Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A SACRIFICIAL REFRACTORY SHIELD ASSEMBLY FOR USE ON A
BOILER TUBE
FIELD OF THE INVENTION
The invention relates to a refractory shield assembly
for protecting a component, such as a boiler tube or
structural member, from an abrasive erosive gas stream
carrying particulates.
BACKGROUND OF THE INVENTION
It is well understood that a boiler operates at high
temperatures and components within the boiler must be able
to withstand extreme conditions that exist therein.
One such component is a stainless steel boiler tube
through which water and/or steam flows.
Hot combustion gases carrying abrasive particulates,
such as fly ash, flow over the tubes. The particulates
will abrade and erode the tubes. As a result, the tubes
will have a short life without protective measures.
It is conventional to protect a boiler tube in the
path of the gas stream with a stack of sacrificial
"shields". These shields are semi-circular, elongate,
stainless steel members that are each secured to the tube
by u-shaped clamps that extend around the back of the tube
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and are welded to the side edges of the shields. It is not
uncommon for as many as three shields to be stacked and
separately clamped on a boiler tube with the objective of
protecting the tube for about a two year or longer period.
It is typical practice to shut down and open the
boiler for inspection and maintenance about every 1 to 1~
years. Typically, one will find on the first shut-down
that one or two of the shields will have been so eroded
that they have fallen off or need to be replaced. This is
then done and the boiler is placed back in service. On the
second shut-down, all of the shields on the tube axe
usually removed and a new stack is applied.
During a boiler shut-down, this replacement of the
shields can be the operation that determines the length of
the shut-down. Removing and replacing the shields involves
considerable welding and consumption of welding manpower.
As a consequence, there has long existed a need to
develop a better sacrificial shield which has extended
durability. It is the objective of this invention to
address this need.
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SUMMARY OF THE INVENTION
In accordance with the invention, a refractory shield
assembly is pzovided for use in protecting a component from
a gas stream carrying abrasive particulates. In the
particular case of a boiler tube, the assembly comprises:
~ an elongate, usually semi-circular, metal shield
adapted t:o overlie the boiler tube;
~ the shie:)_d having a plurality of outwardly protruding,
spaced apart "anchors" welded or otherwise secured to
that portion of the shield's outer surface facing the
gas stre~~m; and
~ the shield further having a layer of an abrasion -
resistant refractory material overlying all or part of
the outer surface, which layer is held in place by the
anchors.
In a preferred feature, the refractory material, which
for example may be aluminium oxide silica, has been pre-
treated, while in place on the shield, by progressively
heating it, f:or example in an oven, to remove moisture with
minimal surface cracking of the layer's front face.
In another preferred feature, the refractory material
covers only a central strip of the shield's outer surface,
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so as to leave bare marginal side areas for welding the
clamp ends thereto. Yet the refractory layer extends
through a sufficient arc (say 120° - 150°) so as to fully
shield or protect the underlying shield from direct contact
by the gas stream.
In still another preferred feature, the anchors are
spot or tack welded at points along their lengths to the
shield so as to reduce the likelihood of separation due to
differential thermal expansion and contraction.
From the foregoing it will be understood that a
feature of the invention is applying a layer of abrasion -
resistant refractory material to a protective sacrificial
shield and securing it in place by means of protruding
anchors. This concept finds particular application in the
case of a boiler tube in the path of erosive gas. However,
it is contemplated to be also applicable to other
components, such as structural support I-beams, which are
also exposed to such gas streams.
Another feature is that the refractory shield assembly
lends itself to being fabricated off-site prior to the
shut-down and then brought to the boiler in a state ready
for securement to the component to be protected.
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In one embodiment, a refractory shield assembly is
provided for protecting a component, such as a boiler tube,
against abrasion and erosion by a stream of hot gas
containing particulates, comprising: an elongate metal
5 shield, preferably configured and dimensioned so as to
conform with the component, said shield having an outer
surface; a plurality of spaced apart anchors attached to
the outer sur~ace so as to protrude therefrom; and a layer
of an abrasion-resistant refractory material overlying at
least part of the shield's outer surface and extending
between the anchors, so that the refractory material is
secured in place on the shield by the anchors.
In another embodiment, a process is provided for
manufacturing a refractory shield assembly for use in
protecting a boiler tube from abrasion and erosion by a
stream of hot gas containing particulates, comprising:
providing a semi-circular, elongate, metal shield; welding
a plurality of outwardly protruding, spaced apart anchors
to a portion of the shield's outer surface that will face
the gas stream; applying a layer of refractory material so
as to overlie the shield's outer surface portion, the layer
engaging the anchors so as to be secured thereby to the
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shield, whereby a unitary assembly is produced; and heating
the unitary assembly at progressively increasing
temperatures to dry the refractory material, prior to
installation on the boiler tube.
DESCRIPTION OF THE DRAWINGS
The invention will now be further described by way of
examples only and with reference to the following Figures
wherein:
Figure 1 is a schematic, fanciful representation of a
boiler showing a boiler tube and a stream of hot combustion
gas flowing through the boiler chamber;
Figure 2 is a perspective view in accordance with the
prior art, showing a stack of shields, each clamped to a
section of the boiler tube;
Figure 3 is a transverse sectional view of the
assembly of Figure 2, taken along line A-A of Figure 2;
Figure 4 is a perspective view of a section of boiler
tube supporting a refractory shield assembly in accordance
with this invention, the assembly comprising a shield,
anchors and a. layer of overlying abrasion-resistant
refractory material;
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Figure 5 is a transverse sectional view of a section
of boiler tube to which is secured a refractory shield
assembly, in which the refractory layer only overlies a
central strip of the shield's outer surface which is in the
path of the ga.s stream;
Figure 6 is a transverse sectional view of a section
of boiler tube having an alternative version of the
refractory sh~.eld assembly secured thereto, showing the
layer of refractory material overlying substantially all of
that portion of the shield's outer surface that would lie
in the path of the oncoming gas stream;
Figure 7 is a side view of the shield showing a
plurality of anchors secured thereto;
Figure 8 is a schematic, fanciful representation of a
IS boiler showing a refractory shield assembly of this
invention secured to the underside of the boiler tube that
is exposed to the stream of combustion gas flowing from
below;
Figure 9 is a perspective view of a boiler tube having
the refractory shield assembly secured thereto by tack
welds and showing the refractory material extending between
the anchors; and
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Figure 10 is a magnified view of the section of Figure
9 within the circle X.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Having reference to Figures 1 and 8, there is shown a
fanciful representation of a conventional boiler 1 having a
boiler tube 2 mounted within its chamber 3. Hot combustion
gas 4, produced by a burner 6 and containing abrasive
particulates, such as fly ash, is produced or introduced
into the base 5 of the boiler chamber 6. The hot
combustion gas flows through the chamber 3 in the form of a
stream 7 indicated by the dashed arrows. The stream 7 of
hot gas passes over the boiler tube 2 and exits the boiler
1 through an outlet 8.
Figures 2 and 3 show a sacrificial shield assembly 9
in accordance with the prior art. The assembly 9 comprises
a stack 10 of. semi-circular, elongate, stainless steel
shields 11 held in place on the boiler tube 2 by welded
clamps 12. The shields 11 are positioned in the path of
the gas stream 7, so as to shield or protect the boiler
tube 2.
Turning now to Figures 4 and 5, there is shown a
sacrificial refractory shield assembly 20 in accordance
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with this invention. Some of the parts of the assembly 20
conform or correspond with parts of the assembly 9 and will
be referred to by the same numerals.
More particularly, the refractory shield assembly 20
comprises an arched shield 11 having a plurality of spaced
apart anchors 21 spot welded or otherwise attached thereto.
The anchors 2:L protrude outwardly from that portion of the
shield's outer surface 22 which faces the oncoming gas
stream 7. The anchors 21 may take a variety of
configurations, but those shown in the Figures form
transverse apertures 23, for a purpose explained below.
A layer 24 of abrasion-resistant refractory material
25 overlies all or part of the shield outer surface 22, as
illustrated in Figures 4 and 5. The refractory material 25
extends between the anchors 21 and preferably extends
through the apertures 23 to engage the anchors.
Various abrasion-resistant refractory materials which
are commercially available can be used. We have used a so-
called "aluminium oxide silica" composition available from
Vesuvius USA under the registered trade-mark ACTCHEM.
This material comes in the form of a coarse powder.
It is mixed with water to form a thick paste having the
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consistency of plasticine. The paste is trowelled onto the
outer surface 22 of the shield 11 so as to extend between
the anchors 21 and to extrude through the apertures 23.
The material can then be firmly packed into place. The
5 resulting arcuate layer 24 overlies the shield's outer
surface 22 and engages the anchors 21. It is capable of
remaining in place on the shield 11 without being bonded to
the shield surface 22. The shield 11 and the applied
refractory material layer 24 secured in place thereon
10 combine to form a unitary assembly 26.
The refractory material 25 is moist. If used in the
boiler in tha~ state, the water will quickly evaporate and
leave cracks in the layer's outer surface 27. These cracks
may result in spalling and erosion by the gas flow.
I therefore pre-treat the assembly 26 by heating it
progressively in an oven at slowly increasing temperature,
to evaporate the contained moisture in a controlled manner,
with the objective of drying the material while minimizing
cracking, prior to installation on the boiler tube 2.
The refractory layer 24 may overlie the entire width
of the shield's arcuate outer surface 22, as shown in
Figures 4 and 6, or only a central strip thereof, as shown
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in Figure 5. In the former case, I have found that the
refractory shield assembly 20 may unduly restrict the
passage of the hot gas stream between adjacent boiler tubes
2. For this reason it is preferable to use the Figure 5
embodiment, wruerein the thick layer 24 extends through an
arc of between 120° - 150° but still shields the underlying
shield's outer surface 22. In addition, the shield 11 is
then left with marginal edge portions 27 which are
available for welding the semi-circular holding clamps 28
thereto.
As indicated, conventional holding clamps 28 are
positioned to extend around the back side of the boiler
tube 2 and are welded or otherwise attached to the shield
11, to secure the refractory shield assembly 20 in place on
the boiler tube, for the purpose of protecting the boiler
tube from abrasion and erosion by the gas stream.
