However, when these abrasive, caustic materials are transported through standard-grade steel pipe, the effect on the inner surface can be devastating. Abrasive wear within these pipes occurs when hard particles are forced against or slide along the wall of the pipe. In more extreme cases this can cause pipe leaks and even failure, or significant maintenance costs and downtime for pipe replacement.
"The material running through these pipes is so abrasive, that standard grade carbon steel pipe wears out too quickly," explains Ralph Wollenberg, Chief Engineer at Ultra Tech, a company that manufactures abrasion resistant steel piping systems and accessories. "It can wear a channel, even a hole, in the bottom of a pipe in a short period of time."
For these highly abrasive applications, mild steel pipe is simply not tough enough to stand up to the abuse for more than a year or two. As a result, maintenance engineers are seeking superior piping alternatives to reduce maintenance and prolong system life, at a price point that will not significantly impact the budget.
Abrasive applications in pulp & paper
The total United States production of bark is estimated to be more than 20 million tons annually. On average, bark comprises about 9 to 15 percent of a typical log by volume. The volume is so great that bark must be continually removed from mill sites.
This bark is not only extremely abrasive, but when pneumatically conveyed, it often includes sand and dirt that speeds the deterioration of steel piping.
"When you transport bark or wood chips pneumatically, you always have the dirt and sand that go along with it," says Wollenberg.
According to Wollenberg, many large processors are using this mill-generated bark, along with a variety of other abrasive fuel sources, to satisfy their own power generation needs on-site.
"This requires transporting bark, wood chips, scrap lumber, black liquor, coal, and other boiler fuels and byproducts to and from the units through steel piping systems," explains Wollenberg.
Black liquor, created during the process of breaking pulpwood into paper pulp for papermaking, is frequently re-used as fuel in recovery boilers. In concentrated form, it contains a high volume of organic dissolved wood residue. Combustion of the organic portion of black liquor in a recovery boiler produces heat, which is then converted to high pressure steam used by a turbine to create electricity.
Although it is abrasive, black liquor also contains minor amounts of impurities such as lime, iron-oxides, sodium chloride, silica and alumina which can cause additional pipe damage.
Finally, limestone slurries are used in boilers for the effective removal of sulfur dioxide. A sorbent, such as limestone, can be added to the fluid bed combustion process with the fuel feed to reduce SO2 emissions.
To deal with the extremely abrasive and caustic nature of these conveyed items, plant maintenance managers are looking to a variety of "abrasion resistant" steel pipe products and accessories to replace standard mild steel pipe.
Abrasion resistant pipe options
Most abrasion resistant pipe options operate on the premise that when two objects meet, the harder object wins out. As such, products are available in a variety of increasing hardness, measured on the Brinell Scale from A-R (abrasion resistant) steel (200 BHN) through iron cast pipe (up to 800 BHN).
Unfortunately, any product that is very hard throughout the wall thickness is also extremely brittle. This brittleness is unacceptable as piping systems are constantly flexing and moving as a result of pressure surges and spikes, and due to mechanical and physical contact at the facility.
However, one type of pipe delivers the best of both worlds: an induction hardened pipe with an abrasion resistant inner surface that tapers to a strong, yet ductile outer surface.
Manufactured by Port Washington, Wisconsin-based Ultra Tech, this unique pipe is produced under the Ultra 600 brand. Ultra Tech begins with a steel pipe manufactured to a proprietary chemistry, followed by induction heating, and finally water-quenching the inner surface to create a single-wall pipe.
At 600 BHN, the inner surface of this induction hardened pipe can withstand most common abrasives, and tapers to a 250 BHN outer surface that is ductile enough to accommodate normal handling during shipment, installation and maintenance. With this proprietary process, pipe can be created in various diameters up to 40", in varying lengths and wall thickness.
Because the outer surface behaves like mild steel, the product can be cut and welded with proper procedure in the field, configured into a variety of fittings, and can accept the standard end options of flanges, weld rings and couplings.
Getting the most wear for your money
According to Wollenberg, the primary concern in selecting the right type of steel pipe for an abrasive application is "getting the most wear for your money."
"If a steel pipe is going to get abused and wear out in several years and need to be replaced, then it might be worth looking into an option that will cost a little bit more but will last, say, 15 years," says Wollenberg. "That would be a good value when compared to the life of the pipe."