The pressure depends on the amount of water the pump passes through the nozzle hole. The smaller the orifice, the higher the pressure. Let's say, with a 100 horsepower pump and a wide bore, you can get the most out of your waterjet at 30,000 psi, but no OEM sells such a product because it's ineffective. On the other hand, 60,000 psi can be achieved with a 5-hp pump, but the application is severely limited, and the orifice will be ridiculously narrow.
The power is proportional to the pressure multiplied by the volume flow rate (p = kp v). For a given pump power, any increase in pressure must be matched by a proportional decrease in volumetric flow. This means that high-pressure pumps must use nozzles with smaller orifices. For example, a 0014-inch 50 hp booster pump. The nozzle orifice limit for 60 ksi is 0010 inches. The orifice is 90 ksi.
For pure waterjet applications without abrasives, greater pressure may result in faster cuts. In fact, smaller diameter jets from high-pressure systems may be more effective in pure water cutting applications, such as food or foam rubber. However, in an abrasive waterjet cutting system, it is the abrasive, not the water, that makes the cut. Conversely, the water accelerates the small abrasive particles in the coherent stream, which erodes the material being cut.
From 10,000 to 60,000 psi, abrasive waterjet cutting speeds have steadily increased. Since the higher psi focuses the particles at a single point, the finish and accuracy are also improved. However, at higher pressures, the direct relationship between psi and cutting speed begins to crumble.