Page 328 - Design of Simple and Robust Process Plants

314 Chapter 8 Instrumentation, Automation of Operation and Control
(Pre)Alarming is applied when the process approaches an undesirable condition
which requests operator action. An example is the danger of an overflowing storage
tank; this requires operator intervention before interlocking becomes activated. In
such a situation the control measurement could provide a pre-alarm, while an inde-
pendent measurement with a higher alarm setting would generate an alarm.

8.3.3.2 Interlocking
Instrumental selection, as was discussed for safeguarding, was based on the analysis
of equipment which in general forms part of a unit operation. Active protection of
the equipment results mostly in interlocking/tripping of any complete unit opera-
tion that has been activated after an alarm. In consequence, the process might be
severely upset or require part or even complete shut-down or an emergency stop.
This problem should be faced during development of the operational strategy, as
discussed previously. Different levels of interlocking or trips include:

± Equipment trip
± Unit trip
± Plant shut-down
± Emergency shut-down (ESD)
A trip may cascade down through a process plant; hence, these ªcascadesº need to be
foreseen and sufficient instrumentation made available to guide the impact of a trip.
In order to enable smooth start-up of the facility after a trip, this (preferably)
would be absorbed in a normal shut-down. An emergency stop of the process is gen-
erally the result of a major mechanical failure with a serious release or with utility
losses that might be manually or automatically activated. The process is automati-
cally blocked, and all actuators are in a foreseen passive failsafe position.
After a trip, emergency block valve (EBV) positions must be confirmed in their
failsafe positions by installation of DIs. This is because EBVs are not in frequent
use, and their correct functioning in an emergency situation is critical.

8.3.4
Observation

For a simple and robust process, surveyability of the process is very important.
Operators are transformed to operation supervisors and need to have limited, albeit
good, information on which to judge whether the process is operating as required.
It is not only data that needs to be communicated ± access to these data is also very
important. Given that important information is received, operation also needs to be
able to analyze the information and to take appropriate action. Therefore, observabil-
ity is an important point to address, and comprises the following items:

. Process measurements for overviewing the process.
. Layered redundant control system.
. Layered overview of the process on the panel or screens within ergonomic
requirements.