Pitot Tube


The Pitot tube functions as an obstruction that changes the cross-section of the flow of the liquid in the pipe or conduit. In a differential pressure flow meter, as the liquid passes through the obstruction, its potential energy is converted into kinetic energy. The velocity of the liquid increases, and is accompanied by a simultaneous decrease in the pressure. When the liquid exits the obstruction, the velocity decreases and the pressure increases again. This pressure drop generated across the obstruction is proportional to the square of the flow rate and is calculated using Bernoulli's equation.

An averaging Pitot tube, also called an Annubar, is based on the traditional Pitot tube design. While the Pitot tube measures the pressure of a liquid at a single point, the more modern averaging Pitot tube measures the average pressure by taking sample values at different points in the pipe. It is inserted directly into the pipe and is perpendicular to the direction of flow. The side of the tube facing the flow of the liquid consists of several impact pressure ports while the opposite side of the tube may have a single or multiple static pressure ports. These ports are connected together to a secondary device such as a differential pressure transmitter.

When the liquid comes in contact with the tube, the kinetic energy of the liquid is converted into potential energy and velocity is reduced to zero. The pressure now measured at the upstream ports is called the total impact pressure which is the sum of the static pressure and the dynamic pressure of the liquid. The impact pressure is directly proportional to the flow rate of the liquid. As the liquid flows around the Pitot tube, the static pressure ports measure the decrease in pressure downstream of the flow. These pressure differentials are then averaged to determine the flow rate of the liquid using the following 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)

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

No industry can progress without reliable and accurate measurement. The key is measurement, simple as that. Measurement can result in two possible outcomes: If the result confirms your hypothesis then you've made a measurement; If the result is contrary then you've found a problem.

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