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Post by mr swarf on Jan 7, 2024 22:23:03 GMT
I am making a pair of Gordon Smith safety valves to the number 2 design. The drawing gives a shroud diameter of .230 to .225. Could someone explain what the difference in the valve performance would be between these two diameters please.
Regards, Paul
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oldnorton
Statesman
5" gauge LMS enthusiast
Posts: 693
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Post by oldnorton on Jan 8, 2024 19:10:08 GMT
Hi Paul, If you posted an image of the drawing we could try to discuss. Otherwise we are relying on memory and/or finding copies of our drawings. Only saying to offer help :-) Norm
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Post by mr swarf on Jan 8, 2024 21:04:25 GMT
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Post by ettingtonliam on Jan 8, 2024 21:18:43 GMT
Presumably at the smaller diameter the valve will pass slightly more steam when it releases. Otherwise those diameters are just the tolerance for the shroud?
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uuu
Elder Statesman
your message here...
Posts: 2,811
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Post by uuu on Jan 8, 2024 21:32:30 GMT
It might affect the mild pop action. Your task, should you decide to accept it, is to make your pair one of each end of the range, test them, and report back on the results.
Wilf
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millman
Part of the e-furniture
Posts: 297
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Post by millman on Jan 8, 2024 22:10:15 GMT
Seems strange to me that the shroud has a five thou tolerance on it when it fits in to a 9/32 un toleranced hole.
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Post by RGR 60130 on Jan 8, 2024 22:10:59 GMT
I’d expect the blowdown period of the valve to vary with the shroud diameter.
With a plain safety valve you will find them tending to constantly feather. The ‘Pop’ feature is to get the valve to open, allow some steam to escape, and close cleanly again. What happens in the design shown is once the valve starts lifting the escaping steam will hit that little flat face either side of the ball, get deflected down to the bottom surface of the body then deflect upwards again and escape to atmosphere.
The steam being deflected down helps open the valve and gives a clean blowdown period.
Reg
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Post by chris vine on Jan 8, 2024 22:35:24 GMT
Apart from that one dimension, all the others are nominal. So, one might start looking at the question as if all the other dimensions are to size and this 0.225 to 0.230" dimension could be varied.
Another way of looking at it is that he has just given a tolerance on that size.
If the diameter is made larger, then the escape passage for the steam is reduced (and the area for the steam to press upwards is increased), so that once the valve has lifted the pressure under that diameter will be greater. This means that the valve will stay open for longer, or until the pressure in the boiler has dropped more. IE, at the larger diameter, the pop action will be more severe. Or the hysteresis (difference between pressure) will be greater.
As Wilf suggests, a little experiment might be interesting. Unless you have access to some sophisticated computer software to simulate it!!
Hope that helps... Chris.
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mbrown
Elder Statesman
Posts: 1,720
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Post by mbrown on Jan 8, 2024 23:03:01 GMT
I am sure that, in one of his articles on safety valves, Gordon Smith explained the tolerance in precisely the terms Chis sets out above. Sorry - not in a position to check the articles until the weekend...
Malcolm
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Post by steamer5 on Jan 9, 2024 2:32:26 GMT
Have tried to copy the relative parts of the articles, but no I’m not going to let you copy that, only the bits I won’t to! 😡😡😡🤬🤬🤬🤬
According to the article the only dimensions that are important are the depth of the cavity in ball shroud ( which then controls the amount of ball stick out) & the diameter of the shroud. If I’ve got it right then increasing the diameter increase the pressure drop ( by increasing the lift) plus decreases the cycle time.
To get the shroud depth correct machine up the shroud, stick a ball in measure the total length, compare against the calculated length in the drawing , adjust as required, it’s going to be easier if the hole is a bit deeper than required to machine off a bit rather than trying to drill to an exact depth. If this doesn’t make sense send me a PM & I’ll send you the article.
Cheers Kerrin
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stevep
Elder Statesman
Posts: 1,070
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Post by stevep on Jan 9, 2024 10:02:32 GMT
When I made new safety valves for my Rob Roy to the Gordon Smith design, I made several shrouds, of differing dimensions. As has been suggested, the difference in diameter affects the hysteresis of the valve - the difference in pressure from where it opens, to where it closes.
I changed between the ones I made until I was happy with the result.
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Post by mr swarf on Jan 28, 2024 21:27:12 GMT
Thanks for your comments. I have now made a pair of valves to the drawing. Testing on air I found the valve opened fine but took a long time to close fully resulting in a large pressure drop. I tried the different shroud diameters shown in the drawing but this had no noticeable effect. I then experimented with increasing the gap between the seat and the shroud and found increasing it gave better results. I kept increasing it a few thou at a time & testing until I had a gap of 30 thou which gave the result shown in the video. Open & fully close within 6psi according to the compressor gauge with no rude noises or needing to tap the spindle to close. The real test will be with steam. Saftey Valve Video by Mr Swarf, on Flickr
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