What is a Venturi Meter ?
A venturi meter is a variable head meter which is used for measuring the flow rate of a fluid through a pipe. In this meter, the fluid is gradually accelerated to a throat and then gradually retarded in a diverging section where the flow expands to the pipe size. A large portion of the kinetic energy is thus recovered (converted back to pressure energy).
Venturi Meter Principle
The basic principle on which a venturi meter works is that by reducing the cross- sectional area of the flow passage, a pressure difference is created and the measurement of the pressure difference (between the inlet of the meter and at a point of reduced pressure) enables the estimation of the discharge/ flow rate through the pipe.
Venturi Meter Construction
A venturi meter consists of
(i) An inlet section followed by a convergent cone/section. The inlet section of the venturi meter is of the same diameter as that of the pipe line in which it is installed which is followed by the short convergent section with a converging cone angle of 15-20o and its length parallel to the axis is approximately equal to 2.7 (D – DT), where D is the pipe diameter and DT is the throat diameter. In the convergent section, the fluid is accelerated.
(ii) A cylindrical throat – the section of constant cross-section with its length equal to diameter. The flow area is minimum at the throat. In general, the diameter of throat may vary from 1/3 to 3/4 of the pipe diameter and more commonly the diameter of throat is 1/2 the pipe diameter.
(iii) A long diverging section/a gradual divergent cone with a cone angle of about 5 - 7o wherein the fluid is retarded and a large portion of the kinetic energy is converted back into the pressure energy.
The convergent section/cone is a short pipe that tapers from the original pipe size to that of the throat of the meter. The divergent cone/section of the venturi meter is a gradually diverging pipe with its cross-sectional area gradually increasing from that of the throat to the original pipe size. At the inlet section and the throat (centre of throat), i.e., at stations 1 and 2 of the venturi meter, pressure taps are provided through the pressure rings/piezometer rings.
A piezometer ring is an annular chamber provided at the pressure taps with small holes drilled from the inside of the tube and is used for averaging out the individual pressures transmitted through the several small holes to a pressure measuring device.
Venturi Meter Working
Fig. : Venturi Meter Principle Working / Venturi meter Diagram
A venturi meter of known coefficient is installed in the pipeline and the pressure taps are connected to a pressure measuring device. Air pockets, if any, are removed from connecting tubings, measuring device, etc. after starting the flow of fluid through the pipeline in which it is installed for flow measurement. In the meter, the fluid is accelerated in the converging cone and then retarded in the diverging cone gradually. An increase in the flow velocity at the throat (a section of minimum flow area) results in a decrease in the pressure at the throat. Due to this a pressure difference is developed between the inlet section and the throat section which is measured/noted with the help of manometer or pressure gauges (connected to the pressure taps) after the steady state is attained. This pressure difference/drop is then related to the flow rate by a mathematical flow equation for the meter.
In the venturi meter, fluid is accelerated in the convergent cone from the inlet section 1 to the throat section 2 and in the divergent cone, it is retarded from the throat section 2 to the end section 3 of the venturi meter. In order to avoid the possibility of flow separation and consequent energy loss, the divergent cone of a venturi meter is made longer with a gradual divergence. Since the separation of flow may occur in the divergent cone of a venturi meter, this portion is not used for measuring the flow rate.
Since there is a gradual reduction in the area of flow, there is no vena contracta and the flow area is minimum at the throat so that the coefficient of contraction is unity.
Pressure Recovery in Venturi meter
Since the angle of divergence in the recovery cone (i.e., in the divergent cone) is small, the permanent pressure loss from a venturi meter is relatively small. In a well designed meter, the permanent pressure loss is about 10 percent of the venturi differential P1 – P2
(i.e., 10 percent of the pressure drop across the meter) which means that about 90 percent of the venturi differential is recovered.
The pressure recovery in a venturi meter is very high and thus may be used where only a small pressure head is available though it is expensive.
Also View: Orifice Meter Versus Venturi Meter
Advantages of Venturi meter
1. Low permanent pressure loss and hence high pressure recovery.
2. High accuracy over wide flow ranges.
3. It can be used for flow measurement of compressible and incompressible fluids.
4. It can also be used where only a small pressure head is available.
5. High reproducibility.
6. Less power loss.
Disadvantages of Venturi meter
1. It is expensive and bulky.
2. It occupies considerable space.
3. Relatively complex in construction.
4. The ratio of throat diameter to pipe diameter cannot be changed.
5. Not suitable for flow measurement of highly viscous slurries.
6. Used only for permanent installations.
7. It cannot be altered once it is installed.