Maintworld Magazine 4/2020

PARTNER ARTICLE
WHAT’S THE RIGHT
Inspection Frequency
for Equipment?
If an inspection frequency
that is too short,
equipment will be over
maintained, and resources
are wasted. If it is too
long, some failures are
going to be missed.
HOW OFTEN DO YOU NEED to inspect
equipment? The short answer is that
it will vary depending on component,
operating context, environment, and
load. But you must understand how
to estimate the correct inspection frequency.
It is not based on criticality, not
the amount of resources you may have
at your plant, and it is not based on life of
the equipment.
Let’s start with defining what we mean
by inspections. Inspections include all
objective and subjective inspections.
• Objective inspections (we measure
something) by observation or use
an instrument. Instruments can
include vibration analyzer, infrared
camera, voltmeter, flow meter, or
ultrasonic.
• Subjective inspections are those
look-listen-feel-smell inspections
In order to set the frequency of your preventive
maintenance inspections, you need
to understand what Failure Developing
Period (FDP) is.
34 maintworld 4/2020
Failure Developing Period
(FDP) (or Pf Curve as many
calls it)
The FDP is the time period from when it is
possible to detect a failure until breakdown
occurs. A failure is when a system or equipment
is operating correctly within given
parameters but has signs of problems.
For example, a centrifugal pump may
be cavitating, but is still providing the
required flow for the operation; this is a
failure, but not a break down. The cavitations
in our example will eventually
develop into a breakdown. The breakdown
occurs when the pump is unable to
perform its intended function.
The FDP is the time difference
between the failure and the break
down. If the pump started to cavitate at
6 am and it broke down 6 pm 6 days later,
the FDP is 156 hours.
So, what’s the Inspection
Frequency?
The theoretical answer to the question
is very simple. The inspection frequency
should roughly be:
For example, if the estimated failure
developing period is 14 days and we need
some time to plan and schedule the corrective
maintenance for that failure to
avoid a break down. A reasonable inspection
frequency is 7 days (FDP/2). If the
inspection frequency is longer than 14
days, we may miss the failure and we will
have a breakdown.
Inspection Tools
changes the FDP
FDP changes when we have access to better
tools. For example, we may be able to
detect a problem with a pillow block bearing
by listening to it with a stethoscope.
This method may give us a warning period
of a few days (on average depending
on situation). However, if we use a vibration
analyzer, we can probably detect the
same failure at least 8 weeks in advance.