Quality Waterjet Newsletter 7/19/2005

Review On Surface Preparation with Waterjets --- Part Three

This part of the review is focused on thermal spray removal using waterjets. An article by Watson (*) described the basic information about thermal spray coatings, removal methods prior to waterjets, and a study on the process of thermal spray removal using ultrahigh pressure waterjets.

Thermal spray is referred to a type of coating that is applied at elevated temperature. Usually metallic or ceramic materials are heated to a molten state and then propelled onto a prepared surface to form the coating. It is often used on jet engines and gas turbine components. The coating is removed and replaced during engine overhaul.

Prior to waterjets, this type of coating was removed with one of these methods: chemical soaking, machining on mills or lathes, or sand blasting. Chemical soaking is hazardous to human beings as well as to environment. Machining is time consuming and often leads to some base material removal and thus shortens component’s life. Sand blasting may introduce unwanted residual stresses as well as waste.

Material removal modes using waterjets include cutting by the stream itself, crack formation from hydraulic pressure, de-bonding due to a tension condition following initial compression state, and an erosion process.

Waterjet tests at up to 55,000 psi were done on coupons that were carefully prepared to match engine’s working conditions. Several process parameters were identified: pressure (thus jet velocity), stand-off, size as well as number of orifices, number of passes/coverage, and angle of attack. The test results show many advantages of waterjets over prior arts. Waterjetting has thus become a highly acceptable process.

A production thermal spray removal system was briefly described. It includes an ultrahigh pressure pump, a protective chamber (cell), motion control, mist collection, and waste filtration as well as disposal.

As the main conclusion, the waterjetting process has revolutionized the thermal spray removal industry.

* Watson, Jeffrey D., “Thermal Spray Removal with Ultrahigh-Velocity Waterjets”, Proceedings of the 7th American Water Jet Conference, Seattle, WA, August 28-31, 1993, Paper 42.

Book Review – Part Two

Trouble shooting of high pressure pumps and systems:

·      Suction Lines: The suction line should be at least as large as the pump suction and one size larger is preferable.  It is advisable to use a flexible hose rather than hard piping when possible. The number of ells, turns and restrictions should be kept to a minimum and an ideal installation would have none at all.  If ells are necessary, they should not be closer than five feet from the pump inlet and no two ells should be closer together than three feet.  Branch laterals and 45-degree ells are preferred in a suction line rather than 90 degree ells. All valves in the suction line should be full open type.

·      Discharge Piping: Discharge piping should extent at least five feet from the pump before any turns or ells are installed and flexible hose is recommended to eliminate line stresses and to dampen vibration. Any ells should be 45-degree type or laterals and two 45-degree ells are preferable to a single 90-degree ell.

Excerpt from Chapter 11, Trouble Shooting, of High Pressure Pumps & Systems by Michael Gracey, with the courtesy of the author.


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