1256
THEORY AND PRACTICE OF ENGINEERING.
Book II.
supply: this may beeffectually done byhaving between the feedpump and the boiler aloaded escape-valve V,its load W being so ad-justed that whenever thestopcock R is turned toimpede the passage of thewater towards the boiler,the force of the feed-pump pushes against the
loaded valve at V, and by it passes through thereturn pipe into the reservoir of supply; thisadjustment requires the attention of an attendant.
Safety-valves are of the utmost importance,and the greatest care should be bestowed ontheir construction ; the earliest consisted simplyof a weight laid on a hole in the top of theboiler: but this plan cannot be adopted whenthe pressure is high and the weight great, be-cause it becomes unsteady : a valve is thereforeused, as in fig. 1989., carefully ground to its seat;a spindle rising upwards carries a series ofweights which may be increased or diminished ;this being attended with inconvenience, a valvewith an internal weight (fig. 1990.) has beencontrived : but in all these modifications, whenthe pressure is high the weight becomes cum-brous. The lever safety-valve was introduced to
Fig.1986.
8AFETY-VALV
Fig. 1987. LEVER VALVE.
Fig. 1989. COMMON SAPETY-VALVB.
Fig. 1991. southern’s safbty-valve.
Fig. 1992.
Fig. 1993. SPRING VALVB.
obviate this (fig. 1987.); a single weight hung on the longer arm ofa lever produces an effect proportional to its distance, and the leverbeing graduated shows the amount of this effect. The form of fig.
1988. indicates still more correctly the point at which the pres-sure of the steam becomes equal to that on the valve which is flator cylindrical, and acted on by a lever having equal arms withweights on each; these rest on light rollers, so as to run downfrom their places and release the steam entirely when the pressurereaches the prescribed limit. Figs. 1991. and 1992. show the formused by Mr. Southern in his experiments on high-pressure steam,the first when closed, the second when open. Fig. 1993. is formedof a series of bent springs placed alternately in opposite directions.
Fig. 1986. is similar in principle, but has a lever interposedbetween the valve and the spring; both are employed in loco-motives. Fig. 1994. was proposed by Mr. Nimmo for steamboat boilers.
n=d
n
3
Fig. 1994. NIMMO’S VALVE.