Page 101 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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& Advantages of a bypass rotameter include low cost. generated and converted to a frequency output pro-
& Its major disadvantage is inaccuracy and sensitivity portional to the flow rate. Rotors are typically made
to material buildup. of a nonmagnetic material, like polypropylene, Ry-
ton, or PVDF (Kynar).
& In the water distribution industry, mechanical-drive
turbine flow meters continue to be the standard.
4.1.4 Mechanical Flow Meters
& These turbine meters use a gear train to convert the
. What are mechanical flow meters?
rotation of the rotor into the rotation of a vertical
& Mechanical flow meters measure flow using an ar-
shaft. The shaft passes between the metering tube and
rangement of moving parts, either by passing isolat-
the register section through a mechanical stuffing
ed, knownvolumes of a fluid through a series of gears
box, turning a geared mechanical register assembly
or chambers (positive displacement) or by means of a
to indicate flow rate and actuate a mechanical total-
rotating turbine or rotor elements.
izer counter.
& All positive displacement (PD) flow meters operate
& The water distribution industry, in recent years, has
by isolating and counting known volumes of a fluid
adopted a magnetic drive as an improvement over
(gas or liquid) while making it flow through the
high-maintenance mechanical drive turbine meters.
meter. By counting the number of passed isolated
This type of meter has a sealing disk between the
volumes, flow measurement is obtained.
measuring chamber and the register. On the measur-
& Each PD design uses a different means of isolating
ing chamber side, the vertical shaft turns a magnet
and counting these volumes.
instead of a gear.
& The frequency of the resulting pulse train is a mea-
& On the register side, an opposing magnet is mounted
sure of flow rate, while the total number of pulses
to turn the gear. This permits a completely sealed
gives the size of the batch.
register to be used with a mechanical drive
& While PD meters are operated by the kinetic energy mechanism.
of the flowing fluid, metering pumps determine the
& Used for clean and noncorrosive fluids.
flow rate while also adding kinetic energy to the fluid.
. What are the applications and constraints in use of
& Turbine flow meter belongs to the category of me-
turbine meters?
chanical flow meters.
& Turbine meters have a low DP and are very accurate
& Other types of rotary element flow meters include
( 0.25% or better).
the propeller (impeller), shunt, and paddle wheel
& Residential and industrial gas and water meters are
designs.
often of the rotary wheel type, that is, turbine type.
& They are exceptionally repeatable, but are restricted
4.1.4.1 Turbine and Paddle Wheel Flow Meters
to clean and low viscosity fluids because of possible
fouling of their rotating parts.
. What is a turbine flow meter and how does it operate?
& They have fast response rate (few milliseconds); high
& A turbine flow meter is a straight flow tube equipped
pressure and high temperature capabilities, up to
with rotating vanes/blades (turbine) mounted at right

345 bar (5000 psi) and 220 C (800 F) with high
angles to flow direction, suspended in the fluid stream
temperature pick coils); and compact rugged
free to rotate on a shaft, supported by one or more
construction.
bearings and located on the center line of the tube.
& They have high pressure and high temperature ca-
Flowing fluid will strike the blades at a small angle to
pabilities with compact and rugged construction.
balance the forces on the rotor and it rotates at an
angular speed that is proportional to volumetric flow & These meters should not be used for slurries or
rate. systems experiencing large, rapid flow or pressure
variation.
& The diameter of the rotor is very close to the inside
diameter of the metering chamber and its speed of & They are sensitive to temperature changes that affect
rotation is proportional to the volumetric flow rate. fluid viscosity.
& Turbine rotation can be detected by solid-state de- & Turbine meters have moving parts that are subject to
vices or by mechanical sensors that include infrared wear.
beams, photoelectric sensors, or very small magnets & One disadvantage in some designs is a loss of lin-
are embedded in the tips of the rotor blades sense the earity at the low flow rates.
frequency of rotation, an electrical pulse is then & Tend to be fragile with high maintenance costs.