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Flow Measurement

Annubar Flow Meter Working Principle

Annubar flow meter are sometimes referred to as Averaging pitots and contain multiple pressure tappings to ‘average’ the flow; this is to try to compensate for a non-ideal flow profile.

The averaging pitot tube is inserted across the pipe as shown below. One side of the bar has pressure taps facing the flowing fluid that are coupled into an “averaging” chamber that measures the total (i.e. static + dynamic) pressure of the fluid.

Annubar Flow Meter

Annubar Flow Meter Working

There may be a single port or multiple tapping ports on the opposite side of the bar to measure the low static pressure in the downstream region.

The difference between the total and static pressures is effectively a measure of the fluid velocity head, which together with the pipe area enables the volumetric flowrate to be determined.

Annubar Flow Meter Principle

Principle of Operation

The  Annubar primary flow element is a device used to measure the flow of a liquid, gas or steam fluid that flows through a pipe. It enables flow measurement by creating a differential pressure (DP) that is proportional to the square of the velocity of the fluid in the pipe, in accordance with Bernoulli’s theorem.

This DP is measured and converted into a flow rate using a secondary device, such as a DP pressure transmitter.

The flow is related to DP through the following relationship.

Annubar Flow Equation

where:

Q = Flow Rate

K = Annubar Flow Coefficient

DP = Differential Pressure

The Annubar generates a DP by creating blockage in the pipe and acting as an obstruction to the fluid. The velocity of the fluid is decreased and stalled as it reaches the front surface the Annubar sensor, creating the impact/high pressure.

The Annubar senses the impact pressure by utilizing a DP transmitter.

Annubar Flow

As the fluid continues around the Annubar sensor, it creates a lower velocity profile on the backside of the sensor, creating the low/suction pressure downstream of the Annubar.

Individual ports, located on the backside of the Annubar sensor measure this low pressure. Working on the same principle as the high pressure, an average low pressure is maintained in the low pressure chamber that connects directly into the transmitter for measurement.

The resultant differential pressure is the difference between the impact (high) pressure reading and the suction (low) pressure reading as seen below.

DP = PH – PL

where:

PH = High Pressure

PL = Low Pressure

The measured DP is used to calculate the flow rate.

Annubar Flow Formulas

Annubar Flow Meter Formulas

Advantages:

  • Can be inserted through a small opening.
  • Can be used to sample the velocity at several points.
  • Low pressure drop, minimal obstruction.

Disadvantages:

  • Single point measurement.
  • Pitot tube is fragile
  • DP signal is low.

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4 comments

V. Parasuraman November 24, 2017 at 6:04 am

Reddy gaaru, what if an 18 inch annubar installed inside a 20 inch dia pipe. Observed showing more flow about 250 SCM per hour. How to compensate this error? Flow totalizer vendor told it was not possible to adjust in totalizer, but to change URV (from 63 mmH2O to 66 mmH2O – sounds unique, isn’t it?!) in the DP transmitter. But no change found. Please suggest. Awaiting your kind response.

Reply
Pedro Takahashi July 20, 2020 at 9:53 pm

Zero calibration

Reply
saroj mandal July 8, 2020 at 12:26 pm

yes, you can do it by recalibrating the DP transitter. Put an offest error to match the actual DP value if annubar were of 20 inch instead of 18 as you have.

Reply
Mohammad August 3, 2020 at 8:12 am

Can we use more than one DP transmitter( Two or three) from single Annubar flow meter and what the disadvantages

Reply

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Annubar Flow Meter Working Principle

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