MEASUREMENT & MONITORING
FLOW MEASUREMENT
BELL FLOW SYSTEMS
A GUIDE TO CHOOSING THE RIGHT FLOW METER
Flow measurement is a critical aspect of plant
and process operation in many industries.
Users choosing equipment to measure the
flow of liquid or gas processes must consider
a wide range of factors to arrive at an optimal
solution. Experience has shown there are
significant differences between flow meter
technologies, with each type of device having
its own advantages and disadvantages. The
following article describes the key criteria in
flow meter selection. It evaluates the most
common instrument designs and offers
guidance in implementing the right solution
for specific applications.
TYPICAL FLOW APPLICATIONS
In modern plants and facilities, personnel
need to make faster and better decisions by
capturing, managing and analysing the right
data at the right time. These facilities rely
heavily on flow processes, and thus accurate
and reliable measurement technology is
vital to the efficiency and safety of their
operations.
Typical flow-metering applications in the
chemical/petrochemical sector for example
include:
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Chemical Batching
Dosing/Blending
Catalyst Injection
Chemical Recovery
Custody Transfer
Steam Flow
Lube Oil Loading
Process Cooling
Pressure Regulation
Leak Detection
Fuel Consumption Monitoring
Fiscal Transfer
Product Load-out
Reactor Feed
Safety Shutdown
Waste Treatment
Emissions Monitoring
Most chemical processing plants have two
primary flow measurement challenges:
accuracy and cost. The goal
is to correctly match the right flow meter
to the right application to achieve the best
performance for the lowest purchase prices
and total cost of ownership.
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PECM Issue 40
COMMON MEASUREMENT TECHNOLOGIES
Flow meters are excellent tools to measure,
monitor, and control the distribution of
a host of process fluids. The question is
which technology to use, since a wide
variety of meter designs are available. Each
type of meter has pros and cons, and must
be properly deployed to achieve optimal
performance.
Coriolis
Coriolis meters contain a
vibrating tube in which a
fluid flow causes changes
in frequency, phase shift
or amplitude. The sensor
signal is fed into the integrally mounted
pc-board. The resulting output signal is
strictly proportional to the real mass flow
rate, whereas thermal mass flow meters are
dependent of the physical properties of the
fluid.
One of the most important features of Coriolis
flow meters is that they directly measure
fluid mass over a wide range of temperatures
with a very high degree of accuracy. Their
unobstructed, open flow design is suitable
for viscous, non-conductive fluids that are
difficult to measure with other technologies.
With no internal moving parts, Coriolis meters
require a minimum amount of attention
once installed. However, they are sometimes
considered too sophisticated, expensive or
unwieldy for certain applications (See Fig 3).
Differential Pressure
Differential Pressure
(DP) meters measure
the pressure differential
across the meter and extract the square root.
They have a primary element that causes a
change in kinetic energy, creating differential
pressure in the pipe, and a secondary element
measuring the differential pressure and
providing a signal or read-out converted to
the actual flow value.
Differential Pressure meters are versatile
instruments, which employ a proven, well-
understood measuring technology that
does not require moving parts in the flow
stream. DP meters are not greatly affected
by viscosity changes, however, they have a
history of limited accuracy and turndown, as
well as complex installation requirements.
Electromagnetic
Electromagnetic meters
employ Faraday’s law of
electromagnetic induction,
whereby voltage is induced
when a conductor moves through a magnetic
field.
The liquid acts as the conductor, with
energized coils outside the flow tube creating
the magnetic field. The produced voltage is
directly proportional to the flow rate.
Electromagnetic meters will measure virtually
any conductive fluid or slurry, including
process water and wastewater. They provide
low pressure drop, high accuracy, high
turndown ratio, and excellent repeatability.
The meters have no moving parts or flow
obstructions, and are relatively unaffected
by viscosity, temperature and pressure
when correctly specified. Nevertheless, their
propensity to foul can cause maintenance
issues. Electromagnetic meters tend to be
heavy in larger sizes and may be prohibitively
expensive for some purposes.
Positive Displacement
Positive Displacement (PD)
meters separate liquid into
specific increments, and the
flow rate is an accumulation
of these measured increments over time.
The rotational speed of a PD meter’s
impeller is a function of the process flow. An
internally coupled counter, either electronic
or mechanical, monitors the measuring
element’s rotations to provide a volumetric
recording of the flow total.
Positive Displacement meters are highly
accurate (especially at low flows) and have
one of the largest turndown ratios. The
devices are easy to maintain as they have only
one or two moving parts.
There is no need for straight pipe lengths as
with other metering approaches. However,
PD meters require clean fluids and can be
large and burdensome to install.