7/28/2019 Boiler Penthouse Ash Accumulation
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Boiler Penthouse Ash Accumulation
written by: Dr V T Sathyanathan edited by: Lamar Stonecypher
updated: 7/16/2010
Ash accumulation the boiler penthouse is a very clear indication
of deviation in operatingparameters of the boiler, ID fan
overloading due to air ingress in the system, and many other
reasons.
Ash accumulation in boiler penthouse can be caused both due to
operational deviations and dueto boiler performance deterioration
or maintenance. The main factor which results in ash
accumulation in penthouse is the positive pressure created at
the top of the boiler furnace. The
rate of ash accumulation depends on the amount of pressurization
and the furnace roof sealingeffectiveness. The upper furnace can
get pressurized if the furnace vacuum maintained is not
adequate. Normally it is a practice to maintain minus 10
millimeter of water column one meter
below the furnace roof. This ensures that the pressure just
below the furnace roof is negative
which means no ash can penetrate the roof sealing and find its
way to the penthouse.
The pressure in the boiler furnace just below the roof can
become positive when the induceddraft fan get loaded fully and more
of the combustion air is send to the furnace. The combustion
air requirement depends on the amount of fuel being burnt to
carry the load in the boiler. One
should not mistake this for the weight of fuel being sent to the
furnace, but it is the million
calories being supplied by the fuel. This can happen when the
boiler is being consciously overloaded due to any reason.
The induced draft fan can get fully loaded when in such boilers
the air ingress to the boilerincreases above a limit. A large
amount of air ingress in boilers happens at many locations,
starting from furnace to induced draft fan inlet. In furnace one
of the major areas is the bottomash hopper design and the sealing
system provided and maintained. The peep holes provided inthe
furnace at various locations to view the flame condition inside the
furnace is also a source of
air ingress if they are not properly closed.
In the other areas of second pass, economizer outlet flue gas
duct, air pre-heater, air pre-heater
flue gas outlet duct to electrostatic precipitator, the
electrostatic precipitator and the flue gas duct
from electrostatic precipitator outlet to induced draft fan are
locations prone for air ingress to the
boiler and over load the induced draft fan. The air ingress into
the boiler at any location can be
evaluated by doing oxygen measurement in the inlet and outlet of
the sections. The percentageair ingress can be worked out from the
equation (O
2outletO
2inlet) divided by (21O
2outlet).
It is normally seen that the expansion bellows given at various
locations are a cause of concern
for air ingress into the system.
Ash accumulation in the penthouse of the boiler can also be
caused from improper combustionleading to pressurization of the
furnace near the roof. It is seen that when the primary air flow
in
pulverized coal fired boiler is just near the settling velocity,
then it can lead to coal particles
