Orifice meter loss coefficient

This meter may thus be considered as an orifice meter with a variable aperture, and the formulae already derived are therefore applicable with only minor changes. Both in the orifice-type meter and in the rotameter the pressure drop arises from the conversion of pressure energy to kinetic energy and from frictional losses which are accounted for in the coefficient of discharge. The pressure difference over the float —AP. is given by ... 

The total friction loss in an orifice meter, after all pressure recovery has occurred, can be expressed in terms of a loss coefficient, ATr, as follows. With reference to Fig. 10-12, the total friction loss is P — P3. By taking the system to be the fluid in the region from a point just upstream of the orifice plate (Pj) to a downstream position where the stream has filled the pipe (P3), the momentum balance becomes... 

Thus, the coefficient C for an orifice meter is much less than it is for a venturi or a flow nozzle, and it also varies in a different manner with Reynolds number. Consider a pressure gradient between points in a pipe fitted with an orifice plate. Point 1 is upstream of the orifice plate. Point 2 is immediately downstream of the orifice, at the vena contracta. Point 3 is further downstream where the flow is normal, as it was at point 1, though at lower pressure owing to the orifice pressure loss. 

The total unrecovered pressure drop, i.e., the friction loss, (Pi - P ), in an orifice meter is characterized by the loss coefficient, K/. 

---