How To Understand Pressure Caps and Header Tanks

[T16]

This thread is mental, literally one of the best ive read in a long time.

A technical analysis and breakdown of all the parts for just two things, that we normally all take for granted.

Quite an eye opener when you think about the complexities of even the smallest of components.

[T-Cut]

We still don't really understand the way the head pressure bleed opens the relief valve. What does the 'pressure chamber' isolated between the O-rings actually do? I've had some further thoughts on it, but I'm really hoping that someone will have an insight.

TC

[Polly]

Originally Posted by T-Cut

We still don't really understand the way the head pressure bleed opens the relief valve. What does the 'pressure chamber' isolated between the O-rings actually do? I've had some further thoughts on it, but I'm really hoping that someone will have an insight.

TC

Excellent post, but perhaps you will allow me to make a couple of suggestions.
1. I don't think the small chamber under the valve assembly has anything to do with opening the valve, but perhaps is intended to prevent a pressure lock under the valve, which might prevent the main valve from closing. I think it's main purpose is to equalise the pressures above and below the diaphragm.
Because the diaphragm is quite small in relation to the main valve, it would require a much higher pressure to open the valve. It has to be noted too, that any pressure from the heads vent pipe will also be applied to the top of the diaphragm via the header tank opening and back up through the annular opening.
2. When considering the action of the pump, it has to be assumed that, at the pump outlet there will be a positive pressure, and at the inlet a negative one. Obviously somewhere on the system there must a point where the pressure is neutral, and that will normally be the point where the feed pipe joins the circulating loop. At all other points, the pressure will vary in accordance to the resistance of the components of the system. This is extremely useful in determining where a system might draw in air, because in order to do so, that component would have be subject to negative pressure, or in this case why water might be pumped into the expansion tank suggests a high positive pressure.
These offerings are based on my training as a gas engineer rather than any knowledge of the subject matter here, but perhaps it can be applied here to help you understand how it all fits together.


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[T-Cut]

Originally Posted by Polly

These offerings are based on my training as a gas engineer rather than any knowledge of the subject matter here, but perhaps it can be applied here to help you understand how it all fits together.

Many thanks for having a read and think about the system. I'll have a look at what you suggest as the purpose of the diaphragm.

Regarding the increased pressure you suggest is needed to lift the diaphragm, it's surface area is not that much different from the area of the main relief valve so I'm not so convinced about it.
Compare them here:



EDIT: I just remembered the interesting variation where earlier caps have the same diaphragm arrangement, but no hole in the chamber wall. (??)


TC

[Polly]

Perhaps you are correct regarding size, but that still leaves the fact that it can vent back down into the header tank.
Perhaps, it just might be there just to make the valve open slowly in the event of surges of boiling water coming up the vent pipe.
If you will allow me to say so, I thought from your article, you were looking at it on the assumption that the system was full of water. I, on the other hand, see it as a combined header tank and what used to be separate spill tank, where, In normal circumstances, the tank will retain an air space at the top. This would be necessary in order to allow expansion of the water to create the pressure.
Who would believe that a radiator cap could be so complicated.


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[T-Cut]

Originally Posted by Polly

- - - still leaves the fact that it can vent back down into the header tank.

Indeed. The larger drilling from the vent channel straight down into the header also ensures it, but we need a good mechanism for the release of heat soak pressure from the head. MGR articles mention the venting of heat soak after the engine's stopped. This is released via the bleed hose into the sealed off 'pressure chamber'. It has be be related to that I think.

Yes, I view the header tank as only part full under normal running conditions. However, I believe the head vent pipe will be more or less full of water after a refill/bleed process. I suspect the 'pumping' of coolant through this pipe when the cap's removed is abnormal, requiring more pressure drop than the water pump can generate. I suspect that's connected with head gasket leakage. As you say, a complex little gizmo.

TC

[Falcon Flyer]

Excellent article. Sadly I cannot add anything technical to it, only a practical solution to cleaning the header tank internally.

The tank needs to be removed to do it, and the lower outlet sealed, but "Steradent" type tablets with boiling water left overnight do a great job of cleaning header tanks, and windscreen washer bottles too. Don't bother with the branded type, supermarket own brand tablets work just as well.

[Tim]

Superb piece of investigative work and write up. You think you know whats going on until reading something like this.

Thank you

[T-Cut]

Originally Posted by Falcon Flyer

The tank needs to be removed to do it, and the lower outlet sealed, but "Steradent" type tablets with boiling water left overnight

Must admit, I've had little success using hypochlorite type bleach (Steradent is a sodium hypochlorite based cleaner). On the other hand, almost any gunge will eventually succumb given enough determination. The pink powdery scale that often forms on the tank walls is an acid/alkali insoluble calcium compound formed by using tap water and OAT antifreeze at too low a concentration. Antifreezes contain calcium sequestrants that keep calcium in solution, but in hard water areas this will precipitate out if the OAT is inadequate strength. The scale absorbs the dye from the OAT or whatever other antifreeze is used after it's formed on the surface. Whether the scale is specifically an OAT generated material is difficult to say. It would require other cars using non-OAT systems to be examined.

A reasonable result can be had by running the header tank alone through a dishwasher cycle. Dishwasher detergents are typically sodium metasilicate (alkaline) formulas. Some people report good cleaning by just shaking the tank containing a dishwasher tablet dissolved in hot water. It certainly removes a lot of gunge, but the underlying scale seems impenetrable.

TC

[COLVERT]

All this Reverse Engineering is very, very interesting.

Just once though I wish that the engineer who invented this would suddenly show up and tell us all his thinking. He must be somewhere about.---