According to Bernoulli’s Principle, the velocity of a fluid decreases as it passes through a constriction. This reduction is accompanied by a rise in pressure.
A correctly sized venturi will measure the full flow range of a system without losing resolution or accuracy. This requires careful design. The following describes how.
The convergent section
A Venturi tube is a fluid flow measuring device invented by Giovanni Battista Venturi. It consists of a main pipe that decreases in size to a throat section and then gradually increases back to its original pipe diameter. The resulting pressure difference can be used to calculate the fluid velocity and flow rate.
The main feature of a Venturi tube is its convergent section that has a conic section shape. When liquid enters the convergent section, its pressure increases due to the decreasing cross-sectional area. The fluid’s velocity also increases as it passes through the convergent section. The turbulence caused by the increased velocity can atomize the liquid into tiny droplets, which helps it transfer mass more efficiently.
However, the convergent section also has an increased risk of leakage because it is a highly constricted area. Therefore, it’s crucial that the meter be constructed with high-quality materials. Cast iron is popular because it’s affordable, durable, and can withstand high temperatures. Other options include stainless steel, brass, and other corrosion-resistant materials.
Venturi meters are designed to work with both liquids and gases, but the gas version is generally more common. Gases are easier to compress than liquids and can be used at higher pressures without damaging the meter. However, it’s important to note that a Venturi meter can only accurately measure flow rates when the pressure is equal to or greater than the atmospheric pressure.
In order to measure the flow rate of a fluid through a Venturi tube, two pressure taps are positioned at the inlet and throat sections. The resulting pressure difference can then be measured by a differential pressure transducer or manometer.
When a fluid passes through the Venturi tube, its pressure is higher at the inlet than at the throat because the convergent section has a smaller cross-sectional area. Then, as the fluid moves through the throat section, its pressure drops because of the increasing cross-sectional area.
This pressure difference is then measured by a differential pressure sensor, which can be used to calculate the fluid’s velocity and flow rate. This method of flow measurement is very accurate and provides a better result than other methods, such as orifice plates and flow nozzles.
The divergent section
The Venturi effect explains why a flow of air or liquid passing through a narrow section experiences a reduction in pressure and increases its velocity. This principle was first derived by Daniel Bernoulli in 1738 and is fundamental to the laws of fluid dynamics. This is why aircraft wings generate lift as they compress the airflow that passes over them. The effect can also be observed in a built environment, explaining why certain regions of cities are windier for their residents.
Venturi tubes are a popular method for measuring volumetric flow rates, using the same principles as orifice plates and other differential pressure flow meters. They are known to create a low permanent pressure drop and feature an inlet or converging section, a throat, and an outlet or diverging section.
As the fluid passes through the convergent section, its velocity increases, creating a differential pressure between the wider inlet and narrower throat sections. The resulting measurement can then be used to calculate the flow rate.
A Venturi tube has an inlet diameter of P1 and a throat diameter of P2. The ratio of these two dimensions defines the device’s discharge coefficient, which is proportional to the Reynolds number and geometry of the inlet and throat sections. The discharge coefficient of a Venturi tube is also affected by the presence of vibration and the friction of the fluid against the pipe walls.
Compared to an orifice, a venturi tube is insensitive to velocity profile effects and requires less straight pipe run as it is essentially a funnel-shaped element. This streamlined design reduces the amount of silt trapped inside it, further contributing to its overall accuracy and reliability. Its contoured nature and self-scouring action make it resistant to corrosion, erosion, and internal scale build-up.
With a high turndown ratio of up to 10:1, a venturi tube can accurately measure both low and high flow rates, which makes it suitable for industrial applications that require both types of fluid. These include water treatment plants, oil and gas pipelines, chemical processing facilities, and even automobile carburetors.
The pressure taps
A Venturi tube is an instrument used to measure fluid flow. The principle of the device relies on a variation of Bernoulli’s law. When a liquid flows through a constricted section of pipe, its velocity increases, but its static pressure decreases. The device is designed to measure this pressure difference in order to calculate the fluid’s flow rate.
In order to do this, the Venturi tube has two pressure taps – the inlet and the throat of the device. These pressure tappings feed into a common chamber, providing an average reading of the pressure differences. This avoids the problem of local turbulence near the constriction, which can cause errors in other devices, such as the orifice plate and venturi nozzle.
The device is designed to work with either a liquid or gas and can be used with either clean or dirty fluids or even solids. It is also highly resistant to corrosion, erosion, internal scale build-up, and clogging, which can occur in other differential flow meters like the orifice plate.
In addition to measuring flow, the device can be used to mix a liquid with a gas. This is achieved by using the liquid’s speed change through the constriction to create a siphon effect. By combining this with a short piece of pipe with a small hole in it, the liquid can be sucked into the hole by the changing pressure, and the gas will escape through the mouth of the Venturi, leaving a mixture of the two liquid and gas.
Another useful application of the Venturi tube is to measure a liquid’s viscosity. Determining the fluid’s viscosity makes it possible to determine how much friction the fluid is experiencing. This can be helpful when designing a system to reduce friction in machinery or vehicles.
The device is easy to use and can be calibrated in a number of different ways. For example, a simple float can be placed in the center of the device and a scale can be read alongside the tube to directly display the volume of liquid passing through that gap in the tube. This makes it an ideal flow measurement tool for applications requiring high accuracy and reliability.
The throat
As the fluid passes through the convergent section and into the throat, its linear velocity must decrease to match the decreased pressure. In physics, this is known as the Venturi effect. It was named after the Italian physicist Giovanni Battista Venturi (1746 – 1822). The fluid pressure must decrease in order for the linear velocity to remain the same. As per Bernoulli’s equation, the loss of kinetic energy in the flow is balanced by a decrease in fluid pressure.
The physics behind the Venturi effect makes it an ideal form of flow measurement. This type of meter is accurate and can also be used in a wide range of applications. It can be used to measure liquid, gas, steam, or other types of non-viscous fluids.
A Venturi tube flow meter consists of a conical pipe with an inlet and outlet section. The difference in the diameters of these sections is what creates the differential pressure, which can then be used to determine the amount of fluid flowing through the meter. Unlike an orifice plate, which requires constant calibration, the discharge coefficient of a Venturi tube remains the same over a large range of line sizes and throats.
Venturi tubes are used in many different types of equipment, including automobile carburetors. When fuel is pumped through the carburetor, air, and fuel are mixed together to create a fine spray of gasoline. This spray allows the carburetor to work as well as possible by maximizing the mixing of air and fuel.
Another advantage of the Venturi tube is that it does not produce significant drag. In contrast, the pitot tube can generate a large amount of friction between layers of fluid due to the sudden change in pressure caused by the change in fluid velocity.
The Venturi effect also allows the use of smaller diameter piping, which makes the device more affordable to manufacture and install than other forms of flow measurement. This can be particularly helpful in areas with limited space or when the installation location is remote from a laboratory for testing and calibration purposes.