KEY FACTORS IN FLOW METER SELECTION
In a typical chemical production facility, fluid
characteristics eg. single or double phase,
viscosity, turbidity, flow profile (laminar,
transitional, turbulent, etc.), flow range, and
accuracy requirements are all important
considerations in determining the right flow
meter for a particular measurement task.
Additional considerations such as mechanical
restrictions and output-connectivity options
also impact the user’s choice.
For most general industrial applications, the
key factors in flow meter selection are:
Process Media
Different flow meters are designed to operate
best in different fluids/gases and under
different operating conditions. That’s why it
is important to understand the limitations
inherent to each style of instrument. The
most important difference between these
two types of media lies in their relative
compressibility (i.e, gases can be compressed
much more easily than liquids). Consequently,
any change that involves significant pressure
variations is generally accompanied by much
larger changes in mass density in the case of a
gas than in the case of a liquid.
Type of Measurement
Industrial flow measurements fall under
one of two categories: mass or volumetric.
Volumetric flow rate is the volume of fluid
passing at a given volume per unit time
eg. Litres per minute. Mass flow rate is
the movement of mass per time unit eg.
Kilograms per minute.
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It can be calculated from the density of the
liquid (or gas), its velocity, and the cross
sectional area of flow. Volumetric measuring
devices, like variable area meters or turbine
flow meters, are unable to distinguish
temperature or pressure changes.
Mass flow measurement would require
additional sensors for these parameters
and a flow computer to compensate for
the variations in these process conditions.
Thermal mass flow meters are virtually
insensitive to variations in temperature or
pressure.
Flow Rate Information
A crucial aspect of flow meter selection is
determining whether flow rate data should
be instantaneous or continuous. A flow rate
has to do with the quantity of a gas or liquid
moving through a pipe or channel within a
given or standard period of time. A typical
continuous flow measurement system
consists of a primary flow device, flow sensor,
transmitter, flow recorder, and totalizer.
Desired Accuracy
In many production processes, accurate flow
measurements can be the difference between
on-spec quality and wasted product.
Flow meter accuracy is specified in
percentage of actual reading (OR), percentage
of calibrated span (CS), or percentage of
full-scale (FS) units. It is normally stated at
minimum, normal, and maximum flow rates.
A clear understanding of these requirements
is needed for a meter’s performance to be
acceptable over its full range.
Application Environment
Flow meters can be employed under a host of
varying conditions in a chemical process plant.
For example, users must decide whether the
low or high flow range is most important for
their metering application. This information
will help in sizing the correct instrument for
the job. Pressure and temperature conditions
are equally important process parameters.
Users should also consider pressure drop
(the decrease in pressure from one point
in a pipe to another point downstream) in
flow measurement devices, especially with
high-viscosity fluids. In addition, viscosity and
density may fluctuate due to a physical or
temperature change in the process fluid.
Fluid Characteristics
Users should be cautious that the selected
flow meter is compatible with the fluid and
conditions they are working with. Many
chemical and petrochemical operations
involve abrasive or corrosive fluids, which
move under aerated, pulsating, swirling or
reverse-flow conditions. Thick and compacted
materials can clog or damage internal
meter components, hindering accuracy and
resulting in frequent downtime and repair.