Confused about header tank cap opening pressure

[morgandoyle]

My boat came to me with two different caps on the header tanks.

One cap is rated 50 KN - 7 lb (I assume to mean 50KN/sq meter - 7 psi).

The other is rated at 75 KPa. (which I make out to be about 11 psi).

Any suggestions on what these ratings mean? I expected the rating to be well above 14 psi (1 atmosphere) for both caps.

Does it mean opening pressure above 14 psi (i.e 21 and 25 psi?)

The engine with the lower rated cap is loosing a bit of coolant and as I can't find a leak I wonder could it be boiling it off coz of low pressure cap (although I haven't seen this either).

Engines are Ford Thornycroft 2722E, about 1985 or so.

 

[cliff]

75 kilopascal = 10.8778304 pound/square inch [absolute] or PSIG.
The pressures quoted are above atmospheric pressure either 7 or approx 11.

This will allow the coolant temp to rise above 100° C before boiling, to 114.744°C for the 7 psi cap and to 119.552°C for the 11 psi cap.

Based on the above I would suggest a 7 psi cap is adequate but the seal may be leaking. Have you tried a new cap? At what temp does/should the overheat alarm go off?
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"Artificial intelligence is no match for natural stupidity"

 

[LittleShip]

Having read Cliffs post and decided it's too technical for me /forums/images/graemlins/smirk.gif I'll go with the last line.............7 psi should be enough for a boat at sea level? /forums/images/graemlins/grin.gif

Tom

 

[cliff]

Duh! physics 101
actually on reflection the 11 psi cap (~120°C boiling point) might be better but can the header take the higher pressure. I still think the 7 PSI cap (~115 °C boiling point) would be adequate - really need to consider the alarm temp and the condition of the header.
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"Artificial intelligence is no match for natural stupidity"

 

[BarryH]

Don't over complicate things. So your loosing a bit of coolant. Is this a continual thing or you loose some then it stops. Are you allowing for expansion when you top up. maybe its a case that the engine cooling is shedding the slight overfill.

 

[davedpc]

You perhaps should have an expansion overflow bottle [usually plastic] which shold suck back when the lump cools. Dave.

 

[omega2]

plastic pipe from header tank overflow, (on filler neck) to plastic bottle, and see how much you collect, we have 180 Sabre's based on the same block as you and eventually had to have the head off to renew the gasket which had blown, but it does not end there, the head also will need skimming, and while your at it have the valves reground. The good news is that we were able to run for 200 hours losing water to the bottle and putting it back when cool, until we bit the bullet and sorted it. Caps should be 7lb, but again with these war horses it is not critical.