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.
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.
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.
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
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.