Page 48 - Design of Simple and Robust Process Plants

32 Chapter 2 Simple and Robust Plant Design
umbrella of ªrobustnessº± which in itself can be subdivided into several types:
mechanical; control; and operational.

2.4.1
Mechanical Robustness

First, we have to consider manual, incorrect operations that challenge mechanical
robustness, for example the wrong lining up of a pump. For instance, an operator
may forget to open the suction valve, leading to cavitation of a pump, but it is
unclear whether the pump is sufficiently robust to withstand this operational error
for a short period. Numerous other examples exist where the robustness of the
design has been improved. In the past, many accidents have occurred because plug
valves were equipped with handles, which incidentally were opened, but today most
of these are equipped with hand-wheels. Another example is the casings of pumps
which, in the past were often designed from cast iron ± a material which cannot
withstand thermal shocks. Nowadays, pumps that may be exposed to fire are de-
signed from cast steel. The design of piping also provided the knowledge that ASA
flanges are more robust than DIN flanges, and resulting in less leakage. By contrast,
the selection of API pumps (which are known for their heavier construction) for
standard applications did not show any major improvement in performance com-
pared with ANSI pumps.
In general, mechanical robustness is reflected in the selection of equipment, pip-
ing items and instruments, but is also applied in control terminology.

2.4.2
Control Robustness

The term ªrobust controlºinfers that a process is able to recover from set-point
changes and disturbances within a wide range, and achieves set-point tracking and
disturbance rejection to its target values within a reasonable time frame. The distur-
bances considered are mainly changes in feed rates or composition, utilities and
environmental conditions. Interruptions in conditions as trips are seldom consid-
ered under robust control. A good control design must achieve a certain robustness
due to the most likely disturbances.

2.4.3
Operational Robustness

The term ªrobust operationºmight be used for the implementation of:
. Automatic start-up, shut-down, and implementation of regeneration se-
quences.
. Recovery of situations where interruptions such as pump trips or alarms
must be anticipated. Such situations must be foreseen and result in an auto-
matic actions, either in process recovery or end-up in a defined standby con-