A Venturi meter is an instrument that measures flow that features a converging piece of pipe to intentionally increase the velocity of the flow and in turn, a pressure drop. From this behaviour the flowrate can be determined.
Venturi meters have been a common tool and have been used in the water supply industry for many years. They are typically used where a permanent drop in pressure is required, and also where the highest amount of accuracy is required. This means they can be used for measuring the flow rates of gases, slurries, suspended solids, water, and dirty liquids.
In this article, we will explore an interesting engineering concept and learn about the calculations which explain its effects. We will study is the ‘Venturi Effect’ and how a Venturi meter can be used, to understand some core fluid mechanics concepts.
Venturi devices can be frequently found in:
- Pipelines of water collection systems and treatment plants to regulate blockage from solids moving in the lines
- Carburettors of combustion engines to measure airflow and regulate the air/gas mixture
- Equipment used in cold or harsh environments due to the lack of moving pieces which leads to a low failure rate
Venturi meter Explanation
The Venturi meter is named after Giovanni Venturi, an early 1800’s Italian physicist and scientist, whose pioneering work in fluid mechanics is still taught to this day in fluid and thermodynamic courses.
In thinking about how a Newtonian fluid flows through a system we can reason that if there is a change in the area through which the fluid flows, then there will be an associated change in flow rate. When connecting two different sized pipes there will be a ‘throat’ or a choke between them, it is at this point where we can find watch the flow rate change.
For example, if a thin pipe is connected to a large one, logic would dictate that the fluid would flow slower once it reaches the large pipe. Likewise, when a large pipe runs into a smaller one, the constant pressure from the big pipe would push the water through a thinner pipe more quickly. The Venturi meter refers specifically to a U-shaped tube which is attached to the pipe. In this device, we can see a change in pressure from the tube to the choke, specifically in the height of the liquid in the tube.
The Venturi meter is built up of three primary parts; the converging section, throat, and diverging section. As a fluid flows towards the throat, pressure will build up behind the throat, pushing the fluid through at a higher rate. In fluid dynamics, the velocity of a fluid will increase as it is passed through a constricted portions, this can be seen in the principle of mass continuity. Static pressure decreases in accordance with the principles of conservation of mechanical energy. Gains of kinetic energy throughout the system will be accrued due to a balance of velocity and pressure, as velocity increases pressure must decrease. This change in pressure can be seen in a tube attached to the piping, by comparing the levels of liquid in each side (see diagram below).
One simple way to demonstrate the ‘Venturi Effect’ is with a piece of tubing or a hose with fluid flowing through it. When you collapse part of the tube and create a ‘throat’ the flow rate at the end of the tube will increase.
Some commercial examples of this effect can be seen in aquariums, where they will use tubes with adjustable Venturi devices in order to aerate the water in the system. Other examples are fire fighting foam nozzles, protein skimmers, Carburetors or water aspirators.
How Does A Venturi Meter Work?
A Venturi meter’s operation is based on Bernoulli’s equation. This states that an incomprehensible fluid’s flow has constant energy at any point. This particular relationship means that the pressure decreases when the velocity increases, so Venturi meters are used where velocity increases are desired. Venturi meters are constructed in such a way the cross-sectional area in the middle of the meter is smaller than that of in the inlet. This increases the velocity of the flow, and means that there is a pressure difference between the inlet and the throat of the meter. Using a differential manometer, this pressure difference can be measured and the flow rate can be determined from this. There is very little pressure loss with a Venturi meter, meaning that it measures flow rate with very high accuracy.
In order to understand how the flow will change, we can calculate the expected changes in the flow rate. Bernoulli’s equation is used as a base to define the system. (Shown below; where p1 and p2 are the pressures, v1 and v2 are the velocity [flow rate] and p is the density of the liquid)
A is the area of the tube the choke and v the velocity of the fluid and p is the pressure.
Using this formula by knowing area and pressure at each point, we can calculate the flow rate before and after the choke.
Why Measure Flow?
In many industries, it is vital to be able to measure the fluid flow rate in a system or even just a part of a system. This is equally important for both liquids and gases, which are essential parts of most processes. Even in plant operation, compress, water, steam, and air are integral parts of the whole plant’s functionality. Venturi meters are typically installed for one of two reasons:
Space or process heating generated by energy needs to be tracked. It is absolutely essential in operations to know where costs are being incurred and what parts can be made more efficient or upgrades to save costs. Installing a Venturi meter allows for these costs to be monitored and allocated to each particular part, product, or system. This means that costs will be significantly reduced as they can be analysed in real time and proactive solutions can be created.
In this case, a Venturi meter will be measuring the rate of flow with which the material or energy moves, to ensure that the process is controlled to an acceptable rate and that the end product is of sufficient quality. A typical application of this would be in the animal feed industry, where steam injection systems are used. This can be a delicate process as if too much steam is used the product will not keep its pellet shape, and if not enough is used the material may not even be able to be processed and could damage some very expensive machinery.
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