ABSTRACT
A: The basic equation for pressure differential in head meters (venturis, nozzles,
orifices) is [1]
W ¼ 359YCd d2o ffiffiffiffiffi hr
pffiffiffiffiffiffiffiffiffiffiffiffiffi 1 b4
q ð1aÞ
where
W ¼ flow of the fluid, lb=h Y ¼ expansion factor, which allows for changes in density of compressible
fluids (for liquids Y ¼ 1, and for most gases it varies from 0.92 to 1.0) Cd ¼ a coefficient of charge do¼ orifice diameter, in. r¼ density of fluid, lb=cu ft b¼ ratio of orifice to pipe inner diameter¼ do=di h¼ differential pressure, in. WC
Cd may be taken as 0.61 for orifices and 0.95-0.98 for venturis and nozzles. It is a
complicated function of Reynolds number and orifice size. The permanent
pressure drop, Dp, across a flow meter is important, because it means loss in power or additional consumption of energy. It is the highest for orifices [2]:
DP ¼ h ð1 b2Þ ð1bÞ For nozzles,
DP ¼ h 1 b 2
1þ b2 ð1cÞ
and for venturis it depends on the angle of divergence but varies from 10% to
15% of h. Q7.04 discusses the significance of permanent pressure drop and the
cost associated with it.
