Flow Measurement

Troubleshooting of DP Type Flow Measurement

What is DP flow meter?

Differential-pressure flow transmitters are very common. Most of the time, a DP-type flow transmitter is used to measure the flow of liquids and gasses, which are clean.

DP flow meter

DP flow solutions have two parts: a primary element that lowers the pressure and a secondary element (a DP flow transmitter) that measures the difference in pressure.

Differential pressure flow meters, which are sometimes called DP flow meters, use laminar plates, an orifice, a nozzle, or a Venturi tube to create an artificial restriction and then measure the pressure loss of fluids as they pass through the restriction. Bernoulli’s principle says that the drop in pressure across a restriction is equal to the square of the flow rate. The flow rate goes up as the pressure drop goes up. These tough, accurate meters are great for a wide range of clean liquids and gasses.

To troubleshoot issues with a DP type flow transmitter, follow the methods listed below

Problem-1: DP-type flow transmitter in an inactive mode without power

Potential Causes:

  1. There are a few things that could cause a DP type flow transmitter to lose power, such as a problem with the cable, fuse, or barrier.
  2. First, check the DP type flow transmitter’s cable connection.
DP-type flow transmitter in an inactive mode without power
  • The right kind of lugs should be used for each connection. Then, use a properly calibrated multimeter to check the voltage.
  • Refer the instrument loop drawing, Check the connections in the junction box and the marshalling cabinet if there is no voltage.
  • If you find a problem, make sure the cables are tightened properly and/or that the plugs are in the right place.
  • Check to see if the fuse in the marshalling cabinet is still good. If the fuse doesn’t work, replace it with another fuse with the same rating.
Fuse in the marshalling panel
  • Check on the barrier. Replace the barrier if it’s broken if it’s malfunctioning. Also, make sure the cable is in good condition.
Barrier in the marshalling panel

Problem-2: Mismatch between field and PLC/DCS flow values

Potential Causes:

  1. The problem of flow value mismatch is often caused by inaccurate range setup on both sides, both in the PLC/DCS/SCADA and the field transmitter.
transmitter range setup menu
  • Check both the PLC/DCS/SCADA and the field transmitter’s range.
transmitter signal range plc program
  • The flow transmitter’s correct range, which should be set up on both sides, will be listed on the data sheet.

Problem-3: The DP flow transmitter does not detect flow

Potential Causes:

  1. There may not be any flow in the pipe, therefore check the process system for sufficient flow.
  2. Possibility of isolation valve(s) being closed or equalization valve being opened, and put the valve(s) in the right place according to the procedure.
  3. Inadequate transmitter setup, and check instrument datasheet for flow configuration.

Problem-4: Unreliable value shown by a DP type flow transmitter

Potential Causes:

  1. Examine the condition of the process isolation valves on the flow transmitter of type DP.
  2. Both the HP side isolation valve and the LP side isolation valve should be open.
DP type flow transmitter1
  • Examine the manifold’s condition. The manifold’s equalization valve should be in the closed position.
DP type flow transmitter2
  •  In the manifold, both of the isolation valves should be open. It is important to correctly fix the vent ports in the manifold.
  • Each tubing connection should be examined. All of the fittings have to be leak-free. There shouldn’t be any ruptured tubes. To find the leak, use a snoop liquid leak detector (or any other leak detector).
  • After properly isolating the DP type flow transmitter, fix any major or minor tube leaks and/or replace punctured tubes.
  • The tubes can also be clogged. To properly clear out the tube, you need to flush or flush with the drain closed.
  • If flushing the tube doesn’t work, you can unclog it with a pump. When using a pump to unclog a tube, the maximum pressure should be 1.5 times the pressure of the process fluid.
  • Check whether the HP side of the transmitter is connected to the HP side of the tapes in the line.
  • Additionally, make sure the transmitter’s LP side is attached to the LP side of the tapes in the line. If detected reversed, the tubing connection must be corrected.
  • The value will not change if any forcing (bypass activation) is applied to either the PLC/DCS or the field transmitter. After receiving the necessary authorization or after completing the interlock restoration process, remove the force.
forced value in plc and dcs
  1. A square root function should only be used on one side. Either the PLC/DCS or the field transmitter side. The flow value will be completely inaccurate if the square root is present on both sides.
Square Root function in transmitter
  1. If the transmitter is still not reacting correctly, you may use a hand pump or a calibration test equipment, depending on the range, to test its responsiveness.
DP type flow transmitter calibration
  1. Examine the condition of the orifice, venturi, pitot tube, or other basic sensing devices that are used to measure pressure. The line must be isolated for this reason.
  2. Utilizing the calibrating equipment, calibrate the DP type pressure transmitter; the device’s response will be found to be satisfactory.
  3. Verify all relevant settings, including LRV, URV, mA range, and device address, if applicable, in accordance with the DP type flow transmitter’s data sheet.

Problem-5: Value fluctuations in DP type flow transmitters

Potential Causes:

  1. Examine the impulse tubes for leaks. The reading of the DP type flow transmitter might potentially fluctuate as a result of minor tube leakage. Repair and arrest the leakage.
  2. Adjust the damping value in the transmitter.
  3. Flush the flow transmitter. The value of the flow will change as a result of any trapped air bubbles in the impulse line.
DP type flow transmitter flushing
  • Verify the flow element since there may sometimes be a partial blockage in the tapping or process pipes.

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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