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    Level Measurement DP type problem

    Hi,
    We are using diaphragm type Differential Pressure transmitters for level measurement of different vessels. Each vessel consists of two transmitters one with PLC loop (HIMA with current isolator card CS4/420) {let name it A} and other with DCS loop {let name it B}. The transmitter A show erratic behavior while transmitter B is relatively stable.
    • Both transmitters are of identical model and make and have exact same configuration & installed at same vessel and diaphragms are located at adjacent locations,
    • The grounding connections were found ok.
    • When transmitter A is forced to any value the trend behaves normal and sets to a fix value
    • The transmitter A was replaced with new one but behavior didn’t changed.
    what could have caused transmitter to behave erratically.

    Thanks in advance

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    Re: Level Measurement DP type problem

    Hi,

    transmitter A show erratic behavior
    This is no error description, therefore we can not use it to solve the problem.

    • When transmitter A is forced to any value the trend behaves normal and sets to a fix value
    Isn't this correct behaviour?
    If not, describe what you expect...and what is not like expected.

    Klaus

    •   Alt20th December 2016, 07:50

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    Re: Level Measurement DP type problem

    Hello,

    1. Interchange the connections of the two transmitters. If the problem shifts to transmitter B, then probably the problem is with PLC or related wiring. If the problem stays with transmitter A, then probably the problem is in the installation of the transmitter A.

    2. For liquid level measurement with DP transmitter, usually dry/wet reference leg installation methods are used. See the picture (dry reference leg) below.



    You can see a drain valve. Your system must have such a drain/fill valve (solenoid/pneumatic etc.) which will be controlled from PLC/DCS. Does that valve work properly?

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    Re: Level Measurement DP type problem

    Quote Originally Posted by KlausST View Post
    Hi,

    This is no error description, therefore we can not use it to solve the problem.

    Klaus
    The erratic behavior means the output is undesirable and unpredictable. Please find attached images

    Attachment 134704
    Attachment 134705
    Attachment 134706
    Attachment 134707


    Quote Originally Posted by KlausST View Post
    Hi,

    • When transmitter A is forced to any value the trend behaves normal and sets to a fix value
    Isn't this correct behaviour?
    If not, describe what you expect...and what is not like expected.

    Klaus
    when transmitter is forced to a fixed value it behaves normally as required, this ensures there is no problem in the loop. Please find the attached images.


    Attachment 134708Attachment 134709Attachment 134710


    in images the green colored is tx B where as blue colored is tx A

    - - - Updated - - -

    Quote Originally Posted by ahsan_i_h View Post

    1. Interchange the connections of the two transmitters. If the problem shifts to transmitter B, then probably the problem is with PLC or related wiring. If the problem stays with transmitter A, then probably the problem is in the installation of the transmitter A.
    Actually the truth is if one is transmitter is working properly and other transmitter is showing Ghost behavior, therefore there is a risk of going completely blind. the transmitter A is replaced with new one but still the problem persists.


    Quote Originally Posted by ahsan_i_h View Post
    2. For liquid level measurement with DP transmitter, usually dry/wet reference leg installation methods are used.
    You can see a drain valve. Your system must have such a drain/fill valve (solenoid/pneumatic etc.) which will be controlled from PLC/DCS. Does that valve work properly?
    we are using diaphragm type for that capillary tube is attached with filled liquid, this maxes it wet leg and completely isolated/sealed.

    Thanks

    •   Alt21st December 2016, 08:00

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    Re: Level Measurement DP type problem

    Measure the output current (4-20 mA) of the transmitter.
    During unstable behavior is the output current stable ?

    Is the unstable behavior is related with any noise source ?
    (for example, the problem may starts when a VFD controlled motor is started)

    Unfortunately the attachments in post #4 can't be seen.

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    Re: Level Measurement DP type problem

    hunting-2.pnghunting-while-process-1.pnghunting-while-process.pngloop-checking-forcing-tx-fix-value-2-.pngloop-checking-forcing-tx-fix-value.pngloop-checking-external-ma-source.png

    - - - Updated - - -

    Quote Originally Posted by ahsan_i_h View Post
    Measure the output current (4-20 mA) of the transmitter.
    During unstable behavior is the output current stable ?

    Is the unstable behavior is related with any noise source ?
    (for example, the problem may starts when a VFD controlled motor is started)
    The current is unstable, it also fluctuates.
    The problem persists indefinitely irrespective to any plant conditions.
    if there was noise problem then the value should have fluctuated when in forced condition.(i think)

    thanks

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    Re: Level Measurement DP type problem

    Is there any external power supply (e.g 24VDC) which is supplying current to all the sensors ? See the picture below.



    You can see a power supply unit. Old SMPS power supply unit may give high ripples (Due to old electrolytic capacitors) at its output and may cause trouble. If there is a common power supply unit, check its output using oscilloscope.

    How do you make a force condition ? Do the transmitters have display and keypad ?

    If possible upload the datasheet of transmitter and wiring diagram which shows how the transmitter, PLC card etc are connected with each other.

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    Re: Level Measurement DP type problem

    Hi,
    apologies for late replying.
    the transmitter is loop powered no seperate/ external power supply.
    forcing is accomplished by using HART, yes tx has display
    Last edited by cheetha; 7th January 2017 at 04:24.

    •   Alt7th January 2017, 04:19

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  9. #9
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    Re: Level Measurement DP type problem

    please find datasheet
    http://obrazki.elektroda.pl/2004730900_1483773204.png


    thanks in advance

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