MARINE ENGINES.

403

In ordinary marine practice there is not often more than 20 lbs.of coal consumed per hour per square foot of fire grate, and thewater evaporated seldom exceeds 9 lbs. per lb. of coal, which corre-sponds to 180 lbs. per hour, or an evaporation of 3 lbs. per minuteper square foot of fire grate. Under those conditions the area ofopening requisite to discharge all the steam a boiler can generatecorresponds to four times the square feet of fire grate, divided bythe absolute pressure; or, let a denote the area of orifice in squareinches, and the absolute pressure—

_4 x square feet of fire grate

Pi

The Board of Trade allowance is half of. one square inch area ofsafety valve for each square foot of fire grate. Hence, the lift ofvalve is proportional to the diameter, and inversely as the pressure.For a discharge of 3 lbs. per minute per square foot of fire grate therequisite lift in inches is twice the diameter of a (flat-faced) valve,divided by the absolute pressure; this, however, does not apply topressures less than 25 lbs.

Take, for example, a valve 5 inches in diameter, I9'6 squareinches in area, which corresponds to 2 x I9'6=39^2'square feet offire grate, which would evaporate 39^2 x 3=117'6 lbs. of water perminute. Then since the area a in square inches, requisite to dis-charge any weight, w in lbs. of steam per minute at the pressure

Pi is—

we would have, by taking the pressure p — 60, and the weightw= 117 6, the area

a =T X - 11 TÜ = 2 '61 square inches,

3 x 60

which corresponds to the opening of a flat-faced valve, 5 inchesdiameter, when lifting 2 -^~ = ‘1667 inches.

The circumference of a 5-inch valve being 157 inches and15'7 X '1667 = 2'6 i square inches of opening, as stated.

When the angle of seat of valve is 45°, the lift required in inchesis—

2'8 x diameter of valvePi

When a boiler is regularly fired, and all the steam generateddischarged through an ordinary safety valve, under a succession of