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Old 01-18-2013, 03:18 PM   #181
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Good info kmulder!

Seems mostly like BMW just opted to have the expansion tank cap act more like an emergency vent valve, and also probably taking into account people using just plain water and at high altitude which lowers everything slightly. It would be nice to know when problems of boiling actually start but that would probably be pretty hard to test.



And also filling to the minimum will be the most dangerous for risking boiling and warping in an overheating situation.


I don't think any government cares about evaporated water.
They might care about the little bit of antifreeze that's released (you've never seen the little bit of dried coolant near the cap?). Net impact is probably minimal, but they do seem to care how much of it makes it into the water eventually. I wouldn't put it past various environmentally inclined lawmakers to demand such measures.


Maybe there's some other reason. But if 2-bar is seldom achieved... Then I can't think of one. Seems like the 1.4 bar caps used on older vehicles would maintain enough of a safety margin for those who don't run the right mixtures.

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Old 01-18-2013, 03:21 PM   #182
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Good info. I'd say keeping the cap stock for environmental purposes is reason alone enough to justify it.
I disagree. The likelihood of it venting anything when everything works properly is minimal. So if something isn't working properly, I would rather protect the engine and ET than a tree.
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Old 01-18-2013, 03:39 PM   #183
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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.
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Old 01-18-2013, 03:41 PM   #184
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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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Old 01-18-2013, 04:03 PM   #185
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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.
Yeah, but after posting, I realized that the water itself is raising the pressure.

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Pressure at this temperature (126 C) was 20.5 PSI.
The vapor pressure of water at 126C (260F) is 35.4 psia, which is about 19.7 pisg (close to what they measured). I would've expected the measured pressure to be bit higher.

I'm going to have to ponder this for a while.
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Old 01-18-2013, 08:46 PM   #186
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...

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.
False. You talk about and explain NPSHA(strangely similar to how engineering toolbox and wikipedia do), but you don't calculate it or even estimate it in the case of an E46, or any car for that matter, so I'm not sure how you come to that conclusion. Further disproving your claim, there are many examples of water cavitating at room temperature. Just look on youtube...
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Old 01-21-2013, 11:46 AM   #187
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False. You talk about and explain NPSHA(strangely similar to how engineering toolbox and wikipedia do), but you don't calculate it or even estimate it in the case of an E46, or any car for that matter, so I'm not sure how you come to that conclusion. Further disproving your claim, there are many examples of water cavitating at room temperature. Just look on youtube...
My source was actually Cameron Hydraulic Data, published by Ingersoll Rand in 1981, which pre-dates both engineering tooxbox and wikipedia, although I use those resources as well.

I don't do any calculations because I don't have enough information. I don't have the pump curves for the water pump, which would tell me what the pump suction head requirement is at different speeds and flow rates. I don't know the vapor pressure of a 50/50 mix of coolant and water at different temperatures. I don't know the pressure loss through the cooling system at different flow rates. I'm speaking in generalities.

I'm not saying that pumps cannot cavitate at room temperature. As you point out, they can. I'm just pointing out that the head available to the pump is dependent on the system pressure and the vapor pressure of the fluid being pumped in a closed system such as an engine cooling system. The other factors, lift and line losses also have an impact, and these can cause a pump to cavitate at room temperature and pressure if the wrong pump is selected for the application.

That being said, I don't think that the coolant in a closed system would ever boil. As the temperature of the water goes up, it's the vapor pressure of the water at that temperature that raises the pressure. As long as everything stays together, the pressure will continue to rise with temperature, and the water will remain water. It's when the relief valve opens, allowing the system pressure to drop, that the water will boil and steam will be created. This is because the system pressure is now below the vapor pressure of the water.

My thinking, simple as it may be, is that when you fill a cooling system at 70F and close it up, the system pressure is 14.7 psia vs. the vapor pressure of 0.36 psia. This is a 14.3 psi difference (33 feet of head, which is a lot for a pump) that will be maintained between the system pressure and the vapor pressure as the water heats up. The air space in the expansion tank allows the air to compress to maintain that pressure differential.

So as long as the pump's suction head requirements are met by this differential (after subtracting the flow losses), the pump will not cavitate.

As always, I'm open to feedback on this.
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Old 01-21-2013, 11:50 AM   #188
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I disagree. The likelihood of it venting anything when everything works properly is minimal. So if something isn't working properly, I would rather protect the engine and ET than a tree.
That's assuming everyone's E46s is working properly. We know that isn't the case and people are overheating all the time and spewing coolant all over. Whether it be air in the system, cavitation, fans that don't turn on, etc. etc. etc.

Again, when everything is replaced at 75-90k intervals using OE/OE-spec'd parts, what exactly is the danger to the engine/ET again?
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Old 01-21-2013, 01:38 PM   #189
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I must say this thread has been one of the more fascinating ones I've read. Don't have a clue as to all the math that was applied in the discussion but the conclusions seem to make sense. I was really entertained with the personalities and egos involved. Like watching a funny reality show.

Seriously though, this thread proves that no matter your experience or education, we're all fanatics when it comes to our vehicles. It's what you take away from the threads that's important.

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Old 01-21-2013, 04:01 PM   #190
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I must say this thread has been one of the more fascinating ones I've read. Don't have a clue as to all the math that was applied in the discussion but the conclusions seem to make sense. I was really entertained with the personalities and egos involved. Like watching a funny reality show.

Seriously though, this thread proves that no matter your experience or education, we're all fanatics when it comes to our vehicles. It's what you take away from the threads that's important.
I'm glad you are enjoying it. I was afraid a few of us had run everyone else off with our technical banter.

I can't help but apply my technical training and background to my Bimmer. I work a lot with hot water systems (used to heat buildings), but they are limited to 180F.

I enjoy the arguments I get from our good members. I read what they say and ignore how they say it. I've learned and re-learned a lot through this one thread. Was it Plato who said that truth is arrived at through dialogue?
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Old 02-08-2013, 05:39 AM   #191
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Any conclusions now that the aftermarket cap has been made available? I'm wondering if there's any merit to claims of increased failure interval for cooling system plastics when switching to lower pressure caps.
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Old 04-08-2013, 01:13 PM   #192
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So based on real world testing, the system never reaches 2 bar. I'm now starting to believe the 2-bar cap might be for emissions purposes or something like that..
If the system never reaches 2 bar then what does the stock cap do?

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Government forcing manufacturers to limit the amount of coolant that can possibly be vented to the atmosphere.
Venting to atmosphere? Isn't that what the ET is for?

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Any conclusions now that the aftermarket cap has been made available? I'm wondering if there's any merit to claims of increased failure interval for cooling system plastics when switching to lower pressure caps.
What claims of increased failure with lower pressure caps? I've not seen any yet.
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Old 04-08-2013, 05:01 PM   #193
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What claims of increased failure with lower pressure caps? I've not seen any yet.
Poory worded - "increased failure interval" should read "longer mean time between failures (MTBF)".
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