a multimeter should be enough. set the valve output in DCS and verify the mA in the field with the multimeter in series. valve can be stroked simultaneously if there is enough inst air.

- item# 1 sourcing or sinking a mA source depends on the DCS I/O module itself. we have few that dont provide loop power - isolated modules for a number of reasons. sourcing the mA S would be needed in this case. in all other cases it would be sinking.

please note im only speaking for 2 wire field instruments tht are loop powered.

Ashwini


Posted by Sam on 27 October, 2010 - 7:51 am
Hi there,

Yes you are probably right about the sinking and sourcing, I can never remember which is which so I just try sinking first and if it does not work switch over to sourcing, either the one or the other normally works but yes you should know what is what and not like me that never bothered to try and remember.

About the mA source usage with a control valve, what I have described is correct and the way it is done on a valve with any EM positioner on. You cannot do a stroke check from the CCR unless you have tested and calibrated the valve and FB positioner first at the valve. You cannot ask the CCR to keep on opening and closing the valve from the CCR while you calibrate in the field. Setting the valve up with the mA source and simulate the signals first will have the result that you only have to bother CCR once the calibration is done for only one quick stroke check from the CCR. They have other jobs as well, especially during first commissioning of the plant.

With SMART positioners it is easiest to leave the power on to the positioner and do a autocal. So with SMART postioners there is no need for the mA source unless you want to stroke check it yourself at the valve first before contacting CCR for the final testing. Sometimes helpful if you are unsure if the autocal was successful or not or when you do some fault finding due to a sticking or binding problem on the valve. Talking on the radio and asking the CCR to open and close for the next half and hour while you try and set the valve up or find a fault is unprofessional and not the way to do it. Solving the problems and get it working properly first with the mA source locally and then only contact CCR for one final stroke check is the way to do it.


Posted by Ashwini on 27 October, 2010 - 3:18 pm
Thxs for the response. I think I see what you are saying. We do stroke and perform auto calibrate on valves (all new ones come with smart positioners) in the shop and then once all is checked, valve is taken out to the field for installation.

When ready to commission, it's stroked again from DCS for a final loop check. I wasn't suggesting using DCS for a full valve calibration check. wouldn't dream of it :)

Ashwini


Posted by sakthi on 25 June, 2012 - 7:51 am
How to do loop checking with HART and SMART for pressure transmitter?


Posted by Jonas Berge on 26 June, 2012 - 11:20 am
Connect a HART field communicator to the device, and command the device to generate 4/8/12/16/20 mA and make sure the reading in the system is 0/25/50/75/100 %

Cheers,
Jonas


Posted by Radwan on 28 June, 2012 - 9:18 am
Dear Friend,

It is wrong to connect the mA source (calibrator) parallel with Pressure Transmitter TX. In this case the calibrator will not work and will give you error message. What you need exactly?

1- If you need to simulate 4-20 mA signal to send it to DCS You have to disconnect the wires from the TX and connect this wires to the loop calibrator and generate output signal (4-20)mA from calibrator to DCS.

2- If you need to test the Tx signal with DCS Then you have to disconnect the Tx from process and apply a pressure steps to the TX While you connect the multimeter in series with the TX to check the TX output if it is varying with pressure applied by hand pump.

3- If you need to check the Tx itself you have to remove it from process to workshop and make calibration check by applying reference pressure according to calibration range of the TX and adjust the TX output (4-20) mA as per the pressure range applied.


Posted by RockWell on 2 November, 2012 - 12:44 pm
Good Day Sir,

Can I simulate a 4-20ma signal to a loop powered device (2 wire transmitter) in the field, remove connection from the transmitter and directly connect my fluke current simulator going to the DCS? The wire removed from the transmitter has a 24VDC supply from the DCS. will this affect or short something in the circuit since you have 24Vdc supply at the same time you will induce 4-20mAdc current from the field?

Thank You


Posted by CSA on 3 November, 2012 - 11:25 pm
Good Day Sir,

What do the instructions provided by Fluke with your instrument, and available on the World Wide Web say?

Does your Fluke have SOURCE and SIMULATE terminals for the leads to be plugged into?

Please write back to let us know what you determine!


Posted by RockWell on 4 November, 2012 - 4:18 pm
Good Day Sir,

Thank You for that hint, that was really help full. I've gone over some manuals and found my answer. Consulting a manual really is that important, my lapses on that one. I will try this on actual one of this days.

Hope to to have more hints from you again.

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