28 • 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.

Basis of Enhanced Performance

A noticeable difference between the reg-

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

with LessPulsed Jet Techniqueby Mohan Vijay

T

Fig. 1

HIGH PRESSURE

Modulation of a regular jet withultrasonic power input. Photoscourtesy of VLN.

CleanerTimes • February 2005 • 29

ph = pVC

Where p = density of water; V =

speed of jet; and C = speed of sound

in water.

On the other hand, the impact

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 =

ph/ps) of the impact pressure on the

target is equal to 2(C/V). For exam-

ple, if the pump were operated at

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 of Effectiveness

The effectiveness of PJT can

readily be demonstrated by some

examples.

Removal of baked enamel—Fig. 2,

shows graphically how the strip of

hard baked enamel indicated by “A”

was removed with the forced pulsed

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

Fig. 2

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

• 2 coats Devoe Bar Rust

• 3 coats International Intervirion Antifoulant

BRA 640 Red.

The average dry thickness of each coating

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

armored vehicles. This work was

conducted both in Canada and

Sweden4. In both cases, PJT proved

to be quite effective.

PJT Systems

The PJT function can be inte-

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.

The design requirements of PJT

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 passTest #3: 40 kpsi Continuous JetTest #2: Pulsed Jet – 8 adjacent passesTest #4: 40 kpsi jet – 32 adjacent passes

Nozzle Diameters:40 kpsi: 0.015-in11 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

CAN’T REMOVE

ROAD FILM?YOU CAN NOW!

SELL AMERICA’S LEADING VEHICLE CLEANER

ETOWAH CHEMICAL would like to send you a

FREE sample (FREIGHT PRE-PAID) of this new

chemical that we manufacture for the washing

industry to REMOVE ROAD FILM. Our road film

removing chemistry enables us to offer this envir-

onmentally safe chemical to take care of your

number one problem.

Call us today, don’t put it off any longer! The call is

free, with no obligation. We just want the opportu-

nity to show you there IS a chemical that WORKS.

Ask for Ben Benefield, National Sales Manager.

ETOWAH CHEMICAL SALES & SERVICE

P.O. Box 4416 • Gadsden, AL 35904

National Watts 1-800-848-8541 • FAX 256-547-7555

We sell “ALKOTA CLEANING EQUIPMENT”

For information circle 170

Fig. 5

Fig. 6

Above: PJT being used for decontamination of aGrizzly armored vehicle. Below: A photograph ofthe forced pulsed waterjet generator (RFM), whichcan be operated at pressures up to 10,000-psi.