Page 333 - Design of Simple and Robust Process Plants

8.4 Control Design 319
. Safeguarding is based on the following philosophies:
± Prevent a potential problem by elimination or minimization of the hazard.
± Prevent loss of containment by equipment failure.
± Prevent the release of hazardous chemicals.
. The safeguarding approach is based on:
± Building different protection layers.
± Designing inherently safer processes based on the keywords: Minimize, Sub-
stitute, Moderate, and Simplify.
. The direct protection of a process is applied in the following order:
± Process design by minimizing, substitution. Moderation.
± Basic control.
± Alarming.
± Interlocking (SIS).
± Physical protection (relief devices).
. Safety instrument system design needs to follow IEC 61508.
. Automated exothermic and gas-releasing reaction systems specifically require
safeguarding during transient operations. The design of these safeguarding
systems requires dynamic understanding and monitoring of the reactions to
enable timely response.
. A layered alerting/alarming strategy was discussed which should have as
important elements, keeping the operator's attention by requesting opera-
tional confirmation or actions, differentiation between pre-alarming and
alarming, and prevention of alarm showers.
. Observation of the process requires:
± Selected measurements for process monitoring in addition to control, operat-
ing and safeguarding instruments.
± Layered instrumentation system divided into: basic control with interlocking,
model-based control and optimization, with the basic control layer function-
ing independently of the higher control layers.
± Observable layered flowsheet continuously updated with latest measurement
readings, and its history.
± Layered verified software with an understandable notation for operation and
specifying the conditions and process limitations during operation.
Overall instrumentation levels can be judged on the ratio of inputs and outputs. An
AI/AO ratio of 3, and a DI/DO ratio of 1 must be achievable.

8.4
Control Design

The achievement of a simple and robust process plants places stringent demands on
process control, as one of the characteristics is hands-off operation with no operator
in the control loop. The demands on controllability is increased where controllability
is defined as: