Steel piping systems used to convey coarse materials, often over long distances, are under constant attack from abrasion. In power plants, the materials are usually coal and limestone slurry. The common industry solution has been to install abrasion resistant (AR) pipe that is much harder on the Brinnell Scale than standard steel pipe. The harder the inner wall, studies have shown, the better it resists the gouging or plowing action of abrasive sliding particle flow.
Unfortunately, this type of abrasion is only one type of wear experienced within the piping system. Impact abrasion, where a slurry meets the inner pipe wall at close to 90 degrees—such as at the bends, elbows, laterals, or tees in the system—has a decidedly unwelcome effect on the inner wall of the pipe.
At sharper angles, abrasive material actively “chips” away at a more brittle inner wall, eroding the surface from the inside out. “When you have a sharp impingement angle, the abrasive material in the slurry can chip away at a brittle inner pipe wall,” explains Ralph Wollenberg of Port Washington, Wisc.–based Ultra Tech Pipe, a company specializing in a variety of abrasion-resistant pipe and pipe accessories for the mining and other industries. “On the other hand, a more ductile inner wall deforms on impact and is better suited to withstand impact abrasion over time.” So extensive is the problem that frequent replacement, repair, and associated maintenance expenses at the bends, elbows, laterals and tees in the system are accepted as the norm (Figure 8).
|8. Always a tradeoff. Abrasive materials in slurry can chip away at a brittle inner pipe wall in pipes with a sharp impingement angle. A more ductile inner wall can deform on impact, better-withstanding impact abrasion over time. Pipe selection must be based on the properties of the material transported. Courtesy: Ultra Tech Pipe|
The solution would appear straightforward: For sliding abrasion, an extremely hard interior pipe wall for the straight lengths of pipe and a more ductile inner wall at the directional transitions. However, it turns out that size transitions and directional changes can experience both abrasive and impact wear.
According to Wollenberg, the variety of considerations that need to be addressed when it comes to abrasive wear illustrate the difference between simply ordering a piece of pipe and the broader field of piping system design.
“Designing a piping system is really a balancing act, and each system is different,” says Wollenberg. “In some cases, if the abrasive material is softer than the steel used for the bends, and it’s not moving very quickly, even an impact situation will not wear away the pipe. If the abrasive material is very hard or being conveyed at a higher velocity, alternative solutions at the directional transitions are required.”
Wollenberg also points out that alternative solutions include induction-hardened pipe bends, installing wear tiles and inserts, lining the bend with various abrasion-resistant materials, and designing in longer-radius bends when possible.
Induction Hardening of the Bends
Strength and wear resistance can be improved by adding alloying elements to the steel during the pipe manufacturing process. This material is commonly referred to as AR200 (abrasion-resistant) piping. Strength and wear resistance can also be increased with secondary processing of the steel, including heat treating by induction hardening. Induction-hardened pipe can extend pipe lift from three to eight times longer than mild steel, with only a moderate price increase (Figure 9).
|9. Prolonging pipe life. Strength and wear resistance can be improved by adding alloying elements to the steel during the pipe manufacturing process or with secondary processing of the steel, including heat treating by induction hardening, which can extend pipe longevity up to eight times. Courtesy: Ultra Tech Pipe|
Several years ago Ultra Tech released its Ultra 600 series induction-hardened pipe, a unique single-wall pipe with a 250 BHN ductile outer surface that tapers to a 600 BHN inner wall surface.
The process begins with a steel pipe manufactured to a proprietary chemistry developed by Ultra Tech, followed by induction heating, and finally water quenching of the inner surface to create the single-wall pipe.
At 600 BHN, the inner wall of the pipe can withstand the sliding abrasion of most common mining slurries, while its more ductile outer surface behaves like mild steel and can be cut and welded in the field, configured into a variety of fittings, and can accept the standard end options of flanges, weld rings, and couplings. Pipe sizes are available in various diameters up to 40 inches and in varying lengths and wall thicknesses. The same technology is used to create the bends, elbows, laterals, and tees with the same hardness profile.
Based on the premise that when two objects meet, the harder object wins out, induction-hardened pipe for both straight lengths and directional transitions is often sufficient for slurries of “softer” material such as coal and limestone. In some cases, mild steel pipe with induction-hardened bends may work as well.
Increasing the Radius of the Bends
Another legitimate technique to mitigate the effects of impact abrasion is using induction-hardened pipe in long-radius bends.
“If there is enough room, you can design the bend with an impingement angle of less than 15 degrees, which is a small enough angle that you no longer have impact abrasion but essentially sliding abrasion,” says Wollenberg. “With a large enough bend, for all intents and purposes you have made it a straight pipe.”
Because Ultra Tech is able to induction harden the pipe during the bending process, it can vary the arc radius of the pipe from 20 inches to 180 inches and handle multiple diameters or thicknesses of pipe.
“The bending process has added benefits, in that it’s much easier to reduce the ovality that one sees so often with a cold bend,” says Wollenberg. “Although you will still get some wall thinning—depending on the radius of the bend—it is much less than a cold bend.”
Wear Tiles and Inserts
A layer of wear tiles can also protect the inside wear surfaces of the pipes. Industrial wear tiles are made from materials that are harder and more abrasion resistant than the base piping.
These tile materials can be cemented carbide, cast high chrome and NiHard (alloyed with nickel and chromium), aluminum oxide, and basalt. Cemented carbide tiles are usually quite small in size (about 1 inch square and ¼ inch thick). Aluminum oxide tiles can be almost any size and have thicknesses over 1 inch. Basalt is a naturally occurring igneous rock that can be molded and cast into shapes while retaining hardness between 8 and 9 on the Moh scale.
Also, if the impact abrasion is particularly severe, thicker inserts can help. “Additional thickness of those wearable material inserts will help out in a lot of those situations,” says Wollenberg.
The bottom line is that the proper selection of component accessories, including the various configurations and alternate materials, can provide the ultimate total system wear life expectancy. By designing piping bends and elbows to withstand high-impact abrasive wear, a reduction in operating costs can be realized in the system.
“If you’ve got more of a critical system where you can’t afford any down time other than scheduled, then it’s worthwhile to spend the extra money for the appropriate abrasion-resistant solutions,” says Wollenberg.
—Contributed by Ultra Tech Pipe (www.ultratechpipe.com).