OBJECTIVE:
(i) To calibrate the instrument (by determining the constants K and n, assuming the actual discharge Qa = K*Hhgn).
(ii) To determine the coefficient of discharge (Cd) of the given orifice meter for different rates of flow.
(iii) To study the variation of Cd and discharge with respect the head by plotting the following graphs.
Cd Vs HHg
Qa Vs Log HHg (to find K and n )
Qa Vs HHg (using K and n values)
EQUIPMENT:
a) Measuring tank of Size 0.6 x 0.6 x 0.8 metre with overflow arrangement, gauge glass, scale arrangement and a drain valve.
b) Stop Watch.
c) Orifice meters fitted onto horizontal pipes of diameters 20mm, 25mm and 40mm with pressure tapping’s and gate valves to regulate flow rate.
d) Differential mercury manometer with wooden scale of 1m length and scale graduations of 1mm to measure the loss of head.
e) The orifice diameter corresponding to the pipe diameters are as follows:
S.No: | Pipe Diameter, mm | Orifice Diameter, mm | |
1.
2.
3. |
20
25
40 |
13.41
16.77
26.83
|
.
BASICS:
Orifice meter or orifice plate is a device used for measuring the rate of flow of a fluid through a pipe. It works on the same principle as a venturimeter. It consists of a flat circular plate which has a circular sharp edged hole called orifice, which is concentric with the pipe. The orifice diameter is 0.5 times the diameter of the pipe. A differential manometer is connected at section 1 which is at a distance of about 1.5 to 2 times the pipe diameter upstream from the orifice plate, and at section 2, which is at a distance of about half the diameter of the orifice on the downstream side from the orifice plate.
The actual discharge,
Where
a – Area of measuring tank in cm2
h – Height differences in piezo meter in cm.
t – Time to collect water for a height difference of h cm, measured in
seconds.
Theoretical discharge,
Where
A1 – The area at inlet side in cm2
A2 – The area at throat in cm2
Hw - Head difference in the manometer, converted to cm of water.
g – Acceleration due to gravity (9.81).
Coefficient of discharge,
Calibration of Venturi meter
The equation
Qa = Cd x Qt can be written as
Where
OBSERVATIONS:
Sl No
| Differential head in cm. of mercury
HHg
| Head difference Hw, in cms of water
| Actual Discharge, Qa,
cm3 / s
| Log Qa
| Log HHg
| Qa = K HHg n
cm/sec
|
1
2
3
4
5
| | | | | | |
Calibration Table
Sl No | Differential head in cm. of mercury HHg | Head differenceHw, in cms of water | Actual Discharge,Qa, cm3 / s | Log Qa | Log HHg | Qa = K HHg n cm/sec |
1 2 3 4 5 |
PROCEDURE:
(i) Close the valves of inlet pipe, Orifice meter pipe line and manometer.
(ii) The gate valve of the pipeline selected for the experimentation is opened.
(iii) The needle valves of the corresponding manometer & Orifice meter are opened.
(iv) Adjust the control valve kept at the exit side of the Orifice to a desired flow rate and maintain the flow.
(v) Note down the readings of manometer & time for 10cm rise in measuring tank.
(vi) Adjust the gate valve and repeat the experiment.
MAINTENANCE:
- After completing the experiment close the inlet valve and open all the gate valves & needle valves then close them.
- Drain the water from measuring tank after completing the experiment
Calibration
Plot the graph log Qa Vs log H Hg ,Y intercept represents log K and slope represents n
The calibration graph Qa Vs H Hg can be drawn .
H Hg =(H1-H2) in Hg manometer.
Find Qa values corresponding to manometer readings
RESULT:
The coefficient of discharge Cd=
Find the value of K and n from graph
INFERENCE :