Quality Waterjet Newsletter – 03/15/2005

The Impact of Pressure Fluctuation on Surface Finish

Pressure fluctuation is common with double-acting intensifier pumps. Because of compressibility of water, the initial 15% of the piston stroke does not produce any output of high-pressure water, which causes pressure fluctuation. By adding a “reservoir” (called “accumulator” or “attenuator”) to the output line, pressure fluctuation can be reduced.

We know that pressure fluctuation can cause fatigue failure in high-pressure components. Does it have any impact to surface finish in abrasive waterjet cutting? A recent experimental study by Karpinski et al.* discovered that it does.

Anyone who has seen a waterjet cut part may have noticed the “rooster-tail-like” striation marks at the lower part of the cut surface (actually you can see similar marks on surfaces cut by any energy beam such as laser, plasma arc, flame, etc). To eliminate these marks, the cutting speed has to be reduced to about 10-30% of the maximum cutting speed. It is no surprise that so many people in this industry want to find out the causes of these marks and find ways to eliminate them without the cutting speed penalty.

Karpinskit et al. used two intensifier pumps with different volume of accumulators (and therefore different levels of pressure fluctuation) in their abrasive waterjet cutting experiments. They compared the pressure signal with the surface profile of the cut parts in terms of amplitude and frequency of variation. They discovered that the striation marks on the parts cut with these two pump settings are similar in frequency, but the amplitude of the striation marks is significantly smaller with the less fluctuated pressure signal. Therefore, a larger accumulator volume helps to produce smoother surface finish. They also discovered that cutting head vibration is caused by micro-level (which may not show up in normal pressure gauges) pressure fluctuation, probably due to fluid turbulence in the high-pressure pipeline.

An accumulator with a larger volume will reduce the pressure fluctuation and improve surface finish. However, it will also significantly increase the cost of the pump system. These two aspects need to be balanced in building or purchasing a pump system.

*Karpinski, et al. "Effect of pressure fluctuations and vibration phenomenon on striation formation in AWJ cutting”, Proceeding of the 17th International Conference on Water Jetting, Mainz, Germany, September 7-9, 2004, pp.123-136.

Beyond High Pressure

· Call for participation: WJTA American Waterjet Conference will be held on August 21-23, 2005 at the Marriott Westchase Hotel in Houston, TX, focusing on advances in HP waterjet technology, new techniques & applications, systems/supplies/ equipment. For details, visit www.wjta.org call (314)241-1445.

· A book with a working title of HIGH PRESSURE PUMPS & SYSTEMS is being written by Mike Gracey in Houston, Texas. If you have some clear photographs showing high pressure pumps, ultra-high pressure pumps and water jetting project, please contact him at mike.gracey@weatherford.com

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