Sticking SRV ‘pip’ seals

With @Twoberth Duncan’s masterly paper on the history of Monitor 17 and 127 stoves as my reference I’ve been fettling a Monitor 127, 1937 predecessor of the popular Monitor 17B of the 1950’s.




Duncan’s research mentions a Safety Release Valve patent of 1939, so the right period, but it’s concerned with a SRV tamper-proof construction and special wrench to remove/set it.




I was interested in the design of the Monitor 127’s SRV incorporated in the fuel filler cap and looked for another patent relevant to it.






I didn’t find a patent, but stumbled upon a Monitor patent published in 1954 which described an interesting phenomenon to do with SRV rubber pips sticking in the closed position.

Patent No. 714760 describes the problem,

The patent’s proposed solution was to machine the valve seats from stainless steel,

I’ve never seen that proposal of a stainless steel valve seat implemented in any SRV I’ve dismantled. The patent also speaks of a different contour for the valve seat to minimise variations in release pressure from the desired pre-set figure, which I’ve not seen either.

The patent drawing shows (left hand column of drawings) the internal construction of a valve very much like that of my 127 stove’s SRV, though the fuel cap depicted is equipped with wings to grip to undo.




Though the patent appears not to have been implemented in SRV’s from Monitor, the problem it outlines is a valid one and is borne out by the number of SRV’s I’ve dismantled where the ‘pip’ seal has in fact bonded itself to the valve seat, rendering the safety function inoperative. Latterly, viton seals appear to be a better bet than the synthetic rubber of previous eras, but it’s a severe test, to be subjected to a constant spring loading together with exposure to heat, tank pressure, fuel and fuel vapour.

Ironically, my Monitor SRV’s original seal was cork. Shrivelled and crushed by decades of spring pressure, I couldn’t rely on it to maintain a seal, let alone expect it to function as a pressure release. However, it hadn’t stuck to its valve seat! I replaced it with a viton pip.




The patent.



John

 

For our purposes today could an easier solution be simply to use a cork pip when servicing replacement is required?

 

Evidently it was back then Ian. As I said, that was exactly what Monitor had incorporated in my Monitor’s SRV.

Now? You gave me some rather splendid NRV cork pips some years ago, which are still functioning perfectly well. So yes, cork may still be of use. Viton though has admirable qualities and doesn’t seem to stick …

 

Interesting reading, thanks.

 

Brilliant informative and interesting information John.

 

@mr optimus I’m thinking that a stuck-closed SRV is no big deal with a paraffin stove but it makes me wonder how many Svea 123R, Optimus 111B and Optimus 8R filler cap SRV’s aren’t functional due to the phenomenon Monitor identified.

 

Very interesting. Thanks John.

 

@presscall one of those posts that open to new and important questions about safety in the stoves world. I am not exaggerating when I underline the importance of this new information you have shared with us.

Thanks John,

Nicola

 

Interesting
Thanks

 

@Nicola Francesco Elia Thanks for your comment Nicola.

I’ve been sceptical for some years now about reliance on SRV’s as an assurance of safety, relying more on the sight, sound and maybe smell of a malfunctioning stove, or one leaking fuel.

Some years ago, frustrated I think with a lengthy members’ discourse about ‘setting’ a SRV to a measured release pressure, measuring SRV spring lengths and such like, I posted a slightly tongue-in-cheek suggestion for Safe Stove Strategies, which culminated in this photo.




It struck me as foolish that some people were even discarding Svea 123R fuel caps in favour of new, old stock ones rather than ‘tamper’ with the SRV suspected of not working and getting the release pressure ‘wrong’ by over-tightening, or not tightening enough, the penta-screw. Checking that the pip seal had not stuck to the valve seat and replacing it with a viton pip would have ensured the SRV would at least function, whether more-or-less on the specified release pressure. Also, a new, old stock fuel cap having sat in storage (spring pressure and time bonding the pip to the valve seat) could not be guaranteed to function without dismantling it to check!

John

 

I'm still a beginner with Euro stoves, but were there any valve seats machined from stainless?

 

Seems like another application for the hypodermic insert repair! This slotted (more tamperable) 8R cap chirped like a nest of baby birds and was replaced. Now it needn’t sulk in the dark indefinitely!

 

None that I know of Jim, and certainly none in my collection of Monitor stoves.

 

Can you explain?
Replaced the cap?
SRV is an easy to service seal.
How does the hypodermic insert fit in?

 

Photo shows ragged, uneven edges of pip mating surface damaged by past tampering. Unable at the time to restore a flat, sharp bite to the sealing tube rim, this cap assembly was then resigned to damaged parts collection.
(Another complete cap assembly fitted with a fresh Fettlebox pip got that stove purring again; no new story there.)

The eureka moment was realization this old abused cap might be returned to service by drilling bore out for press-fitting a stub of perhaps 4mm stainless capillary tube, similar to @presscall ‘s lipstick jet repair.