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Discussion Starter · #1 ·
I recently purchased a "new style pop valve w/adjustable pressure release" from Accucraft (AP-21103) intending to use it on my Mogul. It certainly works, but not as I expected it to.

It came adjusted to pop at about 80psi. It can definetly be adjusted lower. But regardless of what it's adjusted to, when it pops it consistently dumps a full 40psi. Is this the way it is supposed to work, or did I get a faulty one? I had expected it to dump maybe 10-20psi then shut off, not dump my entire head of steam.

[I know how to adjust the high pressure setting... but don't see any way to affect the low pressure setting.]

Thanks,
Joe
 

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I bought an Accucraft pop valve for my K-27 that did the same thing. I replaced the ball with a precision stainless steel ball and the problem went away. I can't remember whether it was a 3 or a 5 mm ball, but I got it from Quisenberry Station. Another thing I did was once I found the sweet spot on the adjustment for 60 psi, I put a small drop of paint on the edge of the screw in thingy so it stayed in place. I noticed that over time, it moved slightly with each pop, reducing the top pressure. Screwing it in raises the pressure and screwing it out lowers the pressure.
 
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In order to operate as a true pop valve, and this is especially true of miniature pop valves, a set of very definite internal proportions needs to be adhered to, and that includes a precision ball, if a ball is used as the valve. Failing to get one or all of the proportions right causes poor behavior, . . . popping off but dribbling shut, or not shutting, dribbling open and shut, and allowing too much pressure drop between opening and shutting. These are all functions of the internal proportions of the chamber, the ball diameter relative to the orifice and chamber, and the spring rate. The safety on my Pannier allows only a 1 to 3psi drop before letting go and will often pop at 5-second intervals when running at full tilt, some pops lasting much less than 1 second. Repeated popping is wasteful of fuel and water, but I don't care, I'm amazed that such a small device works precisely as it's full size counterpart but it does.
 

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Curmudge,
I accept what you say, but you must realize that most of us are subject to the design we purchase and can only make minor adjustments and improvements since we don't have precise machine tools to do otherwise. I merely suggested ONE solution that worked for me so that Joe could try it and not have to return the device or purchase another one.

I recently talked to a person at a steamup and observed his Accucraft K-28 with an Aster pop valve in it that worked as precisely as yours.
 
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Mine wasn't a criticism, just a bit of information, in case anyone wondered why they often don't work as they should. It appears Accucraft got the seats and chambers right, or nearly enough right, and the ball wasn't quite up to snuff.
 

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Discussion Starter · #8 ·
Posted By ETSRRCo on 08/27/2008 6:54 PM
So do all Accucraft pops have this problem? Joe how hard was it to put the pop on the mogul?

I gave Cliff a call. As always, he was very helpful. He quickly agreed that the pop-valve that I received is clearly faulty, and he is going to pull and bench test a replacement for me (pop at 75, close 65-70). Per Cliff, it should only dump 5-10psi max before closing. So apparently my faulty pop-valve is not the norm, but also not unheard of.

Since he's sending a replacement, I'll go ahead and pull the bad one apart and see if I can find a replacement ball for it as an experiment.

Installing the pop-valve is a breeze. On the mogul, you first have to remove the one bolt that holds the whistle arm to the whistle. Once that's out of the way, the entire steam dome unscrews. The pop-valve is inside and just screws into the boiler. I had to use a pair of pliers to loosen the original; I didn't tighten the replacement quite as much since it has an O-ring and seals well with light pressure.

It does take a special tool to adjust the pop point. I machined one from a cheap flat-blade screwdriver using a dremel tool with a cut-off weel. Took maybe 10 minutes to make. But it's quite possible that a new pop-valve will come factory set at a reasonable pop-point and won't need any initial adjustment at all, so that tool might be optional.

Joe
 

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So the dome just screws off? Might be a dumb question but I just bought my mogul second hand and all it had was boiler certificate and gas tank certificate. I tried unscrewing the dome but it wouldn't budge.

-Eric
 

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Eric,
I don't know about the Mogul, but my K-27 has a screw-off dome and my K-28 has one little allen set screw to release the top half of the dome, which then lifts off. I had to use a flashlight to see the set screw on the front and not on the side of the dome about half way up.
 

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Discussion Starter · #11 ·
Posted By ETSRRCo on 08/28/2008 5:06 PM
So the dome just screws off? Might be a dumb question but I just bought my mogul second hand and all it had was boiler certificate and gas tank certificate. I tried unscrewing the dome but it wouldn't budge.

-Eric



The domes are notorious for being tight. Yes, they just unscrew at the base (after you've removed the whistle handle so it doesn't get in the way).




You might want to consider first unscrewing and removing the whistle itself just for safety's sake. So if you slip or get a bit aggressive while breaking the dome loose, you won't accidentally snap off the whistle at it's base. [Been there, done that. /DesktopModules/NTForums/themes/mls/emoticons/sad.gif ] It should only be finger tight and should unscrew easily.




Joe
 

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Thanks for the advice! Like I said I just bought the mogul and I love it! I was thinking of getting a pop valve for it.

Eric
 

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Discussion Starter · #13 ·
Posted By weaverc on 08/28/2008 5:21 PM
Eric,
I don't know about the Mogul, but my K-27 has a screw-off dome and my K-28 has one little allen set screw to release the top half of the dome, which then lifts off. I had to use a flashlight to see the set screw on the front and not on the side of the dome about half way up.



There is no set screw on the Mogul's dome; just screws off. (At least not on mine.)

Joe
 

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Some do and some don't. These things are very small and it is difficult to maintain exact dimensions to the degree of accuracy required... also because the coefficient of expansion exceeds the dimentional accuracy required due to the miniaturization. There is an article in Live Steam magazine this month that describes making the valves and the author says that he can make two to the most exacting dimensions and get them to work well, but if he exchanges parts between the two, one or both will cease to operate properly. Even attempting to duplicate a part, usually the plunger that is what is lifted, to the samed 'mic'd' dimensions will not necessarily make a working pop valve. The suggestion to change the ball is made often as just another ball may be of the needed dimension to make it work better (though it could also make it worse!). I have had one go off with such a pleasing POP and just as quickly shut off, so I know they can work very nicely, but most often they dribble a bit, then leak some more, then vent steam and then either shut off abruptly, or do the reverse of the start up... or vice-versa, go off sharply and vent too much steam and dribble shut. Taking the ball out and putting it back can affect the operation, where just making adjustments do nothing of value... I assume it is that the ball may not be perfectly round and the removal and reinsertion just makes some different dimension in the correct location change things.
 

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In the January/February 2007 Live Steam & Outdoor Railroading Kozo Hiraoka has an article Safety Valves for Live Steam Boilers. This is a superb 11 page article which explains all about Safety Valves and how to construct them.
Regards,
Gerald
 

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Also as a note these small boilers do not need a "pop" type safety, they tend to drop the boiler pressure to much. The larger ones it works ok for, as they have the steam volume for them. The weeping safety's in my mind are the best for the boiler on the mogul.
 
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"Needing" is not the point. We don't need any of this stuff. People go to great lengths and expense to have as nearly an operational replica of a locomotive as they are able, so why not when technology will allow it and it's not all that difficult to do have a prototypically operating safety valve as well? To me this represents a significant accomplishment, overcomming the probem of dynamic and mechanical losses due to miniaturization, at least in one area. There is nothing I dislike more than a dribbly snot-nosed weepy little safety valve.
 

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I know of some people that have attempted to make true scale safety's but they seem to run into the problem, that even with a short burst they release to much steam pressure. The problem seems to be that with the smaller boilers there in not enough horsepower (flash rate) to over come the pop safety valve. They seem to work much better on the larger, higher pressure boilers because with the higher pressure is higher a higher water temp. Thus making the boiler have a greater flash rate and can overcome the pop type safety. I would like to have a scale working valve as well but for now I will take what works the best for the application.
 
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I tried making a few (for several scales) and there's little difference in the outcome if you don't get the chamber and seat geomtry just right, the release behavior will be all over the place. I determined that among other things I was using "ball-park" design information - "Aww make it about this big, about that long . . . that oughta' work!" WRONG. Like miniature injectors, where the days of LBSC's "poke an weensy 'ole through it about that big" are over, there is now good design information available. My primary interest is pretty much the same as many others, getting prototypical ACTION of the valve and being able to closely control the behaviors we've been talking about rather than the appearance, although obviously the smaller the valve the better. I agree that a "scale" safety would run afoul of one of the effects of scaling, ie, the smaller the scale the more difficult it is to get a device which operates on or controls moving fluids to work. The fluids can't be scaled!
 
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