[modeleng] Automatic Drain Cocks

Hi All,
I dont know if any of you saw the recent article in one of the Model 
Engineering magazines regarding automatic drain cocks.I didn't see it myself 
but I was sent a photostat copy of the design.
The article I believe was submitted by someone in South Africa and basically 
consisted of a short horizontal tube with a loose SS ball in it, sealed at one 
end and a hole at the other with a seating. A connection off of the top of the 
tube was made to the cylinder as a normal drain cock would connect.
Now the idea seemed to be so simple and the best thing since sliced bread.
The principle explained was that when the tube filled with water the ball 
dropped off of the seating and allowed the water to drain out,but when steam 
entered the tube it's velocity pushed the ball along the tube and onto the 
seating.
I made a couple of the said beasts and gave them a test by putting a bit of 
plastic tube on the top connection,filling my mouth with water and then 
blowing.I found that the initial water in the cock blew out but as soon as an 
air bubble went through, the ball popped onto it's seating but would not 
release untill all pressure was released no matter how much water was in the 
pipe.
Now this is not good news for a drain cock as even on a exhaust stroke there is 
still pressure in a cylinder,so it seems that these drain cocks would not 
release no matter how much water was in the cylinders once the ball has seated.
In such a small dia tube I do not see the difference in velocity between water 
and steam being much different which is the supposed operating principle.
I did think at the time that how something so simple not been discovered before 
but has anybody else tried playing with these things?

Regards

Clif 
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