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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?
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?