radiator caps, cooling system pressure

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On Mon, 22 Apr 2013 11:23:19 -0700 (PDT), snipped-for-privacy@my-deja.com wrote:

I'm in Phoenix area and my experience is the same as far as overheating but I've never had this low a pressure cap (7 psi) before. A 13 pound cap would give me another 14 degrees before boil over. I just added AC and that seems to have raised the temp baseline by 10 degrees once the weather is over 90.
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the higher pressure cap may give you more margin to boil over but will give you less margin to springing a leak... so take your pick...
here's an idea..
keep a few gallons of water in the trunk ( a good idea anyway in phoenix) along with the high pressure radiator cap. Keep the low pressure cap on the car for normal use which puts the least strain on the old plumbing. You'll have the high pressure cap and extra water in the trunk for backup.
Mark
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wrote:

I was sort of toying with that approach. Probably will be what I wind up doing.
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wrote:

Well, I give each of you half credit. LOL! At a certain RPM, the pump head (outlet-inlet pressure) and flow rate are both are fairly indept of the coolant temperature as long as the coolant stays a liquid (normal condition) when going through the pump. The engine heat raises the coolant temperature and the pressure (remember this from skool... PV=nRT), but (and this is a big butt) as long as the pump turns at the same RPM, the head should stay about the same.
FWIW, here's a decent drawing of a water pump cross-section
http://assets.hemmings.com/story_image/83720-500-0.jpg
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wrote:

Well, I give each of you half credit. LOL! At a certain RPM, the pump head (outlet-inlet pressure) and flow rate are both are fairly indept of the coolant temperature as long as the coolant stays a liquid (normal condition) when going through the pump. The engine heat raises the coolant temperature and the pressure (remember this from skool... PV=nRT), but (and this is a big butt) as long as the pump turns at the same RPM, the head should stay about the same.
FWIW, here's a decent drawing of a water pump cross-section
http://assets.hemmings.com/story_image/83720-500-0.jpg
A couple of comments--PV=nRT is an equation used in gas flow calculations not when the fluid is a liquid. Don't understand where you are coming from. Just for the record, at constant speed the pressure rise across a centrifugal pump does not remain constant. Typically, there is a droop (loss of delta P) as the pump flow demand increases. Relatively insignificant at first but if the flow demand gets large enough, then the Pump Delta P can drop significantly. Flow demand is dictated by the characteristics of the the system in question--that is, how the delta P vs Flow of the system (line losses etc.) matches up with the delta P vs flow of the pump. Where the two intersect will be the operating point of the System. The idea is to match them so that the intersection takes place where the droop in pump deta P is relatively insensitive to flow demand. MLD
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On 5/1/2014 9:39 AM, MLD wrote:

In a CLOSED system FILLED with INCOMPRESSIBLE liquid, the pressure is whatever the pressure is. The pump can't put more pressure on the output side than is on the input side because it's a CLOSED system FILLED with INCOMPRESSIBLE liquid.
Do pump principles applied to OPEN systems really apply?
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wrote:

Don't understand either of your comments. Could you expand your explanations so they make some sense? Do you know what happens if you dead end a centrifugal pump (Zero flow) while its running?? The discharge pressure might not change but you better be prepared to see the fluid temperature skyrocket. You seem to to make a big point out of INCOMPRESSIBLE fluid. Got some news for you, Ready---fluids, aka liquids, are not INCOMPRESSIBLE!!! Have you ever heard of "Bulk Modulus", entrained air or compressible flow as they apply to liquids? Do you know what would happen to the pressure in a closed system if the fluid temperature (say water) was increased but the fluid was not able to expand due to the closed (or fixed) system volume? Hint: Delta P=(BM) x (Delta V)/V Clue: Pressure can increase up to the thousands! Want to try and conduct your own experiment? Close the water inlet shut off valve in your house. Keep all faucets closed and of course, lock the hot water tank relief valve so that it doesn't open. Now just crank up the temperature of your water heater. This ought to seek out your system's weak link. MLD
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On 5/1/2014 4:46 PM, MLD wrote:

in any significant amount. If the pressure exceeds the cap pressure, it vents. The pump didn't add the pressure. In your scenario, the pump added heat. So, get back to the topic. In a closed car cooling system under normal operation can the pressure on the output side of the pump significantly exceed the pressure on the input side?
I think we can exclude any vaporization of the liquid leading to excess pressure. If it did, it would vent and, eventually, there'd be no more liquid to pump.
But thank you for the clue and the hint and the nitpicking.
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Yes, and it looks like a lot of you have no idea how much it can, and does.
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On 5/1/2014 9:08 PM, Rick wrote:

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On Friday, May 2, 2014 3:28:35 AM UTC-4, mike wrote:

It's exactly the same rational as in an open system. If there is no pressure difference in a closed system, why would the pump move water at all?
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Simply put, it's a pump and there is a restriction on the outlet side. An engine cooling system isn't a swimming pool with a pump in the middle of it.
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On Friday, May 2, 2014 10:33:04 AM UTC-4, Rick wrote:

So what? Pumps are made to create pressure and force fluids past resistance in the system. A "restriction" is just like any other resistance in a piping system, eg elbows, the length of the pipe, etc.
An

Simply put, a pump is always a pump and it can only move fluid via a pressure difference between the intake and output side. Otherwise, explain the physics whereby a pump moves fluid without a pressure differential. You can't, not on this planet.
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He asked why the outlet pressure is high, Again, simply put, it's because there is a restrictin downstream. Obviously the outlet pressure has to be greater than the inlet pressure.

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wrote:

Once more---A centrifugal pump--that's what is in your car--the pump discharge pressure is a function of the pump speed squared. Once the thermostat opens there is a flow path from pump to radiator back to the pump inlet. Other than a slight droop in pressure due to the flow demand, the pressure rise across the pump basically constant. Obviously, no venting occurs as long as the pressure at the radiator is less then the cap pressure setting. Pressure at the radiator is established by whatever line loses there are between the pump discharge and radiator. Venting occurs because as the coolant temperature increases it's volume will also increase. Why?? Density=Weight/Volume. Density decreases as temperature increases. Since the weight of the coolant doesn't change, it's the volume that does. As the coolant expands, the pressure in the system increases (remember, trapped volume aka closed system) and when the pressure gets to the cap setting (won't take long), the cap opens and the fluid goes into the overflow bottle. If the radiator cap didn't open--whole new scenario--things would break!! If this doesn't answer your questions, suggest you do a bit of reading on your own. MLD
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On Thursday, May 1, 2014 3:59:51 PM UTC-4, mike wrote:

If the pump can't put more pressure on the output side, then what exactly makes the water flow? Why does the water flow faster the faster the pump runs? Even in an open system, like a pool, the system is under pressure, atmospheric pressure. In an engine cooling system, the overall pressure of the whole system will rise with temp, but there is still a pressure delta across the pump.

Yes, of course they do.
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Great Question. My original question remains unanswered .... Does the water pump ADD another 10 to 30 psi internal pressure (as the sources I originally was looking at claimed) and if it does, how come that extra pressure, on top of the already existing 12 psi heat pressure, not cause the cap to blow off from the excess pressure now totaling from 20 to 40 psi? All I can think is that the pump only adds perhaps 2 to 5 psi and that by the time it "gets" to the cap area, flow resistance has dissipated it down to 1 or 2 psi and it has ceased to be a problem. Or maybe the pump only "adds" negative pressure, i.e. suction at the inlet, or some combo of all that.
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On 5/1/2014 4:36 PM, Ashton Crusher wrote:

somewhere to get the water to pump. If it's coming from the closed system, there ain't none available unless you vaporize some on the source side.
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On Thursday, May 1, 2014 8:05:57 PM UTC-4, mike wrote:

It does have somewhere to get the water to pump, it's through the engine cooling system. And without a pressure difference, by what physics do you explain the movement of the coolant through that system?

Nonsense, the pump generates pressure just like any pump would.
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outlet side into a plugged rad with the cap off. With the cap on, with no air in the system, significantly less. But at best, only a pound or two. Will running the water pump dead headed cause a temp increase? Sure, a very small amount. Inconsequential compared to the heat output of the engine. It will NOT cause a pressure increase in the closed system. The increased pressure increases the boiling point of the coolant. It also helps get and keep entrained air out of the cooling fluid. The only thing that causes the pressure rize is temperature. The pressure reduces back to atmospheric when the temperature drops back to room temperature. Current production vehicles have a "catch tank" that holds excess coolant if any is forced out to regulate the pressure, and it is drawn back in on cooldown.
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