Page 301 - Design of Simple and Robust Process Plants

8.2 Instrumentation 287
. The two top contributors to failures for temperature measurements are the
sensor and electrical connections; these are responsible for over 50% of the
failures.
. For valves the percentage of failures in relation to the installed base was low
compared to flow, level and pressure measurements. The type of failures are
not available, but bad stem travelling and hysteresis are the most frequently
observed phenomena.
These data provide a good indication of the reliability of instruments as designed
during the 1970s and 1980s. Since that time, much effort has been spent (particular-
ly by vendors) to improve the reliability of instrumentation. Progress made in this
field is described below for the most commonly applied instrumentation (Liptak and
Venczel, 1982).

8.2.2
Instrumentation Systems

Instrumentation systems have been through a complete development cycle. During
the 1960s, single loop pneumatic controllers were standard, while off line computers
were first used for engineering calculations and for some specific off-line opera-
tional models. The next step was the introduction of computers for steering sequen-
tial operations, all of which were based on digital actions. In particular, batch opera-
tions (and also regeneration operations) benefited from the consistency in auto-
mated operation, the result being improved quality and increased capacity.
Electronic single loop controllers were first introduced in the 1970s, at which
time small-scale set points were regulated by a supervisory computer system. By the
end of the 1970s, multi-loop computer-based control systems had been introduced
for analog control, based on direct digital control (DDC) technology. Control systems
were further developed in the 1980s and 1990s, when the combination of digital and
analog operational functionality was realized. Subsequently, computer power was
increased and screens were introduced as the operator interface to replace the large
panels. More sophisticated control activities could be implemented in this system,
which included model-based control. A computer system for intensive operational
optimization calculations and set point steering was installed on a higher level of
the control hierarchy. The hierarchy for control is shown schematically in Fig-
ure 8.2.
In the 1970s, the pneumatic signal transmission was to a four-wired electronic
system, though this was later replaced by a two-wired system where the power for
the instrument was combined with the signal. This was all based on analog signals.
The development of a so-called ªfieldbusº ± a core cable for digital data communi-
cation to replace all the individual wiring ± occurred at the end of the 1990s, and
was standardized in IEC 61158. The initial drive for this technology was cost reduc-
tion for wiring. The technique of digital data communication opened a new world in
instrumental design.