Sunday, November 25, 2012
When I first began experimenting with a waterjet system back in 1965 I used a pump that could barely produce 10,000 psi. This limited the range of materials that we could cut (this was before the days when abrasive particles were added to the jet stream) and so it was with some anticipation that we received a new pump, after my move to Missouri in 1968. The new, 60-hp pump came with a high-pressure end that delivered 3.3 gpm at 30,000 psi. which meant that a 0.027 inch diameter orifice in the nozzle was needed to achieve full operating pressure. However I could also obtain (and this is now a feature of a number of pumps from different suppliers) a second high-pressure end for the pump. By unbolting the first, and attaching the second, I could alter the plunger and cylinder diameters so that, for the same drive and motor rpm, the pump would now deliver some 10 gpm at a flow rate of 10 gpm. This flow, at the lower pressure, could be used to feed four nozzles, each with a 0.029 inch diameter. However there were a couple of snags in using this system to explore the cutting capabilities of waterjet streams in a variety of targets. The first of these was when the larger flow system was attached to the unit. In order to compare “apples with apples” at different pressures some of the tests were carried out with the same nozzle orifice. But the pump drive motor was a fixed speed unit which produced the same 10 gpm volume flow out of the delivery manifold regardless of delivery pressure (within the design limits). Because the single small nozzle would only handle a quarter of this flow, at that pressure (see table from Waterjetting 1c) the rest of the water leaving the manifold needed an alternate path.