orificemeter provides a simpler and cheaper arrangement for
the measurement of fow through a pipe. An orificemeter is
essentially a thin circular plate with a sharp edged concentric
circular hole in it.
- The orifice plate, being fixed at a section
of the pipe, (Fig. 15.3) creates an obstruction to the flow
by providing an opening in the form of an orifice to the
Fig 15.3 Flow
through an Orificemeter
- The area A0 of the orifice
is much smaller than the cross-sectional area of the pipe.
The flow from an upstream section, where it is uniform,
adjusts itself in such a way that it contracts until a section
downstream the orifice plate is reached, where the vena
contracta is formed, and then expands to fill the passage
of the pipe.
- One of the pressure tapings is usually
provided at a distance of one diameter upstream the orifice
plate where the flow is almost uniform (Sec. 1-1) and the
other at a distance of half a diameter downstream the orifice
- Considering the fluid to be ideal and
the downstream pressure taping to be at the vena contracta
(Sec. c-c), we can write, by applying Bernoulli’s
theorem between Sec. 1-1 and Sec. c-c,
are the piezometric pressures at Sec.1-1 and c-c respectively.
- From the equation of continuity,
where Ac is the area of the vena contracta.
- With the help of Eq. (15.11), Eq. (15.10)
can be written as,