Stays are sometimes screwed into the tube plates and then riveted over at the outside ends. A better method, however, is to let the ends of the stays receive a nut on each side of each tube plate.
Boiler tubes are secured in their tube plates by being expanded in. This may be done by driving in a taper steel mandrel, and then clinching them over, or by using a tube expander. There are two principal kinds of tube expanders, in one of which small rolls travel around the bore of the tube and expand it, while in the other a number of segments, held together by a spring, are forced outwards by a mandrel driven in by hammer blows.
Too much expanding is apt to weaken the tube close to the inside face of the tube sheet.
Boiler tubes leak first at the end which receives the greatest heat from the fire, the leakage being caused by the expansion and contraction of the tube, which is obviously hotter than the water which causes the tube to expand more than the boiler shell. The remedy is to re-mandrel or expand the tube.
The scale that forms on the face of the tube sheet keeps the water away from contact with the plate, which with an undue thickness of scale will crack between the tube holes.
A tube that is split or that cannot be made steam tight by being re-mandrelled or expanded is plugged up at each end by means of either wooden or iron plugs. The best plan, however, is to use iron discs having a stepped diameter, so that one end will fit the bore of the tube, and the other will form a shoulder that will cover the end of the tube.
Each disc has a hole through its centre, so that a wrought iron rod or bolt may be passed through the hole and receive a nut at each end. Beneath the flange of each disc, a grummet of spun yarn and white lead is placed, so as to make a steam tight joint when the nuts are screwed home. This stays the tube plates as well as stopping the leaky tube.
If wooden plugs are used, they are made a driving fit in the tube bore, and driven through until they have passed the split, and a second wooden plug is driven tightly from the same end of the tube.
