A Venturi tube is a device used to measure the speed of a fluid. It can also be said to be a device used to measure the pressure difference of a fluid. Venturi tube is also often called Venturi meter, Venturi flowmeter, classic Venturi flowmeter, etc. Venturi tubes, orifices, flow nozzles, and flow tubes, like all differential pressure generators, are based on Bernoulli’s theorem.

What is a Venturi Tube?

Venturi Tube Definitions

Venturi tube A venturi tube is a device for measuring the pressure difference of a fluid. It was invented by the Italian physicist G. B. Venturi, hence the name. A venturi is a tube that first contracts and then gradually expand. Measure the pressure difference between the entrance section and the smallest section. The flow rate can be obtained by using Bernoulli’s theorem.

Solve the problem of accurate measurement of various gas flows in current industrial enterprises in low pressure, large pipe diameter, and low flow rate. A fluid measuring device with a wide measuring range and easy installation. The unique structure design and data processing method have a strict fluid mechanics basis. And in the national large-scale key wind tunnel laboratory for real-flow calibration.

The venturi tube is a throttling sensor developed based on the efficiency of the venturi. It is a throttling sensor. The venturi tube structure is divided into standard venturi and general venturi.

According to the different manufacturing methods, the standard (classical) text is divided into the standard venturi with the roughcasting shrinkage section, the standard venturi with the machined shrinkage section, and the standard venturi with the rough welded iron plate shrinking section.

The standard venturi is designed and manufactured according to the national standard GB/T2624-2006. Perform verification according to the national standard JJG640-94.

According to the structure, the venturi tube is divided into a built-in venturi tube and a plug-in venturi tube. Good results have been obtained in the measurement of combustion air, cold air, and gas metering (blast furnace gas, coke oven gas, converter gas) of hot blast stoves in iron and steel plants, and large pipe diameters and low flow rate pipe meters of boiler primary air and secondary air in thermal power plants.

Venturi tube flow meter

A Venturi flow meter is a differential pressure flowmeter. The Venturi flow meter is a combination of a Venturi tube, a differential pressure transmitter, and a valve block. It is often used to measure the flow of pressure pipes.

Venturi flow meters are often used to measure the flow of fluids such as air, natural gas, coal gas, and water. It includes three parts: “constriction”, “throat” and “diffusion”. Install on the pipe where the flow rate needs to be measured.

Venturi flowmeter is a new generation of differential pressure flow measuring instruments. The basic measurement principle is a flow measurement method based on the law of conservation of energy-Berlier equation and flows continuity equation.

The throttling process of fluid flowing through the inner venturi tube is basically similar to the throttling process of fluid flowing through a classic venturi tube and an annular orifice plate.

Extended reading: Fluid flow meter types

Venturi flow meters types:

Classic Venturi:

It is applied to the flow measurement of various media and has the characteristics of small permanent pressure loss, the required long and short front and rear straight pipe sections, and long service life.

Casing type venturi:

It is mainly used in the flow measurement and control of various large-caliber and high-pressure or dangerous media in the petrochemical industry.

Venturi nozzle:

It is suitable for the measurement occasions of various media. It has the characteristics of small permanent pressure loss, a short length of the front and rear straight pipes required, and long life. The installation length of the body is shorter than that of the classic venturi.

Extended reading​: What is a flow nozzle?

If it can be accurately manufactured in accordance with ASME standards, the measurement accuracy can also reach 0.5%. However, the accuracy of the domestic Venturi flowmeter is difficult to guarantee due to its manufacturing technical problems. For the working condition of ultra-supercritical power generation, the use of the equalizing ring at the throat is a very dangerous link under high temperature and high pressure. If the equalizing ring is not used, the standard will not be met. The measurement accuracy cannot be guaranteed. This is a contradiction in the manufacture of high-pressure classic Venturi flowmeters.

The pipe is made of the same material as the inlet and outlet. Fluid scouring and abrasion of the throat are severe. Long-term measurement accuracy cannot be guaranteed. The length of the structure must be manufactured according to regulations. Otherwise, the required accuracy will not be achieved.
Due to the strict structural regulations of the classic venturi, its flow measurement range is the largest and the minimum flow ratio is very small, generally between 3 and 5.

This makes it difficult for Venturi flowmeters to meet flow measurements with large flow changes.

Venturi tube working principle

The venturi tube is composed of the following parts:

①Entrance section: a short cylindrical section with a diameter of D;
②Contracting section: the shape is a tapered tube, the cone angle is about 21°±2°;
③ Throat: a short straight pipe section with a diameter of about 1/3~1/4D and a length equal to the pipe diameter;
④ Diffusion section: Conical tube with a cone angle of 8°~15°. There is a pressure measuring ring at 0.25-0.75D from the end of the inlet section, with at least 4 pressure measuring holes on it, and the pressure ring leads to the pressure gauge.

In addition, in the center of the throat, there is also a multi-channel pressure measuring ring leading to the pressure gauge. The pressure difference between the inlet section and the smallest section (that is, the throat section) can be measured through the scale of the pressure gauge or the automatic recorder.

Suppose the average velocity, average pressure, and cross-sectional area at the entrance section and the throat are v1, p1, S1, and v2, p2, S2; the fluid density is ρ.

Applying Bernoulli’s theorem and the continuity equation and noting that the streamlines of average motion are of the same height, we can get:

Venturi tube working principle 1

The formula for calculating the flow rate Q can be obtained:

Venturi tube working principle 2

After knowing ρ, S1, S2 and measuring p1-p2, the flow rate Q can be obtained according to the above formula.

The main advantage of the venturi tube is its simplicity of installation. Secondly, due to its diffusion section, the fluid gradually decelerates, reducing the turbulence (see turbulence). Therefore, the pressure head loss is small, no more than 10-20% of the pressure difference between the inlet and the throat.

Venturi tube design

According to the manufacturing process and use, the venturi is divided into standard venturi, general venturi, venturi flow tube, small diameter venturi, rectangular venturi, and other structures. The detailed structure is as follows:

Structure type:

The standard (classic) Venturi tube is composed of an inlet cylindrical section A, a conical contraction section B, a cylindrical throat C, and a conical diffusion section E. The diameter of the cylinder section A is D, and its length is equal to D; the contraction section B is conical and has an included angle of 21º±1º; the throat C is a circular cylinder section with a diameter d, and its length is equal to d; the divergent section E It has a conical shape with a spread angle of 7º~ 15º.

Venturi tube design 1

The general-purpose venturi, like the standard venturi, is composed of an inlet cylindrical section A, a conical contraction section B, a cylindrical throat C, and a conical diffusion section E.

The general-purpose venturi adopts the method of changing the contraction angle of the standard venturi and the length of the diffusion section to make it have the advantages of venturi, greatly shortening the length of the body, and effectively reducing the pressure loss.

Venturi tube design 2

The Venturi flow tube is also composed of an inlet cylindrical section A, a conical contraction section B, a cylindrical throat C, and a conical diffusion section E. The Venturi flow tube adopts a special pressure method to make it widely used in the flow measurement of dirty media and mixed-phase flow.

Venturi tube design 3

The small diameter venturi is composed of an inlet cylindrical section A, a conical contraction section B, a cylindrical throat C, and a conical diffusion section E. The small diameter venturi adopts an integrated mechanical processing method to measure the fluid flow of small diameters. At the same time, it can use a variety of materials to meet the requirements of the on-site working conditions and can meet various connection methods such as welding, flange connection, and threaded connection.

Venturi tube design 4

The rectangular venturi is composed of an inlet cylindrical section A, a conical contraction section B, a cylindrical throat C, and a conical diffusion section E. Main technical parameters of rectangular venturi:

  • Nominal diameter: DN=1.13×(WH)0.5≤6000mm
  • Inlet diameter ratio W/H: 0.5≤W/H≤2.0
  • Throat diameter ratio w/h: 0.5≤w/h≤2.0
  • Equivalent β value: 0.44≤β=(w/h)0.5/(W/H)0.5≤0.74
  • Reynolds number range: 2×105≤ReD≤2×107
  • Accuracy: ±1%
  • Repeatability: ±1%
  • Working pressure: 0~25Mpa
  • Working temperature: -100℃~500℃
  • Turndown ratio: 1:10

Rectangular venturi is mainly used in power plant air supply and suction, heating furnace air supply, and suction occasions.

Venturi tube design 5

Extended reading: Pitot Tube vs Venturi Meter

Venturi tube application

Since its development, venturi products have been successfully applied to the measurement of high-humidity natural gas, low-pressure dirty biogas, coke oven gas, gas, steam, hot water, high-temperature hot kerosene, etc., And the practical application range is rapidly expanding.

Besides, the venturi tube has a unique measurement advantage, for measuring conditions such as long straight pipe installation conditions, special high temperature, high pressure, strong corrosive and dirty media, and non-single phase flow measurement.

Of course, in the sewage and wastewater treatment industry, in addition to venturi flowmeters, electromagnetic flowmeters are also the first consideration for many users.
For example, if you need to measure the flow of wastewater in a 2-inch pipe. Then you can refer to Magnetic Flow Meters Guides.

Extended reading: Hot Water Flow Meters Improve Heating-Boiler System

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Related DP flowmeters

Sino-Inst offers over 50 flow meter products. About 50% of these are differential pressure flow meters (like the Orifice flowmeter), 40% are Magnetic Flow Meters, and 40% are Thermal mass flowmeters.

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