In water-tube boilers the covers opposite the three rows of tubes nearest the fire should be taken off once a month, and the tubes thoroughly scraped and washed out; and all the tubes should be thoroughly scraped and washed out at least once in four months. This is for water of average quality. If the water is bad, clean the tubes oftener.

When mechanical hammers or cleaners are employed for removing scale from tubes, the pressure used to operate them should be as low as will suffice to do the work. Do not allow the cleaner to operate for more than a few seconds upon any one spot, and see that it goes entirely through the tube. Avoid high temperatures in the steam or water used to operate the cleaner.

Putting Boiler Out of Service. In putting a boiler out of service, it should be cooled, emptied, and thoroughly cleaned, both inside and outside. The setting should likewise be cleaned in all its parts. Leave the handhole covers and manhole plates off. After washing the interior of the boiler, let it drain well. Then see that no moisture can collect anywhere about the boiler, nor drip upon it either internally or externally. Empty the siphon below the steam gage if the boiler room is likely to be cold, or take the gage off and store it safely away.

Do not allow moisture to come in contact with the outside of the boiler at any time, either from leaky joints or otherwise. Keep the mud drums and nipples, and the rear ends of horizontal and inclined tubes in water-tube boilers, free from sooty matter. If internal corrosion is discovered, notify your employers at

once.

Examine your boilers carefully in all their parts, whenever they are laid off, and keep them as clean as possible, both inside and outside. See that all necessary repairs are made promptly and thoroughly. Keep the water glass and pressure gage clean and well lighted. If any contingency arises that you do not understand, report the matter to your employers at once; and if you think it possible that serious trouble may be impending at any time, shut down the boiler immediately.

Inform yourself respecting any local laws or ordinances relating to the duties of engineers and firemen, or to the plant in which you work. If there be any such, attend to them faithfully.

CHAPTER XIX

HOW TO COMPUTE STRENGTH OF BOILER SHELLS

In order to ascertain by computation the maximum allowable pressure we must first compute the bursting strength of the solid boiler shell, then find the weakest part of this shell, which, of course, will give us the point where the shell would really give way. We next compute the steam gage pressure that would cause the boiler to rupture at its weakest point. This is known as the bursting pressure. It is important to note here the difference between the bursting pressure of the boiler and the bursting strength of the boiler shell. The former indicates the reading of

FIG. 78.-Standard form of test specimen.

In order to thoroughly test out plate material for boilers, a form of standard specimen has been established by the Boiler Code Committee of the American Society of Mechanical Engineers. The above illustration shows the standard form for the tension, cold-bend, and quench-bend test to be made from each boiler plate as rolled.

the steam gage at which the bursting will take place while the latter indicates the unit internal pressure in the boiler material when rupture occurs.

As a working gage pressure for boiler operation a factor of safety of 5 is often used—that is, a gage pressure that of the bursting pressure is considered as the largest gage pressure that may be safely put upon the boiler. It should be noted that when considering the safety of a boiler we always deal with gage pressure and not absolute pressure. The bursting pressure of a boiler is the difference between the pressure inside the boiler and the pressure outside, when rupture would occur, and as the latter is always the pressure of the atmosphere the bursting pressure must be the amount the inside pressure would be above the atmospheric pressure, which is the same thing as gage pressure.

In order to ascertain the breaking strength of boiler material, a sample known as a standard form is put to test. Experimentally it has been found that whether a piece of material is subjected to rupture by tension, compression, or shear, the unit force required to rupture a square inch section, is equal to the total force observed in rupturing the specimen in each particular case divided by the cross-sectional area. This fundamental law enters largely in computation of boiler strength. Let us then proceed to this analysis.

The Strength of the Solid Plate. In the study of gases and vapors it has been experimentally established that the pressures exerted by such substances are felt equally in all directions at any given point under consideration. Let us then consider the most

D

FIG. 79.-A diagrammatic representation of internal boiler pressure.

Since the pressure of a vapor is exerted equally in all directions we should consider that direction which would produce the most active results in tearing apart a boiler when dee ducing expressions for the safe working pressure. In order to ascertain the total pressurtending to burst the riveted section shown in the middle figure above, the pressure should be taken with the direction as shown by the arrows in this figure.

disastrous direction for pressure action. This evidently would be in such a direction as would tend to tear the boiler shell apart. If the length of shell considered be of length p equal to the distance from center to center of the riveted section or what is known as the pitch of the rivets, we have for a boiler of thickness t a resisting area of pt sq. in. If the solid shell will not burst until each square inch of area has upon it a unit force of S, pounds, the total resistive force according to the experimental law stated in the previous paragraphs evidently ptS. Hence if A is the strength of solid plate, we have

Rule I. Multiply the thickness of the plate by the pitch of the rivets and by the tensile strength of the plate. The result is equal to the strength of solid plate.

As an illustration let us compute the strength of the solid plate for a boiler whose thickness of shell is 1/4 in., whose spacing of rivets is 15% in., and whose tensile strength stamped upon the boiler plate is found to read 55,000 lb. per square inch.

Applying Rule I, we have that the strength of solid plate is

A = tpSt

=

0.25 X 1.625 X 55,000

=

22,343 lb.

The Strength of the Net Section.-As in the case of the weakest link determining the strength of the chain, so the strength of the boiler shell is determined by its weakest section. This will evidently be at the point where the shell has been perforated for the insertion of rivets. The actual area that will resist rupture is now no longer pt but since it has been weakened by an area dt wherein d represents the diameter of the rivet hole, B, the net resistive force now becomes

B

=

(pt - dt)St
dt) S1 = (p

(p - d)tSt

(2)

Rule II (a). From the pitch of the rivet subtract the diameter of the rivet hole, then multiply by the thickness of the plate and again by the tensile strength of the plate. This result is equal to the strength of the plate between rivet holes-in other words to the strength of the net section.

Taking as an illustration the same boiler mentioned in Rule. I, we have, if the diameter of the rivet hole is 1116 in., that the strength of the plate B between rivet holes is

B

=

(p

d)tSt

=

(1.625 0.6875) 0.25 × 55,000 = 12,890 lb. Resistance to Shear.-A boiler may not only fail by bursting apart the actual shell material but the rivet itself may give way. Under pressure the riveted boiler seam may pull apart and cut or shear off the rivet similar to the action that would take place by using a huge pair of shears. The area of cross-section of the rivet is evidently the only opposition that such an action would receive over the distance between one set of rivets in case of a single row of rivets, or if there be n rows of rivets, the area resisting shear is n times that for a single row. Hence, the force that would oppose rupture due to shear is evidently n (.7854d2) S., where S, is the pounds pressure exerted over each square inch of cross-section under shear. From results shown by tests, average iron rivets will shear at 38,000 lb. per sq. in. in single shear and 76,000 lbs. in double shear; steel rivets at 44,000 lbs. in single

shear and 88,000 lb. in double shear.

resistance to shear C for a riveted section is

Hence we have that the

(3)

Rule II (b). Multiply the area of the rivet (.7854d2) by the shearing resistance as follows. If iron rivets in single shear, allow 38,000 lb. per sq. in. of section, or if of steel allow 44,000 lb. per sq. in. If the resistance is in double shear add 100 per cent. to the above. The result is the bursting pressure for shear. Continuing the example above cited, we have that the shearing strength C of one rivet in single shear is

CnX.7854d2S, 1 X .7854 X .68752 X 44,000

=

= 16,332 lb. Resistance to Compression. Again the rivet may be forced to give way by having its longitudinal section (dt) actually crushed if the total crushing force of the steam pressure exceed dtSe, where Se is the crushing pressure in lb. per sq. in. over each unit area of the rivet. Hence the resistance to compression D is

Rule II (c). Multiply the diameter of the rivet by the thickness of the boiler plate and then multiply by the unit bursting stress for compression for the rivet, which is taken at 95,000 lb. per sq. in. The result is equal to the strength of the rivet section for compression.

The resistance to compression D for the example above cited is then

Ꭰ

D= dtS, 0.6875 X 0.25 X 95,000 = 16,328 lb.

=

The Efficiency of the Riveted Section.-We now see that the riveted section weakens the solid plate in three ways. In the

Shearing
Point

FIG. 80.-A single riveted lap joint for boiler plates.

By taking into consideration the stresses involved in a sectional distance equal to the pitch of the rivets, P, as shown, we are enabled to deduce the safe working gage pressure for boiler operation.

first place, the boiler may give way more easily because a section equal to the rivet hole has been cut from the solid plate. In the second place, the rivet may be actually sheared in two, and in the third place, it may be crushed longitudinally. The next