<< Jeff Spitzer wrote:
 >
 > This looks like a classic case of inlet cavitation. >>

Thanks, Jeff.  That's not what I wanted to hear, but would rather hear
it now than later.  I was under the impression that I had about 14 psi
(absolute) inlet pressure during the test, and would have (at s/l) 15
psi plus the radiator system pressure on the inlet side, thereby,
reducing the probability of cavitation.  Hadn't considered the temp
effect.

Any suggestions for an easy mod to improve the inlet side?

Thanks, again.

Bob Darrah

Bill Freeman wrote:

Bob,
The things that prevent cavitation are:  increase the pressure
at the inlet and increase the boiling temp/press characteristics
of the fluid. Since you are probably stuck with water/glycol at
a specific temperature, the only obvious thing left is to increase
the inlet pressure.

I'm not a pump expert, but seems like a cooling system with the
lowest point as high as possible (above the pump would be
 good) and with as low flow resistance a cooling loop as is practical
should give the highest inlet pressure possible with a given system.
Any resistance in the system drops the downstream pressure and
reduces the pressure remaining at the inlet.  For example, if the pump
is putting out 15 psi at the outlet, it might be 12 psi at the downstream
side of the thermostat, 10 psi at the outlet of the engine,  6 psi at the
outlet of the radiator, and then lose another 1/2 psi going uphill a
foot to the pump inlet, where the remaining pressure is 5.75 psi.

Removing some of the flow resistance might increase this to
9 psi, a very substantial increase, which could prevent cavitation
if the system was nearly cavitating at the original conditions.
If the radiator was a foot ABOVE the pump inlet, the pump
inlet pressure gains a 1/2 psi - not a really big deal, and probably
not too important with these type of systems where the rad
is unlikely to be very far above or below the pump.

One thing that the above discussion does not address is the pump
characteristics.  Adding a restriction in the pump outlet side that
causes a 2 psi pressure drop might conceivably cause the pump
operating point to shift enough to overcome the resistance and
keep the same flow rate and possibly even keep the same inlet
pressure.  Doesn't seem too likely, but I'm as tructures guy and
it has been a long time since I did anything with pumps and it
was open loop stuff then, which is a lot different than closed loop
stuff, I think.

Note that these are purely hypothetical delta-P numbers and
have NO relationship to any actual pressure drops that may
exist in any of the system components. I don't want to hear
anybody say I said that radiators have a 4 psi pressure
drop.  They might, but I have no idea.

Bill

Jeff Spitzer wrote:

And to this I will add that when we talk about cavitation we are really
talking about *absolute* pressures.  Bill did not cite the reference for
his pressures so all I know is they were guage to something. Someone
pointed out in a recent post that it is best to put the radiator cap near
the pump inlet.  Bill's example helps clarify why.  If a 10 psi cap is
placed at the pump inlet in Bill's example the sea level absolute pressure
would be 14.7+10=19.7 psia.  If instead the cap were placed at the pump
outlet the absolute pressure at the inlet would be 14.7+10-(15-5.75)=15.45
psia.  A significant difference for cavitation.

Jeff Spitzer

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