470
MODERN STEAM PRACTICE.
the following rule, would be of itself sufficient to relieve theboilers:—
A = ^ + area of guides of valve,
^ HS
Or A = - + area of guides of valves.
ME
i Iß SUPbblow oft
This valve should be loaded, say I lb. per square inch, less than
the easing valve.
7th. As experience inthe use of valves of thisdescription is acquired,both may be of this kind,and one of them made toblow into the sea withoutany increase of pressure,as is illustrated by the dia-grams (Figs. 342A—342c)from actual practice; theother to be the easingvalve, and loaded 1 lb. persquare inch in excess ofthe working valve.
8th. If the heating sur-face exceeds 30 feet perfoot of grate surface, the size of safetyvalve is to be determined by the heatingsurface.
9th. As boilers decay from age it isnecessary gradually to reduce the pressureof steam, and the committee recommendthat valves should be made of a size to suitthe pressure to which the boiler may ulti-mately be worked when it becomes old.
loth. Springs should be adopted forloading safety valves, and they should bedirect-acting where practicable.
When levers are used, the friction of the joints will cause anextra resistance, and consequent increase of pressure, when thevalve is rising, and a loss of steam through diminution of pressurebefore it will close.”
Figs. 342A, 342B. — a, Safety valve. b, Spring, cc, Studswith screwed ends. d, Cap. e e, Chest leading to wastepipe and nozzle at the side of ship. f, Hand lever for lift-ing valve.