Dr. Olsen's Lab

Power at the Nozzle

Regardless of the size or type of pump or the size of its drive motor, the real measure of power output is the power at the waterjet nozzle. This is a direct function of the nozzle pressure and the volume flow rate through the nozzle, which can be expressed by the following formula:

HP = 0.58PQ

Where:

1. HP equals the hydraulic power actually delivered through the nozzle in units of horsepower

2. P is the water pressure at the nozzle in units of thousands of pounds per square inch  (for example, use 55 for 55,000 psi). This can usually be closely approximated by the pump output pressure, but watch out for systems that try to operate relatively large nozzle orifices (say greater that 0.014”) using relatively long runs of ¼” ultra-high pressure tubing with many fittings.  The pressure drop between the pump and the nozzle for such systems can be several thousand psi.
3. Q is the volume flow rate through the nozzle, in units of gallons per minute
4. The constant of 0.58 accounts for the units of measure being used in the equation.

This simple equation makes two things very clear:

1. The size of pump motor and the exact design and brand of pump are not in the equation.  All that really matters in determining true nozzle power are the nozzle pressure and the volume flow rate
2. Both pressure and volume flow rate are in the equation and have equal effect.  Power at the nozzle can be increased by increasing pressure or increasing volume flow rate or a combination of both.

So the next time you are trying to compare ultra-high pressure pumps, ignore the size of the drive motor shown on the manufacturer’s spec sheet.  Go further down the spec sheet and find the values of the recommended continuous operating pressure and the corresponding output volume flow rate.  Then grab your calculator and determine for yourself the actual effective output power.

Best regards,

John Olsen