Cooling system pressure

[bostonhatteras]

Anyone have a solid understanding of the 871 cooling system when it comes to the pressurization aspect? The DD cap provides for 7 psi pressurization. My understanding of pressure in an internal combustion cooling system is that under pressurization steam pockets do not form, or are more difficult to form, and water somehow stays where it is supposed to be, in contact with the metal. What I don't really get about DD system pressure and the previously mentioned system theory is that there are no air pockets to speak of in a DD 871. If the heat exchanger cap is pulled and the engine filled with water from that, highest, point then all the parts of the engine have been filled with water, there is no air purge necessary. Is the 7 psi attempting to keep the heads in contact with water IF THE COOLANT IS LOW? These engines seem to be happy at zero psi system pressure. Not looking to exceed 7 psi, just wondering what's the point of it? I do not have external expansion tanks, just an overflow tube which has never overflowed. Although the manual states this under pressure cap section we certainly aren't trying to increase boiling point of the coolant with pressure as we all know, or should know, what happens to DD engines past 210 F. I assume then that the 7 psi is a marginal at best failsafe incase temps get to 212F and the aspect of boiling coolant versus not boiling-but hotter coolant would make the difference between a saved engine and an engine with cracked heads. Another aspect of pressure caps is the readmittance of air to the system once cooled. This aspect of avoiding vacuum I can readily agree with. In a properly filled and operating cooling system that never sees over 185F or so, it would seem the system pressure is somewhat superfluous.

 

[Captain Ralph]

The caps are open till they get warm. That chunk of copper you see in there closes the cap when warm water touches them. The idea is to let any air out as the coolant expands and rises.
If no warm water touches them, they don't close.

Then 7lb regulation comes in; To much pressure, gaskets, O rings and seals start to blow out.

On other DDC models, some items may be taller than the tank top or subject to trap air like big wet turbos. Lil bleed petcocks are used here with lil coolant return lines to help clear that last air bubble.

Always check your coolant tank when cold. With no recovery bottle, then the cold level should be around 1" below the cap neck.

I do recommend recovery bottles installed. When the engine cools, other than drawing air in, it draws coolant back in. Then, no rust in the top of the expansion tank, no coolant in your bilges, less odors.

Keeping steam down?? Coolant (antifreeze) has stuff in it greatly reducing boiling and steaming temperatures. Funny, coolant does not cool the engine, water does. That is why to much coolant is not good either.

Bottom line; Don't get hot.
225°f should be the worse case temp. I think my alarms are supposed to go off around 215°f.
Not much time left if something has already failed.

Hope this helps

 

[JD5652]

Development of steam follows what's called "steam tables". Water under pressure will develop steam at a given temperature above 212 deg F at "X" pressure. It follows the laws of physics.

 

[bostonhatteras]

Not sure there’s more to know past these two answers. Seems the sheer simplicity, relatively, of my 871 N engines belies the need for pressure with other DD designs. The recovery tank and its value in preventing rust and environmental issues is an excellent point, well taken. What lead to the question is 1/4” tapped holes into my heat exchanger tanks that provide for direct water filling via a first valve plumbed to ships potable water system and a second valve that when opened allows to know when heat exchanger is full. Second valve when opened in effect allows for spill over when tank is full from potable water valve. One tapped hole leaks if filled above it, hence one engine never truly develops it cooling system pressure at operating temp. The answers make me think it’s worthwhile to address the slight leak.

 

[Captain Ralph]

Not sure there’s more to know past these two answers. Seems the sheer simplicity, relatively, of my 871 N engines belies the need for pressure with other DD designs. The recovery tank and its value in preventing rust and environmental issues is an excellent point, well taken. What lead to the question is 1/4” tapped holes into my heat exchanger tanks that provide for direct water filling via a first valve plumbed to ships potable water system and a second valve that when opened allows to know when heat exchanger is full. Second valve when opened in effect allows for spill over when tank is full from potable water valve. One tapped hole leaks if filled above it, hence one engine never truly develops it cooling system pressure at operating temp. The answers make me think it’s worthwhile to address the slight leak.

In all my life, and I have been in many, many bilges.
I have never witnessed such an installation.
But this scares the ship out of me; Seals and gaskets will fail on potable water pressures. Thank goodness it's a 71.

 

[Boatsb]

If memory serves I recall an antifreeze and water mix at 7 psi would not boil below 245 degrees. Steam in the system causes lower heat transfer and hotspots where theres no liquid contact. I'm pretty sure the engineers who built the engine knew what they were doing.

 

[bostonhatteras]

Rest assured, there is no added pressure. the valves remain closed 100% of the time. There is about 2" above heat exchanger filler neck with hatches closed so the rational on whoever's part to install these scores of years ago is readily understood. Plus, there is the 7 psi cap to prevent things from going awry.

 

[bostonhatteras]

If memory serves I recall an antifreeze and water mix at 7 psi would not boil below 245 degrees. Steam in the system causes lower heat transfer and hotspots where theres no liquid contact. I'm pretty sure the engineers who built the engine knew what they were doing.

My point was that if the engines self destruct at 220 what's the advantage of the pressure. I wanted theory, not at all trying to prove the oem design wrong.

 

[Boatsb]

Hot spots cause steam pockets. The antifreeze and pressure keep them from occurring. The engines have warmer and cooler spots and the temp y9u see on the gauge does not represent all the engine parts temp. Just the coolant measured at a certain paint.

 

[Timeless72]

Interesting thread. Mine did not have recovery tanks. Instead, the lines ran into the bilge, which seemed messy and weird. I added quart size bottles to each and notice one side will be half full at the end of each season.