So I'm getting ready to do some comprehensive testing on a rebuilt Svea 123. I popped my rebuilt cap into a test rig real quick just to see where the pressures were. Right now with fresh rubber I'm lifting at 65# and re-seating at 43# very consistently. I will let the rubber relax a bit and test again in 24 hours. The cap was lifting at about 30-35# before rebuild with a hissing leak by for about 5# before it lifted. I plan to do a full pressure test on the stove at upper and lower operating temperatures, perform a leakdown test, then I'd like to do some operational characterization through a tank of fuel. I'll post data and graphs of the testing, I'm still putting together the hot/cold box so it'llbe a couple days yet. Also I need a new nitrogen tank so I have a constant pressure source, I used a hand pump today.
It would seem reasonable to me to think that an original fuel cap with a now long deteriorating safety valve in use would always tend to fail on the low side of specification, in the way you’ve described here. In other words, no special safety hazards in use as far as that goes. But, I don’t know this to be the case. What do you think? Is it possible for a potentially dangerous pressure condition to occur?
My only concern would be the deformation in the rubber changing the behavior of the SRV. In theory the spring relaxing over time will lower the lift pressure.
The rubber compound originally used in SRVs to tends to harden over time, and can warp causing leaks and release at lower pressure. There is a possibility that the SRV seal may adhere to it's seat as it degrades, although I have not come across instances of this. This would cause a potentially increase in opening pressure There is also a possibility old or incorrect fuel could cause the SRV seal to adhere to its seat. But none of the above should cause any problems to the competent stove user. Just as nobody should be driving a car with rubber brake hoses and seals older than their recommended service life, the filler cap sea and SRV seal on a petrol/gasoline stove should be changed at a sensible service interval (5-10 years? Debatable, ought to be at least closely monitored at this point). If you don't know that the season we're recently changed, or can't be sure that the materials used were appropriate, they need changing. I personally struggle with the mindset of trying out a newly acquired unknown stove to 'see if it works', or doing a preliminary dunk test- The seals need replacing, or sooner or later you will have at best reliability issues, or at worse safety issues.
Yup it's like old radio, you /could/ just plug it in and see if it works, but you really should just replace the caps and check the transformers before you even try. This stove I'm rebuilding was shipped air mail to me with ancient fuel in it, and full of residue and build up. The SRV rubber disc had a large indent/cut from the seat, and the seal gasket was dry rotted.
I think it is pretty well established there aren’t any safety issues, per se. I mean, if there were we’d have probably heard about them by now. The fuel cap gaskets need replacing, but that’s not news. The Svea I use is from about 1970 - I think the jury has went home on this a long, long time ago.
Established by whom? I think the general consensus shared by the majority of CCS members is that self pressurising stoves burning volatile fuels should be correctly serviced by somebody competent before use. Leaving an aged, hardened, and potentially stuck SRV seal in place isn't competent servicing. A hardened SRV seal will most likely cause reliability issues and nothing more than a candle flame, but if it has stuck to it's seat or if the SRV is otherwise defective then over pressurising can occur, as in this incident. There have been several near misses with self pressurising stoves over the years reported directly by CCS members, not 'friends of friends' or 'heard in the bothy', caused by leaking seals. Any of these could have resulted in fire or ultimately explosion of not dealt with promptly, I if the user was distracted, or panicked.
The proximate cause in that incident, was because the user attempted repairs on the cap in the first place; however, he apparently did not test at what pressure the valve would release! Oops. Thus, in any event the safety cap did not serve for the purpose after his ministrations. This is the point I’m trying to make. When I suggested people who are not mechanically inclined would do well to simply purchase a new fuel cap from Optimus International a few people jumped on this reasoning as evidence of a “throwaway society”.
The 'attempted repairs''- in that thread consisted of the user doing the sensible thing by replacing the SRV seal. He later tested the cap and it did not open until 90psi, and to reach a sensible opening pressure the adjuster had to be backed of fully. Not then down to his maladjustment of the cap, in this unsual case it seems most likely a previous owner had fitted an incorrect spring of increased strength. The same thread also includes an observation of a CCS member describing a hardened SRV seal resulting in a candle flame growing to a metre in length, more than enough to start a fire if the user is momentarily distracted. I stand by my view that not replacing the SRV seal means at best unreliability, and at worse dangerous. Your comments infering a new poster was not mechanically minded seemed somewhat out of place seeing as they had already described successfully dismantling the stove and replacing the wick. There was also no establishment first that the stove was of an age to have been fitted with an adjustable cap. Unlike in the US, a large proportion of 8R models in the UK are old enough to have the early non-adjustable SRV. Servicing this involves unscrewing with a flat bladed screwdriver, swapping the old seal for new, and screwing the cap back on. There is no risk of mistakes and a mechanical mind is not required. While I agree it is always sensible to impart the importance of safety to a newcomer, and to ensure they understand what they are doing, I don't agree with encouraging them to avoid overhauling the original Swedish component for a new Asian version which may be of an entirely different design to the original.
I’m simply pointing out while it makes good sense to replace the now-fossilized rubber bits, with respect to a flaming vessel full of pressurized gasoline, it only makes sense to do a complete competent, workmanlike job. This means of course verifying the safety valve will release according to specification. A lot of people are mechanically inclined just enough to get them in trouble, was what I was alluding to. We here love this kind of stuff. Sort of self-selected oddies and pyromaniacs and inveterate tinkerers here, among many others. We would do well to realize most people don’t want to get this deep into it! Their eyes just glaze over. See it all the time, details do not interest them too much for whatever reason.
I'd suggest that most who join this forum are here because they want to learn, and need support and encouragement from the start rather than making assumptions and writing them off. There are exceptions like you say, a lot of whom give up once they realise the the cost and effort of getting a stove working outweighs the additional money they'll get for it on ebay or marketplace. Here's an Optimus 8r. Note the SRV design. Judging by numbers which appear on eBay, I reckon around one in three 8r's in Britain are older models with this design of SRV. Are you aware of this when you recommend UK newcomers replace their cap with what may be an incorrect and more complex design? This older design of cap has no adjustment and dismantles with a flat bladed screwdriver? It should be obvious this cap design doesn't require much skill or effort to work on. Unserviced, it will be unreliable and is likely to release inappropriately, which is worrying and potentially dangerous. In the unlikely event it is siezed it would fail to release, which is very dangerous. A service removes that possibility. On reassembly there is no testing or setting up required. The later design as we know requires a more specialist tool, and there is scope for adjustment, but due to the wide tolerances simply replacing the screw as was or starting low and increasing if needed are both simple and viable approaches to safe operation.
