Most commonly used flow measuring elements are Orifice,Flow nozzle and Venturi
Thin plate ,concentric orifice can be used for any steady flow of clean ,homogeneous fluid which is in the normal turbulent flow region with pipe reynolds number of 500 or greater
Design consideration : Orifice plates
a)Upstream edge of the orifice must be sharp and square .
b)The thickness of the plate at the orifice edge must not exceed
1/30 of the pipe internal diameter
1/8 of the orifice bore
1/4th of the dame height
c)In measuring steam and liquid flows the orifice taps should be made from the side of the pipe
d)In measuring gas flow the taps should be from on the top of the taps.
A flow nozzle will handle about 60 percent more flow than an orifice plate under the same given condition and its permanent pressure loss will vary from 30 to 80 percent of the differential pressure depending the diameter ratio.
Flow nozzle is more expensive to install than orifice plate .
Flow nozzle can normally handle any liquid fluid which an orifice can handle.
Flow nozzle can be used to handle fluids with some entrained solids.
A venturi tube like a flow nozzle will handle about 60% more flow than an orifice plate but its permanent pressure loss will be only 10 to 20 percent of the differential pressure
A venturi tube is large and heavey
A venturi tube can handle any fluids whixh an orifice and flow nozzle can handle
A venturi can be used on fluids which carry a relatively high percentage of entrained solids
A venturi coefficient is less affected by decreasing reynolds number than either a nozzle or orifice and is therefor more accurate over wider range of flow