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In early fluid processing plants, controlling the process frequently required many operators.
They continuously circulated around each process unit, observing locally mounted instruments
and manipulating the valves. Overall plant operations would often require operators to ‘tour
the plant’ with a clipboard, manually recording key parameters. At the end of the first pass,
following appropriate calculations, the operator would make a second trip to adjust controls.
Technology improved, and the ability to transmit pneumatic signals became a reality, so the
control room came into being at larger plants. Operators began moving the large indicating
instruments to one location, along with some controls that transmitted signals back to valves
in the plant. Operators could now record their readings in logbooks and make adjustments in
the operating process without touring the plant as frequently. They still had to tour the plant
to adjust the more distant valves, dampers, and other end elements.

By Samuel Herb

In early fluid processing plants, controlling the process frequently required many operators.
They continuously circulated around each process unit, observing locally mounted instruments
and manipulating the valves. Overall plant operations would often require operators to ‘tour
the plant’ with a clipboard, manually recording key parameters. At the end of the first pass,
following appropriate calculations, the operator would make a second trip to adjust controls.
Technology improved, and the ability to transmit pneumatic signals became a reality, so the
control room came into being at larger plants. Operators began moving the large indicating
instruments to one location, along with some controls that transmitted signals back to valves
in the plant. Operators could now record their readings in logbooks and make adjustments in
the operating process without touring the plant as frequently. They still had to tour the plant
to adjust the more distant valves, dampers, and other end elements.
Following World War II, electronic controls became more rugged, and new types of sensors
measured parameters not previously measurable. Controllers got smaller, so more of them
could fit on a panel. Further, as computers became less expensive, they became more
common in large facilities. The centralized control room became increasingly common and
steadily more complex.
Video technologies, with their ability to display data or allow the operator to initiate control
actions, made possible the onset of distributed control. Thus, the central control room could
provide centralized information without having all the processing in one location; this
distributed the overall risk to the control system while reducing the cost and complexity of
wiring.
Now operators no longer have to tour the plant. They can literally ‘let their fingers do the
walking’ as they call up each controller or group of controllers on their screens to check the
progress of their process. If necessary, they can easily make set point and output changes
from their keyboard, as well as respond to any alarms if a process is ‘off normal.’
Distributed control systems distribute risk by dispersing control throughout the plant: A
problem at location X needn’t ruin operations at location Y. Further, plants receive data and
act on it for virtually all practical purposes in real time. High-quality operator interfaces enable
a knowledgeable operator to know what’s going on in his plant with little more effort than a
glance.
DCSs have some noteworthy weaknesses, though. First, they are expensive to purchase and
install. What’s more, the operating system and communications protocols are proprietary. The
operator must select wisely lest, as Ben Franklin liked to say, he repent at leisure.

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