Page 102 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 79

& Relatively expensive. & Some corrosion-resistant designs are made from
. Does a turbine flow meter require a minimum straight plastic materials such as PVC.
distance before its installation? . What are the operating flow ranges of turbine flow
& To maintain an even cross-sectional flow, it is re- meters?
commended that there be a straight pipe length of at & Turbine meters should be sized so that the expected
least 10 inner diameters of the meter upstream and at average flow is between 60% and 75% of the max-
least 5 inner diameters downstream of the sensor. imum capacity of the meter.
. Can a turbine meter be used for flow of air or gases? & Flow velocities under 0.3 m/s (1 ft/s) can be insuffi-
& Some turbine flow meters can be used with air or cient, while velocities in excess of 3 m/s (10 ft/s) can
gases. However, if there are air bubbles or vapor result in excessive wear.
pockets in the liquid, the reading will be inaccurate. & Most turbine meters are designed for maximum
There should be a laminar (stable) flow through the velocities of 9 m/s (30 ft/s).
cross section of the pipe. . What is a dual-rotor turbine flow meter?
. What are the plus points in using a turbine flow meter? & A dual-rotor turbine flow meter involves two rotors
& The turbine flow meter is an accurate and reliable turning in opposite directions
flow meter for both liquids and gases. ➢ The front one acts as a conditioner, directing the
& Good accuracy with liquids. Used as a calibration flow to the back rotor.
standard. ➢ The rotors lock hydraulically and continue to turn
& Range of flow rates is 10:1 and response is very fast. as the flow decreases even to very low rates.
& Compact and has the same diameter as the pipe. & Dual-rotor liquid turbines increase the operating
& Signal output available for totalizing flows. range in small line size, under 5 cm (2 in.)
applications.
& Available for low flow rates.
& Easy to install and easy to maintain. . What are the fluid parameters that affect performance of
turbine flow meters?
& Unrecoverable DP is moderate.
& Viscosity affects the accuracy and linearity of turbine
. What type offlow measuring device is used for domestic
water supply lines? meters. It is therefore important to calibrate the meter
for the specific fluid it is intended to measure.
& Turbine flow meter.
& Repeatability is generally not greatly affected by
. What are the limitations of using a turbine flow meter?
changes in viscosity and turbine meters often are
& Sensitive to viscosity changes. Calibration is depen-
used to control the flow of viscous fluids.
dent on viscosity.
& In general, turbine meters perform well if the Rey-
& Straight pipeline required upstream.
nolds number is greater than 4000 and less than or
& Good for clean liquids and gases.
equal to 20,000.
& High cost. & Because it affects viscosity, temperature variation
& Problems with bearings. can also adversely affect accuracy and must be
. What are the materials that are normally used the compensated for or controlled.
construction of turbine flow meters? & The turbine meter’s operating temperature ranges

& Most industrial turbine flow meters are manufactured from 200 to 450 C.
from austenitic stainless steel (301, 303, 304 SS), & Density changes do not greatly affect turbine
whereas turbine meters intended for municipal water meters. On low-density fluids (sp. gr. < 0.7), the
service are bronze or cast iron. minimum flow rate is increased due to the reduced
& The rotor and bearing materials are selected to match torque, but the meter’s accuracy usually is not
the process fluid and the service. affected.
& Rotors are often made from stainless steel and bear- & The linearity of a turbine meter is affected by the
ings of graphite, tungsten carbide, ceramics, or in velocity profile (often dictated by the installation),
special cases of synthetic ruby or sapphire combined the viscosity, and the temperature.
with tungsten carbide. . What are the features incorporated in turbine flow
& In all cases, bearings and shafts are designed to meters for gas service?
provide minimum friction and maximum resistance & Gas meters compensate for the lower driving torque
to wear. produced by the relatively low density of gases.