205CTMag.pdf28 • February 2005 • CleanerTimes
he Pulsed Jet Technique (PJT) concept has been known for more than
30 years. However, the concept described in this article is quite different from
the techniques described in the literature1. The nature of the forced pulsed water-
jet described here2,3 is illustrated in the four photos of Fig. 1, which were cap-
tured by means of a Nd:YAG pulsed laser.
The first photo shows the typical appearance of a jet emerging from a com-
monly used waterblasting nozzle. In the second through fourth photos, the jet
is changed by introducing ultrasonic oscillations in the nozzle just upstream of
the nozzle exit. As the ultrasonic power is
gradually increased, the appearance of the
jet changes as indicated in the photos.
Finally, when the ultrasonic power input
is optimum, fully developed (mushroom-
shaped) pulses of water are formed. Notice,
however, that the pulses are not complete-
ly separated. They are superimposed on
the continuous stream, a process referred to
as modulation.
ular jet and the fully developed PJT is their
diameters. Whereas the diameter of the
former is almost equal to the orifice diam-
eter, the diameter of the mushroom-shaped
pulses can be, depending on the flow and
pressure, as large as 20 times the orifice
diameter. The increase in jet diameter at a
given standoff distance can provide a cor-
responding increase in productivity. An
attraction of the technique is that no mov-
ing parts are used in modulating (pulsing)
the continuous stream2,3.
Impact pressure is a second area of improvement over a regular, non-pulsed
jet. The impact pressure generated on the target to be treated (cleaning, cutting,
or virtually any application) by the pulse is the waterhammer pressure, Ph. The
magnitude of Ph can be calculated using the equation:
Doing More
T
Fig. 1
HIGH PRESSURE
Modulation of a regular jet with ultrasonic power input. Photos courtesy of VLN.
CleanerTimes • February 2005 • 29
speed of jet; and C = speed of sound
in water.
pressure, ps, of a non-pulsed jet on the
target is simply the pump pressure less
the losses in the hoses, fittings, etc.
Therefore, the amplification (M =
10,000 psi, the theoretical impact
pressure would be about 80,000 psi!
In practice however, due to various
inefficiencies involved in the process,
the impact pressure would be about
40,000 psi, equivalent to the generally
used ultra-high pressure (UHP) pump.
A third benefit is the high fre-
quency of impacts, in the present case
20 kHz.
examples.
waterjet at only 4500 psi at a hy-
draulic power of 15 hp. On the other
hand, the strip “B” was removed
using a 30,000 psi UHP waterjet at
the same hydraulic power. The
width removed with the pulsed jet
is clean and is almost four times
that of the UHP jet, confirming the
existence of large diameter pulses.
Removal of thick coatings—Fig. 3
shows even more markedly the
performance of a pulsed waterjet at
11,000 psi compared to a UHP con-
tinuous waterjet at 40,000 psi. Both
delivered the same hydraulic power,
27 hp. Single-orifice nozzles were
used for the comparison. Tests were
conducted on an X-Y gantry, at a
traverse speed of 2000 in./min, at a
fixed standoff distance of 1.5 in.
For information circle 384
Comparison of baked enamel removal by UHP and PJT waterjets.
The steel sample, obtained from the US
Navy, had multiple coating films:
• 2 coats anti-corrosive paint
BRA 640 Red.
film was 5 mils, resulting in a total coating
thickness of 35 mils. Blast profile on the steel
substrate was 2–4 mils.
First, a single pass test was conducted to find
out the width of the swath of paint removed. In
the photograph of Fig. 3, these are identified as
Test 1 (pulse jet) and Test 3 (40,000 psi jet). The much
wider path removed by the pulse jet is apparent.
To achieve a 1-in. wide swath, the pulsed jet re-
quired only eight adjacent passes, compared to
32 by the UHP jet (Tests 2 and 4). The tests result-
ed in area removal rates of 104.2 ft2/hr for the
pulsed jet and 26.0 ft2/hr for the non-pulsed jet.
If one employs an oscillating nozzle, a single-
orifice nozzle is quite sufficient. However, for
many hand-held applications rotating waterjets
are required. A twin-orifice self-rotating nozzle,
which has been thoroughly tested for removing a
variety of non-skid coatings, is depicted in Fig. 4.
Decontamination—Fig. 5 shows PJT used to
address a complex challenge: chemical and
radiological decontamination of
Sweden4. In both cases, PJT proved
to be quite effective.
grated into the design of a machine
consisting of a pump and the pulse-
producing ultrasonic system, or it
can be an add-on/retrofit for exist-
ing equipment. Figure 6 shows an
add-on module in use. All the end-
user has to do is connect a 10,000-psi
pump to the module, and the per-
formance obtained is equal to or
better than that of a 30,000–40,000-psi
UHP pump.
result in a gun such as shown in
Fig. 6 that is quite light and easy to
operate. Both the retrofit module
and the gun illustrated here have
been tested by the Electrical Standard
Association (ESA) in Canada and
certified as safe for hand-held or
robotic applications. For information circle 346
30 • February 2005 • CleanerTimes
Test #1: Pulsed Jet @ 11 kpsi – single pass Test #3: 40 kpsi Continuous Jet Test #2: Pulsed Jet – 8 adjacent passes Test #4: 40 kpsi jet – 32 adjacent passes
Nozzle Diameters: 40 kpsi: 0.015-in 11 kpsi: 0.040-in (pulsed jet) Standoff distance: 1.5-in
Fig. 3
Fig. 4
Removal of hard non-skid coating by a self-rotating pulsed waterjet at 10,000 psi.
Performance comparison between lower pressure pulsed and UHP non- pulsed waterjets for removal of hard, multi-layered marine coatings.
What’s Next
PJT retrofit modules are being evaluated at the sites of two large U.S.
companies. In a subsequent article, I will describe the outcome of these
investigations and other demanding applications such as refurbish-
ing concrete ceilings of underground parking garages, and I will also
describe enhancements to the technology.
Mohan Vijay, Ph.D. has been a part of the international waterjet community
for 30 years and has written extensively on waterjet technology. Dr. Vijay
founded VLN Advanced Technologies Inc. in 1998 after 25 years with the
National Research Council of Canada. He is a past director of the boards of
both the International Society of Water Jet Technology and Water Jet
Technology Association and is a past president and chairman of the board
respectively of these organizations. CT
References
1. Vijay, M.M., “Pulsed Jets: Fundamentals and Applications,”
Proceedings of the 5th Pacific Rim International Conference on Water Jet
Technology, New Delhi, India, 1998, pp. 610-627.
2. Yan, W., A. Tieu, B. Ren and M. Vijay, “High-Frequency Forced Pulsed
Waterjet Technology for the Removal of Coatings, Journal of Protective
Coatings & Linings, Volume 20, Number 1, January 2003.
3. Vijay, M.M., “Ultrasonically Generated Cavitating or Interrupted Jet,” US
Patent No. 5,154,347, October 13, 1992 (International patent pending).
4. Tieu, A., W. Yan, M.M. Vijay, T. Cousins, D.S. Haslip, S.E. Sparkes,
T.A. Jones and D. Estan, “Chemical and Radioactive Decontamination
of Armored Vehicles Using High-Frequency Forced Pulsed Waterjet
Machine,” Proceedings of the 16th International Conference on Water
Jetting, Aix-en-Provence, France, 2002, pp. 609-626.
CleanerTimes • February 2005 • 31
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