Avoid “Cutting” When Cleaning
As higher and higher pressure is increasingly being used in the cleaning industry, there is a concern that the waterjet --- used for cleaning heat exchanger tubes or other steel process lines --- can actually “cut” into the tubes and cause damages. To address this concern, Wright et al* did some tests with both rotary and stationary nozzles and presented the results on the 2005 WJTA conference.
Some of the tests were done with a stationary nozzle shooting at a 1018 steel pipe perpendicularly at varying pressure and time period. At pressure of 250 Mpa (36 kpsi), significant damage was observed for as short as 10 seconds of time period. Damage can be avoided if pressure is 140 Mpa (20 kpsi) or lower and the standoff is small (9.6 mm or .038”). However, a poor quality jet (less coherent) may lower the safe pressure limit to 70 Mpa (10 kpsi).
Standoff distance is another major factor. A test was done with a stationary nozzle (.97 mm or .038” dia.) shooting at a 1018 steel pipe perpendicularly with 105 Mpa (15 kpsi) for 30 seconds. No damage was observed at 9.6 mm (.038”), and depth of damage increased in proportion to standoff, up to about 50 mm (2”). Damage declined as standoff increased beyond 50 mm.
Compared to a stationary jet, a rotating jet can reduce the chance of damage to the steel pipe. However, a rotating jet dwelling at one spot can also cause damage, depending on pressure and dwell time. One of the tests was done with a three-jet nozzle head (.84 mm or .033” dia orifice), rotating at 500 rpm and aiming at 85° to target surface. This test indicated that 10 seconds of dwell time is safe with 140 Mpa or lower pressure, but not with 250 Mpa. So when a pressure of 250 Mpa or higher is needed, the nozzle must be rotating and moving the whole time.
Tests with single-jet rotating heads, running at 250 Mpa, were also done to determine the effect of angel of jet. At 10°, no damage was observed for up to 60 seconds. But as the angle was increased to 20° and beyond, damage started to occur and increase proportionally with dwell time and angle.
Therefore, to avoid damage to the subject steel pipe, one should use caution when selecting pressure, standoff distance, and angle of attack, and avoid having the jet dwelling at one spot. Sometimes damage may be due to corrosion. Usually a corrosion pit has sharp edges while a waterjet-eroded pit has smooth and rounded edges.
On the same conference, the same authors also presented another paper on a similar topic, applied to sewer pipe cleaning with lower pressure.
* Wright, D., Wolgamott, J., and Zink, G., “Safe Waterjet Cleaning of Steel Process Lines,” Proceedings of the 2005 WJTA American Waterjet Conference, Houston, Texas, August 21-23, 2005, Paper 2B-13.
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