Plasma cutting pipe in the water mist
Modern Piping Inc. finds that it can cut pipe more efficiently with a water mist twist in plasma cutting
An Iowa pipe fabricating shop discovers it can eliminate secondary grinding on stainless steel material by adding water misting to its plasma cutting operation.
Cutting-edge technology sometimes takes awhile to make its way into the niche areas of metal fabricating. But when it does, its impact is immediate, like anywhere else.
Modern Piping Inc., a Cedar Rapids, Iowa-based mechanical contractor with its own large-scale fabrication shop, cuts a lot of pipe. So when its Watts W-242 CNC automated plasma pipe cutter it purchased in 2009 hit the shop floor, the new equipment made a real difference in productivity. The shop could place a 20-ft. stick of pipe on the machine; program consecutive cuts in a row; and then watch the machine do its work, moving from one cut to another in a matter of minutes (see Figure 1). Operators no longer had to rely solely on band saws, hand-held torches, or track-style cutting devices that run along the pipe’s exterior.
While cutting had reached a new level of efficiency, postcutting activities still remained a challenge. The typical plasma cutting process left a lot of dross to clean up. For a shop that prides itself on the quality of weld prep that it offers its welders, that translated into a lot of grinding.
“Before, we’d have a $3 grinding wheel in there and go through a wheel per prep cleaning it up,” said Jim Tedrow, the fabrication shop manager.
That all changed recently. A new plasma cutting technology emerged from the mist (see Figure 2).
Just Add Water Mist
The CNC pipe cutting machine came with a 90-amp plasma power source, which is akin to the boxes used with hand-held torches in shops across the U.S. It provided the necessary power to cut the 2- to 24-in.-OD pipe the equipment was designed to accommodate.
About a year ago, Modern Piping approached representatives of Victor Technologies about putting a 300-amp plasma power source on the pipe cutting machine. But this wouldn’t be just a power upgrade. The plasma equipment supplier wanted Modern Piping to use its Water Mist Secondary (WMS®) process for the cutting of 3-D shapes. The cutting-edge technology had worked its way to northwest Iowa.
Now plasma cutting with water mist is not unusual. In fact, the idea of automated plasma cutting with the use of tap water to help generate a shielding gas has been around for three decades. It’s been pushed more aggressively in recent years as a means to cut nonferrous materials more quickly and cleanly with the WMS process.
Nitrogen is used as the primary gas (plasma gas), while the water is used as a secondary gas (shield gas). The water in the torch is divided into its principal components (hydrogen and oxygen) during the cutting process. The hydrogen generated creates a reduced atmosphere in the cutting zone, isolating it from contaminating elements and producing a clean, dross- and oxide-free surface. The main amount of the water used during the process (from 0.3 to 0.5 liters per minute) is converted to principal components, which do not require disposal.
The water helps to do a couple of other things as well. It helps to reduce the heat, when compared to traditional plasma cutting temperatures, which allows the metal to cool faster. The water also reduces the amount of fume and other emissions commonly connected with dry plasma cutting.
The water mist technology has been implemented in other manufacturing settings and seen by hundreds at industry events, such as FABTECH®. Victor Technologies was anxious to have Modern Piping apply it to its stainless steel pipe cutting, and the shop was excited about the technology’s potential impact.
“There’s nobody in the region that fabricates like we do, so this was a big leap,” Tedrow said.
Making It Work
As with any new technology implementation, the fabricator and equipment-maker have to work out the kinks. Coupons were exchanged between Modern Piping and Victor Technologies’ Application Lab, as both parties tried to dial in the correct cutting parameters for the stainless steel pipe cutting. The parties were looking for the right combination of amperage and water to eliminate the excess dross that was a common result of a typical pipe cutting job (see Figure 3).
“Carbon steel is easy to cut. It’s way more forgiving than stainless,” Tedrow said. The stainless steel took a bit more work to figure out.
But when the parties came across the right combination, the result was just like what was possible with 2-D plasma cutting. Dross was no longer an issue.
The next hurdle was to prevent the molten cutting arc spray from adhering to the pipe. During the process of cutting tube and pipe, the molten plasma arc spray contacted the inside wall opposite the torch. This molten material spray became embedded and required a large amount of cleanup.
To solve this problem, an additional source of tap water was fed into the tube. This water would provide a shield between the inside wall and the molten arc spray.
“The water would quench the molten material before it would contact the inside wall of the tube,” said Chris Braudis, application development manager, automated cutting and welding, Victor Technologies. “[The molten metal spray] becomes solid, almost to the quality of fine sand, which then could be easily removed.”
Modern Piping wouldn’t need aggressive grinding action anymore to make the pipes ready for secondary processes (see Figure 4).
“Before the change, there would be so much slag after the cut [see Figure 5], we’d have to get a grinding wheel in there and clean it up because we couldn’t send it out like that,” Tedrow said. “Now we don’t have to touch it for cleaning.”
“We grind stainless now, just to shine it up to make a good weld,” he added.
Braudis said a clean, weld-ready edge is to be expected because hydrogen from the water, released during cutting, creates a reduced atmosphere in the kerf, preventing the oxides from contaminating the metal.
To illustrate just what kind of savings that the water mist technology on the pipe cutting machine has made in terms of production time, Tedrow pointed to the simple task of putting bevels on pipes. If the pipe were thick, say with a wall thickness of 0.75 in., the shop would have to use a band saw to make a straight-down cut and then attach a bevel tool on the cut edge to create the form for a weld joint. The process could take the better part of a day if a large number of pipes had to be prepped. Today the pipe can be placed in the chuck on the table and the torch head tilted to create a simple bevel.
“It puts a bevel on it that’s near machine quality,” Tedrow said.
The Modern Piping team also is getting used to the new plasma power source and its advanced controls (see Figure 6). The machine can operate anywhere from 5 to 300 amps, so it can be used for fine cutting and marking as well. The controls ask for material type and thickness before a cutting program is engaged for the first time, so that the operator knows what consumables should be used for the job; this helps with delivering quality cuts.
The pipe cutting equipment and the rest of the fabrication machinery were headed over to a new 70,000-sq.-ft. facility at the time of this writing. The pipe cutting business is now with the company’s sheet metal fabrication business under one roof.
“We’ve got some exciting things happening,” Tedrow said.
New technology has a way of energizing any fabrication shop.
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.