Vena-Contracta taps

The fifth type of tap is unique in that the downstream tap location varies depending of the orifice P ratio. This tap is located at the vena contracta the location where the stream issuing from the orifice attains its minimum cross section. The location of this tap is defined from the upstream face of the orifice as is the D/2 tap. The downstream tap for corner, flange, and pipe taps is measured from the downstream face of the orifice. Vena contracta taps maximize the measured differential pressure. For modem transmitters this is not an important consideration and this type of tap is no longer widely used. 

Vena-contracta taps. The upstream static hole is one-half to two pipe diameters from the plate. The downstream tap is located at the position of minimum pressure (see Fig. 10-19). 

Permanent pressure loss across a concentric circular orifice with radius or vena-contracta taps can be approximated for turbulent flow by... 

The orifice coefficient shown in Fig. 10-8 is valid to within about 2-5% (depending upon the Reynolds number) for all pressure tap locations except pipe and vena contracta taps. More accurate values can be calculated from Eq. (10-10), with the parameter expressions given in Table 10-1 for the specific orifice and pressure tap arrangement. 

A square-edged or sharp-edged orifice is a clean cut square-edge hole with straight walls perpendicular to the flat upstream face of a thin plate placed crosswise of the channel. The stream issuing from such an orifice attains its minimum cross section (vena-contracta) at a distance downstream of the orifice. This varies with the ratio, p, of the orifice to the pipe diameter. All measurements of the distance from the orifice are made from the upstream face of the plate for the corner, radius, pipe, and vena contracta taps. 

Equation (8.37) is useful for design because Co is almost constant and independent of p provided JV o.o is greater than about 30,000. Under these conditions C may be taken as 0.61 for both flange taps and vena-contracta taps. 

Orilii ( assemblies have generally been installed in the piping systems with I ing-joint and flange connections one-piece orifice plate and metallic O-rings ate used, lather flange or vena contracta taps are used and the pi t - in t i< measured as indicated in the previous section. Mild steel orifice... 

Three types of pressure connections may be used vena contracta taps, l-D and V2-D taps, and flange taps. Appropriate disdiarge coefficients have to be used in each case. 

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