Factors Design Margins

For the designer, the use of a standardized 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 foundation plates, pipe connections, and the selection of the drive motors standard electric motors would be used. 

For an operating company, the standardization of equipment designs and sizes increases interchangeability and reduces the stock of spares that must be held in maintenance stores. 

The use of standards in design is illustrated in the discussion of the pressure vessel design in Chapter 13. Relevant design codes and standards are cited throughout the book. 

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The value of constant k depends on system complexity, design margins, and design objectives for rehabUity, whereas the value of growth factor fi depends on the development effort... 

When too many factors of safety are added to the true required design flows, excess margins may affect compressor stability. When the compressor manufacturer begins the design, therefore, unjustified excess capacity flows exist. Be realistic in regard to building in flexibility or capacity. 

Designers of most structures specify material stresses and strains well within the pro-portional/elastic limit. Where required (with no or limited experience on a particular type product materialwise and/or process-wise) this practice builds in a margin of safety to accommodate the effects of improper material processing conditions and/or unforeseen loads and environmental factors. This practice also allows the designer to use design equations based on the assumptions of small deformation and purely elastic material behavior. Other properties derived from stress-strain data that are used include modulus of elasticity and tensile strength. 

Until about the second World War chemical processes were developed in an evolutionary way by building plants of increasing size and capacity. The capacity of the next plant in the series was determined by a scale-up factor that depended mainly upon experience gained from scale-ups of similar plants. Due to a lack of predictive models for chemical processes and operations, processes had to be scaled up in many small steps. This procedure was very expensive and the results unreliable. Therefore, large safety margins were incorporated in scale-up procedures, which often resulted in a significant unintended overcapacity of the designed plant. 

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