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.