WATER JET CONFERENCE - Waterjet Technology Association

cleaning ability. Therefore, in any analysis of jet cutting performance, it is necessary to
take both the jet velocity and volume into account.
Jet velocity is proportional to the square root of the nozzle pressure that produces
it. Since it is easier to measure the nozzle pressure, it is used instead of the resulting jet
velocity.
In much of the literature reporting jet cutting, the results are analyzed primarily as
a function of the pressure at the pump. This is very misleading. If the nozzle size is
constant and the pressure is increased, then the flow rate through the nozzle also
increases. Therefore, the nozzle's power output has increased from both greater flow and
higher pressure. In many of the cases it is the increase in total power that has improved
the cutting performance. To get a true test of the effect of jet pressure, the nozzle size
should be reduced accordingly to keep the nozzle power constant.
It is important to understand the distinction between nozzle pressure and jet
power. Nozzle pressure is a measure of force per unit area. Most materials require a
minimum nozzle pressure (threshold) for effective penetration. Jet power is a measure of
the total amount of energy per unit of time which is being delivered by the jet. Large
volume jets have a lot of power and can cut or clean many materials very effectively,
even at low pressure.
It may be helpful to refer to Figure 1 which shows the relationships between,
cutting effect and nozzle pressure for two sizes of nozzle.
Using the small nozzle and increasing the pressure from P1 to P2 will increase the
cutting effect from E1 to E2. However, a larger nozzle could have achieved the same
effect with more flow but at the lower pressure Pi. This is assuming that P1 and P2 are
greater than the threshold pressure.
FRICTION LOSS
All of the piping elements in a waterblast system have some resistance to flow.
Each of the elements contribute to the total pressure drop from the pump to the nozzle.
The pressure drop depends on the flow rate and fluid viscosity. Unfortunately, the
standard reference works on fluid flow typically deal with relatively slow velocities.
They are of little value in analyzing waterblast systems. What is needed is a means of
quantifying the potential for pressure drop in each element and a way to add them to give
the total.
The authors have found it useful to employ the flow coefficient, Cv. This term is
normally used to measure the flow restriction in control valves. The Cv for any device
can be determined by measuring the flow rate and corresponding pressure drop through
the device. Table 1 presents the measured Cv values for some commonly used elements
in a waterblast system, and the resulting pressure loss from friction. In most tube cleaning
jobs the lance rod is the major source of pressure loss due to its small diameter.
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