If we were dealing with a full circle or tube this would be an easy matter, but in this case we are dealing with a portion only of a circle, and even that portion receives some support from the bottom row of stays at the sides of the combustion chambers, hence the rule for a full cylinder does not apply in this case, nor is there any direct rule on the subject, the practice being to stay with angle or irons and be sure, that they are ample for the purpose.

THE STAYS BELOW THE FURNACE.

The final calculation with regard to this boiler refers to the staying of the flat surface beneath and between the furnaces, the stay being shown at H in Fig. 38, and at H in Fig. 44.

The area to be supported only refers to the back end shell plate, because at the front end the plate is stiffined by the compensating ring around the man hole door (which is shown in Fig. 38 at E). It is obvious that if the stay power is enough for the back plate it must be enough for the front (double) plate.

The first thing to do with reference to this stay is to find how much flat surface it has to support, and this is done by drawing circles around the nearest stays, as shown in Fig. 44 at a, b, c, d, etc. The diameter of these circles equals the pitch of the stays, and lines g, g, h, h, drawn touching their perimeters, as shown in the figure, gives the boundary line on two sides of the area to be supported. The area between these lines and the stays is supported by the stays.

The third boundary line of the area to be supported is found by drawing an arc m m in the figure distant from the shell to an amount equal to half the pitch of the stays, the area between this line and the shell being supported by the cylindrical shell of the boiler. If it be desired, however, this half pitch may be measured from the end of the round corner of the bend of the end shell plate, as was explained with reference to the end plate of the steam receiver on page 114.

The area in this example about equals a square of 13 × 13 inches, or an area of 182.25 square inches.

First we must calculate whether this area is too great to be supported by one stay, and this has already been done for this plate (in connection with the other stays) on page 132 and found to be 181 square inches, which is sufficiently near to 182 to pass.

The diameter of the stay is 23 inches at the bottom of the thread, and its area is 3.77 square inches.

To find the working pressure of this stay we multiply its area by the constant 9000 and divide by the area to be supported, thus:

We may now find

area the stay supports.

the actual stress per cross sectional square inch of stay by multiplying the area to be supported by the constant 9000 and dividing by the area of the stay, thus:

Area the stay supports

Square inches. Coustant.
= 181.25 X 9000

area of stay = 3.77

4626 lbs. per square inch of stay section.

We have now calculated the strength of this boiler piece by piece, and are enabled not only to know that it is strong enough in all its parts, but also to judge of the uniformity of strength in the whole structure.

It is believed that the reader will have by this time become so familiar with the rules and the reasoning upon which they are based, that he will be able to apply them to other examples of boilers without difficulty.

In this example the boiler had a receiver, hence it has not been necessary to enter into the calculations necessary when the steam passes from the boiler into a superheater.

PART VI.

DRAFT OF A SPECIFICATION FOR A MARINE BOILER.

1st. This is to be a steam boiler to carry 80 lbs. pressure and pass a Board of Trade certificate for passenger service.

2nd.

Shell.

The shell is to be 8 feet 6 inches long, and 7 feet inter nal diameter. The length of the boiler is to be formed in one plate and there is to be no longitudinal rivetted seam below a line struck through the centre of the furnaces.

3rd. The boiler is to be double rivetted throughout, including the circumferential end seams of shell, longitudinal seam in furnace, and longitudinal seam in the steam receiver.

4th. A man-hole door is to be fitted in the back end plate of the boiler immediately above the combustion box, and is to be of sufficient dimensions to admit an ordinary sized man to enter the steam space.*

A second man-hole door is to be cut at the front end of the boiler between the furnaces at the bottom of the boiler. There are also three small doors to be cut in the shell, two at the front end for inspection of the furnace sides (next to the shell of the boiler), and one in the back end at the bottom of the boiler (the latter is the ordinary man or sludge hole), and unless the midship sides of the furnaces can be seen (above the centre of the furnaces), from the man-hole door, an additional peep hole door is to be fitted at a convenient place above the furnaces (amidships). 5th. Furnaces.

diameter.

Two plane steel furnaces 7′ 0′′ length 30" mean

*This door is of much importance, as it permits a man to enter the boiler in a straight position and get a complete view of the water space and stays at the back of combustion boxes, but it should be seen that the deck in the hold will not interfere with his entering.

135

The double rivetted longitudinal seam is to be kept well below the level of the fire-bars.

6th. There is to be a separate combustion box to each furnace, and the sides and backs of the boxes are to be stayed by stays screwed into both plates and fitted with nuts on each end.

7th. The boiler is to be fitted with a convenient number of plane and stay tubes of 31⁄2" diameter (all iron) the plane tubes to be of No. 8. B. W. .G The stay tubes are to be swollen at the front end and screwed into both tube plates, and the back tube plate is to be made of such a thickness as to permit of the usual nuts on stay tube ends in the combustion box being dispensed with, at the smoke box end the usual nuts are to be fitted.

8th. The top of the combustion boxes are to form a segment of a circle (practically true circle), and double angle irons or bars are to be rivetted round from the commencement of the curve at the back plate to the flange at the tube plate, and the corresponding back shell plate is to be stayed to these angle irons, or Tirons, by palm stays, as it is thought that they will afford a better opportunity for a clear view of back water space stays, &c. On the back plate either double angle irons or T bars are to be rivetted, to correspond with the stay holds at the opposite end, unless it is found practicable to screw the opposite end into the shell plate, and fit an internal and external nut in addition or with an internal and external nut to pass the stay through a plane hole at the back shell plate, but in this case the stay is to be caulked steam tight on the outside of shell plate.

9th. The boiler is to be fitted with a cylindrical steam receiver, placed fore and aft, 8′ 6′′ long and 24" diameter, to be made of steel plate double rivetted longitudinally.

The connection of the shell to the steam receiver is to be made by two necks or tubes, 8" diameter each tube, the flanges of which are to be double rivetted, and the plate out of which they are made is not to be less in thickness than the main shell plating, if made of steel, the longi. tudinal seam is to be rivetted, if of iron, it may be welded, but in the latter case it must exceed the thickness of the shell plate. A small peep hole door is to be cut in the receiver at each end, either in the cylinder or the end plate to allow the internal surface to be examined.

10th. No punching of holes allowed whatever.

All rivet holes are to be drilled with the shell plates in place.

11th. The boiler is to be fitted complete with fire-bars, bridges, smoke box and test cocks, the smoke box to be carried up clear of the end plate, the test cocks to be fitted direct into the boiler.

12th. The boiler is to be fitted with a pair of 3′′ spring safety valves with screw easing gear to raise each valve, adjusted to a W. P. of 80 lbs. all complete, approved and passed by the Board of Trade, the valve springs to have not fewer than 14 coils of steel wire.

13th. The steam receiver and upper two thirds of the shell are to be coated with a non conducting cement, to be approved with reference to its quality.

14th. All steam space stays, where passed through a plane hole at one end, with internal and external nuts, are to be caulked steam tight on the outside of shell plate.

To this rough draft may be added such other items as the nature of the case may permit or require, and in this connection the following notes will be of service:

NOTES ON MARINE BOILER CONSTRUCTION.

In the case of steel shell plates, no work should be done upon them under heat in the boiler makers shop, nor indeed upon any other part where the working strain on the plate is in tension.

This brings us to a very important consideration, viz., that of rolling steel plates into circular form. It is preferable under any conditions to roll them cold, but if for want of power in the rolls, it is compulsory to heat the plate to enable the rolling, then great care is absolutely essential in the heating, because each plate must be heated quite evenly all over, and it must be hot enough when it has left the rolls to anneal thoroughly if put in a dry place, and preferably covered up in the usual manner of steel annealing, as by dry ashes or lime.

If there is the slightest question as to the plate being properly annealed, it should be struck in various places by heavy hammer blows, for it is found in practice that plates unevenly or imperfectly annealed are altogether unreliable. It follows, therefore, that a hot rolled plate that has lost its redness at any part when the rolling is completed, can only