Boiler Calculations.

and with butt strap joint of 87 per cent. 1631 pounds bursting pressure.

or

9

We now take the roundabout joint:

tensile strength X thickness X circumference

-X 60000 X 113

16

1018

area of head

= bursting pressure

=3746 lbs.

and taking 54 per cent. for a single-riveted seam, we have a bursting pressure of 2,022 pounds, or 400 pounds greater capacity than the longitudinal seam. If we take 70 per cent. for the double-riveted roundabout seam, we will have 2,622 pounds or 1,000 lbs. greater. There will never be a longitudinal joint made that will need a double-riveted roundabout joint.

Allowing a factor of safety of 5 for the longitudinal joint, we have a safe load of 344 pounds, and allowing a factor of safety of 6 for the roundabout seam we have 347 pounds as the safe load.

Tubular boilers require stays above the tubes. First find the area to be braced. Two inches above the tubes and 3 inches around the shell need not be taken into

account.

The distance between stays should be square root of

6,900

working pressure X diameter of bolts

Instead of 6,900 use of 5,530 for salt water and 5,000 for copper bolts.

Tubes should be of wrought iron. Steel tubes

Too Many Tubes.

require annealing, are too stiff, and will leak sooner than iron. Tubes give a cheap heating surface, and in order to get a boiler of large capacity it is the practice of some builders to put in all the tubes possible, so as to make the horse-power large. For this purpose they put in tubes away above the center of the boiler, reducing the area of the surface of the water for disengaging the steam, and a pulsating boiler is the result. The tubes are simply an economizer and are not as important as some other things.

When the temperature in the furnace is 2,200 degrees the shell will absorb the heat, so that when it enters the flues it is down to 1,000 degrees, and not over one-half of that can be absorbed by the tubes with modern high pressure.

Should an excessive number be put in, the hot gases will only go through a portion of them. Tubes which are too small break up the gases so much that the draft is restricted, and they become easily choked with soot.

Boiler Settings and Fittings.

Water issuing easily from the open end of a vertical pipe will assume the form shown in Fig. 12.

When entering a pipe, water or gas will assume the same form, shown in Fig. 13, so that the volume would be represented by the small cross-section, rather than by the area of the tube.

In putting in large pipes in water powers the pipe can be enlarged at the intake for what is termed the "entry head," and the pipe filled. This cannot be done. with the ends of tubes in boilers. Could it be, the velocity through the tubes would be greater and the deposit of soot less.

Feed Pipes-Circulation.

Tubes should be put in so as to obstruct the circulation of water within the boiler as little as possible. A free and full circulation of water counts for capacity and economy and is more important than a few extra tubes.

Care should be taken that the tubes are of full thickness of metal, also that the material for the shell is the specified thickness at the thinnest part.

The feed pipe should discharge at the coolest part

Fig. 12.

Fig. 13.

Shapes of Water or Gas Entering or Leaving Tubes.

of the boiler, which will be that portion the farthest from the fire.

One reason for this is that the circulation is the least disturbed. The boiler will deliver up the most heat from the fire when water is flowing fast over it, so a rapid circulation means more rapid taking up of heat and easier steaming.

Where water is admitted directly over the fire in a

Boiler Settings.

sheet boiler, it means leaks at the joint at head of boiler and at the first joint near the bridge wall. The correct plan is to put the feed at front head, top of tubes and to one side of boiler. Carry it to the rear of boiler, then across to opposite side and down between shell and tubes. The blow-off pipe should extend down to the floor

level, as shown in Fig. 14. It should be extra heavy iron pipe and a casing of larger pipe put around it. Should the water get to boiling, it can circulate in this vertical pipe, which it would not do with the horizontal pipe shown by dotted lines.

The blow-off valve for high pressures has given a

About Safety Valves.

great deal of trouble. Put on two valves, both extra heavy solid disk gate valves with outside screw.

When using, the valve nearest the boiler is opened first and then the other. When closing, the outside is closed first. This brings all the wear on the outside valve, as the inside is always balanced and moves freely. If preferred, an asbestos packed cock can be used for the outside valve.

Lever safety valves have about gone out of date. They or single-seat spring valves should never be used alone, but there should always be a double seat or "pop" safety valve. The latter, with a rise in pressure of 3 or 4 pounds, will open wide, and no further rise is possible; while with the two first the pressure may rise 20 to 40 pounds before the valve will relieve it. For years to come, in some cases, lever valves will be used.

"Pops" are set before leaving the factory. They can be changed by tightening or loosening the spring, -one side of the hex nut for five pounds, but if this is changed very much the ring at the bottom of the valve wants changing to preserve the sensitiveness of opening and closing. All boilers should have two safety valves.

The rules for area of safety valves are: For "pop" valve allow I square inch area of valve for each 3 square feet of grate. For lever valves allow I square inch for each 2 square feet of grate; or, multiply the weight of water evaporation per hour by .005; the result is area. of valve disc in square inches.

The water gage fittings should all be of a heavy pattern, and the glass gage 34 inch. The water glass gage should have automatic valves in the event of the glass breaking, or else levers on the valve stems, with chains so that the gage can be shut off. In case the