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Waterjet Cutting

Seal of approval

By Gretchen Salois

Above: A 5 ft. by 13 ft. Jet Edge mid rail gantry system at National Metals in DeForest, Wis.

Often a pitfall of the waterjet process, faulty seals get an improved, game-changing design

April 2013 - “Seals are the Achilles’ heel of waterjets,” says Michael Wheeler, research and development engineer at Jet Edge. The St. Michael, Minn.-based company manufactures waterjet systems including precision CNC waterjet gantries, waterjet pumps, accessories and parts. When cutting with higher operating pressures on a waterjet, traditional plastic seals often fall short. Traditional dual intensifier systems have eight seals: four dynamic and four static. If any of these seals develop a leak, the machine must be shut down for maintenance. To address this potential problem, Wheeler developed a new type of metal-on-metal seal to replace the four static seals.

A waterjet cutter uses a thin, high-pressure stream of water to cut anything from foam to steel. Although the results are exceptional, the basic operation is straightforward. “A waterjet pump is like a giant industrial grade syringe,” says Wheeler. “A plunger is pulled back and water is sucked in. When the plunger is pushed forward, the water is pressurized and expelled.” It’s the high-pressure seals in the waterjet pump that prevent the water from escaping the system while the water is pressurized. The cycle can be repeated 40 to 1,000 times per minute. Fluctuating water pressure is responsible for much of the wear and tear on waterjet parts, which eventually fail because of fatigue and cracking. 

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As parts deteriorate, they need to be replaced. According to Wheeler, replacing a failed plastic seal takes an experienced operator about an hour. “This is an hour of lost production,” he adds. “Minimizing the amount of system downtime maximizes the operator’s profits.” 

Metal-on-metal seals are different from plastic seals in that metal seals are part of the cylinder and check tube. Replacing the metal-on-metal seal is the same as replacing a cylinder or check tube. “The procedure takes about an hour, but the average life of a cylinder or check tube is significantly longer than that of a plastic seal, so the need to make repairs is more infrequent,” Wheeler says.

Because the metal-on-metal seals are integral parts of the high-pressure cylinder and the check tube, the seal lasts as long as the components, Wheeler says. “Depending on conditions, a metal-on-metal seal can have five to 10 times the useful life of a traditional seal,” he says. By incorporating  metal-on-metal seals, the pump can be assembled and disassembled multiple times with minimal maintenance.  

While traditional metal-on-metal seals use mating tapered or conic surfaces, Jet Edge’s metal-on-metal seals use convex curved surfaces. “The advantage of this design is that it minimizes contact between mating surfaces,” Wheeler says.

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A static seal is created by two or more objects that contact one another but do not move relative to each other. “An automotive head gasket and a pressure cooker seal are two good examples of static seals,” Wheeler says. A dynamic seal is created by two or more objects that contact one another, but at least one of the objects moves relative to the others, he says. Piston rings in an engine and the rubber knob in a syringe are examples of dynamic seals.

“In a traditional intensifier, both the dynamic and static seals have to be replaced at the same time,” he says, noting that traditional seals cannot be reused. “If the intensifier incorporates the metal-on-metal seal, only one seal requires replacing. Therefore a seal change requires only half the traditional number of consumables.”

Protecting progress

Jet Edge’s X-Stream (XP) pressure intensifier pumps produce cutting pressures of 75,000 psi and have been using the metal-on-metal seal technology since the line was first introduced at Fabtech in 2007. Fabrication and testing of the first prototype by Brian Wallace, a Jet Edge machinist, and former Jet Edge employee Reynold Sacquitne were “critical to validating the concept and the design of the seal,” Wheeler says.

As a seal wears, its ability to prevent the escape of high pressure water from the system slowly erodes. When the water escaping past the seal constitutes a nearly continuous stream of drips, the seal has failed.

Wheeler explains that a traditional plastic static seal can fail one of three ways: The o-ring portion of the seal fails, the seal extrudes out between the face of the cylinder and the check tube, or the seal develops cracks. The failure of plastic seals due to cracking is seen only at pressures of 75,000 psi and above, so it is not as common as failure due to o-ring damage or extrusion. In a conventional high-pressure sealing arrangement, a plastic seal is constrained between a high-pressure cylinder and a check tube. The plastic seal is the weakest part of this assembly, thus the most common point of failure. In a metal-on-metal seal, the high-pressure cylinder and check tube are placed in direct contact to create the seal, completely eliminating the weak link in the normal sealing system.

Jet Edge’s high-pressure fluid sealing mechanism boosts waterjet seal life by using metal-on-metal seals that do not crack like plastic seals. The technology has worked so well for the company that it introduced the metal-on-metal seals to additional product lines, including its Eco-Jet direct drive pumps and some 60,000 psi intensifier pumps.

According to U.S. Patent 8,333,387, issued for Wheeler’s new seal in December 2012, “Embodiments of the present invention provide effective and robust sealing for high pressure fluid systems that include passageways and junctions employed to selectively constrain the high pressure fluid.”

Wheeler says the metal-on-metal seal covered by that patent is the only seal Jet Edge has used to successfully contain 130,000 psi. Jet Edge believes the technology has given the company a competitive advantage. FFJ

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Sources

  • Jet Edge
    St. Michael, Minn.
    phone: 800/538-3343
    fax: 763/497-8701
    www.jetedge.com



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