WATER JET CONFERENCE - Waterjet Technology Association

Subscripts
n - Nozzle
p - Pump
t - Total
Hydraulic Power
HP= K1* P* Q (1)
When HP is in horsepower, P is in psi, and Q is in gpm, this becomes,
HP = (P*Q)/1714 (2)
Definition of Cv
Cv is defined as the flow of fluid in gpm through a device with a 1 psi
pressure drop across it.
Cv = Q*(ρ/62.4*Δp)^1/2 (3 )
For water at approximately 70 F this can be rewritten,
P = (Q/Cv)^2 (4)
The total Cv for a system composed of several elements is,
(Cv total)^-2 = (Cvl)^-2 + (Cv2)^-2 + . .. (Cvn)^-2 (5)
Calculatinq Cv
An equation has been derived empirically from test measurements by the authors,
to predict the Cv for tube, pipe and hose.
Cv=53 * D^2.5/L^1/2 (6)
This equation yields Cv's that are within 5% of the measured values for a large
range of plumbing elements commonly used in waterblast cleaning. Water temperatures
of + 30° from 70°F caused only a + 2% change in measured Cv. When testing relatively
unrestrictive devices it is necessary to use only the pressure drop resulting from friction
loss and not the velocity increase.
Optimum Flow
For any given pump pressure and system Cv, there exists an optimum flow rate
that will produce the maximum nozzle power. This value of Q can be solved for directly.
From earlier discussion we know that the nozzle
HP n=K 1*P n*Q ( 1 )
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