UFC 3 -430-11
14 February 2001
for diaphragm actuators are 1.03 and 2.07 bar (15 and 30 psig). Standard pressure for
piston operators is 6.21 bar (90 psig).
2-3.2
Design Considerations. Piston operators are used for dampers and control
valves. Size pneumatic devices operating on a nominal 6.21 bar (90 psig) air supply so
that they will operate with a minimum pressure of 4.14 bar (60 psig) and will withstand a
maximum pressure of 7.58 bar (110 psig).
2-3.3
Service Tubing Size. Use 9.53 millimeter (3/8 -inch) outside diameter (OD)
copper or stainless steel tubing for signal transmissions of 60.96 meter (200 feet) and
over. For shorter lines, use 6.35 millimeter (1/4 -inch) OD copper or stainless steel
tubing.
2-3.4
Control Distance. Pneumatic control can be used for up to 60.96 meter (200
feet) without any special provisions and up to 91.44 meter (300 feet) if the valve has a
positioner. A volume booster must be used for distances between 60.96 and 91.44
meters (200 and 300 feet) if the valve does not have a positioner. Use electronic
signals for valve control in lieu of pneumatic for distances greater than 91.44 meter (300
feet).
2-3.5
Instrument Air Regulation. Furnish pneumatic instruments requiring an
instrument air supply with individual combination filter -regulators and an output gauge.
Instrument air should be supplied by an oilless compressor to help eliminate control
instrument and device contamination.
2-4
ELECTRICAL CHARACTERISTICS.
2-4.1
Electrical Interface. The electrical interface between instruments and a
digital controller varies depending upon application. Typical analog signal ranges and
levels include the following:
mA direct current (DC) (4-20 mA, 10-50 mA, or 0 -100 mA)
volts DC (0-10 millivolts, 0 -100 millivolts, or 0 -5 volts)
temperature (thermocouple in millivolts, or RTD)
volts alternating current (AC) (120 volts)
Transmitters and control valves commonly use a range of 4 -20 mA DC.
Switches and solenoid control commonly use a switched level of 120 volts AC. For
signal requirements for instrument loops refer to ANSI/ISA-S50.1, Compatibility of
Analog Signals for Electronic Industrial Process Instruments.
2-4.2
Communication Data Bus. Complex boiler control systems may involve
multiple digital control systems (e.g. DCS, PLC, or computer controlled smart
instruments) linked together by a communication data bus. The communication data
bus passes significant data between the digital control systems in a serial format. There
are numerous bus architectures available. One commonly used is the RS -485
hardware bus with Modbus software protocol. Refer to TIA/EIA -485, Characteristics o f
2-2
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