The Venture Meter and Orifice Meter

The energy equation assuming no losses is:

If the venture is mounted horizontally, the elevation head z cancels out and the equation simplifies to:

The continuity equation (mass balance) for incompressible flow is:

The theoretical venture equation is derived by combining the energy balance with the continuity equation.

where h = P/

The discharge coefficient (C) is:

The discharge coefficient is a function of geometry and Reynolds number and has a value of near one for Reynolds number of 200,000 or more.

An empirical venture meter equation which can be used to compute flow rate is where the velocity approach factor is

M = [1 (A2/A1)2]-1/2

For the orifice meters, as much as the pressure differential can be measure as in venture meters that is using the same equation .However, the discharge coefficient is different for the orifice meters. Orifice meters can provide values within +/- 1% of the true discharge coefficient, C. Due to abrupt changes in diameter, the orifice plate is much less efficient than the venture. The discharge coefficient is theoretically about 0.62 for high Reynolds number, but will vary with throat Reynolds number and the degree of sharpness of the orifice.

Test procedure1. Calibrate the venture meter.

Air was purged out of the piping by running the system.

The valves of the venture were made sure to be open. And the first valve was opened before closing the second valve.

At high flow the time was recorded that took to collect 50 gallons of the 165.The pressure gauges and throat was recorded while collecting the water.

The different pressures were recorded together with the amount of water collected and the elapsed time.

The pump flow control valve was then closed to a low setting.

Part c and d were repeated.

2. Calibrate the orifice meter

Air was purged out of the piping by running the system.

The valves of the venture were made sure to be open. And the first valve was opened before closing the second valve.

At high flow the time was recorded that took to collect 50 gallons of the 165.The pressure gauges and throat was recorded while collecting the water.

The different pressures were recorded together with the amount of water collected and the elapsed time.

The pump flow control valve was then closed to a low setting.

Part c and d were repeated.

Results Table 1: Venturi Flow Meter Calibration Flow rate

(in3/s Reynolds Number P1 P2 Pressure loss(P1-P2) Discharge coefficient Time(sec) V(GAL)

2.8 12.3 20 -12 32 3.97 17.8 50 gal

2.47 12.5 16 -10 26 4.197 20.22 50 gal

1.54 12.5 9 -9 18 4.266 32.45 50

0.86 13.3 14 -10 24 4.138 57.8 50

Table 2: Orifice Flow Meter CalibrationFlow rate

(in3/s Reynolds Number P1 P2 Pressure loss(P1-P2) Discharge coefficient Time(sec) V(gal)

2.24 2.119 29 -9 38 41 22.23 50

1.98 2.228 21 -9 30 45.2 25.15 50

1.40 2.198 10 -10 20 46.6 35.59 50

0.887 2.232 16 6 10 45.3 56.34 50

Discussion

From the plotted graphs, it is evident that calibration of venturi meter is more accurate when used to measure the flow rate of a fluid through a pipe than when an orifice is used. This is also evident in comparison of the Reynolds number and discharge coefficient of the two meters

Recommendation

Errors are always expected in an experimental set up. One main causes of error, for this experiment, is due to the presence of bubbles within the hose and is assumed by the experimenter to be negligible. There is a big difference with respect to the rate of flow due to the presence of the bubbles. It is also essential to note that the orifice meter cannot directly measure the vertical displacement of the water stream at its varying distances.

References

Baker, R. (2000). Flow measurement handbook. Cambridge, UK: Cambridge University Press.

Mays, L. (2001). Water resources engineering. New York: Wiley.

Wurbs, R., & James, W. (2002). Water resources engineering. Upper Saddle River, NJ: Prentice Hall.