Venturi Meter or Venturi Tube, a well-known device in fluid mechanics. Measuring the rate of flow of liquid is essential many times and this simple device is widely used to do it precisely.

Venturi meter is a flow measurement device and it works based on Bernoulli’s equation.

How does it work? Let’s explore the whole concept!

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.

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.

The measurement of flow is very much essentials now a day. In all industrial applications, flow measurements can be explained as follows:

**System control**

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.

**Application**

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

- orifice,
- flow nozzle, and
- venturi meter;

In this chapter, we will explore Venturi Meters.

**A Venturi Meter is consisted of**

- Converging part or converging cone
- Throat Part
- 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

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.

There are few assumptions for venturi effects,

- The inner side of the venturi meter is frictionless.
- The fluid is incompressible fluids
- The fluid flow is steady & irrational.

We have already learned Bernoulli’s Equation. So,

P.E + K.E + Pt.E = Constant.

Where,

Here, *ρ* & g both are constant for a particular fluid, hence, constant/*ρ*g = constant.

So, we can write,

Which implies,

Pressure head (P.H)+ Kinetic head (K.H)+ Potential or Elevation Head (Pt.H) = Constant.

Where,

p: Pressure,

*ρ*: Liquid density,

*g*: Gravitational acceleration

h: Height

v: Water velocity

z: Elevation

As per the continuity equation, for specific fluid flow,

Area (a) x Velocity (v) = constant.

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.

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.

Check a NICE ANIMATED video from Muthu Vignesh!

At point-1

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.

At point-2

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.

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.

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.

Applying Bernoulli’s equation at sections 1 and 2

Now applying the continuity equation at section 1 and 2

Discharge

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.

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.

**Condition-1**

**Condition-2**

The liquid in the manometer is lighter than the flowing fluid in the pipe.

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?

**Solution** **in Steps**

**Input Data**

- Mercury specific gravity, gm = 13.6
- Mercury column, hm = 25cm
- Liquid specific gravity, g = 0.9
- Inlet diameter, a1 = 20cm
- Throat diameter, a2 = 15cm

**Calculate Area**

- The area of inlet, a1 = (π d1
^{2})/4 = (π x 400)/4 = 314 cm^{2} - The area of throat, a2 = (π d2
^{2})/4 = (π x 225)/4 = 176.6 cm^{2}

**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

**Calculate Discharge**

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.

**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.

**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.
- High accuracy
- 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.
- Highly efficient.
- High viscosity effect.

**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.

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.

An installed image of the venturi meter is referred here.

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,

- Model No.
- Type of fluid.
- Type of application.
- Flow rate.
- Temperature of fluid.
- Viscosity & specific gravity of fluid.
- Size of connection (Inlet and outlet).
- Differential pressure.
- Value of Beta Ratio.
- Material of pipe.
- Material of venturi meter, etc.

There are a lot of venturi meter manufacturer available worldwide, few of the are listed,

- HydraCheck,
- Aalborg Instruments & Controls,
- RCM Industries,
- Sierra Instruments,
- Blue-White Industries,
- ERDCO Engineering Corporation,
- Intra Automation,
- Delta Engineering,

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,

- Cast Iron
- Carbon Steel
- Stainless Steel
- Mild steel

**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.

Few important differences are captured below to have a basic idea concept of these two,

Description | Orifice Meter | Venturi Meter |

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 |

Pressure Loss | High pressure loss, around 60% | Low-pressure loss, around 10% |

Pressure loss measurement | By pitot tube | By the inlet & throat size |

Suitability | Not suitable for slurries | Suitable for slurries |

Coefficient of discharge, Cd | It less | It is comparatively high |

Cost | Low cost | High |

Pipe design | 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.

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