Page 226 - Design of Simple and Robust Process Plants

5.7.8 Instruments 211
. Avoid multi-functional debates about what is required ± each discipline will
want to add instruments that are ªnice to haveº, but not essential for opera-
tion.
. Select only reliable and robust instruments to fulfill these requirements
. Avoid the use of ªlocalº instruments. Simple and robust processes have lim-
ited field operators. When the local instruments are needed, they often fail or
cannot be trusted. Local instrument connections for analyzing operational
status at initial start-up and maintenance can be sufficient.
. Avoid instruments with long response times and sample loops. Direct, in-
line measurements are essential for robustness and adequate control. Long
sample lines and lead lines are a source for fouling, and will only cause prob-
lems.
. Avoid duplicate measurements, but if these are required for safety or environ-
mental reasons, then install two analog measurements. It is easy for opera-
tion and fault detection software to verify which instrument has failed in case
of two analog elements.
. Triple instruments are a nightmare and essentially the wrong direction to
take. It is better to invest in reliable and robust instruments than in triple
redundancy. In specific cases, for instrumental safety protection, there may
be no alternative, but always search for better solutions. In the ªoldº days, it
was essential for airplanes to have four engines to fly over the ocean, but
nowadays two are sufficient, and the reliability level is the same.
. Avoid manually operated by-passes around control and block valves; a hands-
off operated process should not be operated with manual operated bypasses.
. Avoid too many limit switches to verify a process action. For nonfouling sys-
tems, block valves should be reliable. Limit switches on these valves add to
the unreliability, and are a major cause of nuisance trips. An emergency
block valve needs a switch to confirm its emergency action, and not necessar-
ily its stand-by condition. When limit switches are applied they should have a
much higher reliability than the valves to be watched.
. The ratio between analog inputs and outputs (AI/AO) and digital inputs and
outputs (DI/DO) should be used as a yardstick to compare instrument
designs. For continuous processes, an AI/AO a ratio of 1 is the ultimate mini-
mum, but in practice a value of 3 needs to be achieved.

Although simplification is possible by following these guidelines, it is infinitely bet-
ter to use only single reliable and robust instruments, which are correctly selected,
installed and calibrated.

Summary
Simplification and its ranking with regard to increasing levels of complexity have
been discussed for several systems and unit operations, including: