Page 89 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
66
one critical dimension, the wedge gap. Pressure drop & Repeatability: The ability of a flow meter to indicate
through the unit is only about half of that of conven- the same readings each time the same flow conditions
tional orifices. exist. These readings may or may not be accurate, but
& Integral wedge assemblies combine the wedge ele- will repeat. This capability is important when a flow
ment and the pressure taps into a one-piece pipe meter is used for flow control.
coupling bolted to a conventional pressure transmit- & In general, turndown ratios for orifice meters are
ter. No special piping or fittings are needed to install between 3:1 and 5:1.
the device in a pipeline. . What is a venturi meter?
. What are the considerations in the selection of orifice & A venturi meter consists of a smooth converging
plates? contraction to a narrow throat followed by a shallow
& Concentric square edge orifice plates are recom- diverging section.
mended for general applications in clean fluid with & The meter is installed between two flanges intended
relatively high Reynolds number. for this purpose.
& Concentric quarter-circle orifice plates are recom- & Pressure is sensed between a location upstream of the
mended for applications when low Reynolds number throat and a location at the throat.
values occur, that is with low rate liquid flow or . What are the recommended proportions (structural
highly viscous streams.
details) of a venturi meter?
& Eccentric orifice plates are recommended for appli-
& Entrance cone angle ¼ 21 2 .

cations where the drainage of extraneous matters is
& Exit cone angle ¼ 5–15 .

required.
& Throat length ¼ 1 throat diameter.
& Segmental orifice plates are recommended for appli-
& Upstream tap is located at 0.25–0.5 pipe diameter
cations where heavier or lighter components are
upstream of the entrance cone.
mixed in a given fluid as in the case of two-phase
flow measurement. & Venturi tubes are available in sizes up to 183 cm
(72 in.) and can pass 25–50% more flow than an
. Give the equation for volumetric flow through an ori-
orifice with the same DP.
fice.
& Pipes of up to 25 cm diameter usually utilize ma-
0:5
4
Q ¼ C 0 A 0 ½ð2DP=rÞ=ð1 b ފ : ð4:2Þ chined constrictions. They can be installed in large
diameter pipes using flanged, welded, or threaded
where A 0 is the orificearea and b isthe ratio of diameters
end fittings.
of orifice to pipe.
5
6
& Venturi accuracy is best for N Re between 10 and 10 .
& For corner taps
& Because of the cone and the gradual reduction in the
2:1
C 0 0:5969 þ 0:0312b 0:184b 8 cross-sectional area, there is no vena contracta. The
flow area is minimum at the throat.
2:5 6 0:75
þð0:0029b Þð10 =N ReD Þ : ð4:3Þ & Figure 4.10 gives the constructional details of a
venturi meter.
. What are the three “R”s in assessing an orifice meter?
. Illustrate the effect of Reynolds number on discharge
& Reliability/Uncertainty/Accuracy: The coefficients
coefficients for different types head flow meters.
calculated for flange taps are subject to an uncertainty & Figure 4.11 gives the effect of Reynolds numbers on
of approximately þ 0.5% when the b-ratio is be-
different flow meters.
tween 0.20 and 0.70. When the b-ratio is between
. What are the average values of orifice and venturi
0.10 and 0.20 and 0.70 and 0.75, the uncertainty may
coefficients?
be greater. Minimum uncertainty occurs between 0.2
& Orifice coefficient: 0.61. This is true if N Re at the
and 0.6 b-ratios. Below 1,000,000 Reynolds number,
orifice is above 20,000 and d/D is less than about 0.5,
there will be some small increase in uncertainty with
the value of the coefficient being nearly constant at
the minimum Reynolds number of 4000 being the
0.61. This value is adequate for design for liquids and
limit of the standard. Typical accuracy is about 2–4%
gases. The coefficient is determined experimentally.
of full scale.
For N Re < 20,000, the coefficient increases sharply
& Rangeability: Sometimes called turndown, is the
and then drops.
ratio of maximum flow to minimum flow throughout
& Venturi coefficient: 0.9.
which a stated accuracy is maintained. This is in the
range of 4–1. . What is the significance of discharge coefficient, C d ?