Quality
Waterjet Newsletter – 04/21/2009
Fundamental Aspects in Waterjet Cleaning Louis and Schikorr*
wrote an article back in 1982 to address some fundamental issues in waterjet cleaning. The article still has a great
reference value today. Here a review of the article. The article started with listing
out 15 typical waterjet cleaning applications. It
also sorted out the types of applications by the types of substrates and
layers to be removed. Characteristics of substrates were described in
details. Two noted characteristics of metal substrates are “Wöhler behaviour”(fretting) and corrosion. Layers were classified into
organic (naturally grown and artificial) and inorganic types. The
characteristics of each type of layers were also described in details. The
mechanisms (mechanical, specific, and chemosorption)
of adhesion were also discussed. The article also talked about two
types of free jets: round and flat. Both types are fully turbulent and they
disintegrate into small fluid particles shortly after they leave the nozzles.
Flat jets have a higher tendency of disintegration than the round jets. A continuous jet flowing
perpendicular to the target surface produces a normal force as well as a
shearing force that is parallel to the surface. In the case that the jet is hitting
a kerf, the flow will change direction and a large
portion of the kinetic energy will be converted into pressure, which could
open up a crack. In reality the jet becomes a stream of small fluid particles
which could produce pulses of impact pressures multiple times of the
steady-state pressure. A high temperature in the jet is helpful in removing
layers consisting of oil and grease. Regarding the layer removal
process in waterjet cleaning, the authors suggested
that crack formation is necessary for removing a hard and/or brittle layer
and dynamic loading (impact) is preferable. For ductile types of layers, a
high impact speed is preferable because a ductile material will become more
brittle with higher impact speeds. For viscous and viscoelastic
layers (bitumen, chlorine-caoutchouc, oil-grease),
a quasi-static load is sufficient and a higher temperature of the jet is
helpful. The last session of the article
addressed the topic of process optimization, assuming metals as the substrate
material. Metals can be damaged by the jet after an incubation period because
of the “Wöhler behaviour”.
Achieving the highest productivity without causing damage to the substrate
can be achieved by optimizing these three variables: (1) operating at a
pressure which can cause significant mass-loss for the layer to be removed
but below the incubation pressure of the substrate; (2) loading time should
be below the incubation period of the substrate; (3) a proper operating
distance between the nozzle and the target should be either just above or
just below the peak erosion zone. * Louis, H. & Schikorr, W. (1982) Fundamental aspects in cleaning with
high-speed water jets, Proceedings of the 6th International
Symposium on Jet Cutting Technology, Guildford, England, April 6-8, 1982, pp.
217-228. |
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