thing to do then is to determine the ratio that each one of these factors bears to the strength of the solid plate and adopt the weakest or smallest ratio as the possible point where rupture will take place. Compute these three efficiency ratios for the joint E; as follows:

Ei

Rule III. Divide the strength of the weakest section by the strength of the solid plate. (See Rule I.) The result is the efficiency of the riveted section.

Thus in the example cited we have seen that the strength of the solid plate is 22,343 lb., that its strength between rivet holes is 12,890 lb., that the shearing strength is 16,332 lb. and that the crushing strength of the plate in front of one rivet is 16,328 lb. Hence, the weakest place is in the strength between rivet holes and consequently the efficiency of point E; is

Gage Pressure Necessary to Burst the Solid Boiler Plate.We come now to the most interesting point of our analysis, namely to compute the bursting pressure of the solid plate.

By introducing a number of rows of rivets for riveted lap joints the shearing strength and the crushing strength of the riveted section are proportionately increased, while the tensile strength of the net section remains the same.

In the discussion of the strength of the solid boiler plate we found that the force of steam pressure acting so as to tear the boiler plate apart longitudinally would evidently prove most disastrous in bursting the solid boiler plate. Since the pressure

of steam exerts itself equally in all directions, we shall compute the total pressure available in this particular direction as this would give us the critical pressure for our present consideration.

8

If the boiler is of length 1 in. and inner diameter D in. the area of steam pressure is Dl. Since now the boiler gage pressure is P. lb. per sq. in., the total pressure of the steam would evidently be P, Dl lb. To resist the boiler tearing apart there is a strip of boiler metal on each side of length l and thickness t. Hence the total metallic area of resistance is 2lt. If now the force of resistance offered by the metal is S. lb. per sq. in., we have, when an explosion or bursting apart is about to take place, that this resistive pressure is 2ltSt.

Equating these two pressures, we have

Rule IV. Multiply the thickness of the plate by the tensile strength of the plate and divide by the radius (one-half of the diameter). The result is equal to the bursting pressure of the solid plate.

In the example previously cited we now compute the bursting pressure of the solid shell of the boiler under consideration for a boiler diameter of 36 in. as follows:

This means that a gage pressure of 764 lb. per sq. in. would rupture the given boiler if it existed without a riveted seam.

Bursting Pressure of the Seam.-But our boiler under consideration would evidently burst before the bursting pressure of the solid plate were reached for the riveted section has weakened its total strength. In Rule IV we found that the efficiency of the riveted joint is the ratio of the strength of the weakest point to the strength of the solid plate. Hence we have that the gage pressure P at which the boiler will probably rupture at the riveted joint is

Rule V. Multiply the bursting pressure of the solid plate by the efficiency of the joint. This result is equal to the bursting pressure of the seam.

Thus since the efficiency of the joint E, is found to be .578 and the bursting pressure P, of the solid plate to be 764 lb., we have that the bursting pressure P of the joint which is the weakest part of the boiler construction is

FIG. 82.-A double riveted butt and double strap joint.

In general the butt joint doubles the shearing strength of the joint while the net tensile strength and the crushing strength of the joint remain the same as in the lap joint discussion.

The Safe Working Pressure. Of course the boiler is never allowed to operate anywhere near this bursting pressure. A factor of safety is insisted upon. The U. S. tables are based upon a factor of safety of 3.5 for drilled holes and 4.20 for punched holes, which are the lowest factors allowed in any civilized country. The factor in most European countries is either 5 or 6. In any case, if factor of safety f is used, we have that the working pressure P is found from the formula

w

The rule advised by the Hartford Insurance Company's inspectors is as follows:

Rule VI. Divide the bursting pressure of the seam by the following safety factors: 0 to 125 pounds, 4.2; from 125 to 150 pounds, 4.5; 150 pounds or over, 5. The result is the safe working pressure under which the boiler is to operate. The American Society of Mechanical Engineers in their Boiler Code require a factor of safety of 5 for all new boilers.

Thus in the case at issue the safe working pressure Pu becomes

Recapitulating the discussion of the six rules, we now see in its completeness the method involved in computing the safe working pressure of a boiler. In this particular instance we find that a boiler of 36 in. diameter, with 14 in. plates and a single row of rivets spaced 15% in. apart may safely operate under 105 lb. pressure (gage).

Example of a Lap Joint, Longitudinal or Circumferential, Double-Riveted.-By similar reasoning we may now compute the efficiency of a lap joint which is double riveted whether longitudinal or circumferential. Thus, if the tensile strength of a boiler is stamped 55,000 lb. per sq. in. with thickness of plate 5/16 in., pitch of rivets 2% in. diameter of rivet hole 3/4 in., we have by applying our rules:

CHAPTER XX

FURNACES IN FUEL OIL PRACTICE

FIG. 83.-Interior of a furnace, showing brick work and air-spacing.

ET us now set forth the cycle of operations necessary in the utilization of crude petroleum as an economic factor in the production of steam. The oil in a heated state and under pressure must be sprayed into a heated compartment or furnace so that its particles are in fine globules or even in a gaseous state. Such an operation is known as atomization and this must be accomplished in an efficient and thorough manner. Three methods are utilized in practice to accomplish this. In the first instance

steam under pressure is mixed with the oil and the ingredients thus shot into the furnace.

In the second instance compressed air is used to accomplish this result, and in the third instance, some mechanical device or physical characteristic of the oil is made use of to whirl or thrust the oil into the furnace in a pulverized or atomized state. Literally hundreds of inventions have been made to effect the atomization of oil. It is to be remembered, however, that in the consideration of fuel oil economy, the furnace and its efficient construction are after all the real factors that go toward economic fuel consumption.

Fuel Oil Furnace Operation.-When the oil is atomized, it must be brought into contact with the requisite quantity of air for its combustion, and this quantity of air must be at the same time a minimum to avoid undue heat losses that may be carried away in the outgoing flue gases. To accomplish this result the checker work under the burners that control the admission of air must be properly designed. The proper quantity of air admission as a whole is controlled by means of draft regulation.