variable area flowmeters product guide

BULLETIN M-1
VARIABLE AREA FLOWMETERS
PRODUCT
GUIDE
VARIABLE-AREA FLOWMETERS
DESCRIPTION
A variable-area flow meter, or Rotameter,
is a simple, economical and accurate flow
measuring instrument. It consists of a tapered tube mounted vertically in the fluid
stream with its larger diameter at the top.
Within the tube is a float which is free to
move up and down. Fluid flows through
the tube from bottom to top. As it does,
the float rises until the area between it and
the tube wall is just large enough to pass
the amount of fluid flowing. The height
of the float is therefore a measure of the
flow rate through the tube.
OPERATION
The operation of a Rotameter may be explained by first considering
flow in a vertical pipe. If the
pipe is connected to a water
supply, water will flow up the
pipe and out the open end.
When the valve is opened all
of the way, the height of the
water above the end of the pipe
will increase. In fact, each position of the valve (corresponding to a given flow rate)
will cause the water to rise a discrete distance above the end of the pipe. If we had
some accurate means to measure the
height of the water column we
could use the piece of pipe to
measure flow rate. Now, take a
ball with the same diameter as
the inside of the pipe, drop the
ball into the pipe and open the
valve. The ball will rise until the
space between it and the top of
the pipe is large enough for the
water to flow over the edge.
Again, increase the flow rate by
opening the valve. The ball will
rise even higher so there is a larger space
between it and the end of the pipe. The
height of the ball above the pipe is pro-
portional to the quantity of water flowing
through the pipe.
At this point we are going to substitute
a tapered tube for the piece of pipe. The
tube has its smallest diameter at the bottom, and this diameter is such that the ball
shuts off the flow completely.
As we open the valve,
the buoyant effect of the
fluid tends to lift the ball.
But the density of the ball
is greater than that of the
fluid so the passage remains blocked. Fluid pressure builds up until it, plus
the buoyant effect, is
greater than the weight of
the ball and the ball rises
within the tube.
As the ball rises toward
the upper and larger end of the tube, an
annular passage opens between the ball
and the tube wall. The area of this passage
becomes larger as the ball rises higher in
the tube. When the passage is large enough
to pass the volume of fluid flowing, the
pressure difference across the ball, plus the
buoyant force, just equals the ball’s
weight. The ball now comes to rest in dynamic equilibrium. Its height is a direct
function of the flow rate in the tube.
the fluid stream experiences strong pulsations which can cause the float to chatter
against the sides of the tube. When this
happens, accurate reading of float position
is difficult. In extreme cases, this chattering will erode and may even break the
metering tube.
To overcome this problem,
McCrometer introduced flute-guided
tubes. These tubes have integral flutes, or
beads, formed on the inside of the tube at
the time of manufacture. Although the tube
walls are tapered, the diameter enclosed
by the flutes is constant over the length of
the tube. As flow rate changes, the float
moves freely in a vertical direction but is
restrained from any horizontal motion.
Consequently, readings are highly accurate and repeatable.
Another approach to a guided float design uses a circular section tube with three
flats at 60° to one another. Here the float
is contained inside the flats which remain
COMPONENTS
METER TUBES
Simplest of Rotameter tube designs is the
cone shape. Modern conical tubes are mass
produced by a mandrel forming borosilicate glass. The result is a thick walled tube
of uniform strength. The mandrel precisely
controls the tube’s inner bore, thereby providing the accuracy and interchangeability required of an industrial flow meter.
The conical tapered tube has proved itself in many applications. Under certain
conditions, a modified design may be required. One such condition exists where
Conical
tapered
Fluteguided
Triangular
flat
Guide
rod
parallel over the length of the tube. Operation is basically the same as the fluted
tube. The float is free to move vertically
but is restrained horizontally.
In some types of variable-area flow
meters, the float rides up and down on a
guide rod which is centered in the tube and
anchored at the top and bottom. They are
called rod-guided meters.
FLOATS
The shape of a Rotameter float determines
the effect of viscosity changes on the
meter’s accuracy. Simplest of float designs
is the ball float. This is primarily used for
low flow rates (purge meters) where there
Ball
Streamlined
SemiViscosity
viscosity
compensating
compensating
is little variation in viscosity. Ball floats
are commonly manufactured from stainless steel, black glass, red sapphire, tantalum, tungsten carbide and brass. By selecting the float material, a standard tube may
be adapted to a range of flow capacities.
For those applications demanding high
accuracy, the streamlined float is the most
economical choice. Because of its shape,
these floats provide high flow capacity in
a given tube size. The streamlined shape,
however, does not provide any compensation for changes in fluid viscosity. Consequently, they are best suited where minimum viscosity variations can be expected.
Often, a Rotameter is used for fluids
with widely varying viscosities. For this
type of service, the viscosity compensating float provides the maximum immunity
to changes in viscosity, but may require a
larger tube size for a given flow. Rotameters are also available with semi-viscosity
compensating floats. This float shape is a
compromise between the viscosity compensating float and the streamlined design.
It allows a larger capacity in a particular
tube while providing compensation over
a smaller viscosity range.
SCALES
Rotameters are available with scale
lengths to accommodate any application.
These range from 1½” scales for low-cost,
low-volume meters, through 10” lengths
for general industrial applications, to 24”
scales for high-accuracy laboratory instruments.
For general purpose applications, a
common graduation scheme divides the
scale into millimeter increments. Each
Rotameter is accompanied by a calibration chart to convert scale reading into
quantity of the fluid being metered. Millimeter calibration has several practical advantages. If a change is made in the fluid,
or if the meter is transferred to a different
application, the same tube can be used
with a revised calibration chart. And, because the tubes are standard, replacements
are readily available.
Another widely used calibration system marks the tube in percent of maximum
flow with a factor tag accompanying each
tube. Multiplying by the constant on the
tag converts the reading into units of flow
for the particular fluid being metered.
In many cases, Rotameter scales are
graduated directly in units of flow for a
specific fluid, for example, gallons per
minute or standard cubic feet per minute.
Also, a Rotameter can be specifically calibrated for any fluid, at additional cost. Replacement tubes with the same accuracy
would also require calibration.
Rotameter scales may be calibrated in millimeter increments
or directly in flow units for a particular fluid. Markings are on
a detachable scale plate or directly on the tube.
Glass tube Rotameters are supplied
with scales on detachable plates or integral with the tube itself. Units with integral scales have either a fired-on decal or
are etched directly on the face of the tube.
The more flexible arrangement, particularly if the tube is calibrated in flow units,
uses a detachable metal or plastic plate
mounted next to the tube.
SERIES 20-7050, 20-7050-V, 20-7055
LO-FLO ROTAMETERS
Series 20-7050 Rotameters are designed for use where fluid flows are very low.
Eight different metering tubes can be used for measuring wide flow ranges up to
28 gph water. All tubes are interchangeable within the same housing and are easily removed for cleaning. Unit is constructed from 316 stainless steel. Available
with built-in needle control valve, Series 20-7050-V. For higher flow (up to 4.6
gpm), the Series 20-7055 is available.
20-7050
20-7050-V
SERIES 20-5100, 20-5200
“SAFEGUARD” ROTAMETERS
Series 20-5100 (threaded connections) and 20-5200 (flanged connections) are
designed for measuring rate of flow of liquids and gases. “Universal” end fittings
permit instant change in the field between vertical and horizontal piping orientations. Available with neoprene or teflon packing; stainless steel cases; steel, brass,
stainless steel and PVC fittings. End fittings and floats can be made of most machinable materials for corrosion resistance.
20-5100
20-5200
SERIES 20-1340
BALL FLOW INDICATOR
Series 20-1340 Indicator has a guided stainless steel float that moves within a
tapered glass tube as directed by the flow. Body is brass. Designed for vertical
installation only. Has dual scale - one calibrated for gpm water at 70ºF, one for
cfm air at 14.7 psia and 70°F. Sizes from 1/4” to 2” NPT for flow rates to 190 gpm
water. Also available in 316 stainless steel and PVC.
20-1340
SERIES 20-7510
PURGE ROTAMETER
Series 20-7510 is an inexpensive instrument for indicating small flows, with good
reproducibility and moderate measuring accuracy. Includes back flow check valve
and manual control knob. Body is one-piece aluminum; other parts are stainless
steel. Unit is available in five sizes for flows up to 12 gph water.
20-7510
SERIES 20-7010, 20-7030
PURGE ROTAMETER
This unit features stainless steel construction throughout with plastic protection
windows. Design incorporates positive check valve, with or without needle valve,
and quick removal of tube. This type has a 1½” scale and is used for flows up to
12 gph. Series 20-7030 has a 3” scale and is used for flows up to 20 gph. The
rotameters shown here are equipped with a flow control valve.
20-7010
20-7030
SERIES 20-7050-V, 20-7055-V
HIGHER CAPACITY ROTAMETER
These Rotameters provide greater flowrates (up to 4.6 GPM; 20.5 SCFM with
Model 20-7055-V and up to .48 GPM; 2.0 SCFM with Model 20-7050-V) than
most purge meters, but still have accuracy of up to ±1% F.S. Both are made of 316
stainless steel to resist corrosion and have an easy-to-read 5” scale.
20-7050V
20-7055V
SERIES 20-7030-3200
PURGE ROTAMETER WITH
DIFFERENTIAL REGULATOR
These devices are used to maintain constant flow rate settings regardless of pressure fluctuations in the piping. Two designs are utilized, depending upon whether
the pressure fluctuations are upstream or downstream of the flow meter. Units are
available in brass or stainless steel and they can be mounted on most purge meters
of the proper flow sizing. Flow rates are 1 to 120 scfh air. Ideal for furnace and
liquid level “Bubbler” applications.
20-7030-3200
OTHER McCROMETER PRODUCTS INCLUDE:
Magnetic Flowmeters
Propeller Meters
Propeller Meters
Differential Pressure Flowmeters
Differential Pressure Flowmeters
Differential Pressure Flowmeters
Variable Area Meters
Electronic Instrumentation for Remote Display and Control
FOR MORE INFORMATION CONTACT:
Represented by:
3255 W. Stetson Avenue, Hemet, CA 92545-7799
Phone: (951) 652-6811 Fax: (951) 652-3078
e-mail: [email protected] Web Site: http://www.mccrometer.com
Hours: 8 a.m. - 4 p.m. PST, Monday-Friday
Canadian Patent 1325113
Lit# 24700-12 Rev 1.5/10-04
European Patent 0277121
U.S. Patents 4638672, 4812049, 5363699, 4944190 and 5,814,738
Japan patent 1,858,116
Other U.S. and Foreign patents pending
Printed in U.S.A.
 2004 by
McCrometer, Inc.