How does pressure cooker vent work?

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Hi 
Just found a brand new TOWER pressure cooker at the civic amenity site (dump) from the 1980s totally new, all parts sealed in bags, you have to assemble the handles onto it with 2 screws. I guess they don't sell them unassembled anymore. 

Anyway there is a vent on the top called automatic air vent and looks like this 
https://www.ebay.co.uk/i/333154144753?  
The central pin is loose in the rubber grmit so I am wondering how it manages to even build up pressure in the first place with the pin so loose? 
having built up pressure how does it release pressure automatically? Does it fire the rubber grommit out of the top to smash into your eye or the ceiling? If the pressure has pushed the pin upwards to form a seal ...?  

While I am here does anyone know why my google searches frequently come up with ebay pages that look like this one ? It is not a standard listing page. 

Thanks

George_Michael

Pressure cooker relief valves usually work on the principle of the pressure in the cooker reaching a certain level at which time it lifts the weighted valve to allow the pressure to escape. Once it drops to the required level, the weight drops and forms a seal again.
Looking at the one you have, it appears to be a dual purpose one so if for some reason the pressure builds up and the weight isn't lifted, sooner or later the pressure will reach a point that the whole rubber grommet assemble will pop out and prevent the cooker from exploding.
When in use, the internal pressure will only increase by somewhere in the region of about 12 psi so the valve doesn't have to be too heavy or strong.

ka7e

This type doesn't de-pressurise automatically. You either wait for pressure to drop gradually once you have removed from the heat - for anything that might froth (pulses etc) or I used to leave stews and ham joints to cool slowly, or you can plunge the cooker into a sink of cold water (veggies etc). Under no circumstances try and open the cooker when the vent is still sealed as it means there is still pressure in there and it can propel the hot contents everywhere! I loved my Tower p c, used it for 30 years and am quite bereft that I haven't found one that works efficiently on my induction hob

ariba10

You will find that you cannot open the Pressure cooker accidentally when it is under pressure as the lid locks automatically.

You will know when it has depressurised as the little pin in safety pug will be loose.

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Pressure cookers including this one have 2 pressure relief valves,  there is the big automatic valve in the middle with a big chunky metal hat that goes on top,  this central big valve you can see has a ballbearing on a spring inside the valve the ballbearing seals off the valve to allow the pressure to build up,  if it over pressurises the ballbearing is pressed in and the steam escapes making the little hat turn around.   

The second safety valve is a little metal pin inside a rubber grommet. It is there in case the first valve gets blocked.  the last thing you want is a pressurised container with no automatic valves.   

ariba10 said:

You will know when it has depressurised as the little pin in safety pug will be loose.

This is my question, how can pressure build up in the first place when that pin is a loose fit in the rubber grommet?  
And once it is pressurised, and the main valve fails, what is the little rubber grommet pin valve designed to do when the pressure is too great?   My only assumption is that it is designed to blow out the top of the lid making a hole in your ceiling, which of course is preferable to the alternative of having no kitchen at all.  

clive0510

my mum had one of those when we were kids. everything was cooked in that. spuds veg fish etc. kept us alive actually.

ka7e

The little grommet is dumbbell shaped and is forced up against the rubber to form a seal, and falls back down when the pressure is safe to open the lid. Under normal operating pressure it just sits "high", but too much pressure would force it right through the rubber vent. I only had it happen once. Pureed brussell sprouts EVERYWHERE!

Slinky

clive0510 said:

my mum had one of those when we were kids. everything was cooked in that. spuds veg fish etc. kept us alive actually.

Yes we had one too. Everything cooked in it at the same time and all came out with the same texture - very soft.