A venturi tube also measures flow rates by constricting fluids and measuring a differential pressure drop. Venturimeter is a type of flowmeter that works on the principle of Bernoulli’s Equation.
Blog Contents
- Working Principle
- Parts of Venturimeter
- Venturimeter Equations
- Features of Venturimeter
- Types of Venturimeter
- Applications of Venturimeter
- Advantages of Venturimeter
- Disadvantages of Venturimeter
1. Working Principle
The pipe cross-sectional area is reduced to create a pressure difference which is measured with a manometer to determine the rate of fluid flow.
Venturi meter is a differential head type flowmeter.
In the upstream cone of the Venturi meter, velocity is increased, the pressure is decreased.
Pressure drop in the upstream cone is utilized to measure the rate of flow through the instrument
2. Parts of Venturi Meters
There are two tappings on the venturi meter for pressure measurement.
Upstream pressure tapping is located at a distance of one half of pipe diameter (D/2) upstream of the convergent entry.
Downstream pressure tapping is located in the throat (d/2) as shown in Fig.
Cylindrical Entrance Section:
Venturimeter entrance is a straight cylindrical section with a length equal to 5 to 8 times the pipe diameter.
Convergence Conical Section:
In this section, the venturi meter tube diameter gradually decreases. The conical angle is normally 210 ± 20. While the liquid flows inside the venturimeter, the velocity of fluid increases at the expense of a decrease in pressure.
Cylindrical Throat:
Throat consists of the minimum venturimeter diameter. In the throat section, the velocity is maximum and pressure is minimum. Normally, throat diameter = 1/3 to 1/4th of inlet pipe diameter.
Diverging Conical section:
At this section of venturimeter, the tube diameter gradually increases. So, the pressure is build up again to the original inlet pressure.
The cone angle is 5-70. British Standard BS-1042 specifies two conical angles, 5–70 and 14–150 for the outlet cone.
3. Venturimeter Equations
Bernoulli’s principle states the relation between pressure (P), kinetic energy, and gravitational potential energy of a fluid inside a pipe.
The mathematical form of Bernoulli’s equation is given as:
Where,
p= pressure inside the pipe
ρ =density of the fluid
g =gravitational constant
v = velocity
z=elevation or head
a = cross-sectional area of the pipe
d= diameter of the pipe
4. Features of Venturimeters
Design Pressure: No limitation. Limited by DP transmitter/ pipe press.ratings.
Design Temperature: No limitation. Limited by DP transmitter/ pipe pressure ratings
Sizes: 25 mm to 3000 mm
Fluids/ Applications: Clean Liquids/ clean gases
Limited applications: Dirty /corrosive/viscous Liquids & Dirty gases
Flow range: limited only by pipe size and beta ratio.
MOC: No limitation (cast iron/ carbon steel/ SS/Monel, Titanium, Teflon, Hastelloy, Naval Bronze/haste alloy)
Accuracy : It varies from ±0.25% to ±0. 75% of actual flow. The accuracy of DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
Rangeability is 3:1 to 5:1.
Upstream length/ Downstream straight length is 20 / 5
5. Types of Venturimeters
Normally three types of venturimeters are available:
Horizontal Venturimeter:
This type of venturimeter has the highest kinetic energy and the lowest potential energy.
Vertical Venturimeter:
This type has the maximum potential energy and the minimum kinetic energy.
Inclined Venturimeter:
Both potential and kinetic energy are in between the above two types mentioned.
6. Applications of Venturimeter
Venturimeters find wide application in fluid industries. The major application of venturimeters include7. Advantages of Venturimeters
Lower head losses than orifice plates reducing the capital expenditure on pumping eqpt. / save pump energy costs
No process interruption for the exchange of DP transmitter.
Can be used for temperature extremes
Cryogenics or High Temperatures
8. Disadvantages of Venturimeters
Highly expensive
Larger and heavier to handle.
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Venturi Meters 2 Equations Codes Installation blog contains
- Venturimeter Equations
- Coefficient of Discharge of Venturimeter (Cd)
- Codes and Standards of Venturi meter
- Installation of a venturi meter
- Venturi meter Upstream and Downstream Pipe Straight Leg Requirement
1. Venturimeter Equations
Bernoulli’s principle states the relation between pressure (P), kinetic energy, and gravitational potential energy of a fluid inside a pipe.
The mathematical form of Bernoulli’s equation is given as:
Where,
p= pressure inside the pipe
ρ =density of the fluid
g =gravitational constant
v = velocity
z=elevation or head
a = cross-sectional area of the pipe
d= diameter of the pipe
Suffix 1 and 2 are used to denote two different areas; 1 denotes cylindrical inlet section and 2 denotes throat section.
Now as the pipe is horizontal; there is no difference in elevation of pipe center line; So, z1=z2. Re-arranging the above equation we get the following:
(p1-p2)/ρg = (v22-v12)/2g
(p1 – p2)/ ρg is the difference of pressure heads in sections 1 and 2 which is equal to h that can be measured in the differential manometer. So the above equation becomes
h=(v22-v12)/2g……….eq.1
Now applying continuity equations between the same sections 1 and 2, we get
a1v1=a2v2 or v1=(a2v2)/a1
Putting this value of v1 in eq.1 and solving we get,
This Q represents the theoretical discharge of Venturi Meter in ideal condition.
But in actual practice, there will be always be some frictional loss.
Hence, the actual discharge will always be less than the theoretical discharge.
So, to calculate the actual discharge, the above Q value is multiplied by Cd, called the Coefficient of discharge of venturimeter.
So the actual flow rate through the throat of the venturimeter will be given by the following equation.
2. Coefficient of Discharge of Venturimeter (Cd)
The coefficient of discharge for Venturimeter, Cd is defined as the ratio of the actual flow rate through the venturi meter tube to the theoretical flow rate. So the venturi meter discharge coefficient is given by:
Cd=Qact/Q
As Qactual will always be less than Qtheoretical due to frictional losses, the value of Cd is always less than 1.0.
The typical range of the discharge coefficient of a Venturi meter is 0.95-0.99 but this can be increased by proper machining of the convergent section.
The value of venturimeter discharge coefficient differs from one flowmeter to the other depending on the venturimeter geometry and the Reynolds number.
ISO-5167 code provides the values of venturimeter discharge coefficients. For accurate flow measurement, normally straight length requirement upstream and downstream of venturimeter is specified.
3. Codes and Standards of Venturi meter
The codes and standards that provide guidelines related to venturi meters are
- ISO 5167
- ISO 9300
- AWWA M33
- ISO TR 15377
- BS 1042
- ASME MFC-8M
- ASTM D2458
- AGA 9
4. Installation of a venturi meter
Proper installation of venturi meter is the key for ideal operation. So, the installation of venturi meters must be performed following manufacturer guidelines. Normally, the following guidelines to be followed while installing a venturi meter in a piping or pipeline system:
The flow direction arrow in the venturi meter should be checked and installed to agree with the direction of the flow.
Flanges at the venturi meter ends should be properly aligned with the piping flanges.
Pipe Support should not be placed on venturi meters.
Bolts should not be over-torqued.
Installation tolerances should be within industry standards.
Pressure taps should be oriented horizontally for liquid service applications
5. Venturi meter Upstream and Downstream Pipe Straight Leg Requirement
For proper functioning and accurate results, the flow through the venturi meters should stabilize. This calls for minimum straight pipe length requirements upstream and downstream of the venturi meter.
Depending on the type of fitting, type of venturi meter, and beta ratio (the throat diameter divided by the inlet diameter) the straight leg requirement varies.
The following image provides a sample table that provides typical strength leg requirements while installing a venturi meter in a piping system.
