Hey everyone,
Our shop force just finished changing around 300 stays throughout a boiler and I thought I would share the process with you, since it differs from US practice. Just for fun, they are called støttebolte in danish (stu-te-bol-te).
The first step is to use an acetylene torch to cut out half of the stay bolt from the firebox side (possible because of the copper fireboxes). The stays are cut out in an alternating pattern. After the stays have been partially burned out, an air operated drill is used to drill just under the thread size, knocking the remains into the bottom of the water leg. Later this is fished out with magnets and metal hooks.
Once the hole is evaluated, the appropriate size stay tap is chosen, working up the sizes until the threads are sharp and clean. Stay bolts are 11 threads per inch and can range in diameter all the way from 26mm (1.02") to 36mm (1.41"). The idea of multiple sizes is so that the longest lifespan can be obtained from the sheets on the boiler.
The stay bolts are machined from steel with a Brunel hardness of 120-80. We also use copper stay bolts around the area you would expect flexibles, which contain arsenic for longer lifespan. Generally the stay bolts are machined in rods of two or three at a time, to then be cut apart and machined. After a while, an assembly line forms and production is relatively fast (30-40 on a good day, less with copper). My father ended up creating a really useful home made taper bore tool to help save time.
The interesting part of the whole ordeal comes in sealing the boiler. The tapered cone bore in either end is for the purpose of expanding the material to fill the threads completely. Two medium sized air guns with expanding tools are operated by two guys, with a third person measuring the distance from the sheet to the stay bolt head inside the firebox. This is so there is enough material to be moved and form a round head (similar to rivets). The tool for this purpose is a large air gun with a second piston on the back to help lock the gun in place. One man opens the air valve, while another turns a handle on the forming tool to help achieve the round head. Finally when all of this is done, the process is complete!
Hydro test of the boiler is this Saturday, and I hope there will be no more work necessary! The boiler was already approved by the authorities this spring, but the decision to change almost all of the stays was made a little bit after and is considered maintenance.
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File comment: burning out stays IMG_4078.jpg [ 283.85 KiB | Viewed 5743 times ] |
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File comment: drilling out the other half 11230105_10206116311900309_2038132432992160402_n.jpg [ 109.79 KiB | Viewed 5743 times ] |
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File comment: The drawing all stay bolts are machined too stay bolt drawing.jpg [ 101.78 KiB | Viewed 5743 times ] |