Critical facts on high pressure waterjet

There has been a lot of discussion about the benefits of moving to higher and higher water pressure in abrasive waterjet (AWJ) cutting systems. It has gotten to the point that you now hear statements such as “Everybody knows that the secret to faster cutting is higher pressure.” However actual cutting test data and user experience show that such statements are incorrect and misleading. In fact experienced AWJ users who have tried 90,000 psi (90ksi) systems are now moving back to more traditional 55,000-60,000 psi (55-60ksi) systems at a net increase in production speed with reduced operating costs, reduced system downtime and higher profits. Here is why.

The notion that increased pressure means faster cutting ignores the following two basic facts about the AWJ process:

1. Power is proportional to pressure times volume flow rate (P=kpV). For a given pump power any increase in pressure must be matched by a decrease in volume flow rate. More simply put: “You can’t get something for nothing”. This means that a higher pressure pump must use a nozzle with a smaller orifice. For example a 50 Hp intensifier pump that would use a 0.014” nozzle orifice at 60ksi is constrained to just a 0.010” orifice at 90ksi.  

2. In AWJ cutting systems the abrasive does the cutting, not the water. The only purpose of the water is to accelerate a quantity of small abrasive particles in a coherent stream that can erode the material being cut. The smaller diameter waterjet that comes from a higher pressure system may be more effective in water-only cutting applications (such as food products or foam rubber) but it is not more effective at entraining and accelerating the quantity of abrasive particles needed for AWJ applications.

The net effect

The bottom line effect of these two factors, as shown in actual head-to-head cutting tests, is that there is very little net difference in cutting speed between traditional AWJ pressures in the 55-60ksi range and ultra-high pressures in the 87-90ksi range, as long as pump power and abrasive flow rate are kept the same. This is true over a wide range of materials and a wide range of thicknesses. For some materials the lower pressure systems may actually cut faster by a bit and for some materials the opposite may be true, but in all cases the straightline separation cutting speeds are very close. Table 1 shows some typical test results which are confirmed by actual system users:

Operating and maintenance costs

Experienced 87-90ksi users now confirm what has always been assumed: The operating and maintenance costs of 87-90ksi systems are considerably greater than those for 55-60ksi systems. Not only are pump maintenance costs greater due to shorter component life and the need for more frequent seal changes, but also balance-of-system costs are considerably more than many users anticipated. Surprising to some has been the greatly reduced mixing tube life for 90ksi nozzles. Mixing tube suppliers now typically recommend only their highest quality (and most expensive) mixing tubes for use in 87-90ksi systems and even with that, 30 hours is considered a very good life. Other components subjected to 87-90ksi pressure, such as tubing, valves and fittings, are seeing reduced life and greater susceptibility to unpredictable fatigue failure. This has the dual negative effect of increasing maintenance costs and increasing unplanned system downtime. Worst of all, if a user tries to reduce his maintenance costs by running an 87-90ksi system at lower pressure, he finds that he is still limited to a small nozzle because the hydraulic side of the intensifier is flow-limited and just can’t stroke fast enough to provide the volume needed for a full-sized nozzle. Now he is stuck with an expensive pump operating wastefSNEAKERS