B

Figure 10-11 Expansion factor for orifice, nozzle, and venturi meter. (a) k = 1.3; (b) k = 1.4. (From Crane Co., 1978.)

Figure 10-12 Pressure loss in orifice.

The orifice equation [Eq. (10-14)] can be solved for the pressure drop P. - P2 to give

Eliminating P2 from Eqs. (10-19) and (10-20) and solving for P1 — P3 provides a definition for Kf based on the pipe velocity (Vj):

pV<2(1 — p4)(1 — p2) pV?Kf p1— P3 =-2c2-= pef= —2t" (10-21)

Thus the loss coefficient is

If the loss coefficient is based upon the velocity through the orifice (Vo) instead of the pipe velocity, the p4 term in the denominator of Eq. (10-22) does not appear:

Equation (10-21) represents the net total (unrecovered) pressure drop due to friction in the oriffice. This is expressed as a percentage of the maximum (orifice) pressure drop in Fig. 10-5.

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