flow measurements

 flow measurements and instrumentation

flow measurements devices

  ● Flow measurement is essential in many industries such as the oil, power, chemical, food, water, and waste treatment industries.

● These industries require the determination of the quantity of a fluid, either gas, liquid, or steam, that passes through a check point in their daily processing or operation.
● For many industrial operations the accuracy of a fluid-flow measurement is directly related to profit. Examples of such process are:
◦ Gasoline pump
◦ Water meter at home.

● Flowmeters need to be integrated into existing/planning piping system to be useful. 
●There are two types of flowmeter installation methods: inline and insertion.

●Flow measurements methods may be categorized as:
◦ Primary or quantity methods
● Positive-displacement meters
◦ Secondary or rate devices
● Obstruction meters
● Velocity probes
+●Special methods
Positive displacement meters
●Positive displacement flowmeters, also know as PD meters, measure volumes of fluid flowing through by counting repeatedly the filling and discharging of known fixed volumes.
PD Meter 
◦ Rotating Impeller:

○PD meters advantages:

•Most accurate
•Fast time response
◦ PD meters disadvantages:
•High pressure drop due to its total obstruction on the flow path 
•Very low tolerance to suspension in flow (particles larger than 100 µm need to be filtered before the liquid enters the flow-meter).

Secondary or rate devices
•Obstruction meters Venturi Meters
• Nozzles Meters
• Orifices Meters

Venturi Meters
• A Venturi tube is connected to the existing pipe, first narrowing down in diameter then opening up back to the original pipe diameter.

 The changes in cross section area cause changes in velocity and pressure of the flow. These changes can be used to measure the flow-rate of the fluid.
As long as the fluid speed is sufficiently subsonic, the incompressible Bernoulli's equation describes the flow. Applying this equation to a streamline traveling down the axis of the horizontal tube gives,

•From continuity, the throat velocity Vbcan be substituted out of the above equation to give,

•Solving for the upstream velocity 
Va and multiplying by the cross-
sectional area Aa gives the 
volumetric flow-rate Q,




• A discharge coefficient C is typically introduced to account for the viscosity of fluids,




C is found to depend on the Reynolds Number of the flow, and usually lies between 0.90 and 0.98 for smoothly tapering venturis.






Secondary or rate devices
Obstruction meters
 Venturi Meters
 Nozzles Meters
 Orifices Meters

Nozzles Meters
A nozzle with a smooth guided 
entry and a sharp exit is placed 
in the pipe to change the flow 
field and create a pressure drop 
that is used to calculate the flow 
velocity.



Secondary or rate devices

 Orifices Meters


A flat plate with an opening is 
inserted into the pipe and placed 
perpendicular to the flow stream.




 As the flowing fluid passes through 
the orifice plate, the restricted cross 
section area causes an increase in 
velocity and decrease in pressure.


 It is recommended that location (1) 
be positioned one pipe diameter 
upstream of the orifice, and location 
(2) be positioned one-half pipe 
diameter downstream of the orifice.


Pitot tube

 The Pitot tube (named after Henri Pitot in 1732) measures a fluid velocity by 
converting the kinetic energy of the flow 
into potential energy. 
 The conversion takes place at the 
stagnation point, located at the Pitot tube 
entrance.



Total-Pressure Probes
 A pressure higher than the free-stream 
(i.e. dynamic) pressure results from the 
kinematic to potential conversion. 
 The "static" pressure is measured by 
comparing it to the flow's dynamic 
pressure with a differential manometer.


Pitot Tube
Evaluated at two different points 
along a streamline, the Bernoulli 
equation yields,


 If z1 = z2
and point 2 is a stagnation 
point, i.e., v2 = 0, the above equation 
reduces to,

The velocity of the flow can hence be 
obtained,

 

 

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