Page 59 - Design of Simple and Robust Process Plants

3.3 Design Philosophies 43
ical reliability of these units, including governors for speed control as well as
improved bearings and seals.
Parallel to the development of more reliable machines was the trend to increase
the capacity of single machines/equipment. Larger equipment could be manufac-
tured to satisfy the demand from industry for the design of higher-capacity process
plants, and thus increase the economy of scale. The need to keep these large-capa-
city plants operational was a major incentive to invest in reliability and robustness
of all equipment. The increase of time between overhauls was another requirement
from industry as a means of reducing cost and increasing the availability (up-time)
of the plants.
The result was that techniques came available to increase the mechanical reliabil-
ity and up times of facilities. For example, in the case of rotating equipment specific
improvements were made in bearings, seals, and protective instrumentation, while
the general design trends were to:

. design more reliable equipment;
. design larger equipment; and
. design for longer stand times, and higher availability.
For vessels, heat-exchangers, and fired equipment the increase in size and reliability
were realized through improvements in:

. the selection of materials of construction;
. the mechanical design to reduce and cope with thermal and transient stres-
ses;
. improved operation to reduce thermal and transient excursions;
. improved process design; and
. the prevention or reduction of fouling through chemical means, for example
the use of inhibitors or the development of other catalyst systems.
The demand for larger process equipment was fulfilled not only through application
of the above techniques, but also through the development of new mechanical
designs. The constraints for larger equipment in general were the mechanical
design and the fabrication capability. This not only forced the development of new
equipment designs, it also forced the introduction of new process designs, includ-
ing: (i) the scale-up of glass-lined jacket reactor vessels, which required alternative
means of reactor cooling, such as feed cooling or the development of a reflux con-
denser; and (ii) the scale-up of styrene reactors, which changed from packed vertical-
flow reactors with a tubular heated section to staged radial-flow reactors with inter-
stage heating. Similar solutions were applied to other packed adiabatic reactors,
such as ammonia reactors.
The reliable operation of in particular, smaller components/units ask specific
attention. Generic components are available from several suppliers who service dif-
ferent markets. The development of these components pass a development cycle
that is important to be recognized for application and selection in simple, robust
plants.