- December 12, 2011
Is ultra-high pressure waterjet cutting right for you?
by Jim Barnes
Since its introduction in the early 1970s, waterjet cutting has seen steady increases in pressure. Conventional systems today typically run at 60,000 psi, and many users are adopting ultra-high pressure systems running at 90,000 psi and more. How much pressure is enough?
"Every single manufacturer in the waterjet industry has increased pressure over time," notes Brian Kent, global product manager, Shape Cutting, Flow International. That’s in keeping with a broad trend in metalworking toward increasing power. "The plasma-cutting industry has increased its amperage and the laser industry has increased its wattage," over the same period, he points out.
Jet Edge has seen a "huge increase" in demand for its 90,000 psi X-Stream pressure intensifier pump, says Nancy Lauseng, marketing manager. She cites one customer that traded in its 60,000 psi/50 hp system for Jet Edge's 90,000 psi/100 hp X-Stream unit. "when I asked the operator if he would ever go back to a 60,000 psi pump, he said 'absolutely not.' He needs the X-Stream's faster cutting capabilities to keep pace with his mushrooming workload."
However, the difference between water-only and abrasive waterjet cutting should be noted. "In the abrasive waterjet process, the water does not do any of the cutting,” explains Kent. "The water is used to accelerate the abrasives, which do the cutting." In fact, once the water stream leaves the orifice, the water itself is basically at atmospheric pressure. "However, the higher the pressure, the higher the velocity of the abrasive will be,” he says.
Grace under pressure
"What customers want to know is whether they can cut more parts by the end of the shift,” says Bob Pedrazas, marketing manager, KMT Waterjet Systems. All manufacturers agree that speed is king.
"For every 5,000 psi you go up in pressure, you will get a 12 per cent increase in cutting speed. This is virtually a linear increase," says Kent. "Higher pressure gives you greater power density. You put the same amount of horsepower through a smaller orifice, putting more energy in a smaller area.”
OMAX's waterjet cutting system.
That’s a widely held view, but not all waterjet manufacturers agree. "The issue is, does it cut faster? This is where it gets controversial,” says Laird Parry, senior applications engineer, OMAX Corp.
Pressure and volume flow rate of the abrasive cutting stream are the keys to productivity, says Parry. Using the same pump, though, you can’t raise them both. As you increase pressure in the system by using a smaller orifice, you lower the volume flow rate. If you use a larger orifice, you increase the volume flow rate and decrease pressure. It’s a trade-off.
For example, a conventional system might have a 15 thou. (thousandths of an inch) nozzle and run at 60,000 psi using a 50 hp pump. "If you want to use the same nozzle and increase the pressure to 90,000 psi, you will need a 100 h.p. pump,” says Parry. When you compare 60,000 psi and 90,000 psi performance using the same pump and different nozzles, "the increase in cutting speed is very small. It's more affected by the design of the nozzle than anything else,” he says.
The bottom line for Parry is in test cuts. In the tests he cites, abrasive flow rate and pump power were kept the same, but the pressure and orifice were changed. "By and large, it was a dead heat" in terms of cutting speed, says Parry.
High pressure has an important role to play, says Pedrazas. "The higher pressures are needed for thicker materials, especially titanium, stainless steel, thick aluminum and others… The value is to cut faster and deeper, with less tapering."
He questions whether anyone is trying to cut 12-in. steel sections with 60,000 psi systems. "We know of a company that is cutting 15 in. of steel," with an ultra-high pressure system, he says. The choice of technology is dictated by technical constraints and economics.
"Make sure you know all the variables that go into making a high-pressure cut. Don't just look at pressure and feed rate. You have to look at abrasive usage. If you only look at two out of three variables, you can make the numbers say anything you want," says Kent.
Abrasive is an important consideration. More than half of the cost of running a waterjet comes from abrasive, says Kent. If abrasive cost is not a concern, he says, lower pressure machines can cut faster. "At 60,000 psi, if I want to use 2.4 lb/min. of garnet, I can get fairly close to the speeds of a 90,000 psi machine. But the 90,000 psi machine is going to be using 0.8 lb/min.,” says Kent. "There's basically nothing we can cut at 94,000 p.s.i. that we can't cut at lower pressure…The difference is, you can cut it faster and cheaper."
At 90,000 psi, the biggest orifice you can use is approximately 10 thou., says Parry. The smaller orifices can't handle large quantities of abrasive. "It’s kind of a bottleneck, so they tend to run those at lower abrasive flow rates,” he adds.
Close up of JetEdge's X-Stream 90 to 100,000 psi intensifer and attenuator.
Kent notes that the incremental cost for a higher pressure system might be approximately 10 to 25 per cent higher than a conventional one.
High pressure machines have important differences from conventional ones. "To go from 60,000 to 90,000 psi is a massive change,” says Pedrazas. To get those kinds of pressures, you need a different intensifier and a different system. The water tubing and cutting heads also are different.
While transparent to the user, the controls may be different, too, taking into account the fact that the characteristics of the jet… Acceleration, deceleration, how you do taper models – the software is different,” says Kent. "From the operator's standpoint, though, it's very similar; the machine is just cutting faster.”
Hydraulic intensifier pumps are used for high pressure systems. Conventional systems, such as those manufactured by OMAX, often use simpler, direct-drive pumps. Parry notes that the firm uses direct-drive pumps by choice. It has a history with hydraulic intensifiers: "If we wanted a hydraulic intensifier pump, we could get one pretty easily," he says.
Very high pressure
Manufacturers of high-pressure systems see ongoing interest in higher pressures. Pump technology is being challenged. For example, one pump from KMT Waterjet clocks in at 125 hp. "Such a powerful pump allows you to have two cutting heads working simultaneously at 90,000 psi each. You can double your production very simply, notes Pedrazas. Even though his firm carries a range of pumps, more than half of sales are currently for high-pressure systems.
Flow's waterjet cutting system.
Flow International has comparable pumps in development. "Right now, we run at 94,000 psi. There is nothing magic about that number. Someday, we will come out with a higher pressure pump, but the economics have to make sense," says Kent.
"More pressure, more parts, more profits," says Pedrazas.
Parry remains skeptical about ultra-high pressure. "If you want more speed, get more power. And that can be done more economically at 60,000 psi than at 90,000 psi," he says.
According to Parry, it makes more sense to increase the pump size without changing the orifice size to raise flow rates and boost cutting power. You are going to use more water and abrasive, but you will avoid any potential problems with maintaining a more complex system. "Higher pressure definitely affects reliability and maintenance costs, where increased volume flow rate does not,” he says.
The best way to decide among the alternatives? "Do some test cuts on the materials you usually cut,” says Parry. "You will see for yourself–the time spent, the quality, and the abrasive consumed. Also, try to get a realistic handle on what your operating and maintenance costs are going to be.”
Jim Barnes is a Toronto-based technical journalist with more than 30 years of experience.