Page 98 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 75

larger rotameters are less sensitive to viscosity & Ease of installation and maintenance: Rotameters
effects. mount vertically in the pipe without need for pipe
➢ The float shape does affect the viscosity limit. If taps, connecting lines, seal pots, valves, or require-
the viscosity limit is exceeded, the indicated flow ments for a straight run pipe upstream or down-
must be corrected for viscosity. stream, as is the case for orifice meters.

& Sustained high repeatability: The float moves & Design temperature: Metal tube up to about 450 C.

freely in the metering tube, experiencing no friction Glass tube up to 120 C. Plastic tube up to 60 C.
or hysteresis. The rotameter thus attains the ideal & Design pressure: Metal tube up to 100 bar. Glass tube
design goal of having high repeatability and up to 20 bar. Plastic tube up to about 5 bar.
being easily readable. Repeatability is 0.5–1% of & Ability to handle low flow rates: The small rota-
full scale. meters can measure liquid flow rates down to
& Low pressure drop: Because the area between the 0.65 mL/min and gas flow rates down to 47 mL/min.
float and the tapered tube increases with flow rate, DP & Viscosity: Liquids up to 200 cP.
across the float is low and relatively constant. This . How is the effect offluid density variations taken care of
reduces pumping costs. Also, a meter can be selected in rotameters to variations in operation?
to provide lower DP by using an oversized tube with a & Because the float is sensitive to changes in fluid
light float.
density, a rotameter can be furnished with two floats
& Linear scale: Because area variation is the measure of
(one sensitive to density, the other to velocity) and
flow rate (rather than head or differential pressure), used to approximate the mass flow rate.
the calibration curve is practically a straight line. & The more closely the float density matches the fluid
This means that the meter can have an indicating
density, thegreater the effect of a fluid density change
linear scale with evenly spaced divisions, comparing
will be on the float position.
better for readability than any differential pressure
& Mass flow rotameters work best with low viscosity
flow meter. Rotameter scale readings are not com-
fluids such as raw sugar juice, gasoline, jet fuel, and
pressed (harder to read) at low flow rates, so the same
light hydrocarbons.
degree of accuracy holds throughout the entire flow
. What type of rotameter is used for purge gas flow
range.
measurements?
& Lack of need for electricity: Rotameters used as local
& Low-capacity glass tube rotameters are extensively
indicators require no connection to an electric power
used in purge systems in which service they are
source; hence, there is no need for explosion-proof-
known as purge meters.
ing when flammables may be present. Useful where
the flow measurement must be made in a hazardous ➢ Instrument Society of America (ISA) defines
area where electric power is unavailable, or would be purge meter as a device designed to measure small
potentially dangerous. flow rates of liquids and gases used for purging
measurement piping.
& Special applications: Used as a bypass meter around
a mainline orifice for larger pipe sizes. Very high flow & With the help of a needlevalve purge meters facilitate
rates of liquids or gases can be measured by installing accurate control of low flow rates. For example, for
a rotameter in a bypass line around an orifice plate in water, these rates can be as low as 3.8 L/min (1 gpm)
3
the main line. and for air 56 L/min (<2ft /min).
& Signal: Visual or electronic. . What is a purge flow regulator?
& If a needle valve is placed at the inlet or outlet of a
& Sight glass functionality: A glass tube rotameter can
also serve as a sight glass, eliminating the need for a rotameter and a DP regulator controls the pressure
separate device to show that the process fluid is difference across this combination, the result is a
flowing. purge flow regulator.
& Such instrumentation packages are used as self-con-
& Familiarity: Most users are familiar with rotameters
and can readily understand their operation. tained purge flow meters.
& These are among the least expensive and most widely
& Ease of corrosion-proofing: Because of its design
simplicity, a rotameter can be economically con- used flow meters.
structed of high corrosion-resistant materials. & Their main application is to control small gas or
& Ease of conversion to measure a different fluid: A liquid purge streams.
model installed for service on one fluid can be & They are used to protect instruments from contacting
recalibrated for measurement of another with ease. hot and corrosive fluids, to protect pressure taps from