Hydrocarbon Processing - February 2022 - 20

Process Controls, Instrumentation
and Automation
B. BURTON, Emerson Automation
Solutions, Shakopee, Minnesota
Reduce maintenance costs by optimizing pressure
transmitter calibration intervals
Pressure instrumentation is crucial
for the functionality and safety of hydrocarbon
processing facilities worldwide.
Many of these facilities deploy hundreds
or even thousands of pressure devices to
accurately monitor and maintain optimal
pressure levels in various applications.
With so many of these devices in one
plant, it is essential to minimize servicing
and maintenance costs. Optimizing the
calibration frequency of pressure transmitters
is one way to achieve significant
cost savings, helping producers maintain
affordability and allowing them to achieve
greater operating returns. Unfortunately,
legacy best practices and broadly applied
internal standards have created calibration
maintenance schedules with shorter
than required intervals, resulting in high
and unnecessary maintenance costs.
Shorter maintenance windows generate
high costs for producers in the form
of extra time, money and resources spent,
as well as additional risks to personnel as
technicians spend more time in the field.
When operators can estimate more accurate
calibration frequencies for pressure
transmitters,
these maintenance costs
may be considerably reduced. If operators
take another look at their calibration
schedules, they will find that modern
pressure transmitters do not necessarily
demand such short calibration intervals.
It is possible to estimate a more precise
calibration interval by performing a few
simple calculations.
Pressure transmitter technology has
evolved dramatically in the last two decades.
Advanced instrumentation enables
operators to depend on these devices
for longer intervals without sacrificing
accuracy or reliability. Although these
intervals are not commonly stated in a
pressure transmitter's literature, there are
ways to extrapolate them from published
specifications and data so that operators
can safely maximize the time between
maintenance cycles.
The driving factors and costs of
frequent calibration. The processing
industry's most critical applications rely
on pressure technology. Pressure instrumentation
devices can also be platforms
for level and flow. They must be accurate
and reliable so it is easy for operators to
assume that calibrating them more often
will achieve better performance. However,
current best-in-class transmitter
technology does not require such short
calibration intervals, and frequent maintenance
may do more harm than good.
In addition, the manual routine of calibrating
each transmitter uses considerable
resources and disrupts the processes
that depend on them. Devices are located
everywhere around a plant, in hard-toreach
places, making the work all that
much more difficult. Technicians must
be in the field for more extended periods,
shutting down transmitters to attach
a calibration device and checking if the
reading is accurate.
Older devices were not as stable as the
FIG. 1. Root sum squared tolerance method
used to calculate TPE.
20 FEBRUARY 2022 | HydrocarbonProcessing.com
sophisticated equipment available today.
Many operators use legacy best practices
and standards carried over from older
technology to select the calibration frequency
on their new technology. Applying
broad regulatory requirements or ambiguous
performance parameters written
to cover field devices that have poorer
capabilities can also contribute to operators
overscheduling maintenance on advanced
pressure sensing technology.
How to calculate accurate calibration
intervals. Manufacturers do not
usually recommend calibration intervals
for their products, but published specifications
and application-specific data
can be used to determine a reasonable
estimate. There is a five-step process to
establish the correct calibration interval
for a pressure transmitter.
Step 1. Step 1 involves determining
the performance required. What are the
general monitoring, reporting, recordkeeping
and verification requirements for
the pressure transmitter? Typically, allowable
uncertainties as a percentage of calibrated
span are a function of the criticality
of the application with safety and plant efficiency
at 0.5%, regulatory control at 1%,
supervisory control at 1.5%, and monitoring
and optimization at 2%.
Step 2. This step defines the operating
conditions. Changing conditions
impact performance. Input the temperature
range and variations the device will
be exposed to in ambient conditions.
For a differential pressure measurement,
it is important to input the line
pressure variable.
Step 3. Step 3 is a total probable error
(TPE) calculation utilizing a root mean
squared method (FIG. 1). The square of
the reference accuracy is added to the
squared temperature effect and added
to the squared span static pressure effect.
The square root is then taken to
determine the TPE. For a static pressure
device measuring absolute or gauge pressure,
the pressure effect is zero.
Step 4. This step determines the stability
of the output. The stability specification
is a key variable and the final input
to the calibration frequency calculation.
Stability specifications can vary widely
depending on the performance class of
the instrumentation. Industry-leading
pressure transmitters have stability specifications
of 10 yr-15 yr. The higher the
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