Page 85 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
62
located at a distance of 1 pipe diameter from the
orifice plate.
& Pipe taps: These taps are located at 2.5 nominal pipe
diameters upstream and at 8 nominal pipe diameters
downstream (point of maximum pressure recovery).
They detect the smallest pressure difference and
because of the tap distance from the orifice, the effects
of pipe roughness, dimensional inconsistencies, and,
therefore, measurement errors are the greatest.
& Vena contracta taps: Upstream static hole is 0.5–2
pipe diameters from the orifice plate. Downstream
tap is located at the position of minimum pressure,
called vena contracta. This point, however, varies
with the b-ratio. Usually, upstream tap is located 1
pipe diameter (inside) and 0.3–0.8 pipe diameters
downstream from the face of the orifice plate. They
are not commonly used in other than plant measure-
ments, where flows are relatively constant and plates
are not changed.If the plate is changed,it may require
Typesoforificeplates. (a) Sharp-edged, (b) thick
a change in the tap location. Exact dimensions are
FIGURE 4.4
plate, and (c) thick plate with curved radius.
given in appropriate tables.
. What is beta-ratio? ➢ The vena contracta taps provide not only the
maximum pressure differential but also give rise
& Beta ratio (b) is the ratio of orifice plate bore divided
to noise. Also, in small pipes, the vena contracta
by pipe internal diameter, that is, d/D where d is the
might lie under a flange. Therefore, vena contracta
plate bore and D is the inside diameter of the pipe.
taps normally are used only in pipe sizes exceeding
. What are the different types of taps used in orifice
15 cm (6 in.).
meters?
& Corner taps: Static holes are drilled, one in the
& Flange taps: These taps are located 1 in. (25.4 mm)
upstream and one in the downstream flange, with
from the upstream face of the orifice plate and 1 in.
openings as close as possible to the orifice plate.
(25.4 mm) from the downstream face with a þ 1/64
& Corner taps are widely used in Europe. In-line sizes
to þ 1/32 tolerance. These are more commonly used.
less than 5 cm, they are used with special honed flow
& Figure 4.5 illustrates installation and flow pattern for
meter tubes for low flow rates.
a sharp-edged orifice with flange taps.
& Figure 4.6 illustrates orifice tap location alternatives.
& Radius taps: Radius taps are similar to vena contracta
taps, except the downstream tap is fixed at 0.5D from . Name some methods of reducing upstream calming
the orifice plate. On the upstream side, these are section requirements for a head flow meter.

Sharp-edged orifice meter showing flow pattern with flange taps.
FIGURE 4.5