Flow Measurement by Venturi and Orifice Meters

Objectives:

I. To find the coefficient of discharge for vcnturi meter.

2. To find the coefficient of discharge for ori rice meter.

Venturi and Orificemeters are the most commonly used now-sensing elements in:

industries. The venturitube has a converging conical inlet, a cylindrical throat, and a

diverging recovery cone. It has no projections into the fiuid, no sharp corners, and no

sudden changes in contour.

- - ...... ~toJ--

.~

. .-/..... . ...

The inlet section decreases the :.lrea of the JluiJ stream. causing the velocity to

increase and the pressure to decrease. The low pressure is measured in the center of the

cylindrical throat since the pressure will be at its lowest value, and neither the pressure

nor the velocity is changing. The recovery cone allows for the recovery of pressure such

that total pressure loss is only 10%· to 250/<. The high pressure is measured upstream of

the entrance cone. The major disadvantages of thi.~ type oj' now detection are the high

initial costs for installation and difficulty in installation and inspection.

u , • u

The Venturi effect is the reduction in fluid pressure that results when a fluid flows

through a constricted section of pipe. The fluid velocity must increase through the

constriction to satisfy the equation of continuity, while its pressure must decrease due to

conservation of energy: the gain in kinetic energy is balanced by a drop in pressure or a

pressure gradient force. An equation for the drop in pressure due to Venturi effect may be

derived from a combination of Bernoulli's principle and the equation of continuity.

The basic equation for venture meter is obtained by writing the Bernoulli equation for

incompressible fluids across the upstream cone. If VI and V~ are average upstream and

down stream velocities, respectively and p j>-den0y of the fluid then equation becomes

z z _ Z(P1' - Pz)alVl - azVz - ---- - - - (1). p,

Where al and az are kinetic energy correction factors at position I and 2.

The continuity equation can be written sinrhe density is constant as

V, = (~:fV2./ . f

Where Dz = diameter of throat in meters

Al = diameter of pipe in' mete:'s

Where f3 is the ratio of the diameter of throat to that of diameter of pipe.

Above equation applies strictly to the frictionless now to account for the small friction

loss between locations and 2 equation 3 is corrected by iutroducing an empirical factor

Cv and writing""

\ Cv; /'Z(Pl - Pz)

J 1- f34 'J P

The small effects of the kinetic energy factors at and az are also taken into account in the

definition of Cv .

Where 52 is cross sectional area of throat in m2.

Substituting (PI - P2) = pgH in the equation (5) we get

c-~~I Q;;;= V252 = ] 2gH~~)1 ,)1 _J]4 'v .

\I eJ (JaM .-.1 j1C1111 \;y'f.-J"~ (

4. H = manometric difference* (specific gravity of m.anometric fluid ~ specific gravity of

An orifice plate is a device used to measure the rate of fluid flow. It uses the :'lame

principle as a Venturi meter, namely Bernoulli's principle which says that there is a

relationship between the pressure of the fluid and the velocity of the fluid. When the

velocity increases, the pressure decreases and vice versa.

Differential

ManometrICdifference

\

pipe in which fluid flows. As fluid flows through the pipe, it has a certain velocity and a

certain pressure. When the fluid reaches the orifice plate, with the hole in the middle, the

fluid is forced to converge to go through the small hole; the point of maximum

convergence actually occurs shortly downstream of the physical orifice, at the so-called

vena-contracta point. As it does so, the velocity and the pressure changes. Beyond the

vena contracta, the fluid expands and the velocity and pressure change once again. By

measuring the difference in fluid pressure between the normal pipe section and at the

vena-contracta, the volumetric and mass t10w rates can be obtained from Bernoulli's

equation.

1. Check all the clamps for tightness.

2. Check whether the water level in the ump is sufficient such that the suction

pipe of pump is immersed.

3. Switch on the pump.

4. Open the gate valve and start the t1ow.

5. Open the outlet valve of the venturi meter and close the valve of orifice meter.

6. First open air cocks then open the venture meter cocks, remove all the air

bubbles and close the air cocks slowly and simultaneously so that mercury

does not run away into wate£.

(t 0')8. Close the gate valve of measuring tank and measure the time for 5 liters of

water for discharge, and the manometer difference.

9. Repeat the procedure by changing the discharge and take the@readings.

10. Repeat the same for orifice meter.

01.-.Manometer Time for 5 Lt water Q(actual)

S.No differencedischarge (sec) m3/sech (m)

I23456

S.No Manometer difference Time for 5 Lt water Q(actual)hem) discharge (sec) m3/sec

123456

For venturi meter

Cross sectional area at throat to venturi meter S2 = 1.76 * 10' 4 m2

The ratio of the diameter of throat to that of diameter of pipe {J is 0.4848

For orifice meter

Cross sectional area to orifice meter S2 = 1.54 * 10' 4 m2

The ratio of the diameter of orifice to that of diameter of pipe {3is 0.4904

1. The actual flow rate (Qa) Vs >/ IJP for venturi meter.

2. The actual flow rate (Qa) Vs >/IJP for orifice meter.

1. Book: 'Unit Operations of Chemical Engineering' by McCabe and Smith.

2. www.wikipedia.org (visited on 12 - 07 - 2009).

3. www.instrumentationblog.bJogspoLcom (visited on 12 - 07 - 2009).

4. www.cheresources.com (visited on 12 - 07 - 2009).