Page 240 - Design of Simple and Robust Process Plants

226 Chapter 6 Process Design Based on Reliability
The constant failure rate zone B represents the situation during the normal opera-
tional period of a process ± the mission time. It should be noted that selection of the
component is normally based on the low constant failure rates. The stand time of
the components, before an increase in failure rates due to wear-out is noticed,
should preferably exceed the mission time of the process. This does not mean there
is no wear-out ± only that there is no significant increase in failure rates due to wear-
out.
The wear-out failures zone C represents a zone which is certainly entered by sev-
eral components in process plants. Differentiation might be made between compo-
nents here: (i) those which have very long life times (over 4±10 times the mission
times, or 10 to 20 years), for example power cabling, windings of electric drives,
lines or process vessels; and (ii) components which have a relative short lifetimes,
for example 1.5±2times the mission times. Both types are subject to inspections.
The components with short lifetimes will be subject to repair during an overhaul in
order for them to be considered as new. Consideration might be made to replace the
bearings or seals of machines, or to replace frequently used switches.
The components with long lifetimes will be subject to inspection during over-
hauls or during operation, for example wall thickness measurements. A set of simi-
lar components which is under suspicion of wear-out failure can be identified using
the Weibull analysis of life data (see referred literature). The identification of poten-
tial failures might be used for a maintenance plan to avoid unplanned stops.

6.2.1
Availability and Unavailability

Availability (A(t)) is the fraction of time over a defined period (the mission time) that a
component or system is fully operational.
For a process plant, the mission time is the period between two overhauls (also
called turn-arounds), and must not be confused with the time between planned pro-
cess stops for regeneration or cleaning.
A time line is shown of a process in Figure 6.5, where the turn around is shown
as well as planned stops for regeneration and stops for unplanned failures. The max-
imum time available is now the mission time minus time for planned process stops.
The steady-state value of availability A is formulated as:

MTBF
A= (3)
MTBF‡MTTR
A model for failure often applied is the constant failure rate, which corresponds
to the zone B of the bathtub representation. The constant rate model is mathemati-
cally expressed as:

R(t) = e kt
In this particular case
1
k ˆ
MTBF