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Leaky Safety Valves.

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mgj19/11/2009 23:23:02
1017 forum posts
14 photos
I can't get these darned things to seal - any ideas.
 
Basically they are pretty standard spring and ball type, using a spring preload of 5lbs force - there are two of them. they don't leak badly, but it offends me. Also at present they are bone dry - no oil.
 
Everything is concentric, because all was drilled at the same setting - and reamed.
 
I am told that one should not bevel the edge of the hole in which the ball sits so at present its left reamed sharp for line contact.
 
I could bevel it if thats the right thing to do, but if so at what angle should the seat be cut to.
 
Also in the past I used to do ball valves by giving the ball a sharp tap into the seat with a reasonable knockometer. Use different ball in service, and it should seal. 
 
Or use a viton ball.
 
Add some heavy steam oil?
 
Suggestions please as to the recommended way. Rcomended being the one that will work with least expense and effort.

Edited By meyrick griffith-jones on 19/11/2009 23:25:20

Windy20/11/2009 06:58:51
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910 forum posts
197 photos
Hi Meyrick, regards seating on valves, a method suggested in the flash steamers bible Experimental Flash Steam by J H Benson and A A Rayman on page 101 solder a ball on a piece of tubing of slightly smaller diameter. This is rotated in a drill chuck and the valve seat burnished. No abrasive required less than a minute of this treatment will do the trick. On my flash steamer all the pump valves are stainless steel and the balls have to be non magnetic as the magnetic type balls dont seem to like hot water and steam and the lovely smooth finish disappears. Back to the book I see John Benson and his Son at some of the regattas I compete at a most charming man and allways a pleasure to meet.
   
Windy  
Rob Manley20/11/2009 10:04:16
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71 forum posts
14 photos
Ideally the seating should be bored, both the bore and the seating face as reaming never produces a good enough round hole for sealing purposes.  As you say a light 'tap' just to seat the ball is quite common but the tap should only be very light. 
Circlip20/11/2009 15:17:53
1723 forum posts
Try a centre drilled "Punch" a sliding diameter in the valve body (The Dia. ABOVE the ball hole). If you've tapped the ball and it doesn't seat first time, you have to square off again and try again. Re-read Rob's "Tap", VERY light.
 
  Regards Ian.
chris stephens20/11/2009 17:00:47
1049 forum posts
1 photos
Hi Meyrick,
I do not claim any particular expertise on the subject, other than seating balls in Triumph and Ariel oil pumps. 
The thought occurs, for a decent seating, that the ball you intend to seal the hole must be of a fairly carefully selected size. Intuitively it goes something like; if the hole is small compared to the ball you have effectively a flange type joint(leaky). If the ball is nearer to the size hole, you would have in effect a tapered bung(not leaky). Logic therefore dictates that there is a compromise ratio  of ball to hole size.that will seal effectively. All other considerations excepted, naturally.
If someone can point me in the direction of a paper on the subject, I can follow my own philosophy and research the subject further, for future reference
chriStephens
mgj20/11/2009 18:03:45
1017 forum posts
14 photos
I must admit this is a field I know little about, so I have followed drawing. The ball is 281, and the hole .219.  Reamed and all drilled on the mill. The guides are a light sliding fit in the bronze turrets. The preload is adjustable, but set at a theoretical 5lbf
 
I follow Circlips idea about directing the light tap, accurately, and that seems very sound.
 
If that doesn't seal then I will burnish (ref Windy) the seat that Circlip/Robs tap has made.
 
If that don't work I will set up the valve body in the 4 jaw and bore a thou out and then recalculate area for preload - the gospel according to Rob.
 
If that doesn't work, then I'll go to a redesign in Chris vein - flat plate/disc and Viton O ring. 
 
Grateful to you all for your help. I'll let you know how I get on.
 
 

Edited By meyrick griffith-jones on 20/11/2009 18:05:39

Edited By meyrick griffith-jones on 20/11/2009 18:06:14

chris stephens20/11/2009 19:51:53
1049 forum posts
1 photos
Hi Meyrick,
No, No, I did not suggest flat plate/disc etc. I was hypothesizing about balls in holes. 

I seem to recall a description of a safety valve that was positive in its action, either closed or open , with nothing in between, no half measures. It might have been LBSC writing sometime in the fifties, although someone else must have rewritten it again many times by now. Not that it would be prototypical for your Traction engine. 
chriStephens
mgj20/11/2009 21:33:30
1017 forum posts
14 photos
No, but a plate is merely a ball of infinite radius so your comments got me thinking.
Jens Eirik Skogstad20/11/2009 22:36:31
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400 forum posts
22 photos
Valve seat shall have an angle in the opposite direction (not same as the valve seat as we know, are used in the internal combustion engine) and put the ball down on the valve seat and give a light stroke on the ball, it will shape the valve seat of the ball and will be tight against leakage.
 
I used this method and works well after the idea from the book "Model engineer" by Henry Greenly

Edited By Jens Eirik Skogstad on 20/11/2009 22:37:35

chris stephens20/11/2009 23:15:39
1049 forum posts
1 photos
Hi Meyrick,
The point I was trying to imagine was not so much infinite radius, but the opposite.  The smaller the ball the more it would meet the hole on the balls side, like a wedge. Infinite radius would only put a patch over the hole. Clearly you don't want a wedge, the ball must not get stuck, which would negate it's purpose. Hence the happy compromise requirement, but then what do I know?
chriStephens
PS thinking of infinite radius, made that tool yet?
Martin W21/11/2009 01:20:53
940 forum posts
30 photos
Chris
 
Given the tool some thought but it won't fit on my lathe !!!!!!!!!!!
 
Best regards
 
Martin
chris stephens21/11/2009 03:22:57
1049 forum posts
1 photos
Hi Martin,
???????  
Belay that, i just saw the joke, very funny! 
Did I say "joke", well, one must be charitable once in a while.
chriStephens
mgj21/11/2009 16:49:01
1017 forum posts
14 photos
Bit slow today Chris.
 
Bingo - all works fine at 50 psi off the compressor. Tight as a drum.  Sort of bevelled edge, plus the guided tap, plus a bit of twizzling. (belt and braces!)
 
So if I get a leak now its just a matter of upping the preload to suit.
 
Thank you gentlemen, appreciated.
 
As for the tool - not yet. I'll grind it up next time I do a batch of drills.
 
M.
 
 

Edited By meyrick griffith-jones on 21/11/2009 17:11:03

Circlip22/11/2009 17:09:47
1723 forum posts
Right, now that's sorted, ----about this flamin Ti.

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