Page 307 - Design of Simple and Robust Process Plants

8.2 Instrumentation 293
SIS must be selected based on PFD requirements. The development of a reliability
database, particularly for the recently developed instrumentation, is a priority to
enable the design of simple SIS.

8.2.4.1 Design guides
Instrumentation plays a major role in the design of simple and robust plants. High
instrumentation reliability and availability are major objectives to achieve simplicity
(less redundancy in instrumentation) for operation. Instrumentation is a term in
the complexity formulae (see Section 2.2), which includes the point that reduction
of instrumentation reduces complexity. Numbers of nuisance trips which are not
really appreciated by operation will also be reduced by more reliable and less super-
fluous instrumentation. It is instrument unreliability that is the source behind the
demand for instrument redundancy leading to nuisance trips. Robust process con-
trol will more than benefit from good instruments, and is a precondition for hands-
off operation and control.
For instrumentation of simple and robust process plants several design guide-
lines were outlined in Section 5.7.4 in Chapter 5 These design guidelines are
rephrased and completed in the following points:

. Apply only reliable and robust instruments, and let these measure what you
want to measure. Most of the time we like to measure and control mass
flows. Traditionally, this was done by measuring volume, which only in
exceptional cases was corrected for temperature and density changes.
. Develop good instrument selection and installation procedures that ensure
instruments do not have to ªlie or dieº and are available and in good condi-
tion during start-up.
. Apply instrument fault detection
. Apply preferable instruments with sensors in process lines or in slip streams
(long sample lines cause delay times).
. Avoid lead lines for pressure transducers and level measurements as they are
a source of failures.
. Avoid batch-operated analysis like GC if alternatives are available. GC is accu-
rate, but requires sample handling and injection systems. Sample handling
and the analysis have long response times, while injection systems are sensi-
tive to fouling.
. Apply single instruments with fault detection, and deviate only in case of
safeguarding with SIS. Experience has taught that double instruments are a
nightmare for operation as they cannot easily decide which one is wrong. An
exception might be made for batch processes where accurate dosing of com-
ponents that determines product quality and safe operation conditions are a
firm requirement. An accuracy check is mostly done by load cells installed on
process vessels.
. Avoid local instruments. Simple and robust process plants do not have field
operators to watch the process outside ± it is monitored from the control
room.