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I decided to do a little more tinkering with my Aristo Mikado over the weekend, trying to figure out why the safety valve can't keep up with the steam production.  My first steam test with the new pressure gauge revealed that the boiler pressure was still increasing while the safety was releasing steam.  Although the safety is on top of the boiler, hidden under the sand dome, the escaping steam is diverted downwards through an exhaust pipe, rather than releasing directly into the air.  There's a banjo fitting that goes around the safety valve, a short pipe about 3/8" long going forward off the banjo, which curves down into a T joint with a cross pipe that wraps over the top of the boiler and is open at either end, releasing the steam down towards the track just behind the cylinders.  Presumably the idea is to prevent novice steamers from getting a face full of steam if they're looking too closely when the safety pops, though one of my planned projects is to re-route the exhaust steam from the safety to a more prototypical location and release upwards, as Larry Herget did with his.  

I removed the safety valve and its exhaust pipes, and checked the valve to make sure it wasn't clogged or sticking.  I didn't see anything obviously wrong, but decided to give it a good cleaning anyway, then re-installed it on the boiler, minus the exhaust pipe assembly, for another stationary steam test on the kitchen counter.  This time the test was a success.   Steam started to wisp out at around 40 psi, and at 45 the safety was going full blast with the boiler pressure holding steady.  After watching it for about 5 minutes, still at 45 psi, I turned off the gas and let the pressure drop until the safety had closed again, then re-installed the exhaust pipe assembly.  When I lit her up again the result was exactly the same as before, it started venting at around 45 psi, but the boiler pressure kept steadily climbing until I finally shut off the gas just shy of 80 psi.  So, the problem appears to be that the exhaust pipe is choking off the escaping steam and putting back pressure on the safety.  I'm not sure if it's blocked (if so, it must be in the short section between the banjo and the T because there appeared to be more or less the same amount of steam blowing out of each end of the cross pipe), or if it's a poor design that should have a larger pipe diameter.  Either way, it will be remedied when I re-route it.  Have any other Aristo Mikado owners noticed this problem?
 

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This seems to be a serious safety problem. Obviously the safety valve is sized properly for the boiler and firing rate, but the design did not take into account feeding into the extra piping. I would recommend that owners of the Aristo Mike to reduce or shut-off the fire of their locomotive if the safety starts blowing continuously. Serious Safety Problem!
 

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Richard
No problems thus far with our Aristo Mike.  Was the steam release exiting the down pipes making a whistle like sound?  Just wondering if the velocity of  steam was slowed as the result of back pressure.  Maybe obstruction but not enough to completely close the safetly exhaust line.  Did you clean the exhaust lines?
 

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RW, were you using preheated water in your boiler? This weekend I used just off the stove boiling water to fill my Mikado and then immediately lit the burner, sure enough, steam right a way. The pressure gauge I set to blow off at 50lbs, easily reached that and indeed, the safety continued to release the excess pressure. I attribute this to the use of boiling water (not just pre-heated) I simply placed a few more cars behind the engine and cut back on the fuel. This is the first time I've ever used that hot of water, I don't consider it a problem in my case.
 

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Dan
There is a difference in what you are indicating and the situation presented by Richard.  In his case the boiler pressure kept climbing above 80 lbs despite the safety going off.  Therefore the safety could not move the volume of steam through the exit lines.  As Richard indicated when the exhaust line on the safety were removed then there was no restriction that relates to the continued building of steam via normal steam pressure on the gauge once under steam.
Adding boiling/hot water into the boiler then firing will only result in less time necessary to make steam not cause excess steam pressure due to delivery setup.
 

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Posted By Semper Vaporo on 01/15/2008 12:54 PM
This seems to be a serious safety problem. Obviously the safety valve is sized properly for the boiler and firing rate, but the design did not take into account feeding into the extra piping. I would recommend that owners of the Aristo Mike to reduce or shut-off the fire of their locomotive if the safety starts blowing continuously. Serious Safety Problem!



Charles,

You pretty much hit the nail on the head here!  The issue is that the "cap" Aristo puts on their valve to make the steam flow to the pipework down the boiler,  causes the steam to create a backpressure on the valve assembly and spring (keep in mind that it is a very weak spring here!).  To see what I am talking about, take the cap off of the valve and fire up the engine.  You'll see that the pressure of release has dropped by 5-10 psi.  Water temperature has nothing to do with the operation of the safety valve, as they are lifted by the pressure of steam, about 90% of the time...the other 10% being if the engine has an water feed pump and the boiler has enough regenerative capacity to cause the safety to release with water. 

Yes, the pressure will creep up until the gauge is pegged, however this is a fault of the "safety" system that Aristo implemented to prevent, as was said eariler, newcomers to the hobby from being shot in the face by steam.   Of  which I feel this completely unnecessary, in my 7 or so years of running steam (18 yrs old now), I have never once been scorched or burned by a safety valve on a Gauge 1 locomotive. 

Sure, I have come out with a wet shirt, wet hair, steamed glasses, etc, but never been injured by a safety valve.  Pride, maybe, seriously, no way!  There simply is not enough velocity and mass of the steam (in our scale), saturated or not, to cause bodily harm, unless one purposefully holds their hand millimeteres over the valve (which I would not advise!), most of my experiences of a quick service while leaning over the engine (adjusting the bypass, checking valve packing, etc) have been atleast 3-5 inches above the engine.  Enough ranting about that. 

Richard,

If you test fire the engine with the cap removed from the safety valve, you will see that it is indeed backpressure causing your problem...doing Larry's modification will resolve that, plus it looks very realistic!
 

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Ah ha,  I have been clarified......and would agree with the back pressure concept.  This weekend I think I'll get to work on Larry's mod, seems like the way to go.
 
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