# ORIFICE METER

An orifice meter is a simple device for measuring the discharge through pipes. Orifice meter also works on the same principle as that of venture meter i.e by reducing cross-sectional area of the flow passage, a pressure difference between the two sections is developed and the measurement of the pressure difference enables the determination of the discharge through the pipe.

Orifice meter is a cheaper arrangement and requires smaller length and can be used where space is limited.

ORIFICE METER |

An orifice meter consists of a flat circular plate with a circular hole called orifice, which is concentric with the pipe axis.

The thickness of the plate t is less than or equal to 0.05 times the diameter of pipe.

From the upstream face of the plate the edge of the orifice is made flat for a thickness 0.02 times the diameter of pipe and the remaining thickness the plate is beveled with the bevel angle 45°.

The diameter of the orifice is kept at 0.5 times the diameter of pipe.

Two pressure taps are provided one at section 1 on upstream side of the orifice plate and other at section 2 on the downstream side of the orifice plate.

The upstream tap is located at a distance of 0.9 to 1.1 times the pipe diameter from the orifice plate.

The position of downstream pressure tap, however depends on the ratio of the orifice diameter and the pipe diameter.

Since the orifice diameter is less than pipe diameter as the fluid flows through the orifice, the flowing stream converges, which results in the acceleration of the flowing fluid in accordance with the consideration of continuity.

The effect of convergence of flowing stream extends up to a certain distance upstream from the orifice plate and therefore the pressure tap on the upstream side is provided away from the orifice plate at a section where this effect is non-existent.

However on the downstream side the pressure tap is provided quite close to the orifice plate at a section where the converging jet of fluid has the smallest cross-sectional area (which is known as **veena- contracta**) resulting in the max.velocity of flow and consequently the min. pressure at this section.

Therefore a max. Pressure difference exists between the section 1 and 2, which is measured by connecting a differential manometer between the pressure taps at these sections or connecting separate pressure gauges.

The jet of fluid coming out of the orifice gradually expands from the **veena-contracta** to again fill the pipe. In case of an orifice meter an abrupt change in the cross-sectional area of the flow passage is provided and there being no gradual change in the cross-sectional area of flow passage as in the case of venture meter, there is a greater loss of energy in an orifice meter.

Let p1,p2 and v1,v2 be the pressures and velocities at sections 1 and 2 respectively. Then for an incompressible fluid, applying Bernoulli‟s equation between section 1 and 2 and neglecting losses, we have

Where h is the difference between piezo metric heads at sections 1 and 2. However if the orifice meter is connected in a horizontal pipe, then z1 = z2, in which case h will represent the pressure head difference between sections 1 and 2. From equation (1) we have

In deriving the above expression losses have not been considered, this expression gives the theoretical velocity of flow at section 2. To obtain actual velocity, it must be multiplied by a factor CV, called co-efficient of velocity, which is defined as the ratio between the actual velocity and theoretical velocity. Thus actual velocity of flow at section 2 is obtained as

This gives the discharge through an orifice meter and is similar to the discharge through venture meter.

The co-efficient C may be considered as the co-efficient of discharge of an orifice meter.

The co-efficient of discharge for an orifice meter is smaller than that for a venture meter. This is because there are no gradual converging and diverging flow passages as in the case of venture meter, which results in a greater loss of energy and consequent reduction of the co-efficient of discharge for an orifice meter.