Armoring system for an airfoil centrifugal fan

An armoring system for use with an airfoil centrifugal fan for preventing erosion of the airfoil blades of the fan. The armoring system comprises an armor shield adapted to be releasably secured to at least a portion of the upper surface of the fan blades to prevent erosion thereof. The armoring system further includes mounting means cooperating with the armor shield for releasably securing the armor shield to the blades. The mounting means are positionable within the hollow space formed by the upper and lower surfaces of the blades to structurally reinforce the blades without altering their aerodynamic shape.

The present invention relates generally to systems for preventing erosion 
damage to power plant fans, and more particularly, to an armoring system 
for preventing erosion of the airfoil fan blades of an airfoil centrifugal 
fan. 
Coal-burning power plants are fueled by bituminous coals, subbituminous 
coals and lignites. Each of these fuels produce fly-ash particles which 
can severely erode fans used in a power plant. The susceptibility of a fan 
to fly-ash erosion damage is dependent upon, among other things, the 
manner in which the fan is used in the plant. A typical coal-burning plant 
utilizes a number of fans for various applications. For instance, 
forced-draft fans supply combustion air and push hot gases through the 
coal-fired boiler. Primary air fans inject pulverized coal into the 
boiler's furnace compartment. Gas recirculating fans control the output of 
the boiler. Induced draft fans pull hot, dirty gases from the boiler, and 
booster fans push these gases through air pollution control equipment. The 
primary air and forced-draft fans see relatively clean outside air while 
the gas recirculating, booster, and induced draft fans are exposed to 
combustion gases produced in the boiler. The combustion gases are laden 
with fly-ash particles, and as expected, the fans exposed to these gases 
are the most susceptible to the erosive action of fly-ash. 
As measured by the cost of repairs of fly-ash erosion damage and the cost 
of purchasing replacement power while a plant is shut down for repairs, 
induced-draft fans are the most troublesome for the utility industry. 
Three different types of induced draft fans are commonly used: the 
variable pitch axial fan, the radial tip centrifugal fan, and the airfoil 
centrifugal fan. The airfoil centrifugal fan, to which the present 
invention is directed, comprises a plurality airfoil blades having upper 
and lower surfaces defining an inner, void space therebetween. The airfoil 
blades over which the incoming gases pass are particularly subject to 
attack by the fly-ash particles entrained in the gases. 
An approach used heretofore for reducing the erosion of airfoil blades 
involves the use of wear plates, which are simply steel deck plates, 
welded to the exterior surface of each blade. The fly-ash particles thus 
erode the wear plates rather than the fan blades; accordingly, damage to 
the fan is reduced. However, this approach is undesirable for a number of 
reasons. First, the additional weight added to the fan blade by the wear 
plate increases the centrifugal force imposed on the blade during 
operation of the fan, which significantly increases the likelihood of 
blade failure. Additionally, when the wear plates have to be replaced, it 
is a very time-consuming and expensive procedure to remove them as they 
are welded to the blades. 
The utility industry has also experimented with various metalizing 
processes wherein the wear plates are coated with an erosion-resistant 
coating such as tungsten carbide, a chrome plating or a high chromium iron 
weld overlay. Although this technique is an improvement over the use of 
steel deck plates, it suffers from the same drawbacks in that the wear 
plates carrying the erosion-resistant coating are welded to the exterior 
surfaces of the blades. Thus, the repair costs are significant, if not 
prohibitive, and the possibility of structural failure of the blades is 
enhanced. 
The present invention eliminates the major disadvantages associated with 
the above-described erosion-preventive techniques. With the armoring 
system of the present invention, the armor shield is not welded to the 
airfoil blades but rather is removably secured thereto so that the shield 
may be removed without incurring exorbitant repair costs. Additionally, 
the means by which the armor shield of the present invention is secured to 
the airfoil blades structurally reinforces the blades so that the blades 
are able to withstand the additional centrifugal force imposed thereon by 
the weight of the armor shield. Further, the aerodynamic shape of the 
blades and thus the efficiency of the fan is not effected by the armoring 
system of the present invention. 
Accordingly, an object of the present invention is to provide an improved 
armoring system for reducing erosion damage to the airfoil blades of an 
airfoil centrifugal fan. 
A more specific object of the present invention is to provide an improved 
armoring system wherein an armor shield is releasably mounted on the 
airfoil fan blades in a manner which structurally reinforces the blades 
but which does not effect the aerodynamic shape of the blades. 
According to the present invention, an improved armoring system for use 
with an airfoil centrifugal fan for preventing erosion of the blades 
thereof is provided. The armoring system includes an armor shield which is 
adapted to be releasably mounted to the exterior of each blade about at 
least a portion of the upper surface thereof to prevent erosion of the 
blade. A mounting means is adapted to cooperate with the armor shield for 
releasably mounting the shield to the blade. The mounting means is 
positionable within the interior void space defined by the upper and lower 
surfaces of the blade so as to maintain the aerodynamic shape of the 
blade. The mounting means also structurally reinforces the blade so that 
the blade is able to resist the centrifugal force imposed thereon. 
The mounting means may comprise a plurality of support columns spaced about 
the interior, void space defined by the upper and lower surfaces of the 
blade to extend therebetween and to be affixed thereto. A threaded member 
may extend through respective holes in the armor shield and the exterior 
surfaces of the blade and into the support column for releasably mounting 
the shield on the blade. A means for reducing side loading on the threaded 
member due to an axial or radial offset between the holes in which the 
threaded member is positioned may also be provided. Additionally, means 
for disengagably engaging the threaded member may be provided to secure 
the threaded member in the support column to releasably secure the armor 
shield to the blade.

Referring now to the drawings, attention is first directed to FIG. 1 which 
shows an airfoil centrifugal fan 10 having airfoil blades 12 joined on one 
side thereof to a fan centerplate 14 which in turn is attached to a fan 
hub 17. For purposes of the present discussion, it will be assumed that 
fan 10 is to be used as an induced-draft fan in a coal-burning power plant 
(see FIG. 2). As discussed hereinabove, induced-draft fans are most prone 
to damage by gas-entrained fly-ash particles as these fans are used to 
pull hot, dirty gases from the coal-fired boiler. Airfoil centrifugal fans 
are commonly used at such locations as they have a very high resistance to 
fly-ash erosion damage, at least so long as the fly-ash particles do not 
penetrate the surfaces of the blades. The armoring system of the present 
invention may be used with any airfoil centrifugal fan regardless of the 
fan's particular application. The present invention, however, is 
particularly suited for use with those airfoil centrifugal fans used in 
coal-fired power plants wherein the fans are exposed to extensive 
bombardment by fly-ash particles. For example, in addition to 
induced-draft fans, the present invention may be used with airfoil 
centrifugal fans which function as gas recirculating fans for controlling 
boiler output (see FIG. 2). 
The gas flow through fan 10 is illustrated by arrows G.sub.i (gas in) and 
G.sub.o (gas out). The path of the gas flow is across blades 12 from the 
nose or leading edge 12a thereof to the trailing edge 12b. As shown in 
FIG. 3, each blade 12 comprises an upper or pressure surface 12c and a 
lower or suction surface 12d, defining an interior, void space 19 
therebetween. The blade includes a rib or support member 18 extending 
between the upper and lower surfaces. The fly-ash particles carried by gas 
stream G.sub.i -G.sub.o will impinge on airfoil blades 12 and other parts 
of fan 10 to cause erosion damage unless an appropriate 
erosion-preventative system is incorporated in the fan. 
The degree of damage to the blades is dependent upon the angle, velocity, 
distribution, chemical properties, and size of the fly-ash particles 
bombarding the blades. The greatest erosion damage can be expected to 
occur at the leading edge of the blade near the junction of the blade and 
the fan centerplate. However, the other surfaces of the blade are also 
subject to attack, and if the gas stream G.sub.i -G.sub.o contains a 
significant concentration of relatively large particles, that is, 
particles larger than 200 microns, an extensive amount of erosion damage 
can be expected along upper surface 12c from leading edge 12a to trailing 
edge 12b. Accordingly, it is imperative the blade surfaces be protected 
from the erosion fly-ash particles. 
The armoring system of the present invention is illustrated in detail in 
FIGS. 3-5. In the particular embodiment shown, an armor shield 16 covers 
the entire upper surface of each blade 12 and extends around leading edge 
12a to cover a portion of the blade's lower surface 12d. If the chord 
length of blade 12 is represented by line A--A, then as shown in FIG. 3, 
the armor shield covers 20% of the median arc length on lower surface 12d. 
Of course, armor shield 16 may be adapted to cover less than the entire 
upper surface 12c of the blade, or to cover more or less than 20% of the 
median arc length on lower surface 12d. In this respect, FIG. 3 is marked 
with various median arc length percentages on both the blade's upper and 
lower surfaces to indicate additional armor sizes. An armor shield 
extending from 10% on the lower surface of the blade to 10% on the upper 
surface of the blade would be ideal for protecting a blade where most of 
the erosion damage is expected to occur at the blade's leading edge. 
The particular embodiment shown in FIGS. 3 and 4 provides the most 
protection; however, the weight added to the blade by the armor shield is 
a major disadvantage as it increases the centrifugal force imposed on the 
blade, which increases the likelihood of blade fatigue. It if it assumed 
that armor shield 16 is a 0.125 inch thick stainless steel plate and that 
the diameter of the fan wheel is 154 inches, the weight added to each 
blade increases from approximately 17 lbs. when 10% of the blade's upper 
surface is covered to approximately 90 lbs. when the entire upper surface 
is covered. The additional centrifugal force imposed on each blade at 100 
rpm increases from 27,000 lbs. for the 10% coverage to 152,000 lbs. for 
the 100% coverage--a substantial increase in the force which the blade 
must withstand. 
Armor shield 16 is releasably secured to each blade 12 by a mounting means 
generally indicated at 20. Mounting means 20 is adapted to structurally 
reinforce the blade so that the blade is able to resist the additional 
centrifugal force imposed thereon by the weight of the armor shield. The 
mounting means secures the armor shield to the blade in a manner which 
does not alter the aerodynamic shape of the blade and which accommodates 
the varying profile (from leading to trailing edge) of the blade. As can 
be seen from FIG. 4, a plurality of mounting means 20 are used to 
removably secure armor shield 16 to upper surface 12c of blade 12. 
Additional mounting means, such as illustrated in FIG. 3, releasably 
secure the armor shield to lower surface 12d. 
Mounting means 20 is shown in greater detail in FIG. 5. In the preferred 
embodiment, the mounting means comprises a support column or structural 
stiffener 22, preferably cylindrical in shape, having a counterbore or 
recessed area 24 formed therein. The cylindrical support column is located 
in interior void, space 19 formed by the upper and lower surfaces of the 
blade to extend therebetween. The support column may be welded or 
otherwise appropriately affixed at its respective ends to the upper and 
lower surfaces of the blade. As shown in FIG. 3, support column 22 is 
located within countersunk holes 37 and 38 formed, respectively, in the 
upper and lower surfaces of the blade. A weld 39 secures support column 22 
to the upper and lower blade surfaces. The weld forms a seal between 
support column 22 and the respective countersunk holes to prevent fly-ash 
particles from entering void space 19. In this manner, support column 22 
structurally stiffens the blade to support the centrifugal force imposed 
thereon by the blade's weight and by the weight of the armor shield. As 
each blade is structurally reinforced, the structural integrity of the fan 
is enhanced. 
Mounting means 20 further includes a flat head screw or bolt 26 having 
threads 26a formed at one end thereof. A countersunk head 26b is formed at 
the other end of threaded member 26. Threaded member 26 is adapted to 
extend through a countersunk hole 28 in shield 16 and a respective hole 30 
in upper surface 12c of blade 12. Countersunk head 26b seats within 
countersunk hole 28 with the threaded member extending into recessed area 
24 of support column 22. Where armor shield 16 is attached to the blade's 
lower surface, holes like that of hole 30 in upper surface 12c are formed 
in lower surface 12d for securing the armor shield to that surface. 
The mounting means further includes spherical washers 32a and 32b located 
within recessed area 24 to engage threaded member 26. The spherical 
washers are used to accommodate any axial or radial offset between mated 
holes 28 and 30. The spherical washers assure that countersunk head 26b of 
threaded member 26 is in complete contact with countersunk hole 28 and 
that the threaded member is stressed in pure tension with no side loading 
imposed thereon because of any offset between holes 28 and 30. The 
spherical washers eliminate the need for a close tolerance in the location 
of hole 28 with respect to its mated or respective hole 30. Additionally, 
the spherical washers do away with the necessity of having to plug old 
holes and redrill the blade surfaces each time the armor shield is 
replaced. Also located within recessed area 24 of support column 22 is a 
threaded locking arrangement for engaging threaded member 26. The threaded 
locking arrangement comprises a hexagonal nut 34 for engaging the threaded 
end of the threaded member to secure the threaded member in the support 
column thereby releasably securing armor shield 16 to the exterior surface 
of the blade. A lock nut 36 is also provided to releasably lock hexagonal 
nut 34 to threaded member 26. 
Armor shield 16 may have a number of different configurations. For example, 
as noted above, armor shield 16 may comprise a 0.125 inch stainless steel 
plate. Armor shield 16 may also comprise a 0.0625 inch steel substrate 
electroplated with a hard chrome to a thickness of 0.030 of an inch. Yet 
another configuration for armor plate 16 would be the use of a high 
chromium iron weld overlay on a 0.0625 inch steel substrate. The hard 
chrome and high chromium iron overlay are both erosion-resistant coatings 
which are more durable and hence have a longer life than an uncoated steel 
plate. 
A hard facing 40, such as chrome-carbide weld metal, may also be applied to 
the outer surface of armor plate 16 to provide additional erosion 
protection. To remove armor plate 16 from blade 12, hard facing 40 is 
burned off in the area of countersunk head 26b and each threaded member 26 
is unscrewed and removed from support column 22. The armoring system of 
the present invention thus may be replaced relatively quickly, reducing 
the repair costs associated with an armoring system for preventing erosion 
damage to airfoil centrifugal fan blades. 
As can be seen from FIG. 4, a plurality of mounting means 20 are used to 
releasably secure the protective armor shield to the exterior surfaces of 
the blade. The mounting means are spaced about the interior, void space 19 
defined by the upper and lower surfaces of the blade in the region of the 
blade where armor shield 16 is secured. Location of mounting means 20 is 
critical, as the centrifugal force imposed on the armor shield is 
transmitted to the blade through the mounting means. An area of high 
stress concentration has been found to exist in the corner of the blade 
near the junction of leading edge 12a and centerplate 14. This area is 
shown bounded by dotted lines in FIG. 4 and is designated with the letter 
"B". To prevent failure of the blade, it is very important that area "B" 
not be drilled or have a mounting member located therein. 
Referring back to FIGS. 1 and 4, an additional feature which can be 
incorporated into the armoring system of the present invention is a 
horseshoe-shaped stainless steel plate 50 for preventing erosion damage to 
fan centerplate 14. Plate 50 is welded to armor shield 16. Plate 50 does 
not have to be directly attached to centerplate 14. 
A sealant, which is indicated generally at 60, may also be incorporated 
into the system of the present invention. Sealant 60, see FIG. 5, is 
applied between the edge of armor shield 16 and the exterior surfaces of 
the blade to prevent fly-ash particles from entering any gaps 
therebetween. A silicone sealant may be used, and it can be applied with a 
caulking gun. 
Although the invention has been described with reference to a specific 
embodiment, the description is illustrative of the invention and is not to 
be considered as limiting the invention in any way. Various modifications 
and applications may occur to those skilled in the art without departing 
from the true spirit and scope of the invention as defined by the appended 
claims.