By: M.H. Johnson, Barron Industries LLC
Centrifugal fans used in power generation applications are subjected to abrasive environments. After sufficient time in service, the wearing surfaces of fans require repair or replacement. In planning for and performing weld repairs to these wearing surfaces, there are several important considerations. First, the wear must be found, quantified, and an estimate made of the remaining useful life of the wearing surface. Then, the extent of the repairs and nature of repairs should be determined. Finally, a thorough repair plan should be made and a repair technique developed prior to starting the work. This article will discuss practical methods of working through these steps toward a successful repair.
In some situations, it is not difficult to identify the wear patterns and quantify the amount of material loss. Most of the time, however, a little detective work, testing, and trending are required to produce useful information.
It is necessary to establish a baseline for any wear trending effort to be meaningful. This is most easily done when the equipment is new. Most fan manufacturers will provide a drawing of the fan impeller showing overall dimensions, material types, and component thicknesses. Even if such information is not available from the OEM, or if the equipment has already seen many years of service, it is still possible to establish a baseline condition and trend the wear experienced by the fan.
Wear patterns vary widely, even in “sister” fans, since the wear pattern can be influenced by things such as ductwork configuration, damper settings, and dust distribution in the ductwork upstream of the fan, all of which are external to the fan. In Figure 1, several typical wear patterns can be seen. Visual observation of wear patterns recorded by sketches is one of the most useful records of wear history. Thicknesses can be measured by rulers, calipers, or ultrasonic thickness gauges. In areas where significant wear is observed (more than 0.030 inches/year), trending the wear rate helps predict the useful life of the wearing surface. This is especially important when the wearing surface is one of the structural components of the fan impeller.
Trending, such as that shown in Figure 2, enables predictions of remaining life, and facilitates planning of repairs or replacements. Wear trends that involve no material loss from structural components can usually be dealt with on a common sense basis. If a wear plate was originally 1/4-inch thick and is losing 1/16 inch/year of material in its worst wear area, it should be replaced in the third year before the wear affects the underlying structure. Predictions of life become more difficult when material is lost from the structure of the fan impeller since this can affect the structural integrity of the fan. An experienced fan designer should be consulted when expected wear losses involve structural components.
Wear trending can also reveal the effects of system or operational changes. Figure 2 shows the wear trend for a fan blade that has experienced a noticeable change in wear rate. Assuming that the wear has all taken place in the same material over the life shown in the curve, this indicates that something external to the fan has changed, which will result in a much shorter wear life. Typical problems include changes in fuel, increased production, and loss of gas cleaning efficiency.
The type of repair required generally follows the extent of wear the fan has experienced. The simplest of repairs involve minor build-up of wearing surfaces, either in the structural portion of the fan impeller or in a wear-resistant component, such as a wear plate, that protects the structural portion of the impeller. In theory, so long as the filler metal used to make the repairs is comparable in strength, hardness, and ductility to the metal it is replacing, extensive repairs can be made to impeller components.
Practically, there are several considerations that limit the extent of repairs that should be attempted in place. First, it is difficult to prevent warpage in impeller components that are extensively repaired by welding build-up. Undesirable geometry changes can cause rubbing, loss of performance, and increased potential for wear. Second, it is very difficult to control the exact amount and placement of weld metal deposited during repairs, which almost always leads to significant vibration when the fan is re-started. Third, the time required for extensive welding build-up is usually not available, making other alternatives more attractive.
Figure 3. Spoiler beads.
Another type of simple repair is the replacement of, or addition of, spoiler beads (Figure 3). The most common practice is to use a filler metal that is considerably harder than the structural material in the impeller. The beads are placed perpendicular to the local flow direction so as to raise the dust stream off the surface of the impeller. These beads are applied in areas that don’t quite need a wear plate, but experience enough wear to damage structural components.
It is also possible to replace wear plates or portions of wear plates in place. Wear plates of various sizes and materials are often added to the most wear-prone areas of the fan to protect underlying components from damage. Again, common sense should dictate the extent of replacement. If a small portion of a full blade wear plate has lost enough material to expose the underlying structure, but the remainder of the plate can sustain several more years of operation at the current wear rate, only the small portion affected should be replaced. Care must be taken to blend the replacement part into the remainder of the wearing surface since protruding edges tend to wear more rapidly than the surrounding area. Entire wear plates can also be replaced in place. Welding procedures must be followed carefully, as will be discussed below.
The materials used for these wear plates typically fall into one of three categories: an alloy identical to or similar to the alloy used for the structural components; quenched and tempered steels; and composite materials with a hard wearing surface applied to a weldable or boltable structural base material.
The quenched and tempered materials come in a wide variety of alloys and hardnesses. Most fall into the low alloy steel category and many are considered proprietary alloys (i.e., they are not produced to a recognized set of specifications such as ASTM). Hardness for these alloys ranges from around 300 to 600 BHN. Generally speaking, the higher the hardness, the greater the welding difficulty will be.
The composite materials include everything from a low carbon, medium manganese weld overlay applied to a carbon steel backing to a thin tungsten carbide coating that has been furnace brazed onto a structural backing material. One of the most common composite materials in use today is the combination of a chromium carbide weld applied over a structural steel backing material.
Another strategy for wear resistance is to weld deposit a hard surfacing metal directly to the structural fan components in the affected area. This strategy is limited in that it is very labor intensive, and so, it is usually only applied sparingly.
Thermal spray coatings provide yet another category of wear resisting strategies. Almost any filler metal that can be weld applied can also be sprayed onto a surface. Some spray methodologies allow the use of ceramics and other non-weldable materials.
It is not uncommon to find applications that can benefit from the use of more than one of these strategies. An illustration would be a fan blade protected in the highest wear areas by a chromium carbide overlaid composite, a quenched and tempered plate material in the surrounding areas, and spoiler beads of chromium carbide near the perimeter of the blade.
Provided the damage is not too extensive, portions of the impeller structure can be replaced in place. These types of repairs require careful planning and consideration of the difficulty of welding the structural metals. It is not unusual for replacement of impeller components to be combined with repair or replacement of wearing surfaces. This combination type of repair is the most complex of all.
The caveats associated with any of the repair strategies described above are concerns over the increase in rotary inertia (often expressed as WR2 or WK2) that accompanies any addition of material to a rotating assembly, concerns for increased initial costs, and concerns over decreased structural integrity. Another significant concern for operating companies is the increase in maintenance costs and the decrease in maintainability when sophisticated wear materials or combinations of materials are employed.
Based on predicted wear life, improvements to the wear-resisting system may be needed in addition to repairs. The result of this step should be a final design for the restored or improved wear-resisting system. The design should include the final geometry that will be achieved in the repair and the materials that will be used for the wearing surfaces and welding. Even for simple repairs, a sketch should be made to show the welders and fitters the final design; and, for craftsmen unused to making such repairs, a set of clear written instructions should be furnished, giving step-by-step details of the repair process.
All materials to be welded in the repair procedure must be identified, even if they are only adjacent to the repair area, and a complete welding procedure specification (WPS) must be developed for each weld joint. If the materials permit, the welding procedures should be qualified to AWS D14.6 – the welding code for rotating machinery. Unfortunately, due to the extreme hardness and brittleness of many wear-resistant materials, it is impossible to pass the required bend and tensile tests to obtain a fully qualified WPS. Welding of the materials used in the structural components of the impeller can be qualified under AWS D14.6. At a minimum, welders qualified to AWS D14.6 for the structural material of the impeller should be used to carry out all repairs.
As part of the repair plan, the removal of worn materials and excavation of existing welds and other wear-resisting deposits must be considered. Removing wear plates is relatively simple, and a little grinding of wear-roughened areas prior to build-up will prepare the surface reasonably well. However, weld applied hard surfacing that potentially interferes with structural welds must be given special attention. Since the primary wear resistance mechanism in most fan wear systems is high hardness, the chemistry of weld deposited hard surfacing is very different from that of the structural weld metal. Even minor amounts of dilution of hard surfacing in the structural weld metal can have catastrophic results. It is often necessary to dig beneath the surface of the plate to assure complete removal of all hard surfacing deposits.
Figure 4. Blade wear plate indicating where the replacement repair will take place.
Once all the plans are complete, the repair team should be assembled and the entire plan discussed. Each major step of the plan should be explained, feedback taken, and a materials and equipment list developed for the work. Proper non-destructive testing procedures should be planned into the work schedule as well as an independent quality review upon completion of the project.
To illustrate the principles and ideas outlined above, an example repair plan is developed below. The specific problem addressed in this example is the replacement of a portion of a blade wear plate. Figure 4 shows the wear area and where the replacement part is located. This repair will require the following steps:
- Remove the 10-inch x 10-inch portion of the wear plate that will be replaced using air carbon arc gouging. Be sure to carefully mark and remove the same amount of material from each wear plate to prevent serious vibration problems upon re-starting the fan.
- Grind smooth any wear-roughened areas of the underlying structural blade.
- Using WPS BISM-104, make repairs to the worn areas of the structural blade.
- Grind the weld repaired blade surface smooth and flush with the original blade surface and perform a 100 percent magnetic particle examination of the welded area.
- Fit the replacement chromium carbide overlaid wear plate in place, leaving a sufficient root opening to allow a good weld attachment between the structural blade and the base metal portion of the composite plate. Again, careful placement of parts pre-cut to exactly the same size will help reduce vibration when the fan is re-started.
- Using WPS BISM-200, complete the weld that joins the replacement wear plate to both the underlying structure and the surrounding wear plate. NDE may be performed when the E7018 weld layer is complete, but will not be effective once the final hard surfacing has been applied.
- Perform a final visual examination for smooth transition between the old and new parts, excessively large stress-relief cracking in welds, and overall appearance.
M.H. (Johnny) Johnson is currently the General Manager of Barron Industries LLC, a wholly owned subsidiary of Process Equipment Inc. He has 30 years of experience as a design engineer and metallurgist for mechanical draft fans and ancillary equipment. Johnson holds a bachelor’s degree in engineering from the University of Alabama in Birmingham.