FLOWMETER TYPES AND THEIR PRINCIPLES
Measuring the flow of
liquids is a critical need in many industrial plants. In some operations, the
ability to conduct accurate flow measurements is so important that it can make
the difference between making a profit or taking a loss. In other cases,
inaccurate flow measurements or failure to take measurements can cause serious
(or even disastrous) results.
With most liquid flow
measurement instruments, the flow rate is determined inferentially by measuring
the liquid's velocity or the change in kinetic energy. Velocity depends on the
pressure differential that is forcing the liquid through a pipe or conduit.
Because the pipe's cross-sectional area is known and remains constant, the
average velocity is an indication of the flow rate. The basic relationship for
determining the liquid's flow rate in such cases is:
Q = V x A
liquid flow through the pipe
V = average velocity of the flow
cross-sectional area of the pipe
Other factors that
affect liquid flow rate include the liquid's viscosity and density, and the
friction of the liquid in contact with the pipe.
Direct measurements of
liquid flows can be made with positive-displacement flowmeters. These units
divide the liquid into specific increments and move it on. The total flow is an
accumulation of the measured increments, which can be counted by mechanical or
The performance of
flowmeters is also influenced by a dimensionless unit called the Reynolds
Number. It is defined as the ratio of the liquid's inertial forces to its drag
Figure 1: Laminar and turbulent flow are two
types normally encountered in liquid flow Measurement operations. Most
applications involve turbulent flow, with R values above 3000. Viscous liquids
usually exhibit laminar flow, with R values below 2000. The transition zone
between the two levels may be either laminar or turbulent.
R = 3160 x Q x Gt
R = Reynolds
Q = liquid's flow
Gt = liquid's
D = inside pipe
h = liquid's viscosity, cp
The flow rate and the specific
gravity are inertia forces, and the pipe diameter and viscosity are drag forces.
The pipe diameter and the specific gravity remain constant for most liquid
applications. At very low velocities or high viscosities, R is low, and the
liquid flows in smooth layers with the highest velocity at the center of the
pipe and low velocities at the pipe wall where the viscous forces restrain it.
This type of flow is called laminar flow. R values are below approximately 2000.
A characteristic of laminar flow is the parabolic shape of its velocity profile,
However, most applications involve
turbulent flow, with R values above 3000. Turbulent flow occurs at high
velocities or low viscosities. The flow breaks up into turbulent eddies that
flow through the pipe with the same average velocity. Fluid velocity is less
significant, and the velocity profile is much more uniform in shape. A
transition zone exists between turbulent and laminar flows. Depending on the
piping configuration and other installation conditions, the flow may be either
turbulent or laminar in this zone.
Numerous types of flowmeters are available for
closed-piping systems. In general, the equipment can be classified as
differential pressure, positive displacement, velocity, and mass meters.
Differential pressure devices (also known as head meters) include orifices,
venturi tubes, flow tubes, flow nozzles, pitot tubes, elbow-tap meters, target
meters, and variable-area meters, Fig. 2.
meters include piston, oval-gear, nutating-disk, and rotary-vane types. Velocity
meters consist of turbine, vortex shedding, electromagnetic, and sonic designs.
Mass meters include Coriolis and thermal types. The measurement of liquid flows
in open channels generally involves weirs and flumes.
Space limitations prevent
a detailed discussion of all the liquid flowmeters available today. However,
summary characteristics of common devices are shown in Table 1.
(Click here to see Selection
Brief descriptions follow.
The use of differential
pressure as an inferred measurement of a liquid's rate of flow is well known.
Differential pressure flowmeters are, by far, the most common units in use
today. Estimates are that over 50 percent of all liquid flow measurement
applications use this type of unit.
The basic operating
principle of differential pressure flowmeters is based on the premise that the
pressure drop across the meter is proportional to the square of the flow rate.
The flow rate is obtained by measuring the pressure differential and extracting
the square root.
flowmeters, like most flowmeters, have a primary and secondary element. The
primary element causes a change in kinetic energy, which creates the
differential pressure in the pipe. The unit must be properly matched to the pipe
size, flow conditions, and the liquid's properties. And, the measurement
accuracy of the element must be good over a reasonable range. The secondary
element measures the differential pressure and provides the signal or read-out
that is converted to the actual flow value.
Orifices are the most popular liquid flowmeters in use today. An
orifice is simply a flat piece of metal with a specific-sized hole bored in it.
Most orifices are of the concentric type, but eccentric, conical (quadrant), and
segmental designs are also available.
In practice, the orifice
plate is installed in the pipe between two flanges. Acting as the primary
device, the orifice constricts the flow of liquid to produce a differential
pressure across the plate. Pressure taps on either side of the plate are used to
detect the difference. Major advantages of orifices are that they have no moving
parts and their cost does not increase significantly with pipe size.
Conical and quadrant
orifices are relatively new. The units were developed primarily to measure
liquids with low Reynolds numbers. Essentially constant flow coefficients can be
maintained at R values below 5000. Conical orifice plates have an upstream
bevel, the depth and angle of which must be calculated and machined for each
The segmental wedge is a
variation of the segmental orifice. It is a restriction orifice primarily
designed to measure the flow of liquids containing solids. The unit has the
ability to measure flows at low Reynolds numbers and still maintain the desired
square-root relationship. Its design is simple, and there is only one critical
dimension the wedge gap. Pressure drop through the unit is only about half that
of conventional orifices.
Integral wedge assemblies
combine the wedge element and pressure taps into a one-piece pipe coupling
bolted to a conventional pressure transmitter. No special piping or fittings are
needed to install the device in a pipeline.
Metering accuracy of all
orifice flowmeters depends on the installation conditions, the orifice area
ratio, and the physical properties of the liquid being measured.
(Back to Meter
Venturi tubes have the advantage of being able to handle large flow
volumes at low pressure drops. A venturi tube is essentially a section of pipe
with a tapered entrance and a straight throat. As liquid passes through the
throat, its velocity increases, causing a pressure differential between the
inlet and outlet regions.
The flowmeters have no
moving parts. They can be installed in large diameter pipes using flanged,
welded or threaded-end fittings. Four or more pressure taps are usually
installed with the unit to average the measured pressure. Venturi tubes can be
used with most liquids, including those having a high solids content.
(Back to Meter
Flow tubes are somewhat similar to venturi tubes except that they
do not have the entrance cone. They have a tapered throat, but the exit is
elongated and smooth. The distance between the front face and the tip is
approximately one-half the pipe diameter. Pressure taps are located about
one-half pipe diameter downstream and one pipe diameter upstream.
(Back to Meter
Flow Nozzles, at high velocities, can handle
approximately 60 percent greater liquid flow than orifice plates having the same
pressure drop. Liquids with suspended solids can also be metered. However, use
of the units is not recommended for highly viscous liquids or those containing
large amounts of sticky solids. (Back to Meter Types
Pitot tubes sense two pressures simultaneously, impact and static.
The impact unit consists of a tube with one end bent at right angles toward the
flow direction. The static tube's end is closed, but a small slot is located in
the side of the unit. The tubes can be mounted separately in a pipe or combined
in a single casing.
Pitot tubes are generally
installed by welding a coupling on a pipe and inserting the probe through the
coupling. Use of most pitot tubes is limited to single point measurements. The
units are susceptible to plugging by foreign material in the liquid. Advantages
of pitot tubes are low cost, absence of moving parts, easy installation, and
minimum pressure drop.(Back to Meter
Elbow tap meters operate on the principle that when liquid travels in a
circular path, centrifugal force is exerted along the outer edges. Thus, when
liquid flows through a pipe elbow, the force on the elbow's interior surface is
proportional to the density of the liquid times the square of its velocity. In
addition, the force is inversely proportional to the elbow's radius.
Any 90 deg. pipe elbow
can serve as a liquid flowmeter. All that is required is the placement of two
small holes in the elbow's midpoint (45 deg. point) for piezometer taps.
Pressure-sensing lines can be attached to the taps by using any convenient
difference in pressure on the outside and inside walls, caused by centrifugal
force, can be measured with a differential pressure transducer. Figure 2 shows a
Pressure measurements are
obtained by placing taps at 45- degree angles on opposite sides of the elbow.
The size of each of the two taps should not exceed one-eighth of the pipe
diameter. Flow is calculated according to the following
W = 244 [SQ.ROOT SIGN]
where W = flow in pounds per
r = elbow radius
D = elbow diameter
h = differential pressure
p = density in
(Back to Meter Types
Target meters sense and measure forces caused by liquid impacting on a
target or drag-disk suspended in the liquid stream. A direct indication of the
liquid flow rate is achieved by measuring the force exerted on the target. In
its simplest form, the meter consists only of a hinged, swinging plate that
moves outward, along with the liquid stream. In such cases, the device serves as
a flow indicator.
A more sophisticated
version uses a precision, low-level force transducer sensing element. The force
of the target caused by the liquid flow is sensed by a strain gage. The output
signal from the gage is indicative of the flow rate. Target meters are useful
for measuring flows of dirty or corrosive liquids.(Back to Meter Types
Variable-area meters, often called
rotameters, consist essentially of a tapered tube and a float, Fig. 3.
Although classified as differential pressure units, they are, in reality,
constant differential pressure devices. Flanged-end fittings provide an easy
means for installing them in pipes. When there is no liquid flow, the float
rests freely at the bottom of the tube. As liquid enters the bottom of the tube,
the float begins to rise. The float is selected so as to have a density higher than that of the
fluid and the
position of the float varies directly with the flow rate. Its exact position is
at the point where the differential pressure between the upper and lower
surfaces balance the weight of the float.
Because the flow rate can
be read directly on a scale mounted next to the tube, no secondary flow-reading
devices are necessary. However, if desired, automatic sensing devices can be
used to sense the float's level and transmit a flow signal. Rotameter tubes are
manufactured from glass, metal, or plastic. Tube diameters vary from 1/4 to
greater than 6 in. (Back to Meter
Operation of these units
consists of separating liquids into accurately measured increments and moving
them on. Each segment is counted by a connecting register. Because every
increment represents a discrete volume, positive-displacement units are popular
for automatic batching and accounting applications. Positive-displacement meters
are good candidates for measuring the flows of viscous liquids or for use where
a simple mechanical meter system is needed.
of the single and multiple-piston types. The specific choice depends on the
range of flow rates required in the particular application. Piston meters can be
used to handle a wide variety of liquids. A magnetically driven, oscillating
piston meter is shown in Fig. 4. Liquid never comes in contact with gears or
other parts that might clog or corrode. (Back to Meter Types
Oval-gear meters have two rotating, oval-shaped
gears with synchronized, close fitting teeth. A fixed quantity of liquid passes
through the meter for each revolution. Shaft rotation can be monitored to obtain
specific flow rates. (Back to Meter
Nutating-disk meters have a moveable disk mounted on a
concentric sphere located in a spherical side-walled chamber. The pressure of
the liquid passing through the measuring chamber causes the disk to rock in a
circulating path without rotating about its own axis. It is the only moving part
in the measuring chamber. (Back to Meter
extending perpendicularly from the disk is connected to a mechanical counter
that monitors the disk's rocking motions. Each cycle is proportional to a
specific quantity of flow. As is true with all positive-displacement meters,
viscosity variations below a given threshold will affect measuring accuracies.
Many sizes and capacities are available. The units can be made from a wide
selection of construction materials.
Rotary-vane meters are available in several designs,
but they all operate on the same principle. The basic unit consists of an
equally divided, rotating impeller (containing two or more compartments) mounted
inside the meter's housing. The impeller is in continuous contact with the
casing. A fixed volume of liquid is swept to the meter's outlet from each
compartment as the impeller rotates. The revolutions of the impeller are counted
and registered in volumetric units.
flowmeters consist of two radically pitched helical rotors geared together, with
a small clearance between the rotors and the casing. The two rotors displace
liquid axially from one end of the chamber to the other. (Back to Meter Types
instruments operate linearly with respect to the volume flow rate. Because there
is no square-root relationship (as with differential pressure devices), their
rangeability is greater. Velocity meters have minimum sensitivity to viscosity
changes when used at Reynolds numbers above 10,000. Most velocity-type meter
housings are equipped with flanges or fittings to permit them to be connected
directly into pipelines.
Turbine meters have found widespread use for accurate liquid
measurement applications. The unit consists of a multiple-bladed rotor mounted
with a pipe, perpendicular to the liquid flow. The rotor spins as the liquid
passes through the blades. The rotational speed is a direct function of flow
rate and can be sensed by magnetic pick-up, photoelectric cell, or gears.
Electrical pulses can be counted and totalized, Fig. 5.
The number of
electrical pulses counted for a given period of time is directly proportional to
flow volume. A tachometer can be added to measure the turbine's rotational speed
and to determine the liquid flow rate. Turbine meters, when properly specified
and installed, have good accuracy, particularly with low-viscosity liquids.
concern with turbine meters is bearing wear. A "bearingless" design has been
developed to avoid this problem. Liquid entering the meter travels through the
spiraling vanes of a stator that imparts rotation to the liquid stream. The
stream acts on a sphere, causing it to orbit in the space between the first
stator and a similarly spiraled second stator. The orbiting movement of the
sphere is detected electronically. The frequency of the resulting pulse output
is proportional to flow rate. (Back to Meter Types
Vortex meters make use of a natural phenomenon that occurs when a
liquid flows around a bluff object. Eddies or vortices are shed alternately
downstream of the object. The frequency of the vortex shedding is directly
proportional to the velocity of the liquid flowing through the meter, Fig. 6.
major components of the flowmeter are a bluff body strut-mounted across the
flowmeter bore, a sensor to detect the presence of the vortex and to generate an
electrical impulse, and a signal amplification and conditioning transmitter
whose output is proportional to the flow rate, Fig. 7. The meter is equally
suitable for flow rate or flow totalization measurements. Use for slurries or
high viscosity liquids is not recommended.
(Back to Meter
Meter The principle of operation of a swirl flowmeter is as follows. The
meter body has a set of blades welded at the inlet, called the swirler. These
blades impart a tangential velocity (or swirl) to liquids, gases or vapors. The
fluid is then accelerated by a reduction in the meter body bore. A piezoelectric
sensor is located in the center of the meter at the point of maximum fluid
velocity. Flow is then decelerated as it approaches the meter outlet by an
increase in meter body bore. A deswirler is welded to the meter body near the
outlet. This deswirler eliminates the tangential velocity imparted to the fluid
at the inlet so that other instrumentation downstream of the meter will not be
affected by its operation.
A swirl flowmeter
consists of the flowmeter body and an electronics housing (the electronics can
be remote mounted for safety or convenience). Swirlmeters are only available
with flanged meter bodies. A wafer option is not available, as there is for
vortex meters. Swirlmeters use the same sensors and electronics as vortex
meters, only the meter bodies differ in design. Swirlmeters are most cost
effective with stainless steel construction, although Hastelloy is also offered.
Swirlmeters are available in sizes from ?to 16 inches, and have options for
ANSI 150, 300 or 600 mating flanges.
rotation caused by the swirler has at its core a low-pressure zone. The
low-pressure zone is thrown into a secondary rotation proportional to flow rate.
This rotating low-pressure zone can be likened to a helical coil. At low flow
rates, the low-pressure swirls are farther apart (the helical coil is stretched
out). At higher flows, the low-pressure swirls are closer together (the coil is
compressed). Areas of slightly higher pressure separate the low-pressure swirls.
The sensor will deflect (to the left and then to the right) as a pressure swirl
passes from one side to other. The alternating deflection of the sensor produces
a sine wave voltage output, similar to the output shown in the vortex section.
The frequency of this output voltage is the same frequency as the rotating
low-pressure zones, and is therefore proportional to the volumetric flow
The swirl flowmeter
factor does not exhibit the same deviation at high Reynolds numbers as does the
vortex meter factor. This has been verified by testing on water and air at
independent facilities. For this reason, a swirlmeter factor determined by water
calibration is universally valid for all fluids. The swirlmeter, based on the
water calibration, has a published accuracy of 0.5 percent of rate for liquids,
gases or steam.
As with the vortex
meter, swirlmeter rangeability is fixed by the size of the meter and the fluid
properties. The sensor requires a minimum strength pressure pulse to be able to
distinguish the flow signal from hydraulic noise. While turbulent flow is
required at all times, the swirlmeter does not have the same limitation on
Reynolds number, as does the vortex meter and can generally measure lower flows.
On the other hand, the swirlmeter body presents more of an obstruction to flow
than does the vortex meter, and creates higher permanent head losses under
similar conditions. For this reason, the swirlmeter does not measure flow rates
as high as the vortex meter. The swirlmeter, like the vortex meter, averages
10:1 turndowns or higher on liquids, and 20:1 or higher on gases and vapors.
However, the flow range of a 2-inch swirlmeter will be different than the flow
range of a 2-inch vortex meter under the same operating conditions. Just as for
a vortex meter, you select the swirlmeter size to achieve a desired flow range
given the process conditions, and never to match the process piping. Swirlmeters
have to be downsized (using a 2-inch meter for 3-inch process piping) less
frequently than vortex meters. Free computer software is available from
manufacturers that make sizing swirl flowmeters quick and
Swirl Flowmeter Application
Swirlmeters cost about 50
percent more than the same size vortex meter because of the added complexity and
welding requirements. The rule of thumb is to use the vortex meter whenever
possible for cost, and use the swirlmeter for:
- Tight piping
- More viscous liquids (8 cp
< μ < 30 cp).
- Lower flow measurement
- Higher accuracy on gases and
- When downsizing to install a
vortex meter is not feasible (head loss).
- These meters are compatible
with low viscosity (< 30 cp) liquids, gases and steam.
less affected by as many real world parameters as orifice plates and turbine
meters, and are less sensitive to piping effects than vortex meters. Swirlmeters
require just 3 diameters of straight pipe upstream (regardless of bends, valves,
etc.), and either 1 or 3 diameters of straight pipe downstream (the latter only
required when a control valve is downstream of the meter).
Like vortex meters,
they measure velocity and infer actual volumetric flow rate from the known
geometry of the meter body. Swirlmeters are generally used with flow computers
to measure flow in standard volumetric or mass units (along with external
pressure and/or temperature measurements). The flow computer is not necessary if
the fluid density is constant. A pressure tap is provided on the meter body for
making the pressure measurement (using an external sensor). The temperature
element should be located downstream of the meter if needed.
The swirlmeter does
not measure to true zero flow. There is a flow cut-off point below which the
meter output is automatically clamped at zero (4 mA for analog output). For most
applications, this limitation does not pose a problem, as the swirlmeter has
good low flow capability. However, this can be a draw back for applications
where flows during start-up or shutdown operations, or other upset conditions,
can be greatly different than under normal operating conditions. Users may need
an indication of flow under such upsets, even if they do not need to measure
flow accurately, making use of the swirlmeter questionable.
also be questionable for some batching applications, especially if the pipe does
not remain full between batches. The meter will not register flow as the fluid
accelerates from zero to the cut-off value, and as the fluid decelerates back to
zero at the end of the batch. Swirlmeters are unidirectional and will not
measure or subtract any backflow from the batch total. This may create
significant measurement errors, depending on the system dynamics, and the size
of the batch.
There may be a
potential problem installing swirlmeters on existing processes where the flow
range to be measured is completely unknown. Many times, the instrument engineer
makes an educated guess on flow range. A swirlmeter sized for the wrong flow
range, or wrong process conditions, may need to be replaced by a different size
meter entirely. Other devices, like magnetic flowmeters, orifice plates and
turbine meters, are more forgiving, and can be easily adapted to fit the actual
process conditions after installation.
Measuring gas flows
when the process pressure is low (low-density gases) is less of a problem for
swirlmeters than for vortex meters. Low-density gases can be measured with a
swirlmeter, however rangeability may be less than the 20:1 mentioned previously,
and extreme care must be taken in selecting the correct size
multi-phase flow has lower accuracy than for single-phase fluids. The meter will
measure the flow of all phases present and report it as all liquid or gas
(depending on how the meter is configured). The secondary phase should be
removed, if feasible, before the meter for the highest accuracy. Any secondary
phase should be homogeneously dispersed and should not have any potential for
sticking to or coating the meter. There is no evidence to suggest that a
swirlmeter has any advantage over a vortex meter in regards to measuring
Pressure drop must
also be considered when selecting a swirlmeter. Flashing and cavitation have an
adverse affect on meter accuracy, and can damage the meter itself. It was
previously stated that a swirlmeter produces higher head loss than a vortex
meter under the same conditions, up to five times higher. However, this is not a
fair comparison. In real practice, the head loss for the swirlmeter is about the
same as for the vortex meter, because you generally use a larger size swirlmeter
than vortex meter to handle the same application.
Fluids that tend to
form coatings are bad applications for swirlmeters.
One of the main advantages of
the swirlmeter is its insensitivity to piping effects. It is an excellent meter
for tight piping situations.
Swirlmeters can be
installed vertically, horizontally or at any angle. Allow liquids to flow
against gravity to keep the pipe full. When the liquid is moving with gravity,
elevate the downstream piping above the meter installation level to maintain a
full pipe. Install the meter to avoid standing liquid when the pipe is empty.
Also plan for the installation so as to avoid formation of gas bubbles in liquid
flow. Check valves may be used when installing a vortex meter to keep it full of
liquid when there is no active flow in the process.
Mating flanges on
the process piping must be of the same nominal size as on the flowmeter. Flanges
with a smooth bore, similar to weld neck flanges, are preferred. Do not use
reducing flanges. Most performance specifications are based upon using Schedule
40 or Schedule 80 mating pipe. The mating pipe should be of good quality, and
have an internal surface free from mill scale, pits, holes, reaming scores,
bumps, etc., for a distance of 4 diameters upstream and 2 diameters downstream
of the meter. The bores of the adjacent piping, meter and gaskets should be
aligned to prevent steps.
should be placed no closer than 3 pipe diameters upstream or 3 pipe diameters
downstream of the meter.
The sensor used in
the swirlmeter can be replaced in the field, but does require process shutdown.
The meter should be installed with blocking valves, or in a bypass line, if
process shutdown for maintenance poses a problem.
vibration or process noise can affect measurement accuracy. Mechanical pipe
vibration can be eliminated by placing proper piping supports on either side of
the meter, or by rotating the meter in the process piping so that the sensor is
located in a plane different than the vibration. Process noise (from chattering
valves, steam traps, pumps, etc.) is hydraulically connected to the meter by the
fluid. The swirlmeter uses the same electronics with digital signal processing,
as the vortex meter, to eliminate the adverse effects of vibration and noise,
without sacrificing rangeability.
Effect Flowmeter & Momentum Exchange Flowmeter. While vortex shedding flowmeters are
the most recognized types of oscillating flow measurement devices, less well
known are meters based on the Coanda Effect, and the phenomenon known as
The Coanda Effect is
named after aerodynamicist Henri-Marie Coanda, who discovered that a free jet
emerging from a nozzle or conduit will follow a nearby surface and attach to it.
Fluid flowing through the meter body bends toward and attaches to a sidewall. A
portion of the flow is diverted through a feedback passage, however, and pushes
the stream toward a sidewall on the opposite side of the meter body, which also
has a feedback passage through which a portion of the flow is diverted. The
fluid from this feedback passage pushes the stream back toward the sidewall to
which it was initially attached, and the self-initiating, self-sustaining
process is repeated. A sensor, located in one of the two feedback passages,
detects the presence and absence of flow. The frequency of the pulse signals is
linear with volumetric flow rate.
The momentum exchange
flowmeter is similar to the Coanda model, but relies on a different mechanism to
create oscillations. Unlike the Coanda meter, the momentum exchange meter does
not have sidewalls. The shape of the meter body creates a main flow that passes
through the nozzle and towards one side of the meter body or the other. This
creates a flow pulse in a feedback passage, exerting a force on the main jet and
deflecting it so it exerts a force on the fluid in the opposite passage. The
pattern repeats continuously, creating a self-sustaining oscillation. Like the
Coanda meter, it has a sensor in one of the feedback passages that detects the
pulsing of fluids.
While most oscillating
flowmeters -- including the Coanda fluidic flowmeter -- require turbulent flows
to function, the momentum exchange meter does not, enabling its use with
Like other types of
oscillating flowmeters, benefits include minimum maintenance, high stability and
relatively inexpensive purchase cost. However, they are limited to use on pipes
four inches in diameter or less. Use of larger pipes would create too few pulses
per gallon for accurate measurement.(Back to Meter Types
Electromagnetic meters can handle most liquids and
slurries, providing that the material being metered is electrically conductive.
Major components are the flow tube (primary element), Fig. 8. The flow tube
mounts directly in the pipe. Pressure drop across the meter is the same as it is
through an equivalent length of pipe because there are no moving parts or
obstructions to the flow. The voltmeter can be attached directly to the flow
tube or can be mounted remotely and connected to it by a shielded
Electromagnetic flowmeters operate on Faraday's law of
electromagnetic induction that states that a voltage will be induced when a
conductor moves through a magnetic field. The liquid serves as the conductor;
the magnetic field is created by energized coils outside the flow tube, Fig. 9.
The amount of voltage produced is directly proportional to the flow rate. Two
electrodes mounted in the pipe wall detect the voltage, which is measured by the
Electromagnetic flowmeters have major advantages: They
can measure difficult and corrosive liquids and slurries; and they can measure
forward as well as reverse flow with equal accuracy. Disadvantages of earlier
designs were high power consumption, and the need to obtain a full pipe and no
flow to initially set the meter to zero. Recent improvements have eliminated
these problems. Pulse-type excitation techniques have reduced power consumption,
because excitation occurs only half the time in the unit. Zero settings are no
(Back to Meter Types
can be divided into Doppler meters and time-of-travel (or transit) meters.
Doppler meters measure the frequency shifts caused by liquid
flow. Two transducers(one to transmit and the other to receive signal) are
mounted in a case attached to one side of the pipe. A signal of known frequency
is sent into the liquid to be measured. Solids, bubbles, or any discontinuity in
the liquid, cause the pulse to be reflected to the receiver element, Fig. 10.
Because the liquid causing the reflection is moving, the frequency of the
returned pulse is shifted. The frequency shift is proportional to the liquid's
A portable Doppler meter
capable of being operated on AC power or from a rechargeable power pack has
recently been developed. The sensing heads are simply clamped to the outside of
the pipe, and the instrument is ready to be used. Total weight, including the
case, is 22 lb. A set of 4 to 20 millampere output terminals permits the unit to
be connected to a strip chart recorder or other remote device.
Because solids particles or
entrained gases are required for measurement, Doppler meters are not appropriate
for clean liquids. In general, Doppler flowmeters are less accurate than TOF
flowmeters, however, they are less expensive.
(Back to Meter Types
Time-of-travel(Transit-Time) meters have transducers mounted
on each side of the pipe. The configuration is such that the sound waves
traveling between the devices are at a 45 deg. angle to the direction of liquid
flow. The speed of the signal traveling between the transducers increases or
decreases with the direction of transmission and the velocity of the liquid
being measured. A time-differential relationship proportional to the flow can be
obtained by transmitting the signal alternately in both directions.
A limitation of
time-of-travel meters is that the liquids being measured must be relatively free
of entrained gas or solids to minimize signal scattering and absorption.
(Back to Meter
need for more accurate flow measurements in mass-related processes (chemical
reactions, heat transfer, etc.) has resulted in the development of mass
flowmeters. Various designs are available, but the one most commonly used for
liquid flow applications is the Coriolis meter. Its operation is based on the
natural phenomenon called the Coriolis force, hence the name.
Coriolis meters are true mass meters that measure the mass rate of flow
directly as opposed to volumetric flow. Because mass does not change, the meter
is linear without having to be adjusted for variations in liquid properties. It
also eliminates the need to compensate for changing temperature and pressure
conditions. The meter is especially useful for measuring liquids whose viscosity
varies with velocity at given temperatures and pressures.
meters are also available in various designs. A popular unit consists of a
U-shaped flow tube enclosed in a sensor housing connected to an electronics
unit. The sensing unit can be installed directly into any process. The
electronics unit can be located up to 500 feet from the sensor.
sensor housing, the U-shaped flow tube is vibrated at its natural frequency by a
magnetic device located at the bend of the tube. The vibration is similar to
that of a tuning fork, covering less than 0.1 in. and completing a full cycle
about 80 times/sec. As the liquid flows through the tube, it is forced to take
on the vertical movement of the tube, Fig. 11. When the tube is moving upward
during half of its cycle, the liquid flowing into the meter resists being forced
up by pushing down on the tube.
forced upward, the liquid flowing out of the meter resists having its vertical
motion decreased by pushing up on the tube. This action causes the tube to
twist. When the tube is moving downward during the second half of its vibration
cycle, it twists in the opposite direction.
forced upward, the liquid flowing out of the meter resists having its vertical
motion decreased by pushing up on the tube. This action causes the tube to
twist. When the tube is moving downward during the second half of its vibration
cycle, it twists in the opposite direction. The ammount of twist is directly
proportional to the mass flow rate of the liquid flowing through the tube.
Magnetic sensors located on each side of the flow tube measure the tube
velocities, which change as the tube twists. The sensors feed this information
to the electronics unit, where it is processed and converted to a voltage
proportional to mass flow rate. The meter has a wide range of applications from
adhesives and coatings to liquid nitrogen.
This meter has extremely high
accuracy but can also cause a high pressure drop.(Back to Meter Types
Thermal-type mass flowmeters have traditionally been used for
gas measurements, but designs for liquid flow measurements are available. These
mass meters also operate independent of density, pressure, and viscosity.
Thermal meters use a heated sensing element isolated from the fluid flow path.
The flow stream conducts heat from the sensing element. The conducted heat is
directly proportional to the mass flow rate. The sensor never comes into direct
contact with the liquid, Fig. 12. Through pre-existing built-in
calibrations, the temperature differential is translated to mass flow. The
accuracy of the thermal mass flow device is dependent on the reliability of the
calibrations of the actual process gas or liquid and variations in the
temperature, pressure, flow rate, heat capacity and viscosity of the fluid.
electronics package includes the flow analyzer, temperature compensator, and a
signal conditioner that provides a linear output directly proportional to mass
flow. (Back to Meter
channel" refers to any conduit in which liquid flows with a free surface.
Included are tunnels, nonpressurized sewers, partially filled pipes, canals,
streams, and rivers. Of the many techniques available for monitoring
open-channel flows, depth-related methods are the most common. These techniques
presume that the instantaneous flow rate may be determined from a measurement of
the water depth, or head. Weirs and flumes are the oldest and most widely used
primary devices for measuring open-channel flows.
Weirs operate on the principle that an obstruction in a
channel will cause water to back up, creating a high level (head) behind the
barrier. The head is a function of flow velocity, and, therefore, the flow rate
through the device. Weirs consist of vertical plates with sharp crests. The top
of the plate can be straight or notched. Weirs are classified in accordance with
the shape of the notch. The basic types are V-notch, rectangular, and
trapezoidal.(Back to Meter
Flumes are generally used when head loss must be kept to a
minimum, or if the flowing liquid contains large amounts of suspended solids.
Flumes are to open channels what venturi tubes are to closed pipes. Popular
flumes are the Parshall and Palmer-Bowlus designs.
flume consists of a converging upstream section, a throat, and a diverging
downstream section. Flume walls are vertical and the floor of the throat is
inclined downward. Head loss through Parshall flumes is lower than for other
types of open-channel flow measuring devices. High flow velocities help make the
flume self-cleaning. Flow can be measured accurately under a wide range of
flumes have a trapezoidal throat of uniform cross section and a length about
equal to the diameter of the pipe in which it is installed. It is comparable to
a Parshall flume in accuracy and in ability to pass debris without cleaning. A
principal advantage is the comparative ease with which it can be installed in
existing circular conduits, because a rectangular approach section is not
through weirs and flumes is a function of level, so level measurement techniques
must be used with the equipment to determine flow rates. Staff gages and
float-operated units are the simplest devices used for this purpose. Various
electronic sensing, totalizing, and recording systems are also available.
A more recent
development consists of using ultrasonic pulses to measure liquid levels.
Measurements are made by sending sound pulses from a sensor to the surface of
the liquid, and timing the echo return. Linearizing circuitry converts the
height of the liquid into flow rate. A strip chart recorder logs the flow rate,
and a digital totalizer registers the total gallons. Another recently introduced
microprocessor-based system uses either ultrasonic or float sensors. A key-pad
with an interactive liquid crystal display simplifies programming, control, and
calibration tasks.(Back to Meter