The Pitot tube is named after Henri Pitot who used a bent glass tube to measure velocities in a river in France in the 1700s. Pitot tubes can be very simple devices with no moving parts used to measure flow velocities.
Pitot tubes are a common type of insertion flowmeter. The below animation shows the basics for a Pitot tube, where pressure is generated in a tube facing the flow, by the velocity of the fluid.
This ‘velocity’ pressure is compared against the reference pressure (or static pressure) in the pipe, and the velocity can be determined by applying a simple equation.
Note: In Animation from right to left: The tube inserted in the center of the pipe is used to measure Total Pressure and the next second tube is used to measure the static pressure.
When the flow rate through the pipe changes, the pressures at the total pressure tube and static pressure tube vary with respect to the flow velocities. The difference between the total pressure and static pressure is used to measure the proportional flow rate passing through the pipe.
A DP-type transmitter is used to measure the difference between total pressure and static pressure and it is converted into a proportional flow rate.
In practice, two tubes inserted into a pipe would be cumbersome, and a simple Pitot tube will consist of one unit as shown in the Below Figure. Here, the hole measuring the velocity pressure and the holes measuring the reference or static pressure are incorporated into the same device.
Because the simple Pitot tube (Above Figure ) only samples a single point, and, because the flow profile of the fluid (and hence velocity profile) varies across the pipe, accurate placement of the nozzle is critical. To avoid this type of problem we are using averaging pitot tubes.
Note that a square root relationship exists between velocity and pressure drop (see Below Equation). This limits the accuracy to a small turndown range.
Where:
u1 = | The fluid velocity in the pipe |
Δp = | Dynamic pressure – Static pressure |
ρ = | Density |
There are some practical limitations to the use of a pitot-static tube:
The averaging Pitot tube (Below Figure) was developed with a number of upstream sensing tubes to overcome the problems associated with correctly siting the simple type of Pitot tube.
These sensing tubes sense various velocity pressures across the pipe, which are then averaged within the tube assembly to give a representative flow rate of the whole cross-section.
Reference: spiraxsarco
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Thank you sir for the article , it really helped a lot to clear the concept.
Thanks sir