Page 53 - Instrumentation Reference Book 3E

38 Measurement of flow

Sfafic I’ pressure and static pressure) and the principle of
presgure-Yn] operation is as follows.
taPPml !I If a tube is placed with its open end facing into
the flowstream (Figure 1.52) then the fluid
impinging on the open end will be brought to rest
and its kinetic energy converted into pressure
energy. The pressure build-up in the tube will be
greater than that in the free stream by an amount
termed the “impact pressure.” If the static pressure
is also measured, the differential pressure
between that measured by the pitot tube and the
Figure 1.52 Single hole pitot tube. static pressure will be a measure of the impact
pressure and therefore the velocity of the stream.
In equation (1.15) h the pressure differential or
impact pressure developed is given by
h = (V2212g) - (V;/2g) where V, = 0. Therefore,
h = - V:/2g, Le., the pressure increases by V:/2g.
The negative sign indicates that it is an increase in
pressure and not a decrease.
Increase in head:
h = V:/2g or V: = 2gh Le. VI = &@$
(1.51)
However, since this is an intrusive device not all
of the flowstream will be brought to rest on the
impact post; some will be deflected round it. A
coefficient C is introduced to compensate for this
and equation (1.50) becomes:
VI = c m (1.52)
If the pitot tube is to be used as a permanent device
for measuring the flow in a pipeline the relation-
ship between the velocity at the point of its location
to the mean velocity must be determined. This is
achieved by traversing the pipe and sampling
velocity at several points in the pipe, thereby deter-
mining flow profile and mean velocity.
For more permanent types of pitot-tube instal-
lation a multiport pitot tube (such as an Annu-
bar@) may be used as shown in Figure l .53. The
pressure holes are located in such a way that they
measure the representative dynamic pressure of
equal annuli. The dynamic pressure obtained at
the four holes facing into the stream is then aver-
aged by means of the “interpolating” inner tube
(Figure 1.53(b)), which is connected to the high-
pressure side of the manometer.
The low-pressure side of the manometer is con-
nected to the downstream element which meas-

Figure 1.53 The Annubar, courtesy, Dietrich Standard Division of Emerson Process Measurement.