Page 76 - Design of Simple and Robust Process Plants

60 Chapter 3 Design Philosophies
ering vapor pressure, reactive chemical properties, flammability limits, low-
ering solidification point, sensitivity to other plant components.
. Could it be supplied with another (essential) material that we need so that
handling can be minimized and conditions moderated?
Simplify
. No tank.
. Do we need the process feed pump? (Options are gravity flow, or transport
under pressure difference)
. Did we consider seal-less pumps?
. Could we load under pressure difference instead of by pumping into the
tank?
A typical practical example which came to my attention was a tank with caustic soda
used as a neutralizing agent for a waste water facility. The tank was equipped with a
set of metering pumps, which were adjusted by a pH controller. The metering
pump required a lot of operation attention and maintenance due to fouling. One
day an accident happened when an operator was sprayed with caustic soda and
injured, despite his wearing protective clothing. The incident resulted in a root
cause analysis of the system, the four major questions of which were:
1. Minimization: Do we need caustic storage?
2. Substitute: Can we use another neutralizing agent or can we prevent neutra-
lization?
3. Moderate: Do we need the high concentration?
4. Simplify: Do we really need to pump?
Different solutions were studied, but the final choice was to: (i) decrease the caustic
concentration to meter larger flows with a standard flow meter and feed at gravity
flow; and (ii) remove the pumps ± to achieve this the tank need to be elevated only
1 m. The new design was much safer, simpler, and more robust (Figure 3.7).

Design with metering pumps Safer and simpler design without pumps

SC A A FC
pH pH

Waste water neutralization
Fig. 3.7. Dosing with metering pumps versus gravity flow.