Page 74 - Design of Simple and Robust Process Plants

58 Chapter 3 Design Philosophies
Inherently Safer and Environmentally Sound Perspectives

3.3.11
Inherently Safer Design
The development of inherently safer chemical plants is an objective that was intro-
duced by Kletz (1991) and later extended further and promoted by the Center for
Chemical Process Safety (CCPS, 1996) from AIChE and IchemE. The basic concept
was covered in four guiding words:
1. Minimize
2. Substitute
3. Moderate
4. Simplify
Before the meanings of these words are described in detail, the following example
will serve as an illustration. A storage tank is planned for a rather nasty, liquid mate-
rial (product ªXº) which has the following qualitative properties: a low boiling point;
decomposition which starts around boiling point; solidification which occurs some
degrees below the boiling point; high toxicity; flammability; degradation with oxy-
gen; and exothermic reaction with water.
Let us consider an overview of the safety provisions that would need to be
installed on such a storage tank.
Safety provisions are:
. Measurements resulting in signals/actions on high and high-high, low and
low-low signals on level, pressure, temperature, oxygen detector, water detec-
tor.
. Pressure protection PVRV (pressure valve and relieve valve), ERV (emergency
relieve valve), inert padding, flame arrestor.
. Safe let-down systems for vapors.
. Protection against tank leakage.
. Inhibiting system.
. Insulation.
. Indirect heating to avoid high tank skin temperatures.
. Loading and unloading requires seal-less pumps with dead-heading and tem-
perature protection, excess flow protection, vapor return lines.
. External protection such as fire detector, toxicity measurement, sprinklers,
water-guns, fire brigade at stand-by, emergency plan for the site and the com-
munity, quantitative risk analysis (QRA) available, dispersion calculation
information available on-line.
All the above provisions are add-ons, and accept the situation as it was submitted
(Figure 3.6). Although the above is an extreme example, the point to be made is
applicable to storage of chemicals ± that protection is with add-ons.