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Snifting valve

Where is the snifting valve connected

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David Haynes09/12/2011 22:54:59
168 forum posts
26 photos
Hi Nick,
I'm sorry if I missed the drawing or text, but could you please tell me where the snifting valve pipe connects into the wet header.
Many thanks
Dave Haynes
nick feast18/01/2012 08:26:11
75 forum posts
7 photos
Hello David,
Sorry about the delay in replying, haven't been on the site for some time!
With slide valves most people don't bother, as with water release valves on the cyliders.
If you plan to use your engine a lot it may be worth fitting one to the valve chest, it can be installed in one of the dummy valve covers that go on the front. I did try this initially but it kept leaking steam so I did without. The easiest way out is to keep the regulator cracked open even when slowing down so there is no vacuum to 'snift'.
DMB18/01/2012 11:15:11
1354 forum posts
1 photos
David and Nick,
Hope you dont mind the following comment but I think that if any beginner is reading your posts, they may be a bit mis-lead. On full size, the snifter was connected on the regulator/wet header side of the steam circuit so that cooling air would be drawn through the superheater elements to stop them burning out with the heat of the fire. I suggest that "best practice" would be to follow suit, especially if radiant type were used and even non-radiant if made of copper since copper tends to burn out quickly anyway. There is also the need to prevent ash being sucked down the blast pipe in to the cylinders to cause wear.
GoCreate18/01/2012 12:32:02
387 forum posts
119 photos
May I ask how is it that a vacuum is created. From what I can gather when the regulator is shut off a vacuum can be created between the regulator valve and cylinders. Are the pistons, now not being fed with steam, pulling a vacuum?

Edited By tractionengine42 on 18/01/2012 12:32:45

David Haynes18/01/2012 12:56:42
168 forum posts
26 photos
Hello Nick and thank you very much for responding. The timescale of your response is not that important as this is not for a Charlie I am building but for a locomotive I am drawing and that is still 'in the drawing office'. I am also building a small locomotive. With Charlie, I have found the whole series informative and I have no concern for the small 'health and safety' issues that some people have been picking up as I believe common sense should be the overriding caveat for all published articles. With your series, I have borrowed some of your ideas to put on my drawing but acknowledgement will of course be given, not so much from a commercial (I can't see it being published) as an ethical point of view. The locomotive shares some things with your Q1, but also with several others including LBSC's Mona, Don Young's 4F, Martin Evans' 5" Metro &etc.
My question about snifting valves on your slide valve Q1 related to the actual connection point around the wet header, but as you say, in your case you found that they kept leaking steam,. I suppose if I found the same problem, then removing the valves and coasting with the regulator cracked open may be a solution, but I wonder if a spring in these valves would prevent this problem?
Thanks also to John for his comments.
David Haynes18/01/2012 13:11:16
168 forum posts
26 photos
Hi Nigel, yes you are correct, the coasting cylinders will act as a pump and the subsequent vacuum will try to pull from any opening to a lower pressure and also provide a resistance to the coasting pistons. The generally accepted pressure (vacuum) release valve, also known as snifting valve is installed at the wet header end of the steam circuit. That way, cool air will be drawn though the superheater and to the cylinders. If it is installed on the hot or dry end of the superheater, the air will take the short journey to the cylinders, this will allow the superheaters to get hotter and run the risk of burning out. This is especially true if the superheater is radiant, poking right into the firebox.
GoCreate18/01/2012 16:30:54
387 forum posts
119 photos
Hi David, Thanks for that info.
My Allchins TE that I am building does not have a snifter valve so a couple more questions come to mind.
1. Is a snifter valve used when cylinders have either piston valves or slide valves, or only when piston valves are used. (I am thinking a slide valve can lift to clear a vaccum)?
2. Is the snifter valve only applied when superheating is used for the express purpose of using the vaccum created to cool and protect the super heaters, so if super heaters are not used a snifter valve is not used? (Then any vaccum would be used to brake the engine).
David Haynes18/01/2012 18:06:24
168 forum posts
26 photos
Hi Nigel, when the cylinders are cool, there may be condensation, especially if the steam is not superheated as it is going to condense a lot easier. Water ahead of the advancing piston could do damage as the piston approaches the cylinder cover and the waste water doesn't exhaust properly (that small gap is good with a steam cushion but not with a lump of water). With piston valves this water is allowed to escape through cylinder drain valves which are closed when the loco is moving and the cylinders warmed. With slide valves, as the valve can lift by a wee bit, the water can escape up through the valve. Traction engines nearly always (always?) have slide valves so this trapped water should be less of an issue.

The snifting valve, also known as as pressure release valve, is only there to break the vacuum, but it does seem to have this useful side effect of helping to prevent the superheater burning out when steam is not flowing through it,
Hope that answers both 1 and 2 in a round about way!

Edited By David Haynes on 18/01/2012 18:15:24

Edited By David Haynes on 18/01/2012 18:16:11

Edited By David Haynes on 18/01/2012 18:37:09

GoCreate20/01/2012 15:22:23
387 forum posts
119 photos
Hi Dave
I had been wondering about the workings of the snifter valve, now I understand.
Thanks for you help.

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