462
MODERN STEAM PRACTICE.
The following is an extract from a Report on Safety Valves,drawn up by a committee of the Institution of Engineers and Ship.builders in Scotland , 1 which presents the various features of thequestion of area and method of loading in a succinct form:—
“ Safety Valve Openings. — Since an orifice, with a square-edged entrance, reduces the flow from 12 to 14 per Cent., this allow-ance will require to be made in computing the requisite area, andopening of a safety valve, which cannot be considered as presentinga much better entrance to the steam than a square-edged orifice.In making this 14 per cent. allowance the weight in pounds of steamdischarged per minute per square inch of opening, with square-edged entrance, corresponds very nearly with three-fourths of theabsolute pressure in the boiler as long as that pressure is not lessthan 25-37 lbs. Examples of this are shown in the following table:—
Absolute Pressurein lbs. per SquareInch.
Weight Discharged perSquare Inch ofOrifice, with RoundedEntrance, per Minute.
Weight Discharged perMinute withSquare-edged Orifice.
Three-fourthsof Absolute Pres-sure.
w r .
Ws.
&A.
25-37
22 - 8 i
19-6
19
3°
26‘84
23
22-5
40
35-48
30-5
30
45
39-78
34-2
33-8
5 °
44 *06
37-9
375
60
52-59
45-2
45
70
61 -07
52-5
52-5
75
65-30
56-1
56-2
90
77'94
67
67-5
IOO
86-34
74-3
75
The area of opening, requisite to the discharge of any given con-stant weight of steam, it will be observed, is very nearly in theinverse ratio of the pressure. Thus, while 3 square inches of open-ing, with square-edged entrance, will discharge 3x23=69 lbs.weight of 30 lbs. pressure steam per minute, one square inch ofopening will discharge 67 lbs. per minute of 90 lbs. pressure steam.
The quantity of heat, however, requisite to generate (from waterat ioo°) 67 lbs. weight of steam, at 90 lbs. pressure, is only 1 percent. less than is required to evaporate 69 lbs. at 30 lbs. pressure.
The boiler which will generate 69 lbs. of steam per minute at30 lbs. cannot, therefore, possibly generate more than 67-7 lbs. at apressure of 90 lbs.; but many experiments on record seem to indicatethat the deficiency at the higher pressure is more than ten per cent.
1 See Trans, of that Institution, vol. xviii.