NBR / LNER C15
- Thread starter Bob Reid
- Start date
Bob Reid
Western Thunderer
As usual one question throws up a bunch of others but the most obvious one which I have no answer for...
The two photo's on the previous page show "snifter" anti-vacuum valves (or at least they look like that) on the side of the smokebox, so are they snifter valves? and why on non-superheated loco's?
The two photo's on the previous page show "snifter" anti-vacuum valves (or at least they look like that) on the side of the smokebox, so are they snifter valves? and why on non-superheated loco's?
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Dog Star
Western Thunderer
Bob, (@Bob Reid), "snifters" or anti-vacuum valves are not solely for superheated locos - the idea, at least as far as Swindon was concerned, was to allow clean air to be drawn into the cylinders when coasting with the regulator closed.... otherwise any vacuum created in the cylinders / valve chests could draw smokebox solids into the cylinders and hence lead to scoring of the bores.
Bob Reid
Western Thunderer
Thanks Graham. The LNER (who modified the ex-NBR C15's) predominantly used them on superheated loco's as far as I'm aware and these particular loco's were not fitted with them until the conversion to push-pull - they don't appear on the other plain saturated steam loco's in the class so I'd imagine it was found necessary on the push-pull loco's only.
Martin Shaw
Western Thunderer
Bob
Definitely looks like a standard Gresley LNER snifting valve. It appears that only the P&P fitted C15s were so equipped, but like you I wonder why on a slide valve saturated engine. This is speculation on my part, but is plausible. They worked chimney first up the hill from Craigendoran so the auxiliary regulator would be left open and the engine controlled from the normal regulator. Conversely it is mostly downhill from Arrochar, and some 20 miles largely coasting might have caused a thought that additional anti vacuum measures might be needed, although of course in theory a slide valve falls off the port face thus preventing a vacuum being created in the first place. The other possibility is that it was directly related to the P&P equipment, maybe to vent the steam pipes if the auxiliary regulator was shut but passing. I think my first option is more likely.
Graham
Your quite correct in that any piston valved engine will require snifters, irrespective of whether the boiler is superheated or not, and whilst I am aware of slide valve superheated engines, I haven't come across saturated piston valve locos. No doubt someone can prove me wrong.
Regards
Martin
Definitely looks like a standard Gresley LNER snifting valve. It appears that only the P&P fitted C15s were so equipped, but like you I wonder why on a slide valve saturated engine. This is speculation on my part, but is plausible. They worked chimney first up the hill from Craigendoran so the auxiliary regulator would be left open and the engine controlled from the normal regulator. Conversely it is mostly downhill from Arrochar, and some 20 miles largely coasting might have caused a thought that additional anti vacuum measures might be needed, although of course in theory a slide valve falls off the port face thus preventing a vacuum being created in the first place. The other possibility is that it was directly related to the P&P equipment, maybe to vent the steam pipes if the auxiliary regulator was shut but passing. I think my first option is more likely.
Graham
Your quite correct in that any piston valved engine will require snifters, irrespective of whether the boiler is superheated or not, and whilst I am aware of slide valve superheated engines, I haven't come across saturated piston valve locos. No doubt someone can prove me wrong.
Regards
Martin
Bob Reid
Western Thunderer
Thanks Martin, I'll have to rake around for some old steam locomotive engineering books to get a better idea of how the system worked in practice. I did wonder if the snifter was there to prevent an imbalance in the vacuum regulator when coasting that would cause it to be difficult to re-open. Not that it'll make the model C15 any better!