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How 50 hp at 60,000 psi Outperforms 50 hp at 87,000 psi in Abrasive Waterjet Cutting

May 5th, 2011 3 comments

Head-to-head straight line abrasive waterjet (AWJ) cutting tests show that a 60,000 psi direct drive pump powered by a 50 hp motor will consistently cut as fast as or faster than an 87,000 psi hydraulic intensifier pump also powered by a 50 hp motor. This is true over a wide range of materials and thicknesses when both systems operate at the same abrasive flow rate. It is also true for even larger pumps. The overall bottom line is that for a given electrical power input and a given abrasive flow rate, direct drive pumps outperform hydraulic intensifier pumps in AWJ applications. Table 1 below shows some typical test results which are confirmed by actual system users.

Table 1: Comparison of Separation Cutting Speeds (inches/min.)– 60 ksi vs. 87 ksi with same electric input power and same abrasive flow rate

Cutting Speed

Pump

Pressure (ksi)

Orifice Size (inches)

Abrasive Flow (lb/min)

1″ thick alum.

2″ thick alum.

2″ thick steel

50 hp Direct Drive

60

0.015

0.8

20

7.7

3.0

50 hp Intensifier

87

0.010

0.8

17

6.5

2.6

100 hp Direct Drive

60

0.022

1.4

30

12.4

4.0

100 hp Intensifier

87

0.015

1.4

25

9.4

3.7


Why is this so?

How can a lower pressure system cut faster than a higher pressure system when “Everybody knows that the secret to faster cutting is higher pressure?” The fact is that actual cutting test data and real-world experience show that the benefit of going to ever-higher pressure is a myth. In fact, experienced AWJ users who have tried 87,000-90,000 psi (87-90 ksi) systems are now moving back to more traditional 55,000-60,000 psi (55-60 ksi) systems to achieve a net increase in production speed with reduced operating costs, reduced system downtime and higher profits. Here is why:

Some Basic Physics

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

  1. Higher pressure means Lower Flow: Power is proportional to Pressure times Volume Flow Rate (Power=Constant x Pressure x Flow): For a given electrical input power, any increase in pressure must be matched by a decrease in volume flow rate. In the world of physics 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 normally use a 0.014″ nozzle orifice at 60 ksi is constrained to just a 0.010″ orifice at 90 ksi.
  2. Abrasive does the cutting: 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 jet stream that comes from a higher pressure system might be more effective in water-only cutting applications (such as food products or foam rubber), but it actually carries less momentum for entraining and accelerating abrasive particles than a lower pressure jet of the same horsepower.
  3. Efficiency beats pressure: Direct drive crankshaft pumps are inherently more efficient in converting motor power to nozzle power because they do not suffer from the hydraulic losses of an intensifier pump. This means that a higher percentage of the electrical power that drives the pump actually gets to the cutting nozzle and less power is wasted as heat. A 50 horsepower direct drive pump can deliver about 45 horsepower to the nozzle versus a good intensifier delivering 35 horsepower to the nozzle. This 28% boost in power has a dramatic effect on cutting speed.

These three factors combined are what cause the direct drive 55-60 ksi pump to outperform the 87-90 ksi intensifier in material cutting tests and in actual operation.

Operating and Maintenance Costs

Experienced 87-90 ksi users now confirm what has always been assumed: The operating and maintenance costs of 87-90 ksi systems are considerably greater than those for 55-60 ksi systems. Not only are pump maintenance costs greater due to shorter component life and more frequent seal changes, but also balance-of-system costs are considerably more than many users anticipated. To some, a surprising discovery has been the greatly reduced mixing tube life for 87-90 ksi nozzles. Mixing tube suppliers now typically recommend only their highest quality (and most expensive) mixing tubes for use in 87-90 ksi systems; and even with that installed, 30 hours is considered a very good life for the highest quality mixing tube.

High pressure is the enemy of all the system plumbing, not because of the ultimate strength of the components, but because of the much lower fatigue limit. Components subjected to 87-90 ksi 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 87-90 ksi 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 at low power with an undersized nozzle, resulting in greatly reduced cutting speed. His only real solution to this problem is to purchase a new pump designed for higher flow/60 ksi operation.

Abrasive Costs

One of the myths of 87-90 ksi operation is that abrasive costs can be reduced. The question that should be asked: “Abrasive costs can be reduced compared to what?” The data of Table 1 clearly shows the speed advantage of the 60 ksi system over the 87 ksi system when abrasive flow rate is held constant at a reasonable and economic level. Sometimes a particular project might put a high value on speed, even at some increase in operating cost. For such situations the traditional 55-60 ksi system has the advantage of having a large enough nozzle to accept higher abrasive feed rates if desired. For example, merely increasing the abrasive flow rate from 0.8 to 1.0 lb/min for the 50 hp direct drive pump shown in Table 1 can increase the cutting speed in 2″ aluminum by about 10% in those cases where higher cutting speed is more important than higher operating cost. The user of a 55-60 ksi system is not required to go to greater abrasive flow rates, but he certainly has the ability if desired, and he can very accurately determine the costs of doing so in advance.

What is the real key to AWJ performance?

Test data shows and experienced AWJ users agree that the key to faster more economical AWJ performance is not the use of ultra-ultra high pressure. Indeed, for a given electrical input power and abrasive flow rate the increase in pressure only increases costs and system downtime. Rather, the secret to performance is reliable economical operation using efficient direct-drive pumps at traditional pressures in the 60ksi range for fastest cutting at lowest cost.

Best regards,

Dr. John Olsen