Paper360 - May/June 2017 - 36

techlink | MAINTENANCE

Failure Isn't Just an Option -
It's Unavoidable
In Part 2 of this 2-part series, we look at how to use the P-F Curve
to determine the point of functional failure.
JAY SHELLOGG

In Part 1 (which appeared in the March/
April issue of Paper360°) we looked at changing our worldview about what defines failure.
I explained the concept of "Functional Failure"
and introduced the P-F Curve (see Fig. 1.) So,
how does the P-F curve help us?
We use the P-F Curve to identify what can
be done to detect a pending functional failure
and to determine what the inspection frequency
should be. We base the inspection frequency on
one-half the length of the P-F interval. We do
this to give us enough time to proactively plan
and mitigate the pending functional failure. If
half the P-F interval will not give us enough time
to proactively plan for corrective maintenance,
then we may consider an alternate detection
method with a longer P-F interval or we may
choose a frequency of less than half so we have
ample time to plan.
It is important to note that in no way does
the concept of criticality influence the frequency
of inspection. Criticality's only role is in determining if the inspection is worth doing. Those
two previous sentences are sometimes hard to
swallow because this flies in the face of traditional thinking and probably 95 percent of the
maintenance and reliability industry. But, no
matter how critical a system is considered to be,
if its failure modes that lead to functional failure
have identifiable P-F intervals, why would we
inspect at a rate more frequent than half the P-F
interval? Criticality plays no role in inspection frequency.
If half the P-F interval is not long enough
to prevent functional failure, we may decide
to shorten the inspection interval; however,
we commonly find that frequencies are set at
intervals much shorter than half of the P-F
interval. This is done for a host of reasons: to
make us feel better, in response to pressure to
"never have another failure," because it is "best
practice," or because "it's what everyone else
is doing." The list goes on and on, but none of
36

Paper360º

MAY/JUNE 2017

Fig. 1

these reasons are based on a logical, systematic
approach. The P-F interval must set the inspection frequency-not criticality, not emotions,
not best practices.
ELECTRICAL COMPONENTS
So the P-F Curve is fine for mechanical
devices, but what about electrical components? They either work or they don't, right?
Well, maybe not.
Let's go back to that idea of the functional
failure and instead of a pump, let's consider a
finish products line. It doesn't matter if it is fine
paper converting, tissue, a winder, etc. because
I bet everyone who reads this article at one time
or another has cycled power on a computer,
PLC, or drive to clear a fault. It is also very common to clear that fault and still meet the daily/
weekly/monthly production standard. In this
example, functional failure is not defined at
the drive rest, but when the drive fault disrupts
production to such a degree that we do not meet
our performance standard. Even if we define
functional failure as a loss of our production
standard, we still must define a P-F Interval.
Back to our earlier question. Electrical systems either work or they don't, right? I will bet
again that most of you have experienced a drive
rest only to see the drive fault return-first
within weeks or months, then within days or
weeks, then within hours or days-and finally,

not to clear at all. There you go: a P-F Curve.
From the first time the drive faults, you enter
the P-F Curve at point P. Then, each time you
rest the drive you progress down the curve until
you reach functional failure.
Want more proof? How many times have you
had a drive fault that you have experienced in
the past, and at the first rest you go to the storeroom and make sure you have a spare drive in
stock? I'll bet many times. Without even knowing what the P-F Curve was, you were still acting
on the P-F concept and your experience.
In this discussion, I have not mentioned
criticality; however, there is a clear point of
relevance. Criticality will play a role in the
storeroom stocking decision.
One last point-and I'm very interested in
reader feedback on this. Have you ever heard
a manager state, "I don't want to have another
drive fail!"? I have heard this pronouncement
for many mechanical components, but never
electrical. I wonder if it's because electrical
components lend themselves more easily to the
concept of random failures, while mechanical
failures seem as if they should be time-based-
even though both mechanical and electrical
failures are most likely random in nature.
THE CAUSES OF FAILURE
There is another concept that supervisors/
planners/engineers, as well as maintainers
www.tappi.org


http://www.tappi.org

Table of Contents for the Digital Edition of Paper360 - May/June 2017

Setpoint
Over the Wire
Leadership for a Changing Industry
An Ode to Small
The Graying of the Paper Industry
Suppliers Reach Out to Mill Leaders
Making a Difference: 2017 TAPPI/PIMA Awards
RISI’s Asian CEO of the Year
TAPPISAFE Offers Solid Benefits for Mills, Contractors
Reinventing Varkaus
Failure Isn’t Just an Option—It’s Unavoidable
Blower Technology Proves Its Worth
TAPPI Journal Summaries
Global Study: Most People Highly Value Paper and Print
The Changing World of OCC
TAPPI News
ASPI News
Index of Advertisers
Paper360 - May/June 2017 - intro
Paper360 - May/June 2017 - ebelly1
Paper360 - May/June 2017 - ebelly2
Paper360 - May/June 2017 - cover1
Paper360 - May/June 2017 - cover2
Paper360 - May/June 2017 - 3
Paper360 - May/June 2017 - 4
Paper360 - May/June 2017 - 5
Paper360 - May/June 2017 - Setpoint
Paper360 - May/June 2017 - 7
Paper360 - May/June 2017 - Over the Wire
Paper360 - May/June 2017 - 9
Paper360 - May/June 2017 - 10
Paper360 - May/June 2017 - 11
Paper360 - May/June 2017 - An Ode to Small
Paper360 - May/June 2017 - 13
Paper360 - May/June 2017 - 14
Paper360 - May/June 2017 - 15
Paper360 - May/June 2017 - The Graying of the Paper Industry
Paper360 - May/June 2017 - 17
Paper360 - May/June 2017 - Suppliers Reach Out to Mill Leaders
Paper360 - May/June 2017 - 19
Paper360 - May/June 2017 - Making a Difference: 2017 TAPPI/PIMA Awards
Paper360 - May/June 2017 - 21
Paper360 - May/June 2017 - 22
Paper360 - May/June 2017 - 23
Paper360 - May/June 2017 - 24
Paper360 - May/June 2017 - 25
Paper360 - May/June 2017 - 26
Paper360 - May/June 2017 - 27
Paper360 - May/June 2017 - RISI’s Asian CEO of the Year
Paper360 - May/June 2017 - 29
Paper360 - May/June 2017 - 30
Paper360 - May/June 2017 - 31
Paper360 - May/June 2017 - TAPPISAFE Offers Solid Benefits for Mills, Contractors
Paper360 - May/June 2017 - 33
Paper360 - May/June 2017 - Reinventing Varkaus
Paper360 - May/June 2017 - 35
Paper360 - May/June 2017 - Failure Isn’t Just an Option—It’s Unavoidable
Paper360 - May/June 2017 - 37
Paper360 - May/June 2017 - Blower Technology Proves Its Worth
Paper360 - May/June 2017 - 39
Paper360 - May/June 2017 - TAPPI Journal Summaries
Paper360 - May/June 2017 - 41
Paper360 - May/June 2017 - Global Study: Most People Highly Value Paper and Print
Paper360 - May/June 2017 - 43
Paper360 - May/June 2017 - 44
Paper360 - May/June 2017 - 45
Paper360 - May/June 2017 - The Changing World of OCC
Paper360 - May/June 2017 - 47
Paper360 - May/June 2017 - TAPPI News
Paper360 - May/June 2017 - ASPI News
Paper360 - May/June 2017 - Index of Advertisers
Paper360 - May/June 2017 - cover3
Paper360 - May/June 2017 - cover4
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