UFC 3 -430-11
14 February 2001
3-5.3
Interlock and Safety Requirements. Safety controls are needed to protect
against boiler explosions and implosions. Interlock and other safety requirements for
boilers are given in the following American Society of Mec hanical Engineers (ASME)
and National Fire Protection Association (NFPA) standards. Where local or state code
requirements differ from the ASME and NFPA codes, the more stringent code
requirements must prevail.
ASME CSD-1
Controls and Safety Devices for Automatically Fired
Boilers
NFPA-8501
Standard for Single Burner Boiler Operation
NFPA-8502
Standard for the Prevention of Furnace
Explosions/Implosions in Multiple Burner Boilers
NFPA-8503
Standard for Pulverized Fuel Systems
3-5.3.1 Safety Interlocks. Safety interlocks are required but not limited to those
listed below. Additional safety interlocks may be required to fulfill applicable state and
local codes for the boiler fuel type used.
Prepurge Interlock Prevents fuel from being admitted to a furnace until
the furnace has been thoroughly air-purged to remove residual
combustibles.
High Steam Pressure Interlock Fuel is shutoff upon abnormally high boiler
steam pressure.
Low Airflow Interlock Fuel is shutoff upon loss of a irflow from the
combustion air fan or blower, the induced draft fan or the forced draft
fan.
Low Fuel Supply Interlock--Fuel is shut off upon loss of fuel supply
pressure. The loss of fuel supply pressure would result in unstable
flame conditions.
Loss of Flame Interlock All fuel is shut off upon loss of flame in the
furnace or to an individual burner in the furnace.
Fan Interlock Stop forced draft upon loss of induced draft fan.
Low Water Interlock Shut off fuel on low water level in boiler drum.
High Combustibles Interlock (optional) Shut off fuel on highly combustible
content in the flue gases.
Post Purge Interlock Continues fan operations to remove residual
combustibles from the furnace prior to shutting down the fans.
3-5.3.2 Shutdown Valve Reset. Safety shutoff valves must be a manual reset type
and shall not be operable until all limit controls are in the safe position. Sequence
interlocks for start -up and stop must also be provided. Provide annunciator alarms to
respond to any safety shutdown.
3-5.3.3 Dedicated Safety System. The safety shutdown system must be dedicated
and independent of the one used for boiler control. For example, if PLC controllers are
3-2
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