Venturi Meter or Venturi Tube is a mechanical device used to measure the flow rate by converting pressure energy into kinetic energy.
Venture meters means –
The device measure the velocity or flow rate or discharge of fluid in the pipe.
It works based on Bernoulli’s equation & continuity equation.
Each venturi meter has a cylindrical entrance section, one converging part, a cylindrical throat, and a diverging part.
History of Venturi Meters
The main concept of venturi meter has come from G.B. Venturi and it was further developed with the help of An American Engineer Clemans Herchel. The name venturi meter came from G.B Venturi.
Why flow requirement is essentials?
The measurement of flow is very much essentials now a day. In all industrial applications, flow measurements can be explained as follows:
To control the system, flow control is mandatory. Suppose water is flowing in a chilled water circuit and suddenly flow reduces. What will happen then? The system will be inefficient or may stop. But if there is a flow measuring device that gives the data, this problem will be easily solved.
Energy-saving & Cost Saving
Flowmeter pointed out when the process is running efficiently or inefficiently. It helps to control the proper flow as well as increase efficiency. Hence, it helps us to save energy as well as cost.
The flowmeter application is in various fields. It helps to measure from liquids to gases, like water, steam, carbon-di-oxide, compressed air.
The flow rate is measured by the followings
flow nozzle, and
In this chapter, we will explore Venturi Meters.
Parts of Venturi Meter
A Venturi Meter is consisted of
Converging part or converging cone
Diverging Part or diverging cone
Let’s consider a pipe that consists of or connected to a venturi meter. In the pipe fluid is flowing so first it enters into a converging cone then Throat and then Diverging Cone.
This is the section having the size of a pipe to which it is attached.
It should be proceeded by a straight pipe and the length should not be less than 5 to 10 times the pipe diameter
It should be free from fittings, misalignment, and other sources which creates large-scale turbulence.
This converging part is the starting part of the venturi meter and it is attached to the pipe. It means the area will be decreased. It is starting section of the venturi meter which attached to the inlet pipe. When the water flows through the converging part, naturally the area will be decreased.
Now, as the area decreased – the velocity of the water will be increased
If velocity is increased – pressure energy will be decreased.
The other end of converging is attached to the throat.
The converging takes place normally at 21±2° angle.
The throat is placed in the middle of the venturi meter. This is a small cylindrical piece that connects the converging part and diverging part.
One side of the throat is connected to the inlet pipe.
Normally throat diameter is ¼ to ¾ of the inlet pipe diameter.
The length of the throat part equals to its diameter.
When water flowing through this throat area, the cross-sectional area remains constant
The area is constant means the velocity of flowing water as well as pressure energy remains constant.
This diverging part is the last part of the venturi meter and it is attached to the delivery pipe. It means the area will be increased. This part of the venturi meter is attached to the outlet pipe. When the water flows through the diverging part, naturally the area will be increased.
Now, as the area increased – the velocity of the water will be decreased
If velocity is decreased – pressure energy will be increased.
The angle of the diverging part is 5 to 15 degrees.
The low diverging angle helps to avoid the flow separations as well as eddies formation
Flow separation, eddies formation creates due to loss in energy
Kinetic energy is recovered into pressure energy and loss of energy is less.
The diffuser is angle about 5° to 7° and this low angle helps not to separate the flowing fluid from the boundary.
Look at the basic part of a common venturi meter.
1: Section 1 implies inlet.
2: Section 2 implies Throat,
3: Converging Section,
4: Diverging Section,
5: Outlet Pipe
Type of Venturi Meter
Venturi meter is classified into three types,
Horizontal venturi meter
Inclined venturi meter
Vertical venturi meters
Horizontal means the mounting of this type is horizontal.
Widely used flow meters.
In case of, horizontal pipe, it is used.
Kinetic energy is high, as it flows horizontally.
Potential energy is low as its weight in the particular control volume is less.
Vertical means the mounting of this type is vertical.
In case of verticle pipe, it is used.
Due to its vertical movement, velocity is becoming less.
Less velocity incurs lower kinetic energy (only if the flow is upward).
Potential energy is very high as its weight in the particular control volume is high.
Inclined means the mounting of this type is inclined instead of horizontal or vertical.
In case of an inclined pipe, it is used.
Due to its inclined movement, velocity is within the horizontal and vertical types.
Moderate velocity incurs moderate kinetic and potential energy.
Assumptions of Venturi Meters
There are few assumptions for venturi effects,
The inner side of the venturi meter is frictionless.
As per the continuity equation, for specific fluid flow,
Area (a) x Velocity (v) = constant.
Venturi Principle Description
The principle behind the operation of the Venturi meter is the Bernoulli effect as well as the Continuity equation. In the venturi meter, the fluid flow rate is reduced due to the reduction of the cross-sectional area.
Any point of flowing fluid in the venturi meter, there will be pressure energy, potential energy, and kinetic energy.
Any two points of flowing fluid, some of these energies are the same.
The total energy at any point in the wider diameter is equal to the total energy at any point in the narrow diameter.
Explanation Venturi Effects
We will explain here, how exactly a venturi meter works?
We will explain it with a simple diagram.
Increase in fluid speed results in a decrease in internal pressure.
Look at the image of the venturi meter. Consider two points Point-1 & Point-2 in both the different cross-section. a1 is the cross-section that is connected to the inlet pipe and a2 is the cross-section which is the throat.
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P.H1 + K.H1 + Pt.H1 = Constant
a1 = area of cross-section
p1 = pressure
v1 = velocity
Q1 = volume flow rate = a1 x v1
If P.H1 decrease, (K.H1 + Pt.H1) will increase.
P.H2 + K.H2 + E.H2 = Constant
a2 = area of cross-section
p2 = pressure
v2 = velocity
Q2 = volume flow rate = a2 x v2
As per continuity equation, a x v = constant.
If a decrease, v will increase.
The throat area means the area is small. Hence, the velocity is increased.
If P.H1 decrease, (K.H1 + Pt.H1) will increase.
or, If K.H1 increases, P.H1 decreases.
Explanation with Diagram
Look at this diagram, a fluid is flowing through this venturi meter.
We get the following,
a1 and v1 are the area and velocity of point 1.
a2 and v2 are the area and velocity of point 2.
Area a2 is smaller than area a1.
Fluid flows through this tube hence, the volume flow rate is the same, which is a1 x v1 = a2 x v2.
Velocity v2 is more than v1, to maintain the same volume flow rate.
It follows based on Bernoulli’s equation.
Velocity v2 increases mean K.H increases,
K.H increase implies a decrease in pressure P.H.
Knowing the cross-section, density & the pressure of two locations, the velocity of the fluid can be easily calculated.
U-Tube Manometer is widely used to measure the pressure difference.
A diverging part used after the throat to restores the pressure.
Derivation of Discharge
The derivation has been solved on paper because their several rotations cannot be typed here.
In the figure, the water flowing in the pipe (flow in) comes with section 1 and flow out goes with section 2.
The several notations are,
a1 = Inlet area in m2.
d1 = Diameter of Inlet.
p1 = Pressure at the inlet in N/m2.
v1 = Velocity at inlet in m/sec
d2 = Diameter of the throat.
a2 = Throat area in m2.
p2 = Pressure at the throat in N/m2.
v2 = Velocity at throat in m/sec.
h = Pressure heads.
Cd = Coefficient of Discharge.
Qact= Discharge at actual in m3/sec.
Qthe= Discharge (theoretical) in m3/sec.
Theoretical Discharge calculation
Applying Bernoulli’s equation at sections 1 and 2
Now applying the continuity equation at section 1 and 2
Substituting the value of v2 in the above equation
Q is the theoretical discharge under ideal conditions.
The actual discharge will be little different from the theoretical discharge and it is little less. The actual discharge is given by the formula
Where Cd is the coefficient of venturi meter and its value is less than 1.
Pressure Differential by Manometer
The other way to find h (Pressure heads) by using a differential U–Tube Manometer:
We all know, a manometer is having a liquid that is heavier than the fluid flowing in the pipe. This characteristic is used to measure the pressure differential.
Sh =Heavier liquid specific gravity which is in the manometer.
x = Difference of the heavier liquid column in U-tube
S0 =Flowing fluid specific gravity.
Sl =Liquid liquid specific gravity.
The liquid in the manometer is lighter than the flowing fluid in the pipe.
Venturi Meter Sample Calculation
Let’s try to understand a sample calculation for better understanding,
Let’s a Venturi meter is placed in a horizontal pipe. Diameter of inlet section is 20 cm and throat section is 15 cm. It is installed in a pipe to measure the flow of a liquid. The specific gravity of the liquid is 0.9 and mercury is 13.6.
The level of a mercury column is manometer is 25 cm.
Now, we have to calculate the discharge of liquid in lit/hr?
Mercury specific gravity, gm = 13.6
Mercury column, hm = 25cm
Liquid specific gravity, g = 0.9
Inlet diameter, a1 = 20cm
Throat diameter, a2 = 15cm
The area of inlet, a1 = (π d1 2)/4 = (π x 400)/4 = 314 cm2
The area of throat, a2 = (π d2 2)/4 = (π x 225)/4 = 176.6 cm2
Calculate Pressure Differential
The pressure head shall be
h = hm [( gm / g ) – 1]
h = 25 [(13.6 / 0.9) – 1]
h = 352.8 cm of liquid
From Equation (No. 6) of Discharge,
Q = a1 x a2 /[√(a12-a22) x √(2gh)
Q = 314 x 176.6 /[√(3142-176.62) x √(2x980x352.8)
Q = 177634 cm3/sec
Q = 177.634 lit/sec [1 liter = 1000cm3]
Q = 2.96 lit/hr.
Applications of Venturi Meter
Venturi Meter is used in various field like:
The flow rate of fluid in the pipe is calculated easily.
The flow rate of suspended solids, gases, slurries, dirty liquids can also be measured.
It helps to calculate the chemical dosing rate.
Wastewater treatment flow measuring rate.
Widely used in large-diameter pipes.
Used in the waste treatment process.
High-pressure recovery system.
It can measure the pressure.
It can measure the quantity of liquid or gas.
Medical application, like the flow rate of blood in arteries.
Advantages of Venturi Meter
The advantages of Venturi Meter are:
No moving parts & simple operation
Long term reliability
Low head loss, as well as energy loss.
Tolerance of high solids content
In a high flow rate, it is widely used.
Pressure recovery is high, around 90%.
Rarely affected by upstream turbulence.
Due to the flow, it is normally a self-cleaning device
It is suitable for cleaning as well
Rarely affected by erosion
Rarely get clogged with sediments
High coefficient of discharge.
It can be vertical, horizontal, or inclined to suit at the site.
Very less pressure drop.
Suitable for liquid consists of gas, solids, etc.
This can also be used for a compressible and incompressible fluid.
The pressure recovery is much better for the venturi meter than for the orifice plate.
It is self-cleaning. In addition to that, this device can be easily be cleaned and remove all dirt, entrants, etc.
The maximum to minimum flow rate is about 4 to 1
Very low-pressure loss.
High viscosity effect.
Disadvantages of Venturi Meter
Although there are few disadvantages of Venturi Meter, and those are:
Expensive due to the high installation costs.
The maintenance and as well as cost of maintenance cost is a little high.
Turn down, i.e., maximum capacity to the minimum capacity is poor.
Bulky and space constraints.
Long length requirements.
Venturi meters are not suitable for small diameter. Roughly it is below 76 mm.
Limitation of the lower Reynolds number like 150,000.
Less experimental data are comparative to the orifice plate.
Cavitation may occur, in case of slight design issues.
Cavitation and Venturimeter
Based on the venturi meter design, we have seen that the velocity of fluid at the throat is maximum which incurs a minimum pressure.
Let’s try to understand it,
Less pressure means it can be even negative or below vapor pressure.
If it reaches below the vapor pressure, fluid will start to evaporate.
Bubbles will be formed.
Cavitation will be started.
Hence, the pressure at the throat point is designed in such a way that it will never reach to vapor pressure.
Actual installation photograph
An installed image of the venturi meter is referred here.
Standard Venturimeter Specification
The main specification points of a standard venturi meter is listed below,
Normally the accuracy of a standard venturi meter can vary from (±) 0.25% to (±) 3.0%.
The line sizes are normally from 100mm to 800mm. In many cases, it is customized to suit the project requirements as well.
A standard value of throat diameter to the main pipe diameter ratio (Beta Ratio) is considered in the range of 0.3 to 0.75.
Normally 75000 is used as the minimum Reynolds number.
In addition to above, the following details to be given to select a venturi meter,
Type of fluid.
Type of application.
Temperature of fluid.
Viscosity & specific gravity of fluid.
Size of connection (Inlet and outlet).
Value of Beta Ratio.
Material of pipe.
Material of venturi meter, etc.
Manufacturers of Venturimeter
There are a lot of venturi meter manufacturer available worldwide, few of the are listed,
Aalborg Instruments & Controls,
ERDCO Engineering Corporation,
Materials of Venturimeter
Venturi meters can be made of different kinds of materials. It depends on the type of applications or fluids as well as project requirements.
Few of materials are listed for reference purpose only,
Standards for Venturi Meters
ISO 5167-3:2003 specifies the geometry and method of use of nozzles and Venturi nozzles
ISO 5167-3:2003 also provides background information for calculating the flow-rate
ISO 5167-3:2003 is applicable to nozzles and Venturi nozzles (Subsonic flow, single-phase fluid).
ISO 5167-3:2003 deals with two types of standard nozzles, the ISA 1932 nozzle, and the long radius nozzle, as well as the Venturi nozzle.
BS 1042-1-1.2 is used closed conduits fluid flow measurements.
ASME MFC-3M is used for installation and flowing conditions.
Now, don’t be confused the venturi meter with the orifice meter, these two are different items.
Difference between Venturi meter & Orifice meter
Few important differences are captured below to have a basic idea concept of these two,
Construction & space
Simple design, doesn’t have many parts and space requirement is less
Venturi meter means complex design consisting of different parts and space requirement is large
High pressure loss, around 60%
Low-pressure loss, around 10%
Pressure loss measurement
By pitot tube
By the inlet & throat size
Not suitable for slurries
Suitable for slurries
Coefficient of discharge, Cd
It is comparatively high
Should be straight
It can be angular
Hence, we have learned the basics of the venturi meter, its basic details, along with examples. Please write to us, if you have any doubts.