Intro4u2u

So far, we’ve discussed how you need to make sure you consider
more than just static performance when selecting control valves.
But even if you do your homework and have selected the best
valve from an overall financial performance standpoint, how can
you be sure the valve will sustain that performance over the life of
the plant? This is where your approach to maintenance comes in.
A full spectrum of maintenance approaches can be applied
to control valves. As illustrated in Figure 5, they range from
totally reactive and run to failure, to a full-blown predictive
maintenance program. Most customers these days understand
the value of the more proactive approaches to maintenance
and apply them wherever they can. Most studies conclude that
predictive maintenance is up to five times less expensive than
scheduled maintenance, and 10 times less expensive than corrective
or reactive maintenance.
Unfortunately, until recently it has been tough to adopt a
proactive approach to control valve maintenance for a couple of
reasons. First, it can be difficult to establish performance criteria.
For instance, how do you determine when a valve is not functioning
properly and needs maintenance? Control valves are relatively
robust mechanisms and can continue to function and
respond to an input signal even though their dynamic performance
has already degraded to the point where it is impacting the
bottom line. But, if past practice focused on the static performance
of the valve—as we’ve argued earlier in this article—a valve
might be judged to be performing well enough even after it has
“failed” from a dynamic standpoint. The result is that no clear definition
of failure typically has been established.
Second, there has been no easy way to determine the current
operating state of the valve in question. Nearly all predictive
maintenance programs are based on being able to determine the
current performance levels of a given piece of equipment and
compare it to a baseline, typically referred to as “condition-based
maintenance.” As deviations grow between the acceptable baseline
and the current performance, they can be tracked and maintenance
planned for a period before the deviations become
significant. With control valves it has been difficult to exactly
determine the current performance because there was no easy way
to monitor and report on the condition. As a result, most maintenance
has been either unplanned or tied to some calendar date in
the hopes that regular service would keep most problems from
becoming too severe. In most cases, these approaches cost the end
users dearly since failed valves not only tore themselves up but
resulted in less-than-optimum process control performance.
All that has changed with the advent of digitally based field
devices that feature two-way communication between the device
in the line and the control room. The end user can now monitor
a piece of equipment (such as a control valve) online through a
digital positioner mounted on the valve and determine the overall
operating performance without going to the field or breaking
into the line. Once monitored performance levels drop below a
predetermined threshold level, the diagnostic device can even
provide clues as to the root cause by checking a dozen or so of the
underlying operating characteristics of the valve. As can be seen
in Figure 6, once a problem is detected, an alarm button is triggered
and then built-in software can explain the problem with
recommended corrective action.
This new technology addresses both of the problems just
highlighted in that it permits a full-blown condition assessment
to occur on critical valve characteristics and compares them to
well-established norms based on equipment testing. So we end
up knowing very well what we should expect from our control
valves in terms of performance, and we also have a painless way
of remotely monitoring the equipment so we can determine
when a maintenance intervention might be required. We can
finally predict when maintenance problems might occur and
employ just-in-time proactive techniques that allow us to schedule
maintenance so it has the minimum financial impact on the
operation of the plant while ensuring the performance of our
control valves are up to standards.
Taken to the ultimate level, we can then categorize control
valves by criticality rating and custom design the maintenance
and inventory programs around the rating, focusing maintenance
activities and spare parts inventories on those valves that have the
greatest impact on operations. We can also use our predictive
capabilities to cut down on inventories since we no longer have to
maintain safety stock in case of a reactive failure—another significant
benefit to the bottom line.