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

The challenge in making an accurate part with an Abrasive Waterjet has always been the fact that the jet stream bends as it moves through the material being cut.   As shown in the figure below, the lower part of the jet lags behind and moves differently than the upper part and that movement is a complex function of the jet parameters (such as orifice size, water flow rate, pressure and abrasive feed rate), the material being cut (what it is and how thick it is) and the geometry of the desired part.

For example, to create an accurate inside corner careful attention must be paid to the rate of deceleration of the nozzle as it approaches the corner in order to assure that the lower part of the jet has caught up with the upper part of the jet at the exact instant that the corner point is reached.  Otherwise the corner will tend to be washed out.  Even more important, the acceleration of the nozzle as it leaves the corner must be carefully controlled to avoid the “kickback” effect that can leave a notch in the lower edge of the part and ruin its accuracy.  Similar issues apply to outside corners, holes and curved sections of a part.  In general, the more complex the part geometry and the thicker the material being cut, the more important the motion of the jet and the less important the straight-line cutting speed in determining part production time.  Indeed, in many complex parts the nozzle spends all of its time accelerating and decelerating and never even approaches its maximum straight-line speed.

At OMAX we have spent years studying and modeling the motion of the abrasivejet and have an ongoing research effort to improve that modeling and to develop control strategies and algorithms that allow our control software to optimize part production time.  Our software is continually being improved and upgraded to take advantage of this research.  As a result we have seen production times for many representative parts cut in half over the years by machines that have the same straight-line cutting speed that they have always had.  The improvement has come from they way we move the jet, not by using expensive power to increase its maximum speed.  When evaluating different AWJ systems it is critical that potential buyers do more than just compare maximum cutting speed.  You really owe it to yourself to have test parts made that represent the complexity and desired accuracy of the parts that you plan to make.  Then compare the production times to determine which machine is truly faster.

Best regards,

Dr. John Olsen

Fatigue is a term used to describe the failure mechanism of metals that are subjected to cyclical stress. Cyclical stresses cause very small cracks to form and then grow until the resulting large crack weakens the structure so much that a sudden failure occurs. It is as if the metal became fatigued and gave up. You can witness such a failure in just a few cycles by bending a paper clip back and forth until it breaks.

Every time a pressure-loaded component in a waterjet system is pressurized and then un-pressured it goes through a stress cycle.  If a high pressure component is subjected to its full rated pressure and allowed to remain at that pressure, its life will be virtually infinite.  However, if its pressure is cycled on and off over and over again, it will most certainly fail. The number of cycles required for failure is determined by how close the cyclical stress is to the ultimate strength of the material. The S-N diagram (Stress vs Number of cycles before failure) shown below illustrates this for a typical steel.   When the stress level for each cycle is close to the rated maximum stress of the material, the component may fail after just a few thousand cycles.  However, as the cyclical stress is reduced, the average number of allowable stress cycles before failure increases dramatically.  Indeed if the cyclical stress level is reduced to a level below what is known as the Endurance Limit, the allowable stress cycles before failure in steels become virtually infinite and the fatigue problem goes away.

Generalized S-N Curve for Steels

What can we do to minimize the problem of fatigue?  One trick used by manufacturers of ultra-high pressure components is a manufacturing process known as autofrettage.  This technique was originally developed to strengthen large gun barrels on battleships (an application in which each shot fired represents a stress cycle and where a fatigue failure can be particularly catastrophic!).  Under this technique residual compressive stresses are intentionally created in the inside surface of a hollow cylinder (such as a pump barrel or a length of ultra-high pressure tubing).  These residual stresses have the effect of reducing the effective net cyclical loading and thus increasing the number of allowable stress cycles before failure.  Unfortunately this technique also increases the cost of these components.  In addition, it is not really practical for the complex shapes found in such components as fittings, valves, nozzle bodies, etc.

By far the most effective way to reduce fatigue stress and increase component life is to simply operate the system at lower pressure.  As one can see on the S-N curve, a 30% reduction in cyclical stress can increase the allowable number of stress cycles by a factor of 8 or more.  The primary reason that fatigue has become such an issue in ultra-high pressure waterjet systems is that users want to increase their productivity and are doing this by raising the operating pressures to increase the power at the cutting nozzle.  However, as I have discussed in previous entries to this blog, power at the nozzle can also be increased by simply increasing the flow rate. With greater flow rate, as opposed to greater pressure, higher cutting power can still be realized without the drawback of decreased component fatigue life.  This results in lower maintenance costs and greater system reliability, which in turn means more cost-effective operation and less unexpected downtime.  Indeed, unexpected downtime is the worst part of fatigue.  That nice smooth S-N diagram is statistically correct and can accurately predict the average cycles to failure for a large sample of components.  However, for an individual component the average doesn’t have much meaning.  The slightest scratch or imperfection in a component can speed the initiation of cracks and reduce the fatigue life below the average.  You can be sure the component will fail from fatigue, but you never really know quite when.  That can really mess up your production schedule.

Best regards,

John Olsen

Reference

Elements of Material Science, Second Edition by Lawrence H. Van Vlack,
published by Addison-Wesley Publishing Company in 1964

The abrasive waterjet cutting mechanism is similar to a grinding wheel.  The abrasive particles are moved by the water jet rather than the wheel and chip out small pieces just like the abrasive particles in the wheel does.  By using a hard abrasive such as garnet, an abrasive waterjet can cut a very wide range of metals and other hard materials and thus become a useful technology for machine and fabricating shops.

The key thing to remember is that in an abrasive waterjet the abrasive particles are doing the cutting, not the water.  The role of the water is to entrain the abrasive particles that are introduced at the nozzle and accelerate enough of them to a high enough speed to cut effectively.  Faster cutting is achieved by increasing the speed of the particles and/or by increasing the number of particles entrained in the water stream.

Interestingly enough, this gets us right back to the two basic ways of increasing hydraulic horsepower at the nozzle discussed in my entry of last November– increase pressure or increase water flow rate.  Increased pressure increases the speed of the water passing through the nozzle and has the potential to increase the speed of the entrained abrasive particles.  Increased water flow rate increases the ability of the water stream to entrain more abrasive and thus create a cutting jet with more “teeth”.

Either approach can increase cutting speed and either approach has associated costs.  The “more flow” approach results in an increase in consumption of water and abrasive, but at a known and predictable cost.  The “more pressure” approach results in higher maintenance costs, increased unpredictability of high-pressure component life and reduced system reliability.

Best regards,

John Olsen

1. The pump is of low efficiency and a big motor is really required.
2. The motor is larger than required for marketing or manufacturing reasons.

For the first case, I have written some fairly detailed articles about the difference in efficiency between direct-drive pumps and hydraulic intensifier pumps and how a direct-drive pump with a smaller motor can often outperform a hydraulic intensifier with a larger motor. (Refer to such an article at www.omax.com under “products”/ “direct-drive pumps” if you really want to get into it!). However the source of confusion goes even deeper than just the difference in these two types of pumps. Indeed, pumps of the same basic type can have different levels of efficiency depending on the details of how they are designed and manufactured. When the pump designer determined what size motor he would need to drive his pump he had to take that efficiency into account. He (or she!) started by knowing the desired effective power at the nozzle, based on pressure and flow rate. Then he had to add the power wasted by the inefficiency of his pump design and drive system (mechanical or hydraulic). Finally he had to select a motor that would do the job.

For the second case, the Manufacturing Manager has some input on motor selection because he wants a motor size that is readily available from suppliers or perhaps one that is already stocked for some other purpose. It is not uncommon for a pump manufacturer to use a drive motor that is actually larger than really needed. Moreover, the Marketing folks love it because it gives them “power bragging rights” over other pump manufacturers. Unfortunately the actual user may pay for those bragging rights with added electric utility demand charges and no real performance benefit.

The end result of all of this is that the final selected motor size may be substantially larger than the net effective power delivered to the nozzle. As I mentioned in my last entry, output pressure and flow rate are the true indicators of pump power, not the size of the motor.

Best regards,

John Olsen