 Technical
Information about the Spacing of Equipment in Petrochemical Plants
Equipment spacing in the plant design and piping discipline is one area that
continues to resist automation. Sure there are in-house and third party programs
out there that assist designers in developing plot plans. Most of these programs
are just overlays on CADD products. These 2D/3D CADD overlays may give feedback
on costs of the arrangement - or - these 2D/3D CADD overlays may speed up the
development of the plot by automating the generation of equipment graphics and
structural graphics. However there are no tools out there that will arrange a
plot plan automatically.
The main reason for this lack of equipment arrangement automation is the
inability to develop rules that completely define how a list of equipment shall
be arranged on a plot. The difficulty in defining equipment arrangement rules is
due to the complexity involved.
Here is a partial list of the equipment arrangement design considerations
that must be addressed on each project.
-
 High
hazard operations
- Grouped operations
- Critical operations
- Number of personnel at risk
- Concentration of property and business interruption values
- Equipment replacement and installation time
- Interdependency of facilities
- Critical customer or supplier relationships
- Market share concerns
- Fire and explosion exposures
- Corrosive or incompatible materials exposures
- Vapor cloud explosions
- Sources of ignition
- Maintenance and emergency accessibility
- Drainage and grade sloping
- Prevailing wind conditions
- Natural hazards and climate
- Future expansions
- External exposures
The design consideration for 'Vapor Cloud Explosions' alone is a subject that
fills books. The analytical automation tools for studying Boiling Liquid
Evaporating Vapor Explosions (BLEVE) are few and are proprietary. There is
documentation on how to evaluate BLEVE impacts using hand calculations, such as
the TNT equivalency method. However it is generally accepted that these hand
calculation methods do a poor job of predicting BLEVE.
 And
BLEVEs are just one form of Vapor Cloud explosions! Other types of vapor cloud
explosions require different analytical techniques.
So what does the designer do? Designers the world over generally rely on
their company's (or their client's) table standard that lists how far one piece
of equipment needs to be from another piece of equipment. These tables are
typically generated based on experience - particularly the experience of the
insurance industry. A sample chart from the
Bausbacher/Hunt book is shown above.
The following pages are going to compare and discuss the various spacing
criteria available in the public domain. We will also try to point out where
these criteria conflict, and where they may be weak.
By the way, the picture to the right is of a recent explosion in a plant
where the spacing criteria satisfied an insurance group's requirements for
equipment spacing. The initial explosion caused a domino effect that leveled
three units.
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