VENTURI METER

A venture meter is a device used for measuring the rate of flow of fluid through a pipe. The basic principle on which venture 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 enables the determination of the discharge through the pipe.

VENTURI METER

A venture meter consists of (1) an inlet section, followed by a converging cone (2) a cylindrical throat and (3) a gradually divergent cone. The inlet section of venture meter is the same diameter as that of the pipe which is followed by a convergent cone. The convergent cone is a short pipe, which tapers from the original size of the pipe to that of the throat of the venture meter. 

The throat of the venture meter is a short parallel – sided tube having its cross- sectional area smaller than that of the pipe. The divergent cone of the venture meter is a gradually diverging pipe with its cross-sectional area increasing from that of the throat to the original size of the pipe. At the inlet section and the throat i.e sections 1 and 2 of the venture meter pressure gauges are provided.

The convergent cone of the venture meter has a total included angle of 21° + 1° and its length parallel to the axis is approx. equal to 2.7(D-d), where D is the dia. of pipe at inlet section and d is the dia. of the throat. The length of the throat is equal to d. The divergent cone has a total included angle 5° to 15°, preferably about 6°. This results in the convergent cone of the venture meter to be of smaller length than its divergent cone. 

In the convergent cone the fluid is being accelerated from the inlet section 1 to the throat section 2, but in the divergent cone the fluid is retarded from the throat section 2 to the end section 3 of the venture meter. The acceleration of the flowing fluid may be allowed to take place rapidly in a relatively small length without resulting in loss of energy. 

However if the retardation of the flow is allowed to take place rapidly in small length, then the flowing fluid will not remain in contact with the boundary of the diverging flow passage or the flow separates from the walls and eddies are formed and consequent energy loss. 

Therefore to avoid flow separation and consequent energy loss, the divergent cone is made longer with a gradual divergence. Since separation may occur in the divergent cone this portion is not used for discharge measurement.

Since the cross-sectional area of the throat is smaller than the cross-sectional area of the inlet section, the velocity of flow at the throat will become greater than that at inlet section, according to continuity equation. 

The increase in the velocity of flow at the throat results in the decrease in the pressure. As such a pressure difference is developed between the inlet section and the throat section of the venture meter. 

The pressure difference between these sections can be determined either by connecting differential manometer or pressure gauges. 

The measurement of the pressure difference between these sections enables the rate of flow of fluid to be calculated. For greater accuracy the cross-sectional area of the throat is reduced so that the pressure at the throat is very much reduced. 

But if the cross-sectional area of the throat is reduced so much that pressure drops below the vapour pressure of the flowing liquid. The formation of vapour and air pockets results in cavitation, which is not desirable. 

Therefore in order to avoid cavitation to occur, they diameter of the throat can be reduced to 1/3 to 3/4 of pipe diameter, more commonly the diameter of the throat is 1/2 of pipe diameter.

Let a1and a2 be the cross-section l areas at inlet and throat sections, at which P1 and P2 the pressures and velocities V1 and V2 respectively. 

Assuming the flowing fluid is incompressible and there is no loss of energy between section 1 and 2 and applying Bernoulli‟s equation between sections 1 and 2, we get,