"Siemens" here probably refers to a type of open hearth furnace - the Siemens Martin process - rather than working for Siemens the company.
http://doot.spub.co.uk/code.php?value=278 - mention here of a "plug rammer" in the specific context of the Siemens-Martin process, as someone who blocked up the hole in the furnace after it was tapped using a plug of refractory fireclay (this is a batch process, they had to "tap" the furnace to get the steel out).
http://doot.spub.co.uk/code.php?value=279 - "compo man" here as being someone who produces "compo", a mix of refractory materials for, among other things, lining furnaces.
So I would suggest that his occupation here is something in that ballpark - perhaps something similar to this description of the work of a blast furnace keeper:
http://doot.spub.co.uk/code.php?value=171taps furnace by driving with heavy hammer, steel rod into baked clay plug in pouring hole of furnace, thus breaking up plug, which is swept aside by stream of molden iron (or lead, as the case may be); superintends and may assist at replugging furnace pouring hole; gives instructions when large lump of clay is to be thrown into hole and assists gang of men in driving plug home with long metal rods