Page 29 - Design of Simple and Robust Process Plants

12 Chapter 1 Introduction
Inherently Safer and Environmentally Sound Processes

1.3.9
Inherently Safer Design

This concept is based on four guiding lines (Kletz, 1991; CCPS, 1996) that were
accepted as bases for design by IChemE and the CCPS of AIChE. The approach in-
tended to minimize the risk of a facility through the minimization of hazardous
situations. The key words are; minimize, substitute, moderate simplify.

Minimization of hazardous material This can be realized by storage minimization
which is also in line with JIP. The application of gaseous feeds versus liquid feeds
should also be considered. This can be a realistic option in case of low-boiling point
materials such as chlorine, ammonia, and phosgene. The reduction of inventory in
the process is an opportunity, like the removal of a reflux drum in a distillation
column or application of column packing versus trays.

Substitution of a chemical with a less hazardous material There are many examples
of substitution, including: (i) the selection of a different material ± perhaps to
replace a strong acid with a weak acid, or a toxic material by a less-toxic equivalent;
(ii) the dilution of a material; and (iii) the selection of a different physical phase,
possibly from solid to solution.

Moderation of hazardous conditions This might be carried out in order to minimize
the impact of a release of hazardous material or energy (this is also known as ªAtten-
uationº or ªLimitation of effectsº). Different options that might be considered
include operation at less harmful temperature, pressure, concentration and phases,
and the avoidance of interaction of chemicals.

Simplification of a facility This would be conducted from an operational point of
view in order to avoid operational errors. Items that have a major impact on the level
of complexity include:
. Piping connections: more connections provide opportunities for misunder-
standing, more operator interventions, and potential errors;
. Degrees of freedom (DOFs) for operators: more DOFs give more operational
variables to the operator. All manual valves, switches, controlled and manipu-
lated variables, and analog outputs AOs with manual over-rides are DOFs,
with opportunities for errors and disturbances. The DOFs can be reduced by
a higher level of automation, in combination with the minimization of valves
and manual over-rides.
. Interaction of the process with other sections or external sources, static as
well as dynamic. De-coupling of the interaction through hardware and soft-
ware solutions (robust control) can solve this.