UFC 3 -430-11
14 February 2001
For the expansion of existing controls within a plant use the existing
technology. This may be either digital, analog electronic, or pneumatic
controllers.
Use pneumatic controllers in hazardous areas.
Avoid use of pneumatic controllers in the control room.
When using digital control avoid depending on a single or a few control
devices for the entire plant without having a backup. A redundant
controller might not be required if only a single controller controls one loop.
However, if a single controller controls a large number of loops then
provide redundancy so that if the controller fails another controller will
automatically take over.
4-2.1.1 Process Controllers. Process controllers use one or several of the following
control modes:
On-off
Proportional
Integral (also called reset)
Derivative (also called rate)
Most digital controllers have all of the above control modes included. They
are also usually provided with anti -reset windup. Analog and p neumatic controllers
often do not include all three control modes or anti -reset windup.
4-2.1.1.1 Control Modes. In general, use the following control modes for the indicated
control loop.
Flow -- Use proportional plus integral.
Level -- Use proportional plus integral.
Pressure and Temperature -- Use proportional plus integral. Use
proportional plus integral plus rate when the application requires a quick
response time.
4-2.1.1.2 Testing. Recommended practices on tests to be conducted on digital
controllers are defined by ISA-RP55.1, Hardware Testing of Digital Process Computers.
4-2.1.1.3 Controls. Provide a separate control station for each control loop when
using digital controls. Locate critical controls on the front of the panel. Provide the
following minimum controls.
Automatic/manual selection
Set point adjustment
Output signal adjustment when on manual control
Alarm setting
4-2.1.1.4 Alarms. Provide the following minimum alarms.
4-3
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