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Old 01-18-2013, 04:41 PM   #184
WDE46
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Join Date: Sep 2012
Location: Old Greg's Cavern
Posts: 9,388
My Ride: 2004 330Ci OBM
Quote:
Originally Posted by wildirish317 View Post
Good info on the test.

Since this thread has been resurrected, I'd like to enlighten those who care about pump cavitation. A centrifugal pump like our water pump has a "Net Positive Suction Head Requirement" or NPSHR. Head is pressure measured in feet of water (why do we have so many ways to measure things?). If the pressure available to the pump falls below this point, the pump begins to cavitate. (See post 21 for explanation of cavitation) To compound matters, the NPSHR varies with flow through the pump.

Temperature has a huge impact on NPSHA (Net Positive Suction Head Available) to the pump. NPSHA = Absolute pressure - vapor pressure of fluid being pumped + static height (ft) of the liquid being pumped - all suction line losses.

In a closed system, the static height is not considered due to the siphon effects of a closed system even everything out. The suction line losses can be a factor, but for now, we'll ignore them and assume BMW chose the correct pump in thier design.

This simplifies the equation to NPSHA = Absolute pressure minus vapor pressure.

Water at 60F has a vapor pressure of .26 psia. Water at 180F has a vapor pressure of 7.5 psia. Water at 212F has a vapor pressure of 14.7 psia (which is atmospheric pressure at sea level, which is why water boils at 212F.

So, as the water approaches the boiling point, the vapor pressure approaches the pressure of the system that contains it, and your NPSHA drops closer to zero.

Therefore, as long as you are not within a few degrees of your boiling point, your pump will not cavitate. If your water is boiling, your pump is cavitating.
Seems faily obvious. If you're at 209F and not boiling, a very small reduction in pressure will result in boiling. Good thing our system runs pressurized.
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