Page 30 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

13
INTRODUCTION TO DESIGN
For the designer, the use of a standardised component size allows for the easy integration
of a piece of equipment into the rest of the plant. For example, if a standard range of
centrifugal pumps is specified the pump dimensions will be known, and this facilitates the
design of the foundations plates, pipe connections and the selection of the drive motors:
standard electric motors would be used.
For an operating company, the standardisation of equipment designs and sizes increases
interchangeability and reduces the stock of spares that have to be held in maintenance
stores.
Though there are clearly considerable advantages to be gained from the use of standards
in design, there are also some disadvantages. Standards impose constraints on the designer.
The nearest standard size will normally be selected on completing a design calculation
(rounding-up) but this will not necessarily be the optimum size; though as the standard
size will be cheaper than a special size, it will usually be the best choice from the point of
view of initial capital cost. Standard design methods must, of their nature, be historical,
and do not necessarily incorporate the latest techniques.
The use of standards in design is illustrated in the discussion of the pressure vessel
design standards (codes) in Chapter 13.

1.7. FACTORS OF SAFETY (DESIGN FACTORS)

Design is an inexact art; errors and uncertainties will arise from uncertainties in the design
data available and in the approximations necessary in design calculations. To ensure that
the design specification is met, factors are included to give a margin of safety in the
design; safety in the sense that the equipment will not fail to perform satisfactorily, and
that it will operate safely: will not cause a hazard. “Design factor” is a better term to use,
as it does not confuse safety and performance factors.
In mechanical and structural design, the magnitude of the design factors used to allow
for uncertainties in material properties, design methods, fabrication and operating loads
are well established. For example, a factor of around 4 on the tensile strength, or about
2.5 on the 0.1 per cent proof stress, is normally used in general structural design. The
selection of design factors in mechanical engineering design is illustrated in the discussion
of pressure vessel design in Chapter 13.
Design factors are also applied in process design to give some tolerance in the design.
For example, the process stream average flows calculated from material balances are
usually increased by a factor, typically 10 per cent, to give some flexibility in process
operation. This factor will set the maximum flows for equipment, instrumentation, and
piping design. Where design factors are introduced to give some contingency in a process
design, they should be agreed within the project organisation, and clearly stated in the
project documents (drawings, calculation sheets and manuals). If this is not done, there
is a danger that each of the specialist design groups will add its own “factor of safety”;
resulting in gross, and unnecessary, over-design.
When selecting the design factor to use a balance has to be made between the desire
to make sure the design is adequate and the need to design to tight margins to remain
competitive. The greater the uncertainty in the design methods and data, the bigger the
design factor that must be used.