Page 304 - Design of Simple and Robust Process Plants

290 Chapter 8 Instrumentation, Automation of Operation and Control
The selection of thermocouple materials is now driven by less corrosive materials
such as Ni-CrNi, the emf generated dominating the selection to a lesser extent. The
selection of these less corrosive materials and the improved sealing and compensa-
tion techniques make current thermocouple devices more accurate and reliable.
Recall that the sensors and electronic connections were the major contributors to
failures of temperature measurements (see Table 8.1).
Switches are candidates for frequent failures, while detection of failure is only
experienced during testing. Failures are mostly caused by plugging. Switches on lev-
els of vessels or pressure are replaced by installation of two parallel analog measure-
ments provided with software switches. In case of an element failure, diagnostics of
the analog signals can easily assign the failing element. In the past, frequent failing
of switches installed on on±off valves to confirm the valve position was a nightmare
for operation due to the high failure rates. A standard response was to ignore the
signal, but this is unacceptable in a hands-off operated process. The solution came
from the suppliers who currently deliver totally enclosed sets of switches based on
the inductive principle. This replaced the mechanical switch which was less reliable
for on±off valves, and avoided the impact of weather conditions. As a result, both
the reliability and robustness of the design were significantly improved. The option
to minimize switches on open and closed condition of block valves is still preferred
for nonfouling and low-temperature applications.
Valves ± particularly block valves ± might be subject to sticking by fouling or any
other cause, such as a malfunctioning solenoid. These valves currently will still need
some attention from maintenance. Control valves in general have a low total failure
rate, but are subject to wear that results in hysteresis and pulsed displacements.
These problems need to be monitored, and signal analysis of flow or pressure meas-
urements on the same line can identify this problem to be used for predictive main-
tenance. The selection of type and size of control valves deserve more attention to
achieve good operational performance over its required operational range
Analysis and analyzers have been the subject of breathtaking development. During
the 1960s, nearly all analysis were performed at the laboratory, using wet chemical
analysis. This required samples to be taken by operators and delivered at the labora-
tory ± this had already introduced errors. Next to its time-consuming analytical
effort, this approach was subject to human errors and the low accuracy of analytical
procedures. During the 1970s, automated physical laboratory measurements were
introduced, such as gas chromatography (GC) and infra-red (IR) meters, while dur-
ing the 1980s and 1990s the development and in-plant installation of process analy-
zers was a major factor. The analyzers went through a development cycle to improve
the accuracy and reliability, and the interpretation of results was greatly improved by
the installation of microprocessors. In particular, GC was handicapped by the batch
analytical technique which required sample taking, handling, and injection of very
small quantities (a few l l), and this renders the system vulnerable to failures. In
contrast, the large response time (which was a major handicap for control applica-
tions) was greatly reduced by the use of capillary columns rather than packed col-
umns. The process analyzers were also provided with auto-check, auto calibrate
functionality and fault diagnosis. Process analyzers achieved accuracy levels that