Page 322 - Design of Simple and Robust Process Plants

308 Chapter 8 Instrumentation, Automation of Operation and Control
± Start-up of the combustor is simultaneous with a programmed start-up of the
absorber. The start of the absorber is dependent on the initial condition of
the absorber, which often has an acid profile over the column at start-up, as
the trays are full due to the high weirs and type of trays selected (bubble caps
with cooling spirals for removal of absorption heat).
These examples illustrate the usefulness of the start-up methodology.
The shut-down of a process is preferably performed under controlled conditions.
The shutdown is planned in such a way that the process and its units are placed in a
defined safe state (intermediate or process wait), so that continuation of production
or any planned maintenance can easily proceed. Selection of the specific intermedi-
ate state might depend on the time for which the process is projected to remain in
that state. Controlled conditions refers to the operational sequence, as well as the time
trajectories required to avoid any mechanical damage, for example causing too-great
a temperature transient. The shut-down passways require careful examination, and
are mostly performed in a reverse order of the start-up of the (ir)reversible units.
Procedures may include specific steps, such as washing, steaming, and purging.
The operational strategy needs to be translated into a breakdown of all manipulations
required for the operation of each unit. This results in the identification of automatic
valves and motor switches with their DOs, in addition to the AIs and AOs which were
determined during analysis of the control needs. Due to the unreliability of switches
that come in contact with process fluids, these are only applied in exceptional cases.
Any additional instruments that are required to achieve first-pass prime operation
should be added at this stage. These include the additional instruments required for
batch reactions to verify feed quantities for reasons of product consistency. The total
operational procedures must be written as a basis for operational software develop-
ment.

8.3.3
Instrumental Safeguarding

Instrumental protection, including interlocking to prevent injuries to people, envir-
onmental loads and property loss, is detailed in this paragraph. Safeguarding is
based on the following philosophies:
. Prevent a potential problem by eliminating or minimizing the hazard.
. Prevent loss of containment by equipment failure.
. Prevent the release of hazardous chemicals.
A protection philosophy was described by CCPS which is broadly used in industry
and is based on:

. Building different protection layers (Figure 8.11) (CCPS, 1993).
. Design inherently safer chemical processes, with the keywords: Minimize,
Substitute, Moderate, and Simplify (CCPS, 1993, 1996).