Page 299 - Design of Simple and Robust Process Plants

8.2 Instrumentation 285
8.2
Instrumentation

8.2.1
Instruments

Instruments are the eyes, ears, and hands of the process. During the 1980s, the fol-
lowing striking points were identified:

. 50% of our instruments lie, or die
. Instruments during start-up and restart are unreliable
. Most process outages are caused by instrument nuisance.
It is clear that hands-off operation of a simple and robust designed process, cannot
be achieved under these conditions. It was recognized that the above statements
were an exaggeration of the situation, but the message was clear. The demand on
instruments was increasing in order to achieve a high level of automation and con-
trol, on the route to hands-off operation. The causes of these bad experiences with
instrument performance were multiple, and causes for instrument failures can be
classified as:
. Instrument failure
. Selection of instruments
. Installation of instruments
. Interpretation of signals
. Maintenance of instruments
Although failures will always happen in these classes, it was the number of failures
which needed to be severely reduced.
In order to obtain a more defined insight into the number and type of failures,
these were monitored for several processes ± batch as well as continuous. The
instruments were monitored over a two-and-a-half-year period during the late 1990s.
The instruments tracked were flow, level, pressure, temperature, and valves ± these
being the core of process plant operations. It should be noted that the measure-
ments were installed during the 1970s and 1980s, and by far the majority were
installed to monitor:

. Flow, orifices with a dP cell
. Level, dP cells with lead lines
. Pressure, transducer
. Temperature, a mixture of thermocouples and resistance temperature meas-
urements (PT100)
. Valves, several types of block and control valves
A failure was registered every time an instrument technician was called for any sup-
posed malfunction.