What is this on Tornado and Flying Scotsman?

[ryan125hst]

What is this venting steam just behind Flying Scotsman's chimney? I noticed the same thing on Tornado's boiler. The safety valve is at the cab end of the boiler, and it's not a whistle. Does anyone know what it is?

 

[cadder toad]

Snifting valve or anti vacuum valve for the superheater. It allows cool air into the superheated flues and stops the pistons drawing in smoke box ash when coasting. It shouldn't vent steam except maybe for an instant when the driver opens the regulator

 

[AndrewE]

I saw something similar on Blue Peter and it seemed to me that it was probably a steam-driven air compressor exhaust.

 

[John Webb]

I am sure this is an anti-vacuum (or snifter valve) as Cadder Toad mentioned above. They are described in "Handbook for Railway Steam Locomotive Enginemen" first published in 1957 by the British Transport Commission and reprinted several times in more recent years by Ian Allan. In the location seen on the OP's picture it is close to the superheater 'header' where the air needed to be admitted.
They were normally open but closed by steam pressure when the regulator was opened. It is possible that if the regulator valve had a slight leak, steam could be issuing from this valve as there would be insufficient pressure to close it.

 

[AndrewE]

I am sure this is an anti-vacuum (or snifter valve) as Cadder Toad mentioned above. They are described in "Handbook for Railway Steam Locomotive Enginemen" first published in 1957 by the British Transport Commission and reprinted several times in more recent years by Ian Allan. In the location seen on the OP's picture it is close to the superheater 'header' where the air needed to be admitted.
They were normally open but closed by steam pressure when the regulator was opened. It is possible that if the regulator valve had a slight leak, steam could be issuing from this valve as there would be insufficient pressure to close it.

So

Snifting valve or anti vacuum valve for the superheater. It allows cool air into the superheated flues and stops the pistons drawing in smoke box ash when coasting. It shouldn't vent steam except maybe for an instant when the driver opens the regulator

is irrelevant? I watched Blue Peter for 5 minutes or so and it ran intermittently - which to me says Air-brake compressor - just like the underfloor compressors on the old Cl 304s

 

[John Webb]

I don't know the detailed design of either engine enough to be certain. But it's in the right place for an 'anti-vacuum valve'.

I believe there's a Hayne's Manual for Tornado which might have the detail in of both what this valve is and whether there is any connection with the air-pump it has on board.

 

[ryan125hst]

Thanks all, I also posted my question on the GCR Facebook group and the general consensus is it's a Snifter Valve: https://en.wikipedia.org/wiki/Snifting_valve

A snifting valve (sometimes snifter valve) is an automatic anti-vacuum valve used in a steam locomotive when coasting.

It seems many steam locos have them but on some locos they are located close to the cylinders.

I'm surprised I've not spotted it before to be honest.