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Steel piping systems are widely used at coal-fired power plants for a variety of purposes, including the conveyance of coal ash slurry to nearby settling ponds, the transfer of limestone slurry to absorber spray towers for removal of sulfur dioxide (SO2) and hydrogen chloride (HCl) from flue gases, and for transporting away the calcium sulfate by-product of the flue gas desulfurization process.

However, when these abrasive, caustic slurries are transported through steel pipe, the effect on the inner surface can be devastating. Abrasive wear within these transportation systems is produced when hard particles are forced against or slide along the wall of the pipe. The loss of material is the result of the hard, sharp angular edges producing a cutting or shearing action on the pipe wall, which in more extreme cases can result in pipe leaks and even failure, or significant maintenance costs and dow ntime for pipe replacement.

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.

"The slurry is so abrasive, that standard grade carbon steel pipe just wears out too quickly," explained John Pillard, a senior pipe technician with extensive experience in high abrasion applications.

Abrasive applications at coal-fired plants 129 million tons of coal ash, a byproduct of the coal burning process, is produced in the U.S. every year. Most power plants transport the coal ash in liquid form to large surface impoundments or in solid form at landfills near the facility through lon g lengths of steel piping from several thousand feet to miles. There are almost 900 such landfills and surface impoundments in the U.S. today.

Limestone slurry is also commonly used by coal-fired plants for the effective removal of sulfur dioxide and hydrogen chloride from flue gases. These wet scrubber flue gas desulfurization (FGD) systems utilizes a pumped in limestone slurry through which f lue gas containing SO2 is passed in absorber spray towers. The limestone slurry reacts with the flue gas, removing the SO2. The limestone in slurry is then converted into calcium sulfate. This waste material produced by the process is then pumped out f rom the bottom of the spray towers through steel pipe for further treatment for use in commercially viable by-products such as gypsum board and cement.

To deal with the extremely abrasive and caustic nature of these various slurries, plant maintenance managers are looking to a variety of "abrasion resistant" steel pipe products and accessories to replace standard mild steel pipe in these high wear areas .

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 steel (200 BHN) through iron c ast 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 an d 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 t he 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 propri etary process, pipe can be created in various diameters up to 40 inches 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.

The company can also produce the Ultra 600 pipe in long radius bends to further reduce wear and eliminate the frequent replacement, repair, and associated expense accepted as the norm at the bends and elbows in high wear applications.

Getting the most wear for your money As the senior pipe technician, Pillard said his primary concern is to "get the most wear for your money."

Pillard estimates that in an extremely high wear area an A-R pipe could wear out in as little as a year and a half. He estimates that the Ultra 600 could easily double that duration in the same location. In a lower (yet still abrasive) wear application, Pillard has seen the induction hardened pipe last as long as 6-8 years.

"You don't want a pipe that is three times better, but costs ten times as much. That's not cost effective," said Pillard. "The Ultra 600 induction hardened pipe lasts a lot longer than mild steel, with only a moderate price increase. As a result, more of the induction hardened pipe is taking the place of the A-R pipe simply because it's a better value."

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Publication:Worldwide Energy
Date:Sep 29, 2010

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