A Venturi meter is a device used to measure the flow rate of a fluid in a pipe. It is based on the principle of differential pressure and works by creating an constriction in the pipe, which creates a pressure difference between the upstream and downstream sides of the constriction. The flow rate can be calculated from this pressure difference. Venturi meters are commonly used in industrial and laboratory settings.
Venturi Meter Diagram and Parts
A Venturimeter typically consists of four main parts:
- the inlet section: where the fluid enters the device
- the throat: the constriction in the pipe where the pressure difference is created
- the diffuser: where the fluid expands and slows down as it leaves the throat
- the outlet section: where the fluid exits the device
A typical diagram of a Venturi meter would show a pipe with the inlet and outlet sections, and a smaller section in the middle representing the throat. The diffuser section is typically not shown in a diagram, but it is included in the device itself.
In addition, to measure the pressure difference, one or two pressure taps are placed before and after the throat. The difference in pressure is then used to calculate the flow rate of the fluid.
Materials for Venturi Meter
The materials used for a Venturi meter will depend on the specific application and the properties of the fluid that is being measured.
In most cases, the inlet, throat, diffuser and outlet sections are typically made of metal, such as aluminum, stainless steel, or brass. These metals are chosen for their strength and resistance to corrosion, as well as their ability to withstand the high pressure and temperature of the fluid.
In some applications, plastic materials such as PVC or polypropylene may be used for the inlet, throat and diffuser section, especially when the fluid being measured is non-corrosive and non-abrasive.
For the pressure taps, brass is a common material choice as it is strong and has a good resistance to corrosion.
It is important that the material used should be chemically compatible with the fluid to avoid corrosion and erosion which can cause damage to the venturi meter and affect accuracy of the measurement.
Working Principle of Venturi Meter
The working principle of a Venturimeter is based on the principle of differential pressure, which states that the pressure at a point in a fluid is proportional to the fluid’s velocity. The Venturimeter creates a constriction or narrowing in the pipe, which causes the fluid to speed up as it passes through the throat. This increase in velocity results in a decrease in pressure, creating a pressure difference between the upstream and downstream sides of the throat.
As the fluid flows through the throat, it experiences an increase in velocity, which causes the pressure to drop. As the fluid exits the throat, it enters the diffuser, where it gradually slows down. This causes the pressure to increase again, but to a lower value than the pressure upstream of the throat.
The pressure difference between the upstream and downstream sides of the throat is measured by pressure taps, and this pressure difference is used to calculate the flow rate of the fluid.
By the Bernoulli’s equation, the pressure difference is proportional to the square of the velocity difference, so by measuring the pressure difference, the flow rate can be calculated.
In summary, the Venturi meter works by constricting the flow of fluid in a pipe, creating a pressure difference that is proportional to the fluid’s flow rate. This pressure difference is measured and used to calculate the flow rate of the fluid.
Venturi Meter Equations
There are several equations used to calculate the flow rate of a fluid using a this meter. The most commonly used equations are the venturi meter equation and the beta ratio equation.
The venturi meter equation is:
Q = Cd * A^2 * (2g)^(1/2) * (h1 – h2)
Where: Q = flow rate Cd = coefficient of discharge A = cross-sectional area of the throat g = acceleration due to gravity h1 = pressure head upstream of the throat h2 = pressure head downstream of the throat
The coefficient of discharge (Cd) is a correction factor that takes into account the effects of friction and turbulence in the throat. It is usually determined experimentally and is typically between 0.98 and 0.99 for standard venturimeters.
The beta ratio equation is:
Q = Cd * A^2 * (2g)^(1/2) * (1 – (B^2)) * (h1 – h2)
Where: Q = flow rate Cd = coefficient of discharge A = cross-sectional area of the throat g = acceleration due to gravity B = ratio of the area of the throat to the area of the pipe at the vena contracta h1 = pressure head upstream of the throat h2 = pressure head downstream of the throat
The beta ratio is the ratio of the area of the throat to the area of the pipe at the vena contracta (the minimum area of the fluid flow).
Both equations are used to calculate the flow rate of a fluid using a Venturimeter, but the beta ratio equation is more accurate as it takes into account the contraction of the flow at the vena contracta.
It is also important to note that these equations are based on the assumption of steady-state, incompressible, and isothermal fluid flow, and that the fluid is flowing through the throat of the venturi meter without any significant turbulence.
Types of Venturi Meters
There are several types of Venturimeters, which are classified based on their design and the way they measure the pressure difference across the throat. The most common types are:
- Standard Venturimeter: This is the most basic type of Venturimeter, and it is the one most commonly used in industrial and laboratory settings. It consists of a simple constriction in the pipe, and the pressure difference is measured by pressure taps placed upstream and downstream of the throat.
- Nozzle Venturimeter: This type of Venturimeter uses a nozzle instead of a constriction to create the pressure difference. The nozzle is a converging-diverging shape that causes the fluid to accelerate as it enters the nozzle and then decelerate as it exits the nozzle.
- Pitot Tube Venturimeter: This type of Venturimeter uses a pitot tube to measure the pressure difference across the throat. A pitot tube is a long, thin tube that is inserted into the fluid flow, and the pressure difference is measured by comparing the pressure at the tip of the tube to the pressure at the base of the tube.
- Rotameter Venturimeter: This type of venturimeter uses a rotating float to measure the flow of fluid. The float is placed in a tapered tube and the flow rate is determined by the position of the float.
- Wedge flow meter: This type of venturimeter is similar to the standard venturimeter, but it has a wedge-shaped constriction in the pipe instead of a cylindrical constriction. This design improves the accuracy of flow measurement by reducing the effect of fluid turbulence.
Each type has its own advantages and disadvantages, and the choice of Venturimeter will depend on the specific application and the properties of the fluid being measured.
Applications of Venturi Meter
Venturimeters are widely used in various industrial and laboratory applications to measure the flow rate of fluids. Some of the most common applications include:
- Industrial Processes: Venturimeters are commonly used in industrial processes, such as chemical, petrochemical, and power generation, to measure the flow rate of fluids such as water, oil, and gases in pipelines.
- Water Treatment: Venturimeters are used in water treatment plants to measure the flow rate of water in the distribution system. They are used to measure the flow rate of water in the intake, treatment, and distribution systems.
- HVAC systems: Venturimeters are used in heating, ventilation and air conditioning systems to measure the flow rate of air and to ensure proper air distribution in a building.
- Automotive Industry: Venturimeters are used in the automotive industry to measure the flow rate of fuel in engines.
- Airports and Aircrafts: Venturimeters are used in airports and aircrafts to measure the flow rate of fuel and to ensure that the aircraft’s engines are operating at optimal efficiency.
- Research and Development: Venturimeters are also used in research and development in laboratories to measure the flow rate of fluids for scientific experiments and to study the fluid dynamics of different liquids and gases.
- Medical : Venturimeters are used in medical industry to measure the flow rate of gases and liquids in various medical devices such as oxygen concentrators, nebulizers and inhalers.
Overall, venturimeters are versatile devices that can measure the flow rate of a wide variety of fluids in a wide range of applications, making them a useful tool in many industries.
Advantages and Disadvantages of Venturi Meter
Advantages of Venturimeter:
- High accuracy: Venturimeters provide highly accurate measurements of fluid flow rate, especially when compared to other types of flow meters.
- Low cost: Venturimeters are relatively inexpensive compared to other flow meters, making them an attractive option for many applications.
- Low maintenance: Venturimeters require minimal maintenance and are easy to install and operate.
- Low headloss: Venturimeters have low head loss, meaning that they do not significantly restrict the flow of the fluid, which reduces the energy required to pump the fluid and saves energy costs.
- Wide range of fluids: Venturimeters can be used to measure the flow rate of a wide range of fluids, including liquids and gases.
Disadvantages of Venturimeter:
- Sensitivity to fluid properties: Venturimeters are sensitive to the properties of the fluid being measured, such as viscosity and density, which can affect the accuracy of the measurement.
- Limitations of fluid flow conditions: Venturimeters are designed to operate under specific fluid flow conditions, such as steady-state and incompressible flow, and they are not suitable for measuring the flow rate of fluids that do not meet these conditions.
- Non-uniform flow: Venturimeters are not suitable for measuring the flow rate of fluids that have non-uniform or turbulent flow.
- Not suitable for low flow rates: Venturimeters are not suitable for measuring very low flow rates, as the pressure difference across the throat is too small to be measured accurately.
- Need for regular calibration : Venturimeters need regular calibration to ensure accuracy of the measurement.
It is important to consider these factors when selecting a Venturi meter for a specific application to ensure that it is the most appropriate flow meter for the job.
Codes and Standards of Venturi Meter
Venturimeters are governed by various codes and standards that ensure their accuracy and safety in different applications. Some of the most important codes and standards for Venturimeters include:
- ASME MFC-3M: This standard, published by the American Society of Mechanical Engineers (ASME), provides guidelines for the design, manufacture, and testing of Venturimeters. It covers both standard and nozzle Venturimeters.
- ISO 5167-1: This international standard, published by the International Organization for Standardization (ISO), provides guidelines for the design, manufacture, and testing of Venturimeters. It covers both standard and nozzle Venturimeters, and it is widely recognized and used in many countries.
- ISO 5167-2: This standard provides guidelines for the measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full. It covers both standard and nozzle Venturimeters, and it is widely recognized and used in many countries.
- ANSI/ISA-19.5-2009: This standard, published by the International Society of Automation (ISA), provides guidelines for the design, selection, installation, and maintenance of flowmeters for use in industrial process measurement and control systems.
- ASTM D4032-19: This standard, published by the American Society for Testing and Materials (ASTM), covers the calibration of venturi meters and nozzles.
These codes and standards provide guidelines for the design, manufacture, and testing of Venturi meters, as well as for their use in various applications. Compliance with these codes and standards helps ensure the accuracy and safety of Venturi meter measurements.