Aim:
To determine the coefficient of discharge of orifice meter.
Experimental setup:
The experimental set up consists of a circuit through which the water is circulated continuously. The circuit is having a pipe-line of 25 mm dia connected with a orifice meter. Orifice meter is having a d/D =0.6 and is provided with two pressure tappings one at upstream and other at downstream side (or throat). A U tube differential manometer is provided to measure the pressure difference between two sections of orifice meter. A regulating valve is provided on the downstream side of pipe to regulate the flow. A collecting tank is used to find the actual discharge through the circuit.
Formulae:
1. Actual discharge, `Q_{act}=\frac{AR}{T}` cm3/sec
2. Theoritical discharge, `Q_{th}=\frac{a_{1}a_{2}\sqrt{2gH}}{\sqrt{a_{1}^{2}-a_{2}^{2}}}` cm3/sec
3. Coefficient of discharge, `C_{d}=\frac{Q_{act}}{Q_{th}}`
where,
A = Cross sectional area of the collecting tank in cm2
R = Rise in the level of water in collecting tank in cm
T = Time taken for R cm rise in collecting tank
g = Acceleration due to gravity = 980 cm/sec2
a1 = Cross-sectional area of the pipeline in m2 = `\frac{\pi d_{1}^{2}}{4}`
a2 = Cross-sectional area of orifice in m2 = `\frac{\pi d_{2}^{2}}{4}`
d1 = Diameter of the pipeline in m
d2 = Diameter of the orifice in m
h = Difference in manometer limb readings in cm of water = 12.6(hm)
hm = Difference in manometer limb readings in cm of mercury = h1 - h2
h1 = Left limb manometer reading in cm of mercury
h2 = Right limb manometer reading in cm of mercury
Theory:
Orificemeter is a device used for measurement of rate of flow of fluid through a pipe. The basic principle on which Orificemeter works is that by reducing the cross- sectional area of flow passage, a pressure difference is created and the measurement of the pressure difference enables the determination of the discharge through the pipe.
An orifice meter is a cheaper arrangement for measurement of discharge through pipes and its installation requires a smaller length as compared with other flow measuring devices. The opening in the form of orifice is provided at the centre of the plate.
An orifice meter consists of a flat circular plate with a circular hole called orifice with is concentric with the pipe axis. The upstream face of the plate is be leveled at an angle lying between 30° and 45°. The plate is clamped between the two pipe planges with be leveled surface facing downstream. Two pressure tappings are provided one on the upstream side of plate and other on the downstream side of the orifice plate. The pressure difference exists between two sections which, can be measured by connecting a differential manometer to the two pressure taps. The discharge coefficient can be calculated using formula,
`Q=\frac{C_{d}a_{1}a_{2}\sqrt{2gH}}{\sqrt{a_{1}^{2}-a_{2}^{2}}}`
Where,
Cd is coefficient of orifice meter
a1 is cross- sectional area of orifice
a2 is cross –sectional area of pipe
g is the acceleration due to the gravity
h is the difference of head in terms of water.
Procedure:
1. Note down the relevant dimensions, such as the diameter of the pipe, the diameter of the throat, the area of the collecting tank, etc.
2. The regulating valve of a pipe line is kept open.
3. The pressure taps of a orifice meter are kept open.
4. Open the inlet flow control valve and regulate the valve to allow a steady flow through the pipe. Check if there is any air in the manometer tube. If so, remove it.
5. The flow rate was adjusted to its maximum value. By maintaining a suitable amount of steady flow or near steady flow in the pipe circuit, there is a steady, non-uniform flow in the circuit. Time is allowed to stabilise the levels in the manometer tube.
6. The discharge flowing in the circuit is recorded together with the water levels in the left and right limbs of the manometer tube.
7. The flow rate is reduced in stages by means of a flow control valve, and the discharge and readings of the manometer are recorded for every stage.
Observations:
Cross sectional area of the collecting tank, A = .......... cm2
Rise in the level of water in the collecting tank, R = ........... cm
Diameter of the pipe, d1 = .......... cm
Diameter of the orifice, d2 = .......... cm
Table:
| SNo. |
h1 (cm) |
h2 (cm) |
T (sec) |
hm (cm) |
h (cm) |
Qth (cm3/sec) |
Qact (cm3/sec) |
Cd |
|---|---|---|---|---|---|---|---|---|
| 1 | ||||||||
| 2 | ||||||||
| 3 | ||||||||
| 4 | ||||||||
| 5 |
Graphs:
1. Q Vs H:
2. Log Q Vs log H:
3. Q vs `\sqrt{H}`:
Precautions:
1. Remove all entrapped air from two limbs of manometer.
2. Maintain constant discharge for one set.
3. Piezometric scale readings should be recorded at eye level.
Result:
Coefficient of discharge of orifice meter,
a) From average of calculated values, Cd = ..........
b) From Qact Vs H graph, Cd = ..........
c) From Qact Vs `\sqrt{H}` graph, Cd = ..........
d) From log Qact Vs log H graph, Cd = ..........