SENTINEL VALVE.

Fig. 90.

This, although not a boiler fixture or fitting, is intimately connected with them: it is an appliance fast coming into use both for land and marine engines, to guard against the danger to steam engine cylinders arising from “the priming” of the boilers when the steam is used at a high pressure with high speed of the piston.

The separator is usually placed in the engine room, so as to be well in sight. The steam is led down the pipe round a diaphragm plate and then up again to the engine steam pipe. By this means any priming or particles of water that may be brought from the boiler with the steam will fall to the bottom of the interceptor or catch water, from whence it can be blown out, according to the arrangement of the pipes, by opening the drain cock fixed on the bottom. It has a water gauge fixed on the lower end, so as to show whether water is accumulating; and the engineers attention is required to see that this water is from time to time blown off.

In the illustration,Fig. 90, is shown the simplest form in which the device can be made. The arrows exhibit the direction in which the steam travels, the aperture whence the water is to be blown out and the place for attachment of a water column. In practical construction the separator should have a diameter twice that of the steam pipe and be 21⁄2to 3 diameters long. It is often made with a round top and flat bottom and sometimes with both ends hemispherical. The division plate should extend half the diameter of the steam pipe below the level of the bottom of the steam pipe.

InFig. 91is shown an improved form of a steam separator which consists of a shell or casing in which there is firmly secured a double-ended cone. On this cone there are cast a number of wings, extending spirally along its exterior. On entering the separator the steam is spread and thrown outward by the cone and given a centrifugal motion by the spiral wings. These wings are constructed with a curved surface.

It will be noticed that the steam on entering the separator is immediately expanded from a solid body into an annular space of equal volume to the steam pipe, whereby its particles are removed from the centre and thus receive a greater amount of centrifugal motion. The entrained water or grease, etc., is thus precipitated against, and flows along the shell of the separator, and is collected in a well of ample proportions at base of separator, where it is entirely isolated from the flow of dry steam.

Fig. 91.

It was formerly required for each marine boiler to have a small valve loaded with a weight to a few pounds per square inch above the working pressure, so that in case of the safety valves sticking fast and the gauge being false, an alarm might be given when there was an excess of pressure. Such valves were about3⁄4inch in diameter and sometimes as small as3⁄8. An arrangement of a small safety valve attached to a whistle has been introduced, but with advances in other directions relating to safety these specialties are now getting to be only known by name.

These are well-known devices for so controlling the draught of the chimney that the steam pressure in the boiler will be increased or decreased automatically, that is, without the aid of a person. The regulator shown inFig. 92, which is one of many excellent forms on the market, has the power to move the damper in both directions by water pressure, exerting a force on the end of the lever of nearly 200 lbs., thus compelling a certain and positive motion of the damper when a variation in the boiler pressure takes place. It will open or close the damper upon the variation of less than one pound of pressure. The close regulation affords a test for the correctness of the steam gauge.

Fig. 92.

This regulator, by using the water pressure from the boiler as a motive power, becomes a complete engine without the connecting rod and crank, having a balanced piston valve, the valve stem of which is enlarged where it passes through the upper end of the chest into a piston of small area, working in a small open ended cylinder cast on the chest. The pressure forcing this piston outward is counterbalanced by weights as shown in illustration.

The differential motion is accomplished by the device shown at the top of small cylinder.

This device, shown inFig. 93, is designed to utilize the waste products of combustion as they pass from the furnace to the chimney. Its use permits a high and consequently efficient temperature under the boilers and yet saves the excess of heat. It acts also as a mechanical boiler cleaner, furnishing a settlingchamber for the deposit of the impurities separated by the heat which nearly equals that of the live steam in the boiler. This device adds largely to the water capacity of the boiler, frequently containing one-half the weight of the water held in the boiler itself.

It will be readily understood that the openings between the vertical tubes are ample for the chimney flue area and that the device is located between the chimney and the boiler, with the waste furnace heat passing between the tubes.

Fig. 93.

The economizer shown inFig. 93consists of sections of vertical 41⁄2″ boiler tubes fitted to their top and bottom headers by taper joints. The top headers are provided with caps over each tube to permit cleaning out the sediment and remove and replace any tube that may become damaged. The several top headers are connected together at one end by lateral openings and the bottom headers are also connected as shown in cut, having hand holes opposite each bottom header to provide for cleaning out.

Mechanical scrapersare made to travel up and down each tube to keep them clear of soot. These are controlled by an automatic mechanism and driving head, as shown inFig. 93.

The important features about the economizer are, 1, its adaptability to any type of boiler, 2, the saving attained by utilizing that heat which has necessarily been an almost total waste, 3, the purifying of the water by means of the intense heat and slow circulation of the feed water.

Fig. 94.(Sectional View.)

The safety valveis a circular valve seated on the top of the boiler, and weighted to such an extent, that when the pressure of the steam exceeds a certain point, the valve is lifted from its seating and allows the steam to escape. Safety valves can be loaded directly with weights, or the load can be transmitted to the valve by a lever. Again, the end of the lever is sometimes held down by a spring, or the spring may be applied directly to the valve seat.

Fig. 94(2 views) exhibitsa spring loaded safety valve. These are generally provided with a reaction lip, surrounding the seat, which causes them to open much further, and thus enables them to discharge a larger volume of steam than a lever valve of equal diameter.

The operation can be easily understood by examining the figures. As soon as the steam pressure is high enough to lift the valve disc clear from its seat, the steam will escape around the valve seat as in an ordinary lever safety valve, but instead of escaping directly into the atmosphere, the current of steam is turned downward against the reaction lip, by the curved projection on the valve disc, which can be seen in the figure. The steam pressure is thus assisted in holding the valve open, as well as raising it much higher, giving a larger opening than would be the case if the valve were lifted by the pressure alone.

Spring loaded valves are mostly used on marine boilers, locomotives and portable boilers, and wherever outside disturbances interfere with the action of a weight.

A “pop” safety valveis a common form of safety valve and takes its name from the fact that it takes a little more pressure to raise it off its seat than what it is set at, consequently it releases itself with a “pop.”

Fig. 95.

Fig. 95shows a form of dead weight safety valves whenais the valve which rests on the seatingb.

The valve is attached to the circular casting A, A, A, so that both rise and fall together. The weights W, W, etc., are disposed on the casting in rings, which can be adjusted to the desired blow off pressure. Owing to the center of gravity of the casting and weight being below the valve, the latter requires norequires no guides to keep it in position. This is a great advantage as guides frequently stick, and prevent the valve from acting. Another advantage of this form of valve is, that it is difficult to tamper with. For instance, a four-inch valve, intended to blow off at 100 lbs. per square inch would require weight of over 1,200 lbs., which require a considerable bulk. An unauthorized addition of a few pounds to such a mass would make no appreciable addition to the blowing off pressure, while any effectual amount added to the weight would be immediately noticed. It is quite different with the lever safety valve about to be described, a small addition to the weight at the end of the lever is multiplied several times at the valve.

Extract from rules and regulations passed and approved Feb. 25, 1885, by the United States Board of Supervising Inspectors of Steam Vessels:

Section 24.“Lever safety valves to be attached to marine boilers shall have an area of not less than one square inch to two square feet of the grate surface in the boiler, and the seats of all such safety valves shall have an angle of inclination of forty-five degrees to the centre line of their axis.

“The valves shall be so arranged that each boiler shall have one separate safety valve, unless the arrangement is such as to preclude the possibility of shutting off the communication of any boiler with the safety valve or valves employed. This arrangement shall also apply to lock-up safety valves when they are employed.

“Any spring-loaded safety valves constructed so as to give an increased lift by the operation of steam, after being raised from their seats, or any spring-loaded safety valve constructed in any other manner, or so as to give an effective area equal to that of the aforementioned spring-loaded safety valve, may be used in lieu of the common lever-weighted valve on all boilers on steam vessels, and all such spring-loaded safety valves shallbe required to have an area of not less than one square inch to three square feet of grate surface of the boiler, and each spring-loaded valve shall be supplied with a lever that will raise the valve from its seat a distance of not less than that equal to one-eighth the diameter of the valve opening, and the seats of all such safety valves shall have an angle of inclination to the centre-line of their axis of forty-five degrees. But in no case shall any spring-loaded safety valve be used in lieu of the lever-weighted safety valve, without first having been approved by the Board of Supervising Inspectors.”

The following size “Pop” Safety Valves are required for boilers having grate surfaces as below:

2 inch “Pop” Valve for9.42square feet of grate surface.21⁄2inch “Pop” Valve for14.72square feet of grate surface.3 inch “Pop” Valve for21.20square feet of grate surface.4 inch “Pop” Valve for37.69square feet of grate surface.5 inch “Pop” Valve for58.90square feet of grate surface.6 inch “Pop” Valve for84.82square feet of grate surface.

Professor Rankin’s Rule.—Multiply the number of pounds of water evaporated per hour by .006, and the product will be the area in square inches of the valve.

The U. S. Steamboat Inspection Law requires for the common lever valve one square inch of area of valve for every two square feet of area of grate surface.

United States Navy Department deduced from a series of experiments the following rule: Multiply the number of pounds of water evaporated per hour by .005, and the product will be the area of the valve in square inches.

Rule adopted by the Philadelphia Department of Steam Engine and Boiler Inspection:

1. Multiply the area of grate in square feet by the number 22.5. 2. Add the number 8.62 to the pressure allowed per square inch. Divide (1) by (2) and the quotient will be the area of the valve in square inches. This is the same as the French rule.

The maximum desirable diameter for safety valves is four inches, for beyond this the area and cost increase much more rapidly than the effective discharging around the circumference.

There should not be any stop valve between the boiler and safety valve.

The common form of safety valve is shown inFig. 96.

Here the load is attached to the endBof the leverA,B, the fulcrum of which is atc. The effective pressure on the valve, and consequently the blowing off pressure in the boiler can be regulated within certain limits, by sliding the weightWalong the arm of the lever. In locomotive engines, as well as on marine boilers, the weight would on account of the oscillations, be inadmissible anda springis used to hold down the lever.

In the calculations regarding the lever safety valve, there are five points to be determined, and it is necessary to know four of these in order to find the fifth. These are: (1) The Steam Pressure, (2) The Weight of Ball, (3) The Area of Valve, (4) The Length of Lever, (5) The Distance from the Valve Centre to the Fulcrum.

Fig. 96.

In making these calculations it is necessary to take into account the load on the valve due to the weight of the valve-stem and lever. The leverage with which this weight acts is measured by the distance of its centre of gravity from the fulcrum. The centre of gravity is found by balancing the lever on a knife edge, and the weight of the valve-stem andlever can be found by actual weighing. This load can also be found by attaching a spring balance to the lever exactly over the centre of the valve stem when they are in position. The following examples will be computed under these conditions: (1) Steam Pressure, 120 pounds; (2) Weight of Ball, 100 pounds; (3) Weight of Valve and Lever, 60 pounds, weighed in position; (4) Length of Lever, 45 inches; (5) Length of Distance from Valve Centre to Fulcrum, 5 inches; (6) Area of Valve, 8 square inches.

To find the area of the valve:

Rule.—Multiply the length of the lever by the weight of the ball, and divide the product by the distance from the valve centre to the fulcrum, and to the quotient add the effective weight of the valve and lever, and divide the sum by the steam pressure.

Example.

45inches, length of the lever,100pounds, weight of the ball,Fulcrum, 5 in. )450090060pounds, weight of valve and lever,Steam pressure 120 lbs. )960(8 square inches, area of valve.960

To find the pressure at which the valve will blow off:

Rule.—Multiply the length of the lever by the weight of the ball; divide this product by the distance from the valve centre to the fulcrum, and to the quotient add the effective weight of the lever and valve, and divide the sum by the area of the valve.

Example.

45inches, length of lever,100pounds, weight of ball,Fulcrum, 5 in. )450090060pounds, weight of valve and lever,Area of Valve 8 )960120pounds, pressure at which valve will blow.

To find the weight of ball:

Rule.—Multiply the steam pressure by the area of the valve, and from the product subtract the effective weight of the valve and lever, then multiply the remainder by the distance from the valve centre to the fulcrum, and divide the product by the length of the lever.

Example.

120pounds, steam pressure,8inches, area of valve,96060pounds, weight of valve and lever,9005inches, fulcrum,Length of lever, 45 in. )4500100pounds, weight of ball.

To find the length of lever:

Example.

120pounds, steam pressure,8inches, area of valve,96060pounds, weight of valve and lever,9005100)4500(45 length of lever.

Every boiler should be provided with two safety valves, one of which should be put beyond the control of the attendant.

Safety valves that stick will do so even though tried every day, if they are simply lifted and dropped to the old place on the seat again.If a boiler should be found with an excessively high pressure, it would be one of the worst things to do to start the safety valve from its seat unless extra weight was added, for should the valve once start, it would so suddenly relieve the boiler of such a volume of steam as would cause a rush of water to the opening, and by a blow, just the same as in water hammer, rupture the boiler.

Such a condition is very possible to occur of itself when a safety valve sticks. The valve holds the pressure, that gets higher and higher, until so high that the safety valve does give way and allows so much steam to escape that the sudden changing of conditions sets the water in motion, and an explosion may result.

The noise made by a safety valve when it is blowing off may be regarded in two ways. First, by it is known that the valve is capable of performing its proper function, and that there is, therefore, a reasonable assurance that no explosion will result from excessive pressure of steam or other gas, and on the other hand too much noise of this kind indicates wasted fuel.

The hole of the safety valve may be 2, 3 or 4 inches; that does not say that the area is 3.1416, 7.06 or 12.56 square inches, but the area is that which is inside of the joint. The valve opening may be, say 2 inches, butthe circle of contact of valve to seatmay be of an average diameter of 21⁄8inches, if so, all the close calculations otherwise will not avail. In the first place, the area of 2 inches equals 3.1416; that of 21⁄8diameter equals 3.5466, showing a difference of .4 square inches.

Very extended rules issued by the U. S. Government for calculating the safe working pressure, dimensions and proportions of the safety valves for marine boilers are reprinted in “Hawkins’ Calculations” for engineers.

When a safety valve is described as a “2 inch safety valve,” etc., it means that two inches isthe diameterof the pipe; hence the following rule and examples for finding the area.

Rule for finding Area of Valve Opening.

Square the diameter of the opening and multiply the product by the decimal .7854.

Example.

What is the area of a three inch valve? Now then:

3 × 3 = 9 × .7854 = 7.06 square inches, Ans.

Note.—A shorter method of calculating by .7854 in larger sums is to multiply by 11 and divide by 14, for decimal .7855 = the fraction11⁄14th. Note: .7854 is the area of a circular inch.

When valves rise from their seats under increasing steam pressure they do so by a constantly diminished ratio which has been carefully determined by experiment and reduced to the following table.

Pressure in Lbs.Rise of Valve.121-36201-48351-54451-65501-86601-86701-132801-168901-168

The following useful table was prepared by the Novelty Iron Works, New York.

Boiler Pressurein Lbs. Above theAtmosphereArea of Orificein Sq. In. forEach Sq. Ft. ofHeating Surface.0.25.0227940.5.0211641..0185152..0148143..0123454..0105825..00925910..00569820..00322130..00224440..00172350..00138960..00117670..00101580..00089290..000796100..000719150..000481200..000364

Fig. 97.

There are two forms of feed water heaters: (1)The closed heater, where the feed water passes through tubes, which are enclosed in a shell, through which the exhaust steam passes. (2)The open heater, in which the steam and water come into contact. In the latter the water is sprayed into a space, through which the exhaust steam passes, or is run over a number of inclined perforated copper plates, mingled with the exhaust steam.

The original feed water heater called a “pot heater,” consisted of a vessel so constructed that the feed water was sprayed through the exhaust steam into a globe formed tank, from the bottom of which the heated water was pumped into the boiler; its name was originally the “pot heater,” but as it was open to the air through the exhaust pipe, it was, with its successively improved forms called the open heater.

All the heat imparted to the feed water, before it enters the boiler, is so much saved, not only in the cost of fuel, but by the increased capacity of the boiler, as the fuel in the furnace will not have this duty to perform. There are two sources of waste heat which can be utilized for this purpose: the chimney gases and the exhaust steam. The gases escaping to the chimney after being reduced to the lowest possible temperature contain a considerable quantity of heat. This waste of heat energy may be largely saved by the device illustrated on page186.

Fig. 98.

How much saving is obtained under any given condition is a question requiring for its solution a careful calculation of all of the conditions which have a bearing on the subject. Exhaust steam under atmospheric pressure only has a sensible temperature of 212 degrees, but exhaust steam contains also a large number of heat units which are given up when the steam is condensed into water; for this reason it might be thought possible to raise the temperature of the feed water a few degrees higher even than the sensible temperature of the exhaust steam. But this should not be expected, on account of the radiation of heat that would occur above that of the steam.

The steam which escapes from the exhaust pipe dissipates into the atmosphere or discharges into the condenser over ninetenths of the heat it contained when leaving the boiler. This can be best utilized byexhaust feed water heaters, for the use of live steam heaters represents no saving in fuel, as all the heat imparted to the feed water by their use comes directly from the boiler. The purpose for which they are used is to elevate the temperature of the feed water above the boiling point, so as to precipitate the sulphate of lime and other scale forming substances, and prevent them from entering the boiler. Neither does the heat in the feed water introduced by an injector represent saving, as it comes from the boiler and was generated by the fuel.

It is important to note these two statements: 1, That neither live steam feed water heaters, nor 2, injectors save the heat from the escaping steam.

It is also well to remember that it requiresa pound of waterto absorb 1.146 heat units, and that this quantity of heat is distributed through the whole quantity of water, andas a pound of steam is the same as a pound of water, it may be understood that at 212° each pound of exhaust steam contains 1,146 heat units; ten pounds of steam contain 11,460 heat units distributed through the mass, etc.: thus, to explain still further:

To evaporate water into steam, it must first be heated to the boiling point, and then sufficient heat still further added to change it from the liquid to the gaseous state, or steam. Take one pound of water at 32 degrees and heat it to the boiling point, it will have received 212° - 32° = 180 heat units. A heat unit being the amount of heat necessary to raise one pound of water through one degree at its greatest density. To convert it into steam after it has been raised to the boiling point, requires the addition of 966 heat units, which are called latent, as they cannot be detected by the thermometer. This makes 180 + 966 = 1146 heat units, which is the total heat containedin one pound of watermade into steam at the atmospheric pressure. And at atmospheric density the volume of this steam is equal to 26.36 cubic feet, and this amount of steam contains 1,146 units of heat, distributed throughout the whole quantity, while the temperature at any given point atwhich the thermometer may be inserted is 212 degrees. If two pounds of water be evaporated, making a volume of 52.72 cubic feet, then the number of heat units present would be doubled, while the temperature would still remain at 212, the same as with one pound.

If by utilizing the heat that would otherwise go to waste, the temperature of the feed water is raised 125 degrees, the saving would be125⁄1146of the total amount of heat required for its evaporation, or about 11 per cent. Thus it can be seen the percentage of saving depends upon the initial temperature of the feed water, and the pressure at which it is evaporated.

For example, a boiler carrying steam at 100 pounds pressure has the temperature of the feed water raised from 60 to 200 degrees, what is the percentage of gain?

By referring to a table pressure of “saturated steam,” it will be seen that the total heat in steam at 100 pounds pressure is 1185 heat units. These calculations are from 32 degrees above zero, consequently the feed must be computed likewise.

In the first case, the heat to be supplied by the furnace is the total heat, less that which the feed water contains, or 1185 - 28 = 1157 heat units. In the second case it is 1185 - 168 = 1017 heat units, the difference being 1157 - 1017 = 140, which represents a saving of140⁄1157or about 12 per cent.

Where feed water is heated no more than 20 degrees above its normal temperature the gain effected cannot amount to more than 2%, not sufficient to pay for the introduction and maintenance of a feed water heating device, no matter how simple, but if the temperature of the water can be increased 60 degrees the gain will be in the neighborhood of 5%. To make feed water heating practical and economical it would be necessary to increase the temperature of the water about 180 degrees at least, and to do this, using the exhaust from a non-condensing engine without back pressure, would require such a capacity of heater as would give fully 10 square feet of heating surface to each horse power of work developed, and to raise the temperature above this would require a certain amount of back pressure or an increased capacity of heater, so that the subjectresolves itself into a question of large capacity of heater, or a higher temperature of the exhaust steam, which could only be obtained through a given amount of back pressure.

In the same way has been calculated the following table, showing percentages of saving of fuel by heating feed-water to various temperatures by exhaust steam, otherwise waste:

Percentage of saving.(Steam at 60 pounds gauge pressure.)

FinalTemp.Fahr.Initial Temperature of Water (Fahrenheit).32 Deg.40 Deg.50 Deg.60 Deg.70 Deg.80 Deg.90 Deg.602.391.719.86…………804.093.432.591.740.88……1005.795.144.323.492.641.77.901207.506.856.055.234.403.552.681409.208.577.776.976.155.324.4716010.9010.289.508.727.917.096.2618012.6012.0011.2310.469.688.878.0620014.3613.7113.0012.2011.4310.659.8522016.0015.4214.7014.0013.1912.3311.64100 Deg.120 Deg.140 Deg.160 Deg.180 Deg.200 Deg.60………………80………………100………………1201.80……………1403.611.84…………1605.423.671.87………1807.235.523.751.91……2009.037.365.623.821.96…22010.849.207.505.733.931.98

A good feed-water heater of adequate proportions should readily raise the temperature of feed-water up to 200° Fahr., and, as is seen by inspection of the table, thus effect a saving of fuel, ranging from 14.3 per cent. to 9.03 per cent., according as the atmospheric or normal temperature of the water varies from 32° Fahr. in the height of winter, to 100° Fahr. in the height of summer.

The percentage of saving which may be obtained from the use of exhaust steam for heating the feed water, with which the boiler is supplied, will depend upon the temperature to which the water is raised, and this, in turn, will depend upon the length of time that the water remains under the influence of the exhaust steam. This should be as long as possible, and unless a sufficient amount of heating surface is employed in the heater best results cannot be expected.

It does not necessarily require all the exhaust steam—or the whole volume of waste steam passing from the engine to bring the feed water up to the temperature desired, and the larger the heating appliance the smaller proportion is needed—hence heaters are best made with two exits nicely proportioned to avoid back pressure and at the same time utilize enough of the exhaust to heat the feed water.

An impression prevails among many who are running a condenser on their engine that a feed water heater can not be used in connection with it; large numbers of heaters running on condensing engines with results as follows: the feed water is delivered to the boiler at a temperature of 150° to 160° Fahr., depending on the vacuum: the higher the vacuum the less the heat in the feed water.

A heater applied to a condensing engine generally increases the vacuum one to two inches.

When cold water is used for the feed water, the saving in fuel by the use of the heater is from 7 to 14 per cent.

When feed water is taken from the hot well, it will save 7 to 8 per cent.

Where all the steam generated by a boiler is used in the engine and the exhaust passed through a heater it is found by actual experiment, where iron tubes are used in the heater, that approximately ten square feet of heating surface will be required for each 30 lbs. of water supplied to the boiler at a temperature of 200 degrees Fahr.

Ten square feet of heating surface in the feed water heater also represents one horse power.

The following table gives the capacity of cisterns for each twelve inches in depth:

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