Hydrostatic Boiler Testing
The boiler on the model Trevithick Dredger Engine was ready, I thought, for the next boiler inspection.
So far the boiler inspector has approved:
- The plans
- The boiler materials
- The machined materials
- The results of the first bronze brazing session
- The results of the first silver soldering session
Next was the hydrostatic test of the assembled boiler components. This is a test which involves pumping water into the boiler, and holding it there for 20-30 minutes, at double the maximum operating pressure.
I intend to operate the boiler at a maximum of 50psi, but the minimum in the AMBSC code is 60 psi, so the test will be done at 120psi.
So I assembled the boiler, with a gasket under the big flat flanged end, and tested it in my workshop. It pumped up OK to 120-140psi, but there were several leaks. Most of the leaks were fixed fairly easily with teflon tape, but there was a persistent ooze of water from the big flat end gasket. It was not holding pressure.
Next step, a thicker gasket. Some improvement, but not enough. I was still noting a drop of water every 3-4 seconds.
Next step, gasket goo on the gasket. Maybe some further improvement, but still not enough.
OK, what next? Maybe the big flange, or the big flat end plate was not perfectly flat? So an hour or so, rubbing the flange on 600 grit wet and dry on a flat plate. There did appear to be some distortion. The plate had been carefully lathe turned, but maybe the heat from the brazing caused some distortion. A further reassembly then a test still showed more oozing than I was happy about.
Finally the penny dropped!
The flange was held in place by 24 stainless steel threaded rods with square nuts holding the flat plate in place. Some of the holes in the flat plate had evidence of the threaded rod in the form of threading marks made by the threaded rod. These were acting like threaded holes rather than relief holes. So I drilled them all by a further 0.5mm. Problem solved!
Pressure held to 140-160psi, with very slow pressure drop. The drop was caused by slight ooze from the pressure pump taped joins. See the videos. The occasional drip which appears in the video is coming from the leak in the pump delivery hose.
So now back to the boiler inspector….