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STEAM SPACE OF A BOILER.

To find steam space of a horizontal boiler of any size.
RULE.

1. Multiply the area of the end of the boiler in inches by the length of the boiler in inches; the answer will be in cubic inches.

2. To reduce to cubic feet divide by 1728.

EXAMPLE.

How much steam space is there in a boiler 5 feet in diameter and 18 feet long?

The area of 5 feet (per table) is 282704÷3=9423 The length of the boiler, 18 feet, made into inches 216 Now divide:

1728 (12)2035368

12)169614

12)14134

=

56538

9423

18846

2035368

1,17719th cubic ft. Ans.

NOTE.

These results are not absolutely accurate owing to discarding small fractions and not allowing for thickness of iron, but the calculations are sufficiently near to suit all practical purposes.

THE SAFETY VALVE.

The safety valve is a circular valve seated on the outside of the boiler and weighted to such an extent that when the pressure of the steam exceeds a certain point the valve is lifted and allows the steam to escape.

Safety valves can be loaded directly with weights, in which case they are called dead weight valves, or the load can be transmitted to the valve by a lever. An unauthorized addition of a few pounds to the weight of the former would make no appreciable addition to the blowing off pressure while a small addition to the weight at the end of the lever is multiplied several times at the valve.

In the case of locomotive and marine boilers the lever is weighted by means of a spring, the tension of which can be adjusted.

It may be defined, as also applying to all valves, that the seat of the valve is the fixed surface on which it rests or against which it presses, and the face of a valve is that part of the surface which comes in contact with the seat. The spindle is the small rod which projects upwards or downward from the middle of the valve, and so arranged that it causes the valve to raise and drop evenly upon its seat.

The effective pressure on the lever safety valve can be regulated within certain limits by sliding the weight along the arm and in the spring safety valve the pressure can be regulated by altering the tension of the spring.

Every boiler should be provided with two safety valves. The size of the opening into the boiler depends upon its steam producing qualities, the object to be attained being to reduce the pressure within the boiler to its safety point as quickly as possible.

SAFETY VALVE CALCULATIONS.

This is a subject, while old, is ever new to the engineer, and the following rules are given in such a manner that any one who can add, subtract, divide and multiply and read decimals can

LEVER SAFETY VALVE.

soon acquire familiarity with the rules, and thus make them his own to use when he needs them, to adjust a safety valve or for other uses.

Reference is first made to the principles, rules and examples heretofore given relating to the lever (the safety valve is a lever of the third order) the rule of three, and to decimals. Next, in all problems it is well for the engineer to draw, roughly, if need be, a diagram of a safety valve somewhat after the form given in Fig. 111.

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W denotes the weight on the lever in pounds; L, distance from center of weight to fulcrum in inches; w, weight of the lever itself in pounds; g, distance between center of gravity of lever and fulcrum in inches; 7, the distance between center of valve and fulcrum in inches, and F the fulcrum.

In working out the problems the figures and dimensions as soon as known should be put upon the drawing so that the eye may assist in the calculations as they proceed from step to step.

LEVER SAFETY VALVE.

To find the weight of the valve, spindle, lever, etc., proceed as follows:

Take out the valve and spindle and weigh them and make a note of it, then put them back in place, connect the lever and drop it in place resting on the valve spindle, tie a string to the lever directly over the spindle, hook on the scales to the string and weigh the lever, to the weight of the lever add the weight of valve and spindle, or the weight may be found approximately by computation, by use of rules elsewhere given in this work under Mensuration, etc.

The following rules were recently issued by the United States board of supervising inspectors, on account of changes in the rules for granting licenses to engineers of steam vessels.

To find the weight required to load a given safety-valve to blow at any specified pressure.

1. Measure the diameter of the valve, if it is not known, and from this compute its area exposed to pressure.

2. Weigh the valve and its spindle. If it is not possible to do this, compute their weight from their dimensions as accurately as possible.

3. Weigh the lever, or compute its weight from its dimensions.

4. Ascertain the position of the centre of gravity of the lever by balancing it over a knife-edge, or some sharp-cornered article, and measuring the distance from the balancing point to the fulcrum.

5. Measure the distance from the center of the valve to the fulcrum.

6. Measure the distance from the fulcrum to the center of the weight.

Then compute the required weight as follows:

1. Multiply the pressure in pounds per square inch at which the valve is to be set by the area of the valve in square inches; set the product aside and designate it "quantity No. 1."

THE LEVER SAFETY VALVE.

2. Multiply the weight of the lever in pounds by the distance in inches of its center of gravity from the fulcrum; divide the product by the distance in inches from the center of the valve to the fulcrum, and add to the quotient the weight of the valve and spindle in pounds; set the sum aside and designate it "quantity No. 2."

3. Divide the distance in inches from the center of the valve to the fulcrum by the distance, also expressed in inches, from the center of the weight to the fulcrum; designate the quotient "quantity No. 3."

4. Subtract quantity No. 2 from No. 1, and multiply the difference by No. 3. The product will be the required weight in pounds.

To find the length of the lever, or distance from the fulcrum at which a given weight must be set to cause the valve to blow at any specified pressure.

The area of the valve in square inches, the weight of the valve, spindle and lever in pounds, the position of the center of gravity of the lever, and the distance from the center of the valve to the fulcrum, must be known, as in the first example. Then compute the required length as follows:

1. Multiply the area of the valve in square inches by the pressure in pounds per square inch at which it is required to blow; set the product aside, and designate it "No. 1.”

2. Multiply the weight of the lever in pounds by the distance in inches of its center of gravity from the fulcrum; divide the product by the distance in inches from the center of the valve to the fulcrum; add to the quotient the weight of the valve and spindle; set the sum aside, and designate it "No. 2."

3. Divide the distance in inches from the center of valve to fulcrum by the weight of the ball in pounds, and call the quotient No. 3."

66

66

4. Subtract No. 2" from "No. 1," and multiply the difference by "No. 3"; the product will express the distance in inches that the ball must be placed from the fulcrum to produce the required pressure.

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