Installation Requirements
Planning a flow meter installation starts with
knowing the line size, pipe flow direction,
material of construction, and flange-pressure
rating. Complications due to equipment
accessibility, valves, regulators, and available
straight pipe run lengths should also be
identified. Many types of flow meters must
be installed with a run of straight pipe before
and after their mounting location. Where
this is not possible, a flow conditioner can be
used to isolate liquid flow disturbances from
the flow meter while minimizing the pressure
drop across the conditioner.
Power Availability and Hazardous Areas
Pneumatic instrumentation was once used
in most hazardous area applications, since
there was no power source to cause an
explosion. Today’s ATEX installations normally
call for intrinsically safe instruments, which
are “current limited” by safety barriers to
eliminate a potential spark or Explosion
proof solutions which feature approved E xd
enclosures. Self-powered flow meters offer
an advantage in such environments or where
a power source is not available. They do not
require external power to provide a local rate/
total indicator display for a field application,
and instead rely on battery-power for
displaying, logging and transmitting data.
Solar-powered systems can also be used in
remote areas without power.
Users should closely evaluate their process
conditions, including flow rates, pressure and
temperature, and operating ranges. Do not
be swayed by lower priced alternatives that
would be applied outside of their capabilities.
All flow meters are affected to some extent
by the flowing medium they are metering
and by the way they are installed.
Consequently, their performance in real
world conditions will often be different from
the reference conditions under which they
calibrated.
For the lowest uncertainty of measurement,
positive displacement meters are generally
the best option.
Electromagnetic meters provide for the some
of the widest flow “turn-down” ranges and
turbine meters are usually the best choice for
the highest short-term repeatability vs price.
bends sometimes suffer from abrasive media
wear and blockages. Changes in temperature
also affect the internal dimensions of the
meter and require compensation.
The need to recalibrate a flow meter depends
on how well the instrument fits a particular
application. If the application is critical, meter
accuracy should be checked at frequent
intervals. Otherwise, recalibration may not be
necessary for years because the application is
non-critical, or there are no factors which will
change the meter’s performance.
No matter the chosen flow meter technology,
overall system accuracy will not exceed that
of equipment used to perform the meter
calibration. The most precise flow calibration
systems on the market employ a positive
displacement design. This type of calibrator,
usually directly traceable to National
standards, provides total accuracy of at least
0.05 percent.
Necessary Approvals
UK firms are obliged to comply with strict
standards set by the EU. Approvals for the
use of flow measurement equipment in
hazardous locations are covered by the ATEX
directive. Standards such as the Measuring
Instruments Directive (MID) apply to fiscal
and custody transfer metering for liquids
and gases. In terms of environmental
emissions, industrial flow meters must meet
the Electromagnetic Compatibility (EMC)
Standards EN55011:1992 and EN61326-
1:1997.
Output/Indication
Flow meter users must decide whether
measurement data is needed locally
or remotely. For remote indication,
the transmission can be analogue,
digital, or shared. The choice of a digital
communications protocol such as HART®,
FOUNDATION Fieldbus™ or Modbus also
figures into this decision. In a large industrial
facility, flow readings are typically supplied to
an industrial automation and control system
or SCADA system for use in process control
and optimization strategies.
OTHER IMPORTANT CONSIDERATIONS
Companies purchasing flow meters should
remember that accurate instruments cost
more based on their capabilities. It is better
to locate the type of meter suited to a specific
application before sacrificing features for cost
savings.
Despite their high initial cost, Coriolis
meters are ideal for measuring particularly
viscous substances and anywhere that the
measurement of mass rather than volume is
required.
Flow meter users should also take care to
examine long-term ownership costs. A flow
meter with a low purchase price may be very
expensive to maintain. Alternatively, a meter
with a high purchase price may require very
little service. Lower purchase price does not
always represent the best value. Generally
speaking, flow meters with few or no moving
parts require less attention than more
complex instruments. Meters incorporating
multiple moving parts can malfunction due to
dirt, grit or grime present in the process fluid.
Meters with impulse lines can also plug or
corrode, and units with flow dividers and pipe
SUMMARY
Choosing the right flow measurement
solution can have a major impact on
operational and business performance.
For this reason, companies anticipating
a flow meter purchase should consult
with a knowledgeable instrumentation
supplier in the early stages of a project.
The effort spent learning about basic
flow measurement techniques, and
available meter options, will ensure
a successful application once the
equipment is installed.
www.bellflowsystems.co.uk
Issue 40 PECM
135