Page 330 - Design of Simple and Robust Process Plants

316 Chapter 8 Instrumentation, Automation of Operation and Control
The performance of the process covers questions for which, in the past, the answer
was hidden from operators. Supervisors performed calculations by hand at their
desk, and this was not the concern of operators. Nowadays, the role of the operator
is shifted to operational supervisor. Consequently, the information that he/she
requires must provide the answers to questions such as:
± What is the compressor efficiency?
± What is the vibration level of the compressor?
± How does the steam balance appear? Do we need to modify the situation by
changing turbine operations?
± What is the overall performance of the process expressed in performance
terms? (See Chapter 9 for process performance (profit) metering.)
The required information, in turn, requires some additional instruments. For process
observation, these are AIs and DIs, and the instrumentation to provide this informa-
tion is in addition to the instruments needed for control, operation, and safeguarding.
Note that the installation of (as standard) two switches (DIs) to detect valve posi-
tion for observation is recommended not only in specific cases (e.g., safety), but also
in case of unreliable valve operation (e.g., fouling and high temperature gradient
situations). High-voltage switches are, due to their specific nature, provided with
DIs in the off position.

8.3.4.1 Instrumentation Systems
These form the basis for observability of the process. The design of these systems is
undertaken by excellent companies which spend a great deal of development time
and money in order to maintain standards. The requirements for this include:
. Current trends to operate several plants (or a total site) from one control
room accelerates the need for commonality in instrument systems and soft-
ware design.
. Avoid the installation of different workstations for different activities, or this
can ± and will ± lead to errors by operation.
. Instrument system should have highly powered floating point processors
with programmable operation stations for process observation.
. Instrument systems must perform sequence control, as well as all their con-
troller functions.
. Ergonomics requirements must be respected in the control room to improve
observability.
. The reliability of these systems is high, and the technical abilities to provide
good on-screen process overviews are excellent. However, attention must be
paid to redundancy on power supply. You will never forget being in a control
room where all the screens went black due to a power outage!
. The SIS and BPCS (basic process control system) are largely physically sepa-
rated. Currently, some overlap is considered to be an advantage due to an
increased level of knowledge in BPCS. An example is the alarming and inter-
locking of reactive exothermic systems based on process simulations.