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Plasma Technology

Water works

By Nick Wright

Above: Advance Tank’s new automated plasma table uses the Water Mist Secondary cutting process.

A water-based shield gas system eliminates power washing of smoke-blackened plate

November 2014 - Power washing a deck can be one of the most satisfying chores to complete. Power washers are loud, powerful and leave behind a good-as-new deep clean. The downsides? It can take all day. Plus, it must be done with a steady hand lest your power washer become a power etcher. The fact that it has to be done at all is the biggest pain.

Now, imagine spending about 50 hours a week power washing. Surely, there are other things you could be doing—if you can still feel your arm.

A similar scenario long burdened production workers at Advance Tank and Construction Co., Wellington, Colorado. Up until a few months ago, employees spent about 200 man-hours each week power washing smoke residue from plasma-cut stainless steel plate that is later fabricated into above-ground welded tanks.

It usually required six workers to clean plate when dealing with a full workload, which pulled welders and other laborers away from their primary duties. When power washing alone wouldn’t work, they used Compound 302 stainless cleaner. This all took place after workers arduously hand-ground plasma cut edges to get a weldable surface. How could this possibly be efficient?

“We were using track torches and doing our own layout,” says Troy Sauvageau, Advance Tank’s shop superintendent. “It was an old school method of doing it. It was time consuming and inaccurate as far as what we needed.”

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To solve these issues, which affected multiple levels of Advance Tank’s workflow, the company turned to a new plasma cutting system from Thermal Dynamics, the plasma cutting arm of Victor Technologies, St. Louis. At the core of the new system is the same element behind the power washers—water—only deployed at an earlier stage of the process.

Advance Tank now uses the Thermal Dynamics Ultra-Cut 200 XT plasma cutter equipped with the Water Mist Secondary (WMS) cutting head, which uses nitrogen as the plasma gas and regular tap water as the shield gas. By using water instead of open air as a shield, Advance Tank’s stainless steel parts now have minimal heat tinting and leave behind a near-waterjet quality edge, so to speak, according to Cole Miller, shop foreman. 

Most importantly, the plasma cutter leaves behind almost no smoke residue, says Sauvageau. “Of course there’s a little bit of smoke, but it’s all within tolerance of our requirements where we don’t have to wash the plate,” he says. The WMS has reduced the man-hours crews spent power washing plate from 200 per week down to one or two hours.

Big tanks

Advance Tank fabricates stainless and carbon steel tanks for markets including oil and gas, agriculture and fertilizer, and any atmospheric tank a client wants. They’re all made to various American Petroleum Institute standards that govern the fabrication, welding and inspection processes. Oil and gas companies require a lot of X-rays and other quality control tests for such tanks. “On the field side, we do lots of vacuum box testing and hydropneumatic testing,” says Sauvageau.

The stainless steel pieces are generally cut at Advance Tank’s Colorado plant a few miles north of Fort Collins and shipped to the field for assembly. The stainless shop is about five years old—relatively new for the three-decades-old company. Those tank components, which range in thickness from 3⁄16 in. to 1.25 in., are anywhere from 6 in. wide by 20 ft. long, and up to 36 ft. long. The stainless grades Advance Tank uses include 2205, 2304, 304L and 316L. 

When parts are delivered on-site to be erected, they must be ready to weld or assemble. Before using the WMS, sometimes there would be fit-up issues with plasma-cut parts because operators had to grind dross off the edge. Inadvertently, by removing material the edge would be weldable, but not perfect.

After cutting, Cole says he had to “take a 1⁄4 in. wheel on a 9 in. grinder and grind every spot that you cut to make the edge, get it back to weld quality.”

Now, the cuts are clean and don’t require extra man-hours for grinding. That’s especially valuable when Advance Tank is running full throttle, as it is for a current order for 13 large fertilizer tanks to be shipped to locations in Louisiana and Iowa. The cylindrical tanks are sizable, between 22 ft. and 26 ft. shell radius. Most of them are about 40 ft. tall. Plate gauges vary from 3⁄16 in. to 1.25 in. thick on these tanks with some intricate internal piping. Because of the plate condition improvements and the ability to achieve more reliable welds, “we haven’t had nearly as many problems with [the results of] X-ray [testing],” says Sauvageau.

Made with the mist

Using water to generate the shield gas provides numerous benefits. First, during cutting, it minimizes heat input and cools metal faster, which reduces dross. Dirk Ott, Thermal Dynamics’ global plasma automation brand manager, says the WMS process produces no dross when cutting stainless steel and aluminum ranging from 10 gauge to 3⁄4 in. thick. Second, low thermal input minimizes distortion and preserves mechanical properties. Third, water delivers the hydrogen necessary to prevent oxidation on the cut surface—an important consideration as almost all of Advance Tank’s stainless parts get welded.

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It seems overly simple to boil down Advance Tank’s solution to the introduction of water at a critical stage of plate processing. Looking more closely, the process behind the WMS is commonly known as nitrogen-water plasma cutting. During cutting, energy from the plasma gas divides the water in the torch into hydrogen and oxygen, according to Thermal Dynamics. The hydrogen creates a reduced atmosphere in the cutting zone, protecting it from contaminating elements and leaving a clean, oxide-free cut.

Most of the water used during the process, about 4 to 8 gallons per hour, is converted to hydrogen and oxygen, thus requiring no disposal. The water also helps reduce fume and nitrogen oxide emissions to less than that of dry cutting, but more than that of underwater cutting. In that regard, WMS can be considered a semidry process, says Ott. Using nitrogen, whether paired with water or other gases, makes for a straight cut with a narrow kerf. It also lengthens electrode life and helps keep operation costs low. 

Advance Tank has realized longer consumables life as a result. For example, on one set of consumables, Advance Tanks cut two 3⁄8 in. plates and had more than 400 pierces using the 200 amp setting. Miller says that’s comparable to its previous plasma setup, but what sold them was the cut quality and the elimination of the smoke. “This is the first high-definition plasma we’ve had here, so it’s pretty amazing to see it run,” he says. The pierce count at Advance Tank results from the long cuts made to square the plate. In other applications, the WMS process provides a parts life of 800 to 1,000 pierces with a 20-second cut.

Dollars and sense

While the working environment has improved and precious, post-processing man-hours reduced, the dollars saved by investing in the WMS speak volumes. In northern Colorado where Advance Tank is located (the company also has an Alabama fabrication facility), the going rate for fabrication laborers is $14 to $16 per hour, and $17 to $20 per hour for welders, says Sauvageau.

Using $15 per hour as an example, multiplied by 200 hours Advance Tank workers spent power washing plate, the figure comes out to $3,000 savings per week. For the Thermal Dynamics Ultra-Cut 200 XT plasma system, which costs roughly $30,000, payback occurs in 10 weeks.

“That’s why making this purchase was a no-brainer,” Sauvageau adds. Advance Tank is realizing savings in cutting thicker plate, a task that was outsourced before the WMS system. “With 1-in.-thick plate, we couldn’t come close to cutting that with a track torch,” he says.

Advance Tank no longer worries about the logistics of washing plate, either. The shop wasn’t set up for power washing indoors, a necessity when brutal Colorado winters made doing it outside prohibitive.

When we describe a technology as being on the cutting edge, we refer to its most developed, most advanced iteration. Although Advance Tank has used WMS for less than a year, it’s already gaining the savings, benefits and simplicity of a technology based around an unlikely and untapped element—water. FFJ

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