311. Q.—­May not explosion occur in marine boilers from the accumulation of salt on the flues?

A.—­Yes, in marine boilers this is a constant source of danger, which is only to be met by attention on the part of the engineer.  If the water in the boiler be suffered to become too salt, an incrustation of salt will take place on the furnaces, which may cause them to become red hot, and they may then be collapsed even by their own weight aided by a moderate pressure of steam.  The expedients which should be adopted for preventing such an accumulation of salt from taking place within the boiler as will be injurious to it, properly fall under the head of the management of steam boilers, and will be explained in a subsequent chapter.

CHAPTER VI.

PROPORTIONS OF ENGINES.

* * * * *

STEAM PASSAGES.

312. Q.—­What size of orifice is commonly allowed for the escape of the steam through the safety valve in low pressure engines?

A.—­About 0.8 of a circular inch per horse power, or a circular inch per 1-1/4 horse power.  The following rule, however, will give the dimensions suitable for all kinds of engines, whether high or low pressure:—­multiply the square of the diameter of the cylinder in inches by the speed of the piston in feet per minute, and divide the product by 375 times the pressure on the boiler per square inch; the quotient is the proper area of the safety valve in square inches.  This rule of course supposes that the evaporating surface has been properly proportioned to the engine power.

313. Q.—­Is this rule applicable to locomotives?

A.—­It is applicable to high pressure engines of every kind.  The dimensions of safety valves, however, in practice are very variable, being in some cases greater, and in some cases less, than what the rule gives, the consideration being apparently as often what proportions will best prevent the valve from sticking in its seat, as what proportions will enable the steam to escape freely.  In Bury’s locomotives, the safety valve was generally 2-1/2 inches diameter for all sizes of boiler, and the valve was kept down by a lever formed in the proportion of 5 to 1, fitted at one end with a Salter’s balance.  As the area of the valve was 5 square inches, the number of pounds shown on the spring balance denoted the number of pounds pressure on each square inch of the boiler.

314. Q.—­Is there only one safety valve in a locomotive boiler?

A.—­There are always two.

315. Q.—­And are they always pressed down by a spring balance, and never by weights?

A.—­They are never pressed down by weights; in fact, weights would not answer on a locomotive at all, as they would jump up and down with the jerks or jolts of the train, and cause much of the steam to escape.  In land and marine boilers, however, the safety valve is always kept down by weights; but in steam vessels a good deal of steam is lost in stormy weather by the opening of the valve, owing to the inertia of the weights when the ship sinks suddenly in the deep recess between the waves.