Page 46 - RCM - A practical Guide_V1

RCM - A Practical Guide

In this instance, in order to achieve the desired probability of multiple failure the operator should test
the protective device once every 400 operating hours.

FFI Calculation Woes

The difficulty with FFI calculations isn’t necessarily understanding the formulae, no, it’s understanding
the data with which the formula is populated. Failure or fault data is collected in the context of an
asset but determining whether that failure has contributed to the loss of the specific function in
question (or another function to which the asset contributes).

Redesign
Unlike FF maintenance, which is available only to those failure modes contributing to hidden
functional failures, the option to redesign is open to both hidden and evident.
There are, of course, two variations. The driving difference between the two is whether the failure is
considered contrary to safety (or other important factor) or not. The types are:
• Mandatory
• Optional
Mandatory redesign (RM)
RM must either reduce the probability of failure or multiple failures to an acceptable level or reduce
the severity of the perceived consequences. When it comes to redesign action the possibilities are
almost endless and, for RM, cost may only be used as an argument to decide between redesign
options and not to decide against action at all.
Desirable redesign (RD)
Many of the above mentioned modifications are equally applicable to RD. The difference here is that
the justification for going ahead with the modification is likely to be economic in nature and hard
fought.
The main aims of such a modification should be to:
• Prevent the failure occurring or reduce the number of occurrences
• Reduce the consequences of the failure
• Make the item more maintainable and hence reduce the cost of maintenance
Determination of what makes a modification cost effective is by no means obvious and will be
determined not only by the cost of the modification process but also by the amount of remaining life
of the equipment and operating costs etc.
If you’re having to compose a justification for a RD recommendation, then it may be useful to take
account of the following:
Remaining useful life
Is the remaining useful life of the equipment high?
If it is calculated that the cost of the modification will not show benefits within the remaining useful
life of the equipment then it may not be feasible to go ahead. Many modifications are discarded on
these grounds. If the consequences of not carrying out a modification are severe it may be more
beneficial to retire the equipment early rather than risk them happening or incurring the cost of
modification.
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