Efficient Plant October 2017 - 31

feature | reliability strategies

the equipment in a short period of time
prior to being put into actual service to force
any potential infant-mortality failures
to occur. This removes some, but not
necessarily all, of the infant-mortality
risk. If such testing is sufficiently
thorough, however, the remaining
small failure rate for Zone I may
be less than the risk at some point
during normal operation of the
equipment, i.e., in Zone II.
Zone III end-of-life failures
often proceed exponentially
through their failure stages. Because
internal positive-feedback mechanisms
kick in once a certain threshold has
been crossed, initial small deviations from
normal can progress rapidly from a barely
detectable precursor to complete catastrophic failure.
Thus, trying to squeeze out that last bit of
service life from equipment before total failure occurs is a risky move. The steep increase
in the curve in Zone III indicates that the
failure rate changes exponentially, and rapidly increases in a short amount of time. This
introduces an increasing amount of uncertainty in any time-to-failure projections.
That said, the Zone III end-of-life-failures
item noted above basically sums up the previously stated point of this article: A regular
condition-monitoring schedule that's capable
of tracking and predicting equipment performance in Zone II is inadequate when the
equipment is operating in Zone III.
In Zone II, the failure rate is stable and
predictable with only occasional random
failures occurring. Regular condition
monitoring, performed every three months,
six months, or annually, may be sufficient
to verify that the equipment is slowly and
predictably showing the expected effects of
age and wear. This data can then be readily
extrapolated to predict what the values of
the monitored parameters will be when the

OCTOBER 2017

four hours. There's a
significant problem in this
approach, however.

Most organizations
don't respond quickly
enough to modify the
monitoring period
as needed to match
the exponentially
"moving target"
failure rate in
Zone III.

equipment is next monitored with reasonable
accuracy. Simple linear extrapolation often
works reasonably well in Zone II.
In Zone III, however, the failure rate increases exponentially with time. Catastrophic
failure can occur in a short period of time:
from no indication at all to complete failure
in a just a week, a couple of days, or even just
a few hours. The data collected from regularly spaced condition monitoring in Zone II is
generally insufficient to accurately indicate
impending end-of-life failure in Zone III
unless the monitoring periods in Zone III are
also exponentially shortened to match the
exponentially increasing failure rates. This is
the significance of the β shape factor being 3
instead of 1.
Keeping up with the rapidly changing
failure rate in Zone III is difficult. Some
companies attempt to do this by approximating exponentially changing failure
rates with a series of step functions. For
example, they might shorten a quarterly
condition-monitoring period to a monthly
period, then shorten that to a weekly period,
and, perhaps, eventually shorten it to every

WHAT'S THE PROBLEM?
Most organizations don't respond
quickly enough to modify the
monitoring period as needed to
match the exponentially "moving target" failure rate in Zone
III. Since non-linear extrapolation methods have to be used to
predict equipment conditions in
Zone III, and since small errors in
numerically determining the exponential function produce large errors
in the resulting prediction of when failure
may occur, failure may still occur when it is
not expected.
Consider, for example, how high-voltage
oil-insulated current transformers, (HVCTs)
fail. Such units can degrade over time,
primarily due to thermal, electrical, and environmental effects. Many have a useful service
life of 30 to 40 years. When they do fail, however, a significant number explode and catch
fire. Those explosions, in turn, often produce
flying debris that can cause collateral damage
to nearby equipment and personnel.
HVCT-condition-monitoring methods
include partial discharge, gas in oil, and power factors. All of these techniques provide a
good picture of the HVCT condition when
it's in Zone II. But, when the failure rate begins to rapidly change, as it does in Zone III,
the usual sampling periods associated with
these methods are often too long to detect
and warn of impending failures that can
occur in a matter of hours.
A common failure mechanism in an
HVCT is dielectric breakdown of the oil. In
Zone II, the dielectric properties of the oil
change more or less regularly with service
time and can be adequately monitored by
oil-in-gas analyses. With continued service
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