Note.

Very extended rules issued by the U. S. Government for calculating the safe working pressure, dimensions and proportions of the safety valves for marine boilers are reprinted in “Hawkins’ Calculations” for engineers.

When a safety valve is described as a “2 inch safety valve,” etc., it means that two inches is the diameter of the pipe; hence the following rule and examples for finding the area.

Rule for finding Area of Valve Opening.

Square the diameter of the opening and multiply the product by the decimal .7854.

Example.

What is the area of a three inch valve? Now then:

3 × 3 = 9 × .7854 = 7.06 square inches, Ans.

Note.—A shorter method of calculating by .7854 in larger sums is to multiply by 11 and divide by 14, for decimal .7855 = the fraction ¹¹⁄₁₄th. Note: .7854 is the area of a circular inch.

When valves rise from their seats under increasing steam pressure they do so by a constantly diminished ratio which has been carefully determined by experiment and reduced to the following table.

Pressure in Lbs.	Rise of Valve.
12	1-36
20	1-48
35	1-54
45	1-65
50	1-86
60	1-86
70	1-132
80	1-168
90	1-168

The following useful table was prepared by the Novelty Iron Works, New York.

Boiler Pressure in Lbs. Above the Atmosphere	Area of Orifice in Sq. In. for Each Sq. Ft. of Heating Surface.
0.25	.022794
0.5	.021164
1.	.018515
2.	.014814
3.	.012345
4.	.010582
5.	.009259
10.	.005698
20.	.003221
30.	.002244
40.	.001723
50.	.001389
60.	.001176
70.	.001015
80.	.000892
90.	.000796
100.	.000719
150.	.000481
200.	.000364