Page 91 - Boiler plant and distribution system optimization manual
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76 Boiler Plant and Distribution System Optimization Manual
1. An ignition source could be introduced into SAFETY INTERLOCKS
a furnace that contains air and accumulated
fuel vapors that could become an explosive • Automatic start up sequencing has been
mixture. designed to insure fuel air and combustion
occur under controlled and safe conditions.
2. Fuel is discharged into the furnace during
Each boiler must be equipped with an ap-
start up without proper ignition taking place.
proved safety system which has the capa-
3. The burner flame is extinguished during bility to shut it down in the event an unsafe
normal operation without the fuel supply condition develops during start up. This
being shut off. system coordinates with safety interlocks for
4. A major malfunction in the burner or feed- shutdown if an unsafe condition is detected.
water systems. Shutdown is accomplished by stopping the
start up sequence and closing the Safety Shut
5. Variations exist in fuel pressure which could Off Valves (SSV).
cause flame instability.
• Loss of flame interlock; if there is no flame in
Purging Combustible Gasses on Light-off the combustion zone, the boiler must be shut
The start-up condition presents perhaps the down immediately to prevent the accumu-
greatest danger from explosion. It is essential that lation of combustible vapors, which when
the furnace be purged of all combustible gases conditions are right, could cause a powerful
before any source of ignition is introduced and explosion. Pilot flame ignition, main flame
that once fuel flow is initiated, ignition take place ignition and main flame monitoring are all
quickly. important to the prevention of combustibles
Purging the furnace is usually accomplished build up. The Safety Shut Off Valves (SSV)
with the combustion air fan. A purging air timer close to stop fuel supply to burners when the
does not allow ignition until the fan has been op- flame scanners do not sense a flame at the
erated for a specific period of time. For watertube burner.
boilers the purge airflow must be sufficient for at
least eight air changes to occur. • Purge interlock; the combustion products
For firetube boilers, the purge is conducted and possible accumulations of explosive fuel
with wide open dampers and four air changes is vapors must be purged from the boiler be-
normally required. In either case, air flow is ver- fore each light off and after each shut down.
ified by providing limit switches on the dampers
and a pressure switch at the fan discharge, or by • Low air interlock; if air supply is less than
providing airflow measurement devices such as required for safe combustion, the boiler must
differential pressure switches. be shut down to prevent an accumulation of
Once fuel has been initiated to the pilot a fuel rich and possibly explosive mixture in
flame or the main burner, flame verification is the boiler.
usually achieved by means of a flame scanning
device. These flame scanners may be either the • Low fuel supply interlock; loss of fuel de-
infrared flicker type or the ultraviolet type. livery can cause flame instability, erratic
Recently manufacturers of flame safety combustion, poor atomization of some fuel
control components have introduced integrated oil equipment and the possibility of an ex-
flame safety systems based on microcomputer plosion. If low fuel pressure is detected the
technology with self diagnostic features. They boiler is shut down.
are programmed to perform boiler start up and
shutdown sequences, alarm communication and • Low water interlock; The combustion flame
perform energy conservation functions. may be more than 3,000 F which is above the
