Modern instrumentation and control systems are allowing
many users in the process industries to adopt true,
predictive maintenance regimes for the first time. However,
says Mark Allinson of ABB, this strategy can only work if the
frontline instruments are properly applied and maintained

These days,
maintenance is
increasingly about
heading off trouble before it
starts. This predictive
approach is possible due
largely to the advent of more
intelligent control systems and
instrumentation. Predictive
maintenance is not just about
preventing catastrophic failures. Across the
entire range of process industries, where
product specifications are extremely tight
and the pressure to minimise operating
costs enormous, the ability to spot early
signs of deteriorating performance or outof-
specification products can make the
difference between profit and loss.

Controlling and measuring process
variables is just one part of the picture,
however. The condition of the field
instruments and their ability to provide
accurate and repeatable measurements will
also have a bearing on plant performance
and product quality. For this reason, it is
advisable to use systems that not only gather
as much information as possible from the
process, but which can monitor the status of
field instruments and, if necessary, flag up
the need for maintenance.

This is where fieldbus technology comes
into its own. Fieldbus is a generic term that
refers to a single-cable network that can
replace conventional means of transporting
data between field devices and a control
system, such as the traditional mA loop. A
digital fieldbus provides communication
between industrial control systems and field
instruments.

Today’s smart
field instruments
contain valuable
status and
performance
information that
cannot be accessed
via an analogue
link. When a
fieldbus is used,
however, these
variables can be
retrieved and a
whole new world
of possibilities opens up for predictive
maintenance, advanced diagnostics, asset
optimisation and management.

For example, in an analogue architecture,
device failure can, at best, manifest itself as a
‘limit violation’ alarm somewhere in the
process, which could either show a true
process excursion or an instrument failure.

Fieldbus status signals allow operators to
immediately identify if the alarm is due to a
malfunctioning device, and obtain an
indication of the cause.

However smart your controls are, any
preventive maintenance will be meaningless
unless the readings coming back from the
field instruments are reliable. Choosing the
right devices, installing them correctly and
treating them with respect is essential.

Calibration is a good example. Many
pressure transmitter vendors claim their
products require recalibration only every five
years, but the figures quoted are based on a
specific set of conditions that may have little
to do with conditions on site.

Top tips for maintenance
Different types of instrument will require the
user to make different kinds of periodic
checks and adjustments, but there are some
general rules worth bearing in mind to help
keep the need for field instrument
maintenance to a minimum.

Always try to install instrumentation for
easy access, so it can be cleaned, calibrated,
repaired or replaced as easily as possible.

Install instruments in a suitable
environment. Factors such as high
temperatures and humidity will often
shorten their working life.

Be on the lookout for low-maintenance
options. Some types of equipment are
designed with built-in protection against
arduous environments. For example, it may
be worth opting for a type of electronic
transmitter which is ‘fully-potted’, meaning
that the unit is completely encapsulated in,
for example, epoxy resin. This makes the
transmitter less vulnerable to vibration,
contamination ingress and changes in
ambient temperature that can cause drift in
some other models.

Another key maintenance tip is to conduct
regular performance surveys of your plant.

Checking the performance of compressed air
plant and in particular keeping an eye out for
leaks, for example, has been estimated to
typically help UK companies to save 30% in
energy costs.

As more companies move towards
predictive maintenance, advanced
instrumentation and control systems provide
the early warning systems that enable users
to apply automatic condition monitoring to
many production lines for the first time.

Investing in this technology makes sense if
you consider the potential cost of unplanned
downtime if things go wrong. However, it’s
vital to protect your investment by ensuring
that your planned maintenance programme
includes the instruments themselves.

ABB offers a range of high quality
instrumentation to help deliver top levels of
plant performance, as well as highly trained
personnel to help maintain it. Specialist sales
and application engineers can guide you
through the options to ensure you get a
suitable instrument for the job in hand.