UFC 3 -430-11
14 February 2001
CHAPTER 2
TYPES OF CONTROLS
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GENERAL . Control systems may be pneumatic, analog, digital, or a
combination of the three. Older designs utilized pneumatic control for local control
loops. Analog control systems were an industry standard for a short time before quickly
being replaced by digital control systems. The present industry standard is for a
distributed control system (DCS), a programmable logic controller (PLC), or a direct
digital controller (DDC) to provide digital control logic based on information gathered
from electronic sensor inputs and responding with electronic control of pneumatic or
electric powered valves and dampers.
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CONTROL SELECTION . Choose the type of control that will do the job most
economically. This includes total cost over the lifetime of the equipment. Also consider
compatibility with controls used in the existing plant, ease of operation and
maintenance, and plant personnel familiarity and training. Consider unique situations
such as a high EMF (electric magnetic field) where either EMF shielding, fiber optic data
transmission, or pneumatic controls are required. Obtaining the latest technology
should not be used as the main criteria in the selection of controls.
Use the following as a guideline in selecting the type of control.
Microprocessor digital type of controls (e.g. PLC, DDC, or DCS) should be
used for most new designs and in particular where a large number of
control loops are involved. Consideration should also be given to using
microprocessor digital type controls to replace pneumatic control systems
that have exceeded their life expectancy.
Pneumatic control devices should be used in hazardous areas. They may
also be used in the expansion of existing designs or as actuators for the
final control elements in the design. In general, the pneumatic actuation of
larger valves and large number of valves is more cost effective than using
electric actuation.
Analog logic devices (e.g. pneumatic receiver -controllers or electric
potentiometer controllers) should not be considered for new designs.
Consider the environmental conditions (e.g. temperature and humidity) in
which the controls will be installed.
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PNEUMATIC CHARACTERISTICS. Pneumatic characteristics are included
primarily for the pneumatic power requirements of valves and dampers. In most cases,
new designs should incorporate electronic control signals. Standard operating and
supply pressures for pneumatic instruments are defined by ANSI/ISA-S7.0.01, Quality
Standard for Instrument Air.
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Air Supply Pressure. Plant and instrument air is typically available at 6.21
bar (90 psig) pressure. Instruments typically operate from a 1.38 bar (20 psig) supply.
A standard pneumatic control signal is 0.21-1.03 bar (3-15 psig). Standard pressures
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