87. The Safety-valve.-The safety-valve has been used since the earliest days of boiler construction for reducing the pressure when it reached or exceeded a certain limit. It has been built in various forms, but in every case has consisted essentially of a valve opening outward and held in place by a weight or a spring. One form in common use consists of a valve held in place by a weight on the end of a lever, shown in Fig. 139 in section and in Fig. 140 in elevation. In this form of safety-valve the force required to lift the valve

can be regulated by sliding the weight to different positions on the lever. The form shown in Fig. 141 consists of a single weight suspended from the valve and hanging in the upper

FIG. 141.-DEAD WEIGHT SAFETY-VALVE-WEIGHT INSIDE OF BOILER.

part of the boiler. This form is to be commended, since it cannot be adjusted without opening the boiler.

A form used very extensively for low-pressure heatingboilers consists of a single weight resting on a valve, as shown in Fig. 142; its principle of operation is the same as that of the

other valves.

A form much used on power-boilers, and frequently called, from the suddenness with which it opens, a pop-valve consists of a very quick-opening valve held in place with a spring, one form of which is shown in Fig. 143.

FIG. 142.-EXTERNALLY WEIGHTED
SAFETY-VALVE.

FIG. 143.-SECTION OF SPRING OR POP
SAFETY-VALVE.

It is desirable that the safety-valve be made in such a manner that the engineer or attendant to the boiler cannot manipulate it at pleasure so as to maintain a higher pressure on the boiler than prescribed.

Serious accidents have been caused by excessive weighting of the safety-valve through ignorance or carelessness on the part of the attendants, and for this reason a class of valves should be selected which cannot be tampered with. Some of the safety-valves are provided with an external case which can be locked, and others are provided with internal weights, as already described. The lever safety-valve offers the most temptation for extra weighting and should rarely be used.

The area of a safety-valve must be sufficiently large to effectually reduce the boiler pressure when the valve is open and when a brisk fire is burning on the grate. It may be computed from the following considerations:

The steam which will flow through one square inch of opening in one hour of time was found by Napier* to equal in

*Rankine's "Steam Engine."

pounds nearly 50 times the absolute pressure of the steam; further, it has been found by experiment that the safetyvalves in ordinary use open only to such an extent as to make of the total area of the valve effective in reducing the pressure. From these considerations it will be seen that the area of the safety-valve in inches should be the weight of steam generated per hour, divided by the absolute pressure. Considering that 100 lbs. of steam can be generated from each square foot of grate per hour, this would be equivalent to the following rule: The area in square inches is equal to 18 times the grate surface in square feet, divided by the absolute pressure.

The following table gives the area of safety-valve in square inches per square foot of grate required on marine boilers by the English Board of Trade:

area of gauge

The following formula gives results very closely in accord with the English Board of Trade table. Let A safety-valve in square inches, P= absolute pressure pressure plus 15, G = number of square feet of grate surface.

Various rules quite different from the above are given in treatises on boiler construction, but it is believed that the above table represents the best practice of to-day and forms a safe guide for estimating the size of safety-valves.

Safety-valves are liable to stick fast to the seat, through corrosion, in which case they fail to raise with excess of press

ure; for that reason they should be periodically lifted from their seats and otherwise inspected.

In case the area of the valve required is greater than 4 inches in diameter, two safety-valves should be provided for each boiler.

88. Appliances for showing Level of Water in the Boiler. -In the first boilers constructed floats were used, and such

appliances are still common on European boilers. In this country water-gauge glasses and try-cocks are now used, to the exclusion of all other devices. The water-gauge (see Fig. 144), consists of two angle-valves, one of which is screwed into the boiler above the water line; the other is screwed about an equal distance below, and these are connected by means of a glass tube usually to inch external diameter and strong enough to withstand the steam-pressure. When both angle-valves are open the water will stand in the gauge-glass the same height as in the boiler, but if either valve is closed the water-level shown in the glass will not accord with that in the boiler. Three try-cocks are usually put on a boiler in addition to the watergauge. The try-cocks are made in various forms, one kind being shown in Fig. 145, these are located so that one is above, the other below, and the third at about the mean posiItion of the water-line. When the top one is opened, it should show steam; when the bottom one is opened it, should

FIG. 144.-GLASS
WATER-GAUGE.

FIG. 145.-REGISTER GAUGE-cock.

show water. Both try-cocks and gauge-glasses should usually be put on boilers, so that the reading as shown in the watergauge glass can be checked from time to time. This is necessary, because if dirt should get in the angle-valves or passages

leading to the gauge-glass the determination would be inac

curate.

Water-columns attached to the boiler by large pipes, both above and below the water-line, and fitted with try-cocks and water-gauge as shown in Fig. 146, are often provided. These columns frequently contain floats (Fig. 147), so arranged that steam is admitted into a small whistle if the water falls below or rises above the required limits, and thus gives an alarm.

89. Methods of Measuring Pressure.-The excess of pressure above that of the atmosphere is measured by some form of manometer or pressure-gauge. Where the pressure is small in amount, a siphon, or U-shaped tube filled with some liquid is a very convenient means of measuring pressure. The method of using a simple manometer of this character is shown in Fig. 148, in which a U-shaped tube, G FED, has one branch attached to the vessel containing the fluid whose pressure is to be measured; the other, as at D, is open to the air.

a

b c

E

FIG. 148.-U-SHAPED MA

NOMETER.

If water, mercury, or other liquid be placed in the U-shaped