Avg pitot tube flowmeter instalation.


Thread Starter


Situation: encountering installation obstruction for pitot tube flow sensor for measuring liquid (water). the pipe is just barely above ground with no allowance to install the flowmeter at bottom half of the pipe, as per manufacturer guidelines for liquid.

Ques: what are the affects on flow measurements if fluid is approx fully filled and that we install from top at 12 o'clock or 45 deg from 12 o'clock - such as following measuring gas application ? has anyone install it, such way ?

Assuming a full pipe, installation at 45 degrees from the top is going to work okay. Installation at the 12 o'clock position (vertically in the top of the pipe) is less likely to be acceptable.

The biggest issue is whether or not the pipe is full. If it isn't full, it doesn't matter what the velocity reading from the annubar is going to be--the flow calculation will be wrong.


Walt Boyes, Fellow, International Society of Automation, ISA
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Rohit Chandak

You can very well install it from top of the pipe at 12 O Clock position on liquid service provided you make necessary arrangement for air venting. Take the LP/HP tube in air, provide a Tee which will help to bifurcate the signal. The air which comes along with signal will take the upper end of the tube while the other tube should have slope wherein liquid leg will fill the tube going towards transmitter free from air entrapment.

Angle to the sensor will help to keep most of the pressure ports in liquid stream to avoid air collection in instrument tube.
A regular pitot tube will have a problem with air in the line. I once had an application to measure flow in a large half buried water line with massive amount of air. The insertion mag meters I purchased for the job didn't work and we had no way of getting a replacement in time so I ended up making pitot tubes to fit the mag meter connection. The ports were arranged to go down to the tip then back up to the DP cell so the air couldn't get up to the transmitter. I was able to calibrate by doing a drop test on the water tank.

I went back many years later, they were still working and the instrument tech had never looked at them.