Page 27 - Instrumentation Reference Book 3E

12 Measurement of flow

Installation requirements As already indicated, pressure drop is measured by pressure tappings
installation requirements for differential-pressure located upstream and downstream of the gate as
devices are quite critical. It is advisable to install shown in Figure l.l3(a). The position of the gate
primary elements as far downstream as possible is indicated by a scale. As the rate of flow through
from flow disturbances, such as bends, valves, and the orifice increases, the area of the orifice is
reducers. These requirements are tabulated in increased. If all other factors in equation (1.21)
considerable detail in BS 1042 Part 1 1964 and except area A? are kept constant the flow through
are reproduced in part in Appendix 1.1. It is the orifice will de end upon the product A2 E or
critical for the instrument engineer to be aware A2/ e (AZ/A~)~
As
A1
[l
(AI/A~)~].
increases,
-
that these requirements are “rules of thumb”, increases and [l - (A./A,)’] decreases and there-
and even slavish adherence to them may not pro-
duce measurement free from hydraulics-induced fore 1 / J m increases.
error. From a practical point of view, the best The relationship between A2 and flow is not
measurement is the one with the longest upstream linear. If the vertical movement of the gate is to
straight run, and the longest downstream straight be directly proportional to the rate of flow, the
run. width of the opening A? must decrease towards
the top as shown in Figure l.l3(a).
The flow through the meter can be made to
depend directly upon the area of the orifice A2 if
1.3.1.5 Variable-orifice meters instead of the normal static pressure being meas-
ured at the upstream tapping the impact pressure
So far the devices discussed have relied on a con- is measured. In order to do this the upstream tap
striction in the flowstream causing a differential is made in the form of a tube with its open end
pressure varying with flow rate. Another category facing directly into the flow as shown in Figure
of differential-pressure device relies on maintain- l.l3(b). It is in effect a pitot tube (see section on
ing a nominally constant differential pressure by point-velocity measurement).
allowing effective area to increase with flow. The The differential pressure is given by equation
principal devices to be considered are: rotameter, (1.15), where h is the amount the pressure at the
gate meter, and Gilflo. upstream tap is greater than that at the down-
stream tap:
Rotameter This is shown schematically in
Figure l.l2(a). In a tapered tube the upward (1.39)
stream of fluid supports the float where the
force on its mass due to gravity is balanced
against the flow force determined by the annu- Now, at the impact port, V2 = 0
lar area between the float and the tube and the
velocity of the stream. The float’s position in therefore lzl = V,’/2g
the tube is measured by a graduated scale and
its position is taken as an indication of flow
rate. where 11, is the amount the impact pressure is
Many refinements are possible, including the greater than the normal upstream static pressure.
use of magnetic coupling between the float and Thus the difference between impact pressure and
external devices to translate vertical movement the pressure measured at the downstream tap will
into horizontal and develop either electrical be 112 where
transmission or alarm actuation. Tube materials
can be either metal or glass depending on appli- 112 = I1 + 12,
cation. Figure 1.12(b) shows an exploded view of
a typical rotameter. (1.40)

Gate meter In this type of meter the area of the Therefore the velocity V, through the section
orifice may be varied by lowering a gate either A? is given by Vz = dm. The normal flow
manually or by an automatically controlled elec- equations for the type of installation shown in
tric motor. The gate is moved so as to maintain a Figure 1.13(b) will be the same for other orifices
constant pressure drop across the orifice. The but the velocity of approach factor is 1 and flow