|Figure 1. The orifice plate is a
popular DP element.
specifications for a flowmeter are accuracy and turndown (or
rangeability). The accuracy of all flowmeters depends to some degree on
the circumstances of their application. These effects include pressure,
temperature, fluid dynamic influences, and external influences.
Pressure and temperature cause dimensional and physical changes. Fluid
flow and pipe roughness can shift meter performance. Mass meters and
displacement meters are the least affected, while turbine, vortex, and
ultrasonic meters are the most affected. The upshot is that a traceable
and complete calibration in a lab is no guarantee that the same
performance and accuracies will be obtained or maintained in the field.
A flowmeter’s turndown is the ratio of the maximum to the minimum
flowrate capable of being measured at a specified accuracy, providing a
measure of rangeability. High values of turndown can be irrelevant
because typical processing flow velocities often fall within relatively
narrow ranges. For example, most practical liquid pipeline flowrates in
the processing industries range from 0.5 to 12 ft/sec, a turndown ratio
of 24-to-1. Lower rates are difficult to measure accurately and higher
rates result in high pressure drops, pumping energy costs, and erosion
(if solids are present). In the case of pipelines carrying gases in the
processing industries, the practical flow velocities range from 15 to
200 ft/sec — a turndown ratio of about 13-to-1. Many actual
applications have flowrate ranges well within these extremes.
|Figure 2. The Venturi DP element minimizes pressure loss.
Instrument engineers commonly apply volumetric flowmeters to
measure liquids. When the process fluid is a gas or steam, measuring
mass flow becomes especially important. Because these fluids are
compressible, their density changes with pressure and temperature.
Volumetric flow measurement is often meaningless in these cases. But
volumetric flowmeters work if the application also includes measurement
of temperatures and pressures at the meter or these values are known
quantities. Simple calculations based on the three variables can infer
Flowmeters based on
differential pressure insert an element within the piped flow. The
unrecoverable pressure loss across the element — typically an orifice
plate, Venturi, nozzle, pitot tube, or wedge — is a measure of the
|Figure 3. The nozzle has Venturi-like properties.
Flowmeters based on differential pressure represent a popular
choice in the processing industries, constituting nearly 30 percent of
the installations. They have good application flexibility since they
can measure liquid, gas, and steam flows and are suitable for extreme
temperatures and pressures with moderate pressure losses. Accuracy
ranges from 1 percent to 5 percent. Compensation techniques can improve
accuracy to 0.5 percent to 1.5 percent.
On the other hand, DP flowmeters are expensive to install and
have limited rangeability (turndown) compared to other types. They
require a separate transmitter to compute the volumetric flowrate and
develop a standard signal. Changes in density, pressure, and viscosity
can significantly affect accuracy of DP flowmeters. And while they have
no moving parts, maintenance can be intensive.
Instrument engineers can employ a variety of elements to create the
differential pressure, with the orifice plate being the most common.
Orifice plates are inexpensive and available in a variety of materials.
The turndown ratio, however, is less than 5-to-1, and accuracy is
moderate — 2 to 4 percent of full scale. Maintenance of good accuracy
requires a sharp edge to the upstream side. This edge will wear and
degrade over time. Pressure loss for orifice plates is high relative to
other DP elements.
|Figure 4. The pitot tube converts fluid kinetic energy into pressure.
element finds use primarily in water and wastewater applications. It
handles dirty fluids and does not require upstream flow profiling.
Pressure loss is minimal, making it a good choice when little pressure
head is available. Rangeability, while better than orifice plates, is
less than 6-to-1, with accuracy of +/-1-to-2 percent of full scale.
Viscosity effects on accuracy are high. Flow must be turbulent
(Reynolds numbers > 10,000). The Venturi has limited acceptance
within the processing industries.
Characteristics of nozzle elements mimic those of the Venturi. Nozzles
come in three types: ISA 1932 nozzle (common outside of the United
States); the long radius nozzle; and the Venturi nozzle.
|Figure 5. The wedge DP element handles tough, dirty fluids.
Pitot tube: This
element is another low-cost DP element used to measure fluid flow,
especially air flow in ventilation and HVAC systems. Pitots commonly
measure the speed of airplanes. This element works by converting the
kinetic energy of the flow velocity into potential energy (pressure).
Engineers can easily insert the pitot tube into existing piping,
minimizing installation costs. It cannot handle dirty flow and has
Like an insertion meter, the pitot tube makes a measurement at a
point within the pipeline or ductwork. An exception is the annubar
pitot tube, which contains multiple orifices. This element sees the
dynamic pressure across the velocity profile, providing an averaging
|Figure 6. Electromagnetic flowmeters require conductive fluids.
Wedge: This element
consists of a V-shaped restriction molded into the meter body. In
profile the wedge looks like a segmental orifice plate to the incoming
fluid, but it’s more expensive than an orifice plate. This basic meter
has been on the market for over 40 years, demonstrating its ability to
handle tough, dirty fluids. The slanted faces of the wedge provide
self-scouring action and minimize damage from impact with secondary
phases. Rangeability of 8-to-1 is relatively high for a DP element and
accuracies are possible to +/-0.5 percent of full scale.
|Figure 7. AC magmeters have good signal-to-noise ratios, but require frequent calibrations to eliminate zero drift.
flowmeter, however, is not approved for flow measurement by the
American Gas Association (www.aga.org
) or the American Petroleum
). Despite this, it’s a popular choice in oil and
gas applications, especially in production fields. For difficult fluids
it can be equipped with a pair of remote seals that effectively isolate
the metered fluid from the DP transmitter without affecting accuracy.
flowmeters (magmeters) are a popular choice among instrument engineers,
making up about 20 percent of flowmeter installations. Faraday’s law
says that a conductor moving through a magnetic field produces an
electric signal. In this case the fluid is the conductor and
electromagnetic coils surrounding the meter body generate the magnetic
|Figure 9. Coriolis mass flowmeters are ideal, but relatively costly.
Magmeters have no moving parts. They offer wide rangeability and
an unobstructed flow path. Magmeters are ideal for slurries, and
perform well with corrosive and erosive fluids. Engineers can pick a
nonconductive liner material for the flow tube — Teflon, Tefzel,
rubbers, ceramic, etc. — to suit the fluid measured. Magmeters require
minimum lengths of straight pipe to condition the flow profile. On the
downside, the fluid must be conductive, which rules out most
petroleum-based flows. Magmeters also have physical pressure and
|Figure 10. Thermal mass flowmeters excel at measuring low flow rates.
Magmeters have rangeability up to 1000-to-1, depending on the
maximum tolerable measurement error. For flow velocities of 0.5 to 50
ft/sec, accuracies are usually stated as a percent of rate, for lower
velocities, accuracies are stated as a percent of span. Typical
velocities measured range from three to 15 ft/sec for water and clean
chemicals, three to six ft/sec for abrasive fluids, and six to 12
ft/sec for coatings and liquids with entrained air.
The voltage developed across the electrodes is in the millivolt
range and should be carried via shielded cable to a nearby converter.
The converter near the flow tube boosts the signal to a standard (four
to 20 mA) or frequency output (zero to 10,000 Hz) for transmission to a
display or controller.
|Figure 11. Vortex meters have no moving parts, but may be affected by vibration.
The coils producing the magnetic field may be excited by AC, DC,
or pulsed DC. DC excitation accounts for most magmeter installations
today. With AC excitation,
line voltage powers the magnetic coils, and the resulting signal has a
sine wave shape of line frequency whose amplitude is a linear function
of flow velocity. AC designs produce a high coil current (typically 3.2
amp) and a good signal-to-noise ratio. They are relatively insensitive
to media noise. However, they consume much power and can be plagued
with zero drift caused by eddy current noise. To compensate, operators
stop the flow and set the transmitter output to zero. Frequent
operations to eliminate eddy current noise are sometimes necessary to
maintain accuracy, which is about +/-1 percent of rate.
|Figure 12. Turbine meters are well-accepted in the processing industries.
With DC excitation designs, a lower frequency DC pulse excites
the magnetic coils. The converter reads both the flow and noise signals
during a pulse. In between pulses, however, it sees only noise,
permitting noise cancellation after each cycle. So zero drift is not a
problem for pulsed-DC designs. These magmeters offer 0.15 percent to
0.5 percent accuracy and low power consumption. The low coil current
(0.1 to 0.5 amp) comes with smaller signal and low signal-to-noise
ratio. Low frequency designs have longer response times.
|Figure 13. Doppler ultrasonic flowmeters work best on slurries and dirty liquids.
A newer DC design develops a stronger flow signal by increasing
the DC coil excitation current (0.7 to five amp). This design improves
the signal-to-noise ratio, but increases power consumption and cost.
Flow tubes must generally be less than 20 inches in diameter. Another
new design employs dual excitation, pulsing the coils at about seven Hz
for zero stability and also at 70 Hz for a stronger signal and improved
response times (0.1 sec). But this design raises costs and applies only
to flow tubes smaller than 16 inches in diameter. Eddy current
instability increases proportionally as sensor size increases.
|Figure 15. Transit-time ultrasonic flowmeters work best on clean fluids.
directly measure mass flowrate, as compared to other flowmeter types,
which measure flowrate based on volumetric principles. The two primary
types of mass flowmeters, accounting for about 18 percent of
installations, are Coriolis and thermal mass flowmeters.
Coriolis flowmeters represent the ideal flowmeter. They can
be applied to virtually any fluid — liquids, gasses, and slurries. They
are immune to changes in viscosity, pressure, temperature, or density
of the piped fluid and are unaffected by flow profile disturbances.
They offer high accuracy (+/-0.1 percent) and rangeability (100-to-1),
and provide an additional measurement of fluid density. But their
purchase and installation costs are relatively high, as well as their
sensitivity to vibration. In addition, they come in a limited selection
of sizes, rarely over two inches.
|Figure 16. Positive-displacement flowmeters are unaffected by poor flow profiles.
In these meters, the fluid measured runs through one or two
vibrating tubes. Tube designs take on a multitude of shapes, depending
on the manufacturer. Coriolis forces resulting from the vibration and
flowing fluid act on the tubes, causing them to twist or deform.
Sensors pick up the change in tube shapes. Sensor signals may be
related directly to mass flowrate.
The thickness of the tubing walls varies considerably from design
to design. But even the sturdiest tubing will be thinner than the
process piping. Long, bent tubes twist more easily than do short,
straight tubes, and so they will generate stronger signals under the
same conditions. Curved-tube designs provide wider rangeability (up to
200-to-1), while straight-tube meters are limited to 30-to-1 to
50-to-1, with lower accuracy. Straight-tube meters are more immune to
pipeline stresses and vibration.
Thermal mass flowmeters add heat to the flow stream to make
measurements. One or more temperature sensors measure how quickly this
heat dissipates. One technique maintains a heated sensor at a constant
temperature, measuring the current required. Another way is to measure
the temperature difference between a sensor in the flow stream and a
heated sensor outside the stream. Higher flowrates result in increased
cooling. The cooling effects are a function of mass flow.
These meters are compact, and offer wide rangeability (50-to-1)
and good repeatability. They have a fast response time, especially for
gasses, and excel at measuring low mass flowrates. They assume a
constant specific heat and thermal conductivity of the fluid. Changes
in density cause calibration shift, and coating of the sensor can cause
drift. Additionally, they measure a point within the pipe diameter, so
the flow profile must be known. Accuracy levels typically range from 1
percent to 3 percent. However, manufacturers expect to improve their
accuracy, which will widen their application.
Place a bluff body
within a fluid pipeline and the stream will alternately produce
whirling vortices on either side of the body. The spacing between the
vortices remains constant, regardless of the fluid velocity. Within a
pipeline, the vortice effect attenuates a few pipe diameters downstream
from the bluff body. Typically, piezoelectric or capacitance sensors
count the pressure oscillations caused by the vortices. The
vortex-shedding frequency provides a measure of fluid velocity.
Like a vortex meter, swirl meters detect pressure pulsations,
whose frequency depends on the flowrate. Vanes within the meter cause
the incoming fluid to take the shape of a rotating helical coil of
relatively low-pressure zones. At low flowrates the low pressure swirls
or coils are far apart. As flowrates increase, the coils become
compressed, coming closer together. Detectors within the meter measure
the frequency of the swirling low-pressure zones as they pass through
the meter. A deswirler at the meter’s exit eliminates any effects on
Engineers can apply these meters to liquid, gas, or steam, but
should avoid dirty, corrosive, or abrasive fluids. The meters have no
moving parts, have wide rangeability, and are immune to pressure,
temperature, or density changes. These meters, however, lose
effectiveness as viscosity increases. They are also affected by
vibration or pulsation.
Swirl meters are less sensitive to flow profiles than vortex
meters. While vortex meters may require a minimum straight-pipe run of
15 or more pipe diameters upstream and five pipe diameters downstream,
swirl meters require only three diameters upstream and one diameter
downstream. Both may be installed horizontally or vertically, but must
be kept full of fluid during operation. About half of all vortex meter
installations require “necking down” oversized process piping by
concentric reducers and expanders.
A piped fluid that
acts on turbine vanes will rotate the turbine with a rotational rate
proportional to flowrate. Turbine meters make up about 8 percent of the
installed base. Rangeability can reach 100-to-1 if the meter measures
the rate of a single fluid at constant conditions. Accuracy is high and
may top +/-0.1 percent.
Turbine meters are suitable for extreme temperatures and
pressures and can be applied to liquids and gasses. They install easily
and are well accepted in processing applications, especially the oil
and gas industries. On the other hand, turbine meters suit only low
viscosities and are sensitive to flow profile and vibration. Having
moving parts, they usually require frequent calibration. Partially open
valves upstream from a turbine meter can cause significant errors.
Typically, turbine meters require flow upstream straightening vanes.
Additional considerations include:
• Possible flashing
or cavitations from pressure drops. Flashing causes the meter to read
high while cavitations result in rotor damage. The downstream pressure
must equal 1.25 times the vapor pressure plus 2X the pressure drop.
• Small amounts of air entrainment (<100 mg/l) will make the meter read high, while large quantities can destroy the rotor.
• Solids entrained
in the fluid can damage the meter. In these cases, a flushing strainer
or filter must be installed upstream of the flowmeter.
limited to liquids, send an ultrasonic beam into the fluid and look at
reflections from discontinuities such as entrained particles or
bubbles. Reflections from the moving objects will experience a
frequency shift that is a measure of the flowrate. Transit-time
flowmeters have beam transmitters and receivers separated by a known
distance. They look at the differences in time of flight for ultrasonic
beams moving with and against the flow. Doppler meters work best on
slurries or dirty liquids, while transit-time meters work best on clean
These flowmeters have no moving parts and offer an unobstructed
passage for fluid flow. Ultrasonic flowmeters have wide rangeability,
and some designs can simply be strapped on to the outside of pipeline.
Typical accuracy of these clamp-on flowmeters is about 5 percent.
Clamp-on units generally will not work well if the pipe has a lining or
if the pipe material is stainless steel, PVC, or fiberglass. In
addition, the inside of the pipe must be free of sound-absorbing
material, such as dirty grease or scale. Multi-beam, transit-time units
in direct contact with the fluid, however, now have sufficient accuracy
to be used for custody transfer of gases and liquids.
Published data on ultrasonic flowmeters is limited. They have a
relatively high initial cost and performance is sensitive to multiple
installation factors. Deposit buildup on the pipe walls can
significantly affect accuracy, as can extreme vibrations.
Commonly known as
rotameters, these devices provide practical solutions for many
flowmetering applications. They are simple and inexpensive, performing
as well or better than many high-tech flowmeters on the market. Every
rotameter basically consists of two components — a tapered metering
tube made of glass, metal, or plastic and a float that rides within the
tube. These components come in a wide variety of shapes, sizes,
weights, and materials of construction, making them adaptable to a wide
range of application needs. They are easy to install and maintain, but
must be mounted vertically and plumb. Some designs can handle high
pressures and viscosities. Today, most manufacturers have taken all the
factors and data involved in sizing a rotameter and incorporated them
in a sophisticated software program.
Accuracy, however, is relatively low (+/-2 percent) and depends
on precise knowledge of the fluid and process. Rotameters are
susceptible to vibration and plugging by solids and are affected by
density and temperature changes. They apply primarily to flowrates
below 200 GPM and pipe sizes less than three inches.
This meter inserts
a physical target within the fluid flow. The moving fluid deflects a
force bar attached to the target. The deflection caused by the force
depends on the target area, as well as the fluid density and velocity.
These flowmeters have no moving parts such as bearings and can measure
fluid flowrates for any line size above 0.5 inches. Engineers can adapt
the flowmeter to different fluids or flowrate ranges by changing the
target size and material. Rangeability is about 15-to-1. One
disadvantage is that the target meter’s calibration must be verified in
rotors measure fluid flow in positive-displacement flowmeters. The
volumes between the rotors are known and fixed. The rotor rotation
varies directly with the volume of the fluid being displaced. Counters
monitor the number of rotations and a transmitter converts the counts
to volumetric flowrate. Principal types include:
• Single or multiple reciprocating piston meters.
• Oval-gear meters with synchronized, close-fitting teeth.
• Movable nutating discs mounted on a concentric sphere located in spherical side-walled chambers.
• Rotary vanes creating two or more compartments inside the meter’s housing.
are applicable to a wide range of nonabrasive fluids, including those
with high viscosities. Examples include heating oils, lubrication oils,
polymer additives, ink, and many more. They are unaffected by poor flow
profiles. Accuracy may be up to +/- 0.1 percent of full scale with a
rangeability of 70-to-1 or better. Moreover, they require no power and
can handle high pressures.
Positive-displacement flowmeters cannot
handle solids, entrapped air in liquids, or entrained liquids in gases.
They are expensive to install and maintain, having many moving parts.
Pressure drop across the meters is high.
This is the fifth article in a five-part series on the history and operation of flowmeter technology.
Greg Livelli is a
senior product manager for ABB Instrumentation, based in Warminster,
Pa. He has more than 15 years experience in the design and marketing of
flowmetering equipment. Mr. Livelli earned an MBA from Regis University
and a bachelor’s degree in Mechanical Engineering from New Jersey
Institute of Technology. Mr. Livelli can be reached at
or 215 674-6641.