Feel the force of a fundamental element

Water forced through a small aperture at extremely high pressure can be highly formidable.

Those of you familiar with Lothar G Buchheim’s ‘Das Boot’ may recall ‘a jet of water only finger thin, but capable of sawing a man if half.’

Proof indeed that water forced through a small aperture at extremely high pressure can be highly formidable and it is a phenomenon that has not gone unnoticed by manufacturers of water cutting machinery.

Industries employing cutting machinery seem, however, to be largely unaware of the benefits of abrasive water-jet machining.

This may seem somewhat mind-boggling, given the apparent benefits of water-jet cutting.

The surface of a material cut by water-jet is left relativey unmarked, and there is no distortion, even around the edge-zone. A water-jet can also handle reflective materials that might deflect light into the mirror system of a laser-cutting machine. ‘Sadly, few people in industries where cutting is an important part of their business actually know the full potential of water-jet cutters,’ says Colin Gee, of Gee Graphite, a company that has installed the prototype TGD2000 water-jet cutting machine. The TGD2000 is the first 700 bar direct injection abrasive water-jet cutting system of its type to be built in the UK.

The TGD2000 is the result of a ‘triple alliance’ between Gee Graphite, end user of the new machine; Diajet, manufacturer of suspension abrasive water-jet equipment; and machine table manufacturer Techserv Engineering.

Abrasive water-jet methods can be divided into entrainment systems and abrasive slurry systems. The key difference between the two technologies rests on how the abrasive is mixed with water.

Entrained systems pressurise water straight into a jet, and then pull abrasive particles into the rapidly moving stream as it enters the mixing tube. The mixture of water and abrasives is also focused by the mixing tube and directed at the work piece. This arrangement is, in effect, a jet pump or ejector.

However, according to Diajet, entrainment using the ejector principle is an inefficient method of mixing abrasive into water since the mixing process consumes much of the energy in the system. In addition, air is entrained in the cutting beam, which expands after exiting the nozzle, causing the beam to expand and precision to suffer.

A ‘direct injection’ abrasive slurry system mixes water and abrasive in a pressurised container before the slurry is delivered. This avoids the energy loss that entrainment systems endure with their reliance on a jet pump/ejector for mixing.

The TGD2000 appears to have resolved the problems associated with abrasive slurry systems, namely that lower pressures, very high material removal rates, and less expensive equipment are offset by wear on all parts of the system.

‘Normally, what people do with abrasive water-jets is accelerate the water flow at very high pressures, typically 2-4000 Bar, through the nozzle’ says Robin Smoult, General Manager of Diajet.

‘The way we do it is mix the abrasive in before it’s accelerated through the nozzle; the advantage being that you don’t have a mixing area that also mixes air and water,’ explains Smoult. ‘Mixing air and water at high velocities can be highly turbulent and relatively inefficient, so we mix the abrasive in the water so we have a pressure vessel full of abrasive and water. We inject that into the water flow, which can travel any distance, then goes out through the nozzle.’

This two-phase mixture exits as a much more coherent jet, and it stays coherent for longer whether it is in air or under water. This property enables the jet to cut thicker sections with better precision and less taper than conventional jetting tools.

However, mixing water and abrasive prior to the nozzle can result in a number of valves operating at 700 Bar, with abrasive water going through them at high velocities. This in turn can lead to a significant unreliability and represented a fundamental problem for Diajet.

‘We’ve got a way of injecting the abrasive so that you don’t need to control it through a valve,’ says Smoult. ‘We developed a jet pump method of doing this: we divert water flow into the vessel, which pushes the slurry out into the main flow. This means you’ve got no valves operating in a slurry flow of pressure. Essentially, we took the jet pump away from the nozzle, as well as develop a new ‘top discharge device.’

Consequently, Diajet has got the TGD2000 to operate at around 10KW at 700 bar, rather than the more normal 45KW at 3000 bar. With the Climate Change Levy set to be implemented in April 2001, it would appear that the TGD2000 will make a timely, cost effective addition to water-jet cutting.

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