Patent Application
20060137865
The invention related 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.
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 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 1½ 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 are 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.
In accordance with the invention, a refractory shield assembly is provided for use in protecting a component from a gas stream carrying abrasive particulates. In the particular case of a boiler tube, the assembly comprises:
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, for 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, 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.
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 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 surface 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 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.
The invention will now be further described by way of examples only and with reference to the following Figures wherein:
Having reference to
Turning now to
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 21 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
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 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 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 combine to form a unitary assembly 26.
The refractory material 25 is moist. If used in the boiler in that 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
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.
