Field Instrumentation

How to Safely Zero a DP Transmitter with 3-Way Valve and 5-Way Valve Manifolds ?

The 5-valve manifold is critical for relieving, balancing, and venting of a DP transmitter. Setting the transmitter to zero helps in getting actual values since it eliminates any potential differences. Follow the steps below to safely zero the transmitter:

Initial Configuration of 5 way-Valve Manifold (Normal Operation of DP Transmitter)

Under normal operating conditions:

  • Process and transmitter connectors to process, block valve B1 and B2 are open.
  • Equalizing valves E1, E2 are closed while drain/vent valve is also closed.

In order to find the location of the valves, refer to the diagram above.

Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 5 way -Valve Manifold  1
  • First, shut down the valve B2 that disconnects the transmitter from the low-pressure or LP side of the process.
  • This helps in protecting the process fluid within the transmitter during calibration from that within the LP side.
Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 5 way -Valve Manifold  2
  • Gradually crack open the valve E1 situated at the HP side of the transmitter.
  • Important: First of all, always open the high-pressure equalizing valve. Using the low-pressure equalizing valve (E2) first may lead to over pressurization of the transmitter and its consequent damage.
Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 5 way -Valve Manifold  3
  • When E1 is fully opened, open the second valve, E2, the low-pressure equalizing valve.
  • This action equalizes the high-pressure (HP) and the low-pressure (LP) sides of the transmitter. At this point, the transmitter is essentially ‘zeroed’ since the pressure differential across the transmitter is zero.
  • Check that the transmitter output is correct with the help of a HART communicator or any monitoring tool. It should read zero and should not show any pressure differentials at this stage.

Click here for  HART transmitter calibration procedure – For pressure transmitter

Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 5 way -Valve Manifold  4
  • After zeroing is finished, shut the valve E2 to isolate the LP side again as was outlined earlier.
Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 5 way -Valve Manifold  5
  • Then, close valve E1 in order to isolate again the high pressure side as earlier observed during the procedure.
Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 5 way -Valve Manifold  6
  • Finally, reopen valve B2 (low-pressure block valve). This restores the transmitter to its normal operating state.
  • Confirm that the transmitter is functioning correctly by checking the process readings.
  1. Safety First: Ensure you are familiar with the process and safety protocols before performing any maintenance.
  2. Monitor Output: Always check the transmitter’s output during and after zeroing to confirm calibration.
  3. Avoid Over-Pressurization: Opening valves in the wrong sequence can cause damage to the transmitter. Follow the steps carefully.

By following these steps, you can safely and effectively zero a differential pressure transmitter using a 5-valve manifold.

Without taking the transmitter out of service, zeroing a transmitter guarantees that the output signal accurately reflects the zero differential pressure condition. This procedure is not the same as using a 5-Valve manifold. To prevent overpressurizing or harming the transmitter, carefully follow these instructions.

How to Zero a Differential Pressure Transmitter using a 3 way -Valve Manifold ?

In normal operation of a 3-valve manifold:

  1. The process and transmitter are connected via open block valves (B1 and (B2).
  2. By closing the equalizing valve (E), the transmitter’s high-pressure (HP) and low-pressure (LP) sides are isolated. 

Refer to the above diagram (if available) for visual guidance.

Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 3 way -Valve Manifold  1
  • Shut off valve B2 to separate the transmitter from the process’s low-pressure (LP) side.
  • By performing this step, process fluid cannot affect the transmitter while zeroing.
  • Important: To ensure safety and protect against transmitter damage, always close the low-pressure side first.
Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 3 way -Valve Manifold  2
  • To enable pressure to equalize between the transmitter’s high-pressure (HP) and low-pressure (LP) sides, slowly open the equalizing valve (E).
  • At this point, the transmitter signal should show 0 and there should be no pressure differential across the transmitter.
  • Confirm Zero Output: Make sure the transmitter output shows zero by using a HART communicator or another monitoring tool.
Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 3 way -Valve Manifold  3
  • Close the equalizing valve (E) to isolate the HP and LP sides once zeroing is finished.
  • Performing this action guarantees that the transmitter is prepared to resume its usual operation.
Step-by-Step Procedure for Zeroing a Differential Pressure Transmitter using a 3 way -Valve Manifold  4
  • In order to restore the process connection on the low-pressure (LP) side, reopen valve B2.
  • The transmitter is fully operational again and ready to monitor differential pressure. 
  1. Sequence Matters: To prevent damaging the transmitter or overpressurizing it, always adhere to the exact order of activities.
  2. Safety First: Make sure you have the required equipment and safety precautions in place, and that you understand the process system.
  3. Monitor Output: To ensure correct calibration, continuously check the transmitter output both during and after zeroing.
  4. Prevent Unexpected Movements: Slowly open and close valves to avoid giving the transmitter pressure shocks.

By following these steps, you can effectively and safely zero a differential pressure transmitter with a 3-valve manifold.

Zero trim is a calibration procedure used to fix any zero-shift brought on by the system’s static pressure or the transmitter’s final installation position. It guarantees that the transmitter measures pressure accurately beginning at zero. The equalizing valve must remain open during zero trim, which is carried out after the transmitter is completely mounted.

Click here for Step by Step Pressure Transmitter Installation Procedure

To zero a DP transmitter, follow these steps:

  1. Locate the zero-adjustment screw inside the transmitter cover.
  2. Use a slotted screwdriver to turn the adjustment screw.
  3. Make sure there is no difference between the high-pressure (HP) and low-pressure (LP) sides of the transmitter to equalize the pressure.
  4. Turn the screw clockwise to increase the output or counterclockwise to decrease the output until it reads zero.

The steps below must be followed in order to equalize a DP transmitter:

  1. To balance the pressure on the transmitter’s HP and LP sides, open the equalizing valve.
  2. Next, open the block valve for high pressure.
  3. After balancing, shut off the equalizing valve.
  4. Lastly, to put the transmitter back into action, open the low-pressure block valve.

The following actions should be taken for gauge/absolute pressure transmitters:

  1. Close off the transmitter’s tap valve, also known as the main valve.
  2. To ensure that the transmitter is only subjected to the seal liquid pressure, loosen the fill plug and release any pressure that may be present.
  3. Use the provided adjustment mechanism to change the zero point.
  4. After making the necessary adjustments, secure the fill plug and slowly open the tap valve again to resume regular operation.

Click here for Differential pressure Flow Transmitter Output Calculator

Zero adjustment ensures that the transmitter outputs the correct baseline signal typically 4 mA for a 0 Bar input in a standard configuration. This adjustment is crucial for accurate readings and is performed to align the transmitter’s baseline output with the actual input conditions. In certain situations, zero adjustment also fixes any shifts brought on by installation.

  • Zero Offset: The transmitter’s signal at zero input pressure. Any deviation from the expected signal (e.g., 4 mA) at zero pressure is considered a zero offset.
  • Span Tolerance: The difference between the transmitter’s signal output at zero and full-scale pressure. This parameter is usually expressed as a percentage of the full-scale span.

Both zero and span offsets can be adjusted during installation or calibration to ensure the transmitter operates within acceptable tolerances.

Click here for What are the major problems in a pressure transmitter and how to troubleshoot it?

The term “zero drift” describes the steady variation in the transmitter’s output over time under zero input pressure. Environmental variables such as temperature fluctuations, the aging of transmitter parts, or shifts in system pressure might cause it. Zero drift can be decreased with routine calibration and maintenance.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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