Why 2 pressure Caps????
Discussion
I have been looking at the cooling sysytem on my S2 and am struggling to understand it,there is an expansion Bottle with a presure cap on it,so presumably it is a sealed system that expands and contracts into this bottle,why then do I have some form of swirl pot with a pressure cap on it???? does it need 2 pressure caps or could I seal off the one on the swirl pot (Its getting wafer thin so I want to make a stainless one) and put a simple bleed valve on the top of the swirl pot instead.I am wondering if maybe it should just have the pressure cap on the swirl pot venting into a reservoire without a presure cap??????? I know there are fundamental issues with pressure differentials in heating systems by-passing via pressure releifs and that it is possible to vent off the expanded water without it getting back into the system when the system cools,is that why there's 2 pressure caps??????? Any thoughts or past solutions welcome.
You should have a metal swirl tank with a blanking cap on, and a small pipe from the top of that to the bottom of a plastic expansion tank with a pressure cap on. The blanking cap is where you fill the system from. The pressure cap is where it all comes out again.
Or, depending which religion you support, the caps absolutely MUST be the other way round otherwise the skes will fall in on us. Hey, if we didn't have two caps what would we all find to talk about?
Or, depending which religion you support, the caps absolutely MUST be the other way round otherwise the skes will fall in on us. Hey, if we didn't have two caps what would we all find to talk about?
Being off work I have had a chance to look at this further and have joined the religion that believes the swirl tank has to have a pressure cap not a blank (based on the reasoning that the pipe to the Expansion Bottle is only functional when the pressure cap has opened, due to its location in the filler neck
I am assuming the swirl pot has to be filled to the top (due to the location/height of the top hose (35m), what I want to know is:-
1)The cap on the swirl pot is rated 13psi, is that correct.
2)Should the Expansion bottle have a pressure cap on, if so presumably it has to be higher rated than the swirl pot cap??, if so what was it originally.
3)When filling is it necessary to bleed air out of the pipe to the expansion bottle, I don’t think it is as its pressure operated not gravity fed ,but have doubts.
I can see this has been discussed before, so apologies to those of you who have done it to death, but there must be a definitive answer to this. I am asking because the water level in the swirl pot drops to about 60% of its height, then stays there and works fine, but I don’t believe that’s how I should be (as it must drop the corresponding water level in the engine to the same level) and I don’t understand why its doing it.
I am assuming the swirl pot has to be filled to the top (due to the location/height of the top hose (35m), what I want to know is:-
1)The cap on the swirl pot is rated 13psi, is that correct.
2)Should the Expansion bottle have a pressure cap on, if so presumably it has to be higher rated than the swirl pot cap??, if so what was it originally.
3)When filling is it necessary to bleed air out of the pipe to the expansion bottle, I don’t think it is as its pressure operated not gravity fed ,but have doubts.
I can see this has been discussed before, so apologies to those of you who have done it to death, but there must be a definitive answer to this. I am asking because the water level in the swirl pot drops to about 60% of its height, then stays there and works fine, but I don’t believe that’s how I should be (as it must drop the corresponding water level in the engine to the same level) and I don’t understand why its doing it.
Without wanting to shake the tin of worms too hard, and knowing the TVR propensity for making it up as they went along, I have started to wonder if all of the expansion tanks are plumbed in the same way - could some be plumbed in to work "under pressure" and some just as "overflows"? In the first case, the pressure cap would be on the bottle and the blank on the swirl pot - in the second, the caps would have to be the other way round.
It's a TVR - so this seems a perfectly rational (and peace-seeking) theory.
Mine definitely works better with the pressure cap on the swirl pot - one of the first changes I made whan I got the car, the previous owner warned me that it tended to boil, and after I changed the caps over it was fine.
It's a TVR - so this seems a perfectly rational (and peace-seeking) theory.
Mine definitely works better with the pressure cap on the swirl pot - one of the first changes I made whan I got the car, the previous owner warned me that it tended to boil, and after I changed the caps over it was fine.
The normal layout is with a small pipe from the top of the metal swirl tank to the bottom of the plastic header tank.
If everything is in good working order, this system works best with the blanking cap on the swirl tank and a pressure cap on the header tank. With it this way round, you get a steady slow tidal flow between the header tank and the swirl tank which gradually pumps bubbles into the header tank and nice pure water back into the swirl tank, keeping it full to the brim. The other way round works too, it just means you don't get this extra degassing effect and the pressurised part of the system has less volume and less water. The more volume and water you have the better, in general.
What is far more important than which way round the caps are, is that the all the pressurised parts of the system are sealed and hold the required pressure. If any part of the transfer pipe or header tank/pressure cap are damaged or won't hold pressure for any reason, you are better off putting the pressure cap on the swirl tank so that the header tank etc are unpressurised. You'll also get cap/neck combinations that just don't work properly for whatever reason, so sometimes swapping the caps over on a specific car will cause, or cure, a problem.
But, if everything is in good working order and you have this design of system, the pressure cap is best on the swirl^D^D^D^D^D header tank.
If everything is in good working order, this system works best with the blanking cap on the swirl tank and a pressure cap on the header tank. With it this way round, you get a steady slow tidal flow between the header tank and the swirl tank which gradually pumps bubbles into the header tank and nice pure water back into the swirl tank, keeping it full to the brim. The other way round works too, it just means you don't get this extra degassing effect and the pressurised part of the system has less volume and less water. The more volume and water you have the better, in general.
What is far more important than which way round the caps are, is that the all the pressurised parts of the system are sealed and hold the required pressure. If any part of the transfer pipe or header tank/pressure cap are damaged or won't hold pressure for any reason, you are better off putting the pressure cap on the swirl tank so that the header tank etc are unpressurised. You'll also get cap/neck combinations that just don't work properly for whatever reason, so sometimes swapping the caps over on a specific car will cause, or cure, a problem.
But, if everything is in good working order and you have this design of system, the pressure cap is best on the swirl^D^D^D^D^D header tank.
Edited by GreenV8S on Wednesday 23 August 20:11
Wow !! - this is a juicy one, I remember when I bought my 'S' I asked the same question and was flooded with replies from almost everyone including Hydraulic engineers and those used to Psychrometric charts.
Briefly - its a TVR thing, if it works don't fix it - the norm. is for the Pressure relief cap (the one with the spring) to go on the Swirl Tank (the long metal tube)and the similar looking cap but no spring onto the plastic expansion tank. If boiling or overheating is experienced its more likely to be a faulty radiator or blocked hose connection.(assuming water pump is OK)
At the time I was sorting mine out I noted the following - Swirl Tank cap is 15psi. Halfords 601 or Quinton Hazel FC35 sometimes the cap doesn't reach or sit properly on the Swirl tank in which case you need a cap with a longer reach - Quinton Hazell FC49 (which is 10psi but OK).
Some TIV's have the arrangement the other way around and run perfectly so don't panic.
Briefly - its a TVR thing, if it works don't fix it - the norm. is for the Pressure relief cap (the one with the spring) to go on the Swirl Tank (the long metal tube)and the similar looking cap but no spring onto the plastic expansion tank. If boiling or overheating is experienced its more likely to be a faulty radiator or blocked hose connection.(assuming water pump is OK)
At the time I was sorting mine out I noted the following - Swirl Tank cap is 15psi. Halfords 601 or Quinton Hazel FC35 sometimes the cap doesn't reach or sit properly on the Swirl tank in which case you need a cap with a longer reach - Quinton Hazell FC49 (which is 10psi but OK).
Some TIV's have the arrangement the other way around and run perfectly so don't panic.
Thanks for the replies, I am running it currently with the pressure cap on the swirl,from your feedback I have a feeling I should be looking closer at the sealing on the connections between the two units, as green V8 says keeping it all pressurised is paramount and a leak here would cause the problem I have as it would "vent to air" on an open pressure cap.
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