Modern Machine-Shop Practice Part 246

Among the most frequent questions asked in an engineer's examination are those relating to the safety valves of boilers.

These questions may be easily answered from a study of the following:

The safety valve is a device for relieving the boiler of steam after it has reached a certain pressure.

This it accomplishes by letting the steam escape after it has reached the required pressure.

At what pressure the safety valve will blow off depends upon the position of the weight on the safety valve lever.

The calculations referring to this part of the subject are, finding how much weight will be required to be placed at a given point on the lever, in order, with a given sized valve, to blow off at a given pressure.

Finding the position on the lever of a given amount of weight, in order to blow off at a certain pressure.

Finding, with a given sized valve and a given weight, how to mark off the lever and where the notches must be cut for given pressures.

In each of these calculations there are three elements: first, the area of the valve and the steam pressure, which const.i.tute the effect of acting to lift the valve; second, the amount of the weight and its position upon the lever, which acts to keep the valve closed; and third, the weight of the lever and of the valve, which act to keep the valve closed.

[Ill.u.s.tration: Fig. 3357.]

In Fig. 3357 we have a drawing of a safety valve shown in section, and if there was no weight upon the lever, the pressure of steam the valve would hold in the boiler would be that due to the weight of the valve and of the lever upon the valve.

To find out how much this would be, we would have to put the valve itself and the pin _a_ on a pair of scales and weigh them.

Then put a piece of string through the hole at _a_ in the lever, and see how much it weighed when suspended from that point.

Suppose the valve and pin to weigh 2 lbs. and the lever (suspended by the string) 10, and the total will be 12 lbs.

Next we find the area of the valve, and suppose this to be 8 square inches; then we may find how much pressure the valve would keep in the boiler, by dividing the area of the valve into the weight holding the valve down, thus:

Lbs.

Weight of valve and pin, 2 " " lever, 10 -- Area of valve, 8 ) 12 --- Pressure the valve would hold, 1.5 lbs.

The area of the valve is that part of its face receiving the steam pressure when the valve is seated, so that if the smallest part of the valve diameter is equal to the diameter of the seat bore, the diameter from which the valve area is to be calculated will be that denoted by D in the figure, and cannot in any case be less than this. But if the smallest end of the valve cone is of larger diameter than the smallest end of the seat cone (which should not, but might be the case), then it is the smallest diameter of valve cone that must be taken in calculating the area, because that is the area the steam will press against.

Now suppose we rest a 20 lb. weight on the top of the valve that is on the point denoted by I, and there will be 32 lbs. holding the valve down, thus, weight of valve 2 lbs., of lever 10, and weight added, 20 lbs., and to find how much pressure this would hold in the boiler, we divide it by the valve area, thus:

Weight on valve.

Valve area = 8 ) 32 -- 4 = pressure valve will hold.

But suppose we put the weight on the lever, in the position shown in the figure, which is six times as far from the fulcrum F of the lever as the valve is, and its effect on the valve will be six times as great as it would if placed directly upon the valve, so that leaving the weight of the valve and of the lever out of the question (as is commonly done in engineers' examinations), we may find out what pressure the valve will hold, as follows:

_Rule._--Divide the length of the lever by the distance from the centre of the valve to the centre of the fulcrum. Multiply by the amount of the weight in lbs., and divide by the area of the valve.

_Example._--The area of the valve is 8 inches, the distance from the centre of the fulcrum to the centre of the valve is 4 inches, and the distance from the fulcrum to the point of suspension of the weight 24 inches, the weight is 40 lbs., what pressure will the valve hold?

Length of lever.

From fulcrum to valve, 4 ) 24 -- 6 40 amount of weight.

--- Area of valve, 8 ) 240 --- 30

Lbs. per square inch the valve will hold = 30.

The philosophy of this is clear enough when we consider that as the weight is six times as far from the fulcrum as the valve is, and each 1 lb. of weight will press with a force of 6 lbs. on the valve, hence the 40 lbs. will press 240 lbs. on the valve, and as the valve has 8 square inches, the 240 becomes 30 lbs. for each inch of area.

_Example._--The area of a safety valve is 8 inches, the distance from the fulcrum to the valve is 4 inches, and the weight is 40 lbs., how far must the weight be from the fulcrum to hold in the boiler a pressure of 30 lbs. per square inch?

In lbs.

From fulcrum to valve, 4 ) 40 amount of weight.

-- 10

Area of valve, 8 square inches.

Pressure required, 30 --- 10 ) 240 --- 24

Answer = 24 inches from the fulcrum.

_Example._--The diameter of a safety valve is 4 inches, the distance from the centre of the fulcrum to the valve is 3 inches, a 50 lb. weight is 30 inches from the fulcrum, what pressure will the valve hold?

3 diameter of valve.

3 -- 9 .7854 ----- 36 45 72 63 ------ 7.0686 = area of valve.

3 ) 30 -- 10

50 = weight 10 = leverage of weight.

------- Area of valve, 7.068 ) 500.000 ( 70.7 = lbs. pressure per sq. in.

49476 ------- 52400 49476 ----- 2924

HEAT.

The heat unit, or the unit whereby heat is measured, is the quant.i.ty of heat that is necessary to raise 1 lb. of water from its freezing temperature (which is 32 Fahrenheit) 1, and this unit is sometimes termed a _thermal unit_.

The reason that some specific temperature, as 32 Fahrenheit, is taken, is because the quant.i.ty of heat required to heat a given quant.i.ty of water 1 increases with the temperature of the water; thus, it takes more heat to raise 1 lb. of water from 240 to 245 than it does to raise it from 235 to 240, although the temperature has been raised 5 in each case.

The whole quant.i.ty of heat in water or steam is not, however, sensible to the thermometer, or, in other words, is not shown by that instrument.

The heat not so shown or indicated is termed _latent_ heat.

Water obtains latent heat while pa.s.sing from a solid to a liquid state, as from ice into water, and while pa.s.sing from a liquid to a gaseous state, as while pa.s.sing from water into steam, and the existence of latent heat in steam may be shown as follows:

If we take a body of water at a temperature above freezing, and insert therein a thermometer, the decrease in the temperature as the water becomes frozen will be shown by the thermometer. If, then, its temperature being say at zero, heat be continuously imparted to the ice, the thermometer will mark the rise in temperature until the ice begins to melt, when it will remain stationary at 32 so long as any ice remains unmelted, and it is obvious that all the heat that entered the water from the time the ice began to melt until it was all melted became latent, and neither sensible to the sense of feeling nor to the thermometer. Similarly, if the water, after the ice is all melted, be heated in the open air, the thermometer will mark the rise of temperature until the water boils, after which it will show no further rise of temperature, although the water still receives heat. The heat that enters the water from boiling until it is evaporated away is the latent heat of steam. The latent heat of water is 143 Fahrenheit, and that of steam when exposed to the pressure of the atmosphere, or under an atmospheric pressure of 15 lbs. (nearly), is 960, which may be shown as follows:

If a given quant.i.ty of water, as say 1 lb., has imparted to it a continuously uniform degree of heat sufficient to cause it to boil in one hour, then it will take about 5-1/3 more hours to evaporate it all away, hence we find the latent heat by taking the difference in the amount of heat received by the water, and that shown by the thermometer thus:

Degrees.

Temperature by thermometer at boiling point 212 Less the temperature of the water at first 32 --- Heat that entered the water in the first hour 180 Hours that the water was subsequently heated 5-1/3 ------- 900 One-third of 180 = 60 --- Heat that entered the water during the 5-1/3 hours 960 degrees.

This, however, is not quite correct, as it would take slightly more than 5-1/3 hours to boil the water away, and the heat that entered the water after it commenced to boil would be about 966 degrees.

If the steam that arose from the water while it was boiling were preserved without increasing the pressure under which it boiled, and without losing any of its heat, it will have a temperature the same as that of the water from which it was boiled, which is a temperature of 212, so that neither the steam nor the water account, by the thermometer, for the 966 of heat that entered the water after it boiled, hence the 966 became latent, const.i.tuting the latent heat of the steam when boiled from and at a temperature of 212.