Chemical Engineering June 2019 - 44

Endress+Hauser
Periodic review
Define process
limits and
tolerances
Fundamental
to determine
calibration
specifications
Measuring loop
inventory
Installed base of
instruments and
measuring loops
Criticality assignment
and calibration
specification definition
Definition of criticality,
MPE and calibration
interval according to
process tolerance
Definition of
calibration
procedures
Number of setpoints,
reference instrument
needed, conformity
assessment, and so on
To be documented
in SOP
FIGURE 3. An approach to calibration is outlined here
Calibration intervals
In many instances, maintenance
managers, when inquiring about the
appropriate calibration intervals, expect
a standard answer that applies
equally to every instrument in the installed
base. While this corresponds
well to the scheduled maintenance
and service downtimes, it does not
support the overall goals of calibration.
Even the relevant standards do
not specify exact intervals. Instead,
they merely suggest, in the words of
ISO 9001, that " measuring equipment
shall be calibrated or verified,
or both, at specified intervals, or prior
to use, against measurement standards
traceable to international or
national measurement standards. "
Consequently, the organization is responsible
for determining and establishing
suitable calibration intervals.
When determining the appropriate
calibration intervals, it is very important
to realize that a " one fits all "
strategy does not work. To determine
the adequate interval for each
instrument, the following aspects
must be considered:
* Device type and its sensitivity to drift
* Criticality of the measuring point
* MPE
* Aging of instrument
* Individual process and environmental
characteristics
Consequently, calibration intervals
cannot be the same for all cases
and cannot be static over the instrument's
lifetime.
Indeed, it is not uncommon for older
instruments to tend more towards
drifting. Also, process conditions, such
as a high-temperature environment
that impacts electronic components,
can also cause the measurements to
be less stable. In chemical processes,
it is often the products themselves
44
Deployment
IT, process and
tools; training; data
management; reference
tool management
Operation
that put a heavy strain on the instruments,
for example, when they are
highly corrosive. This clearly shows
that only dynamic intervals can factor
in all those aspects. In that respect,
measured values during calibration
shall be taken into consideration and
not only the conformity statement, as
they reflect the behavior of the measurement
instrument.
Setting the calibration frequency
for each measuring device is a
matter of risk management in finding
the optimal balance between
cost and risk. Calibration is therefore
not a pure burden, as it is perceived
by many plant operators.
Instead, it represents more of an
opportunity, because at the end of
day it leads to improved and more
reliable processes.
Performing calibrations depends on
an even deeper understanding of the
installed base. Although that should
be obvious, it's often not the case.
Interval optimization
Keeping a comprehensive inventory
and description of each and
every instrument and measurement
point is the first stage of the process.
Once the installed base of
instruments has been inventoried
and documented, including the
required calibration intervals, it is
still too early to begin the calibration
itself. Of major significance for
proper calibration processes are
the standard operating procedures
(SOPs), which must be created, reviewed
and if necessary, modified.
This is not only important for the
audits. What must be determined
is the number of respective calibration
points, as well as the measurement
uncertainty that can be tolerated.
Which calibration method
will be selected? Which reference
instruments will be used? Which
requirements regarding conformity
should be considered? Within the
entire calibration process, there is
often a lot of hidden potential for
increasing efficiency. The logistics
and the workflow management related
to calibration should not be
underestimated. Furthermore, calibration
managers must make sure
that the employees who will be carrying
out the actual calibration are
thoroughly trained.
Calibration managers who are
not only able to provide the auditor
with a thick stack of certificates,
but have also understood why
they chose specific instruments,
calibration intervals and calibration
methods, will be well-prepared for
the next audit. They can thus avoid
embarrassing scenes like this: The
auditor asks a question about the
calibration of a specific instrument,
after which the calibration
manager begins to feverishly look
for the corresponding certificate
in the hope of finding the answer.
However, those who have pursued
these tasks with a high level of expertise
in the field of measurement
technology and metrology and
taken the aforementioned recommendations
to heart, will not have
to hesitate to provide the auditor a
competent answer.
Deviation management
Deviation management means
triggering a response in cases
where calibration indicates that
the measurements are out of tolerance
or that significant drift is
occurring. At a minimum, the affected
instrument needs to be readjusted
and recalibrated. Under
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Chemical Engineering June 2019

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