The Pitot tube is a differential pressure device that works by creating an obstruction in the flow of liquid inside a pipe. As the liquid passes through the obstruction, its potential energy converts into kinetic energy, increasing the velocity and reducing the pressure. Once the liquid exits, velocity decreases and pressure recovers. The generated pressure differential is proportional to the square of the flow rate, calculated using Bernoulli’s equation.
An averaging Pitot tube, also called an Annubar, improves on the traditional Pitot design by taking pressure samples from multiple points across the pipe diameter. It is installed perpendicular to the flow, with impact pressure ports on the upstream side and static pressure ports on the downstream side. These ports connect to a differential pressure transmitter for measuring the average pressure drop.
This design offers accurate, reliable flow measurement while minimizing energy losses and allowing easy installation without major pipeline modifications.
Pitot Tube measurement formulas
To calculate the volumetric flow rate:
QA = N * K * D2 * Faa * (√DP/GF)
(For liquids)
QA = N * K * D2 * Faa *(√DP/ρF)
(For gases)
To calculate the mass flow rate:
QMASS = N * K * D2 * Ya * Faa * √(P*DP) /T
(For gases and steam)
where,
DP = differential pressure
N = units conversion factor
D = internal diameter of the pipe
Faa = thermal expansion factor
GF = specific gravity factor
ρF = fluid density
Ya = averaging Pitot tube expansion factor
P = static pressure
T = temperature of fluid