Banjo dome, why used and how does it work ?

Many LNER locos were fitted with Banjo domes and the two new boilers being made for Tornado and the P2 will have them. Why were they fitted (because of priming ?), how do they work and what is different to a standard dome and why did other companies not use them ?

Here is a picture of where it will go on the P2 new boiler being built in Germany

 

My understanding was that it was to keep the dome flatter at the top of the boiler barrel so that the diameter could be increased to the maximum allowable within the loading gauge. The large penetration through the shell still allowed access to the internals.

 

That explains the wide diameter round part, but what about the long narrow part corresponding to the neck of a banjo? I assume that has something just underneath it, but what?

 

105 small holes.
Pat

 

OK, but what else? A steam pipe between those holes and the long part of the dome? The thread title asks "How does it work?"

 

There’s a photo here of Flying Scotsman’s dome:



What I’m not sure about is what advantage the shape provides over just having the regulator under a circular dome, though conceivably the perforated bit of the barrel helps stop froth being drawn into the regulator, ie reduces priming when the regulator intake is only a small distance above the water surface.

Tom

 

Does this help?

 

Page 135 in the Omnibus edition? It was about collecting the steam at a greater height above water level, some say the maximum height and not only for the regulator. Priming- something worth taking extra efforts to avoid or minimise.

 

Yes in answer to the question.

 

That's, what, about 10 milli-AlbertHalls?

Just to confirm from the drawing and the pic, is the regulator valve in a steam-tight bowl in the top of the boiler, so the banjo holes are the steam collection point which then goes "down" in the regulator valve. Clever way of ensuring that you get steam from the highest possible point.

What did the LMS do as an alternative? (as chief other exponents of the loading-gauge-filling boiler)

 

Thank you for your reply.

 

Banjo dome a creative and funny nickname! I like that.

Many norwegian steam locomotives were equipped with this (ugly) dome ,nicknamed "Flosshatt dampdom" ("silk hat dome")
https://digitaltmuseum.no/021018430...-bergen-for-norsk-jernbaneklubb/media?slide=0

Knut

 

It looks like the banjo dome gives additional surface area for the separation of water and steam, with the water draining back down through the smalll holes into the boiler.
As a guess I would think that the passage any foam coming up through the small holes could help break it down into water and steam. Presumably this was all fully tested and assessed at the time.