A sharp edge orifice (see square edged orifice discussion beginning on p620 Doeb

Question

A sharp edge orifice (see square edged orifice discussion beginning on p620 Doebelin) is to be used as the sensing element to measure flow of water in a pipe with a 0.6 in ID and flow rates up to 2.5 gpm. The pressure gage that will measure delta P (change in pressure) has a range of 0 to 55 in H20. This flow sensing system should measure flows from about 30% of maximum to the maximum expcted flow. The temperature of the water is about 70 degrees fahrenheit. Determine the diameter (in) of the orifice. Make sure assumptions are clearly noted and all calculations include units and units are easy to check.

The answer is provided as this: orifice diameter = 0.307in; Assume Cd= 0.61 - Why?; Check N(R) at full flow and at 30% of max flow rate to check assumption re: Cd

Thanks for the help!

Explanation / Answer

Q= Cd*a*(2gh')1/2

water density at 70o F = 0.99802 g/cm3   

flow= 2.5gpm=0.334201 ft3/m =0.005557 ft3/sec

0.005557 =0.61*A*(2*32.174*55/12)1/2

A=0.000530 ft2

d= 0.0259 ft =0.31 in

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