Aim: To determine the co-efficient of discharge for the given orifice meter and obtain its variation with the flowrate.
Apparatus: Orifice meter with two pressure tapping, manometer, stopwatch,bucket.
Theory:
Flow meters are used in the industry to measure the volumetric flow rate of fluids. Differential pressure type flow meters (Head flow meters) measure flow rate by introducing a constriction in the flow. The pressure difference caused by the constriction is correlated to the flow rate using Bernoulli’s theorem.
If a restriction is placed in a pipe carrying a stream of fluid, there will be an increase in velocity, and hence an increase in kinetic energy ,at the point of restriction. From an energy balance as given by Bernoulli’s theorem, there must be a consequent reduction in pressure. Rate of discharge from the restriction can be calculated by knowing this pressure reduction, the area available for flow at the restriction ,the density of the fluid and the coefficient of discharge Cd. Coefficient of discharge is the ratio of actual flow to the theoretical flow and makes allowances for stream contraction and frictional effects. Venturi meter, orifice meter, and Pitot tube are widely used head flow meters in the industry.
The Pitot-static is often used for measuring the local velocity in pipes or ducts. For measuring flow in enclosed ducts or channels, the Venturi meter and orifice meters are more convenient and more frequently used. The Venturi is widely used particularly for large volume liquid and gas flows since it exhibits little pressure loss. However, for smaller pipes orifice meter is a suitable choice. In order to use any of these devices for measurement it is necessary to empirically calibrate them. That is, pass a known volume through the meter and note the reading in order to provide a standard for measuring other quantities.
Procedure:
- Connect the manometer to the two pressure tapping of the orifice meter to the manometer. make it sure that, there is no air bubble in the tapping lines. ensure that the manometer indicates zero reading at no flow through the orifice meter.
- Open the valve opening and wait till the flow stabilized. his is indicated by the safety levels of the manometric fluid in the limbs of the manometer.
- Note down the manometer readings.
- Measure the flow rate of water by collecting it in a bucket for a known period of time.
- Repeat (2),(3) and (4) to obtain atleast 10 evenly spaced readings. The last reading should be taken at the maximum permissible manometer reading.
Graphs:
- Plot manometer reading Rm vs. Volumetric flowrate.
- Plot Co vs. NRe.
Results:
Conclusions:
Observations:
- Inside diameter of the pipe, (dp)= cm.
- Diameter of the orifice, (do)= cm.
- Average temperature of water during the run,t= °C.
- Viscosity of water at average temperature,µ= kg/m.s.
Observation Table
Sr. No. | Volume collected,
m^{3} |
Manometer reading (Rm)
m |
Vol. flow rate (Q)
m^{3}/sec |
Time of collection of water, t sec |
Table for Calculated Results:
Sr. No. |
Pressure head across orifice meter,ho ,m |
Velocity of water through orifice meter, uo, m/s |
Reynolds number NRe |
Orifice meter discharge co-efficient Co |
Calculation:
- Cross-sectional area of orifice meter, So=Π/4 * (do*do)= m^2.
- Volumetric flow rate of water, Q= m^3/s.
- Velocity of water through orifice meter, uo= Q/So= m/s.
- Pressure head across orifice meter, ho= Rm*(ρm-ρf)/ρf.= m.
- Reynolds number at orifice meter NRe,o= (do*uo*ρf)/µf
- Discharge coefficient,
Q= uo*So= {Co*So/(√1-β^4)}*√2gho, β=do/dp
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