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ADVANCED MATERIALS & PROCESSES •
JUNE 2014
47
HTPRO
11
Rotameter types
The glass or plastic rotameter (Fig. 3a),
is the most widely used because of its
low cost, low pressure drop, relatively
wide range, linear visual flow indica-
tion, and simplicity of operation. To
pass through the tapered tube, fluid
flow must raise the float. The greater
the flow rate, the higher the float is
lifted. In liquid service, the float rises
due to a combination of the liquid’s
buoyancy and its velocity. With gases,
buoyancy is negligible, so the float re-
sponds mostly to velocity.
The float moves up and down in pro-
portion to the fluid flow rate and the
annular area between the float and the
tube wall. As the float rises, the size of
the annular opening increases. As the
area increases, the differential pressure
across the float decreases. The float
reaches a stable position when the up-
ward force exerted by the flowing fluid
equals the weight of the float. Thus,
every float position corresponds to a
specific flow rate for a particular fluid’s
density and viscosity.
TABLE 1 — COMMONLY USED FLOW MEASUREMENT INSTRUMENTS BY TYPE
Industrial
Disassembly
Sensitivity to
Robust
flowmeter type
Style
Manufacturer (a)
without repiping dirty fluids spare parts
Variable area,
Metal tube
Waukee Engineering Co. Inc.
Yes
Moderate
Delicate
including rotameters
Metal cylinder
Meter Equipment Mfg. Inc.
Yes
Low
Moderate
tube
Glass or plastic
Fisher-Porter
No
Sensitive
Moderate
tube
Brooks Instrument
King Instrument Co.
Dwyer Instruments Inc.
Key Instruments
Vane type
Universal Flow Monitors Inc.
No
Moderate
Moderate
Erdco Engineering Corp.
Orange Research Inc.
Moving orifice
Hedland, Div.
No
Moderate
Robust
Racine Federated Inc.
Piston
Insite (Universal Flow
No
Moderate
Delicate
(with spring)
Monitors Inc.)
Differential pressure/
Orifice
Lambda Square Inc.
No
Moderate
Robust
Orifice
Flowell Corp.
Venturi
Flowell Corp.
No
Moderate
Robust
Fox Valve Development Corp.
Turbine/Impeller
Rotary impeller
Roots (BNC Industrial Co. Ltd.)
No
Sensitive
Moderate
TokicoTechno Ltd.
Turbine
Hoffer Flow Controls Inc.
No
Sensitive
Delicate
Sponsler Inc.
Great Plains Industries Inc.
Thermal mass
Thermal mass
Sierra Instruments Inc.
No
Sensitive
Delicate
MKS Instruments
Brooks Instrument
(a) Not all-inclusive
Features and advantages
Advantages of variable-area flowmeters include:
• Mechanical flow measurement with just a single moving part,
ensuring measurement reliability
• Application versatility and availability of a variety of construction
materials, inlet and outlet sizes, and types
• Easy installation with generally no straight pipe requirements
• Low pressure drops
• Linear scales, allowing easy flow measurement interpretation
• Electronic output availability, preserving the mechanical flow
measurement
Advantages of tapered-tube rotameters include:
• Low instrument cost (when glass or plastic metering tube is used)
• Can be used for very low flow rates
Advantages of slotted-cylinder flowmeters include:
• Flow measurement accuracy determined by the precision of the slot
manufacturing operation; good flow range of 25:1 results
• Instrument specifications can be changed by field replacement of the
slotted tube and float without having to re-pipe the flowmeter vessel
• Ability to handle high flows and pressures
• Improved immunity to pulsating flows, with no minimum backpressure
Limitations common to both tapered-tube and slotted-cylinder variable-area
flowmeters include the requirement of vertical mounting aand the fact that they
contain moving parts.