Chemical Engineering October 2021 - 40

Engineering Practice
Field Calibration of DP Flowmeters:
Best Practices
Improved understanding of differential pressure (DP) flowmeter operation, along with
best calibration practices, reduces maintenance errors
Ned Espy and Roy Tomalino
Beamex
U
tilizing an orifice plate installed
in a pipe and accurately
measuring the differential
pressure (DP) to
calculate flow is one of most common
methods in the chemical process
industries (CPI) (Figure 1). DP
technology is unique among flow
measurement technologies in that
the relationship between the flowrate
is the square root of the input
pressure. This phenomenon presents
some challenges to both the
vendors that supply DP transmitters
and to the technicians that need to
calibrate these transmitters.
While new technology available
to measure flow can deliver amazing
results, and differential pressure
flow measurement is widely utilized
across many industry sectors,
proper use and calibration of these
instruments remains important (Figure
2). An improved understanding
of the basics of DP flow measurement
technology, along with knowledge
about the suggested best
practices for calibration, will help
minimize maintenance mistakes
and calibration errors in the field.
Flowmeter requirements
In DP flow measurement, obtaining
an accurate result involves more
than simply taking the pressure measurement
and calculating the output.
The accuracy of a DP flow measurement
is
based on
the entire
flow-measurement
system,
including
the
following:
straight
runs
(ups
t r e a m
and downstr
eam);
flow element
(orifice
plate in
good condition);
and
transmitter
(consider
" low-flow
cut-off " and proper test points).
A " straight run " within the piping
upstream of the orifice plate
is required for a homogenous and
stable flow of fluid through the
plate. Likewise, a downstream
" straight run " is also required. The
orifice plate itself must be clean,
with sharp edges and no distortions
- inspection of the element
should be conducted to verify it is
in good condition. The mounting
of the plate itself must be carried
out with care to ensure a repeatable
measurement. Assuming that
these physical components are
sufficiently addressed, the focus
can shift to reviewing the calibration
of the DP transmitter to make
sure it is accurate.
Normally, a pressure
transmitter
operates in a linear
mode from zero to
100% (as
an
FIGURE 1. Differential pressure (DP) flowmeters measure the velocity of
fluids by reading the pressure loss across a pipe constriction, such as an
orifice plate like the one shown here
40
example,
at the 50%
input pressure level,
the output is 50%,
or 12 mA).
However, a DP
transmitter operates
FIGURE 2. Following best practices for calibrating DP flowmeters allows plants to take
advantage of the capabilities of new flow-measurement technologies
in a square-root mode. A common
method to verify that a DP transmitter
is operating in the square-root
mode is to look for a 50% output
(12 mA) when the input pressure is
25% of range, using the relationship
shown here:
√0.25 (input ) = 0.50 (output) -or0.25
(input) = 0.502 (output)
Keep in mind that the square-root
calculation is sometimes done in
the DCS with the transmitter programmed
in a linear mode. So make
sure the square root is not being
taken twice: once in the transmitter
and again in the DCS. Check for
that scenario if the numbers are not
adding up.
Low-flow conditions
One other issue to consider is
when there are low-flow conditions.
When the flowrate drops below
10% of the output range of a transmitter
(5.6 mA), the input pressure
becomes so low (less than 1% of
range) that any small change or
noise in the pressure input is amplified
by a factor of 10 or more. Due
to this characteristic, DP flow meaCHEMICAL
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