Page 46 - Design of Simple and Robust Process Plants

30 Chapter 2 Simple and Robust Plant Design
2.3
Why Higher Reliability?

Reliability is defined as: the probability that a component or system performs its function
over a projected time period if used in a proper way, given that the component or system
was to be considered new at time zero.
The ªprojected time periodºused in process plants must at least meet the time
between turnarounds for repairable components, while for other components their
lifetime might be applicable.
The ªproper wayºmeans usage within its design limits.
The achievement of reliability is becoming increasingly important, and incorpo-
rates facets such as:

. savings on maintenance cost;
. longer standing times of components, up and until the time between turn-
arounds (4±6 years) to increase plant availability; and
. design for single components (capital saving and simpler designs).
The mechanical reliability of components is increasing as a result of evolution.
Nowadays, vendors not only receive requests regarding the reliability of compo-
nents, they also sometimes are responsible for maintenance of the equipment sup-
plied. Situations also occur where vendors deliver services such as refrigeration or
(co-)generation of energy. These companies ± who historically are often suppliers of
the equipment ± deliver this type of service logistical spread over a large area. Their
services might include:

. an operational contract;
. a minimum defined reliability and availability level; and
. a maintenance contract.
This means that it is in the company's own benefit to install more reliable compo-
nents and units in order to suppress operational, unavailability and maintenance
costs. These are opportunities to invest in more reliable components.
A striking example is the development of more reliable jet engines for airplanes.
Nowadays, the new designs of airplane are equipped with two engines for long-dis-
tance flight over the oceans, whereas in the recent past, four engines were consid-
ered the standard. These newly designed aircraft, as well as being considerably
cheaper to operate, needed to achieve the same reliability level as the older designs,
and this could only be achieved by designing more reliable engines.
Another example is the considerable improvement that has been achieved with
rotating equipment such as compressors. One such development was the introduc-
tion of gas seals as replacement for liquid seals, while another more recent develop-
ment has been the introduction of screw compressors where neither the screws nor
gears physically touch each other. In this situation, each screw has its own electric
driver where the driving frequency has been synchronized very accurately.
That reliable components can be built and operated is also seen in a domestic
application such as a central heating system. The heating system circulation pump