Ball Valves

[John WeightParticipant @johnweight34532]

Hello fellow Model Engineers,

I am hoping that someone might be able to give me a few tips on how to make ball valves steam or air tight. I have been trying for longer than I like to admit, to stop the ball valves ( there are three) in the water pump on my Minnie Traction Engine letting air or steam by. I can get them to almost stop leaking but not completely. Any advice or tips would be gratefully received, thanks.

John

[John WeightParticipant @johnweight34532]

[mgjParticipant @mgj]

Well there is no doubt you can get them completely steam tight.

Easy way – write to Polly models and use viton balls, which are fine so long as they cannot rise far enough to block an outlet (dead ends the pump,. and bends conrods ! But they are effective, and seem to last. (also make very good injector steam stop valves with a flat ended doohickey on the handwheel instead of a cone, and less hassle than an O ring.)

Ohter way, with stainless balls. Use a D bit (Tracy tools if you are lazy or make one) to get a niice flat bottomed hole – never had a lot of luck with a slot drill or endmill, but thats me. Centre drill and drill the hole, and I always ream to get a good sharp edge. Take a spare SS ball of the right size, and seat in hole, and then with a brass drift and small hammer, give it one good smart twat to give a seating. (Brass drift in bronze/gunmetal and you don't need to chuck that ball away, but that is the normal advice, which I don't follow).

Some burnish the seat, using a ball superglued into a carrier and then use Brasso/toothpaste and an electric drill. If you are nice and square with the drift and firm with the light hammer, you shouldn't have to. (A guide for the drift can be useful and I have made them for the standard sizes so I do use one) Squeeze the ball into place is another option.

Has this lot been running – not possibly scaled up is it? Or is it new?

Set pump type NRVs for about .030 lift max, injector clacks for quite a lot.( .1 of an inch I use) since oscillation and bounce is not a problem.

Hope it works out.

[John WeightParticipant @johnweight34532]

Hello MGJ,

Many thanks for your reply to my query which confirms most of what I have been trying, always with much the same results. I think I shall have to try a bit harder with the seating, your idea about lapping the seat is one I have never tried so will give that a go as soon as I get back from a few days on the Yorkshoire Moors. It is definately not scaled up as itis almost unused so far.

Regards,

John

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

The main cause of problems with ball valves is in the mechanical design . The ball needs to be guided so that it drops on to the seat in the right place every time . Most model designs show the ball as moving in a relatively huge hole which allows large amounts of unpredictable sideways movement .

Other things to watch for are too small and wrongly placed side connections which cause steam or water to generate flow forces strong enough to push ball sideways off seat .

Michael Williams .

Edited By MICHAEL WILLIAMS on 11/06/2012 10:55:29

[mgjParticipant @mgj]

With respect – no.

I have no huge experience to draw on, but on the clacks on my TE, (4 in all) and on MM (3 working) none are guided particularly. I have seen designs which control the ball up and down and stop it rolling around, – while they may show an advantage in particular applications, the common or garden hole with a ball at the bottom of fairly indeterminate size works pretty well.

As long as the distance from the edge of the hole, or seat to be precise to the wall is just about 1/2 the ball dia, or a touch less, the ball will roll back in automatically.

I did actually knock one up these guided jobs for the TE when I was having injector problems (different issue altoghether) and it showed no advantage or disadvantage.

Personally I'm sure its just a matter of a deep enough seat, into which the ball will drop/roll very neatly. Pretty quickly too – especially as on track or on a TE the while thing is being jiggled about anyway.

Its the same thing as the plug on your bath – that gets dragged into place from some distance, and quite accurately, and for the same reasons.

[Author]

Posts