Steam, Its Generation and Use Part 41

Before placing a new boiler in service, a careful and thorough examination should be made of the pressure parts and the setting. The boiler as erected should correspond in its baffle openings, where baffles are adjustable, with the prints furnished for its erection, and such baffles should be tight. The setting should be so constructed that the boiler is free to expand without interfering with the brickwork.

This ability to expand applies also to blow-off and other piping. After erection all mortar and chips of brick should be cleaned from the pressure parts. The tie rods should be set up snug and then slacked slightly until the setting has become thoroughly warm after the first firing. The boiler should be examined internally before starting to insure the absence of dirt, any foreign material such as waste, and tools. Oil and paint are sometimes found in the interior of a new boiler and where such is the case, a quant.i.ty of soda ash should be placed within it, the boiler filled with water to its normal level and a slow fire started. After twelve hours of slow simmering, the fire should be allowed to die out, the boiler cooled slowly and then opened and washed out thoroughly. Such a proceeding will remove all oil and grease from the interior and prevent the possibility of foaming and tube difficulties when the boiler is placed in service.

The water column piping should be examined and known to be free and clear. The water level, as indicated by the gauge gla.s.s, should be checked by opening the gauge c.o.c.ks.

The method of drying out a brick setting before placing a boiler in operation is described later in the discussion of boiler settings.

A boiler should not be cut into the line with other boilers until the pressure within it is approximately that in the steam main. The boiler stop valve should be opened very slowly until it is fully opened. The arrangement of piping should be such that there can be no possibility of water collecting in a pocket between the boiler and the main, from which it can be carried over into the steam line when a boiler is cut in.

In regular operation the safety valve and steam gauge should be checked daily. In small plants the steam pressure should be raised sufficiently to cause the safety valves to blow, at which time the steam gauge should indicate the pressure at which the valve is known to be set. If it does not, one is in error and the gauge should be compared with one of known accuracy and any error at once rectified.

In large plants such a method of checking would result in losses too great to be allowed. Here the gauges and valves are ordinarily checked at the time a boiler is cut out, the valves being a.s.sured of not sticking by daily instantaneous opening through manipulation by hand of the valve lever. The daily blowing of the safety valve acts not only as a check on the gauge but insures the valve against sticking.

The water column should be blown down thoroughly at least once on every shift and the height of water indicated by the gla.s.s checked by the gauge c.o.c.ks. The bottom blow-offs should be kept tight. These should be opened at least once daily to blow from the mud drum any sediment that may have collected and to reduce the concentration. The amount of blowing down and the frequency is, of course, determined by the nature of the feed water used.

In case of low water, resulting either from carelessness or from some unforeseen condition of operation, the essential object to be obtained is the extinguishing of the fire in the quickest possible manner. Where practicable, this is best accomplished by the playing of a heavy stream of water from a hose on the fire. Another method, perhaps not so efficient, but more generally recommended, is the covering of the fire with wet ashes or fresh fuel. A boiler so treated should be cut out of line after such an occurrence and a thorough inspection made to ascertain what damage, if any, has been done before it is again placed in service.

The efficiency and capacity depend to an extent very much greater than is ordinarily realized upon the cleanliness of the heating surfaces, both externally and internally, and too much stress cannot be put upon the necessity for systematic cleaning as a regular feature in the plant operation.

The outer surfaces of the tubes should be blown free from soot at regular intervals, the frequency of such cleaning periods being dependent upon the cla.s.s of fuel used. The most efficient way of blowing soot from the tubes is by means of a steam lance with which all parts of the surfaces are reached and swept clean. There are numerous soot blowing devices on the market which are designed to be permanently fixed within the boiler setting. Where such devices are installed, there are certain features that must be watched to avoid trouble. If there is any leakage of water of condensation within the setting coming into contact with the boiler tubes, it will tend toward corrosion, or if in contact with the heated brickwork will cause rapid disintegration of the setting. If the steam jets are so placed that they impinge directly against the tubes, erosion may take place. Where such permanent soot blowers are installed, too much care cannot be taken to guard against these possibilities.

Internally, the tubes must be kept free from scale, the ingredients of which a study of the chapter on the impurities of water indicates are present in varying quant.i.ties in all feed waters. Not only has the presence of scale a direct bearing on the efficiency and capacity to be obtained from a boiler but its absence is an a.s.surance against the burning out of tubes.

In the absence of a blow-pipe action of the flames, it is impossible to burn a metal surface where water is in intimate contact with that surface.

In stoker-fired plants where a blast is used, and the furnace is not properly designed, there is a danger of a blow-pipe action if the fires are allowed to get too thin. The rapid formation of steam at such points of localized heat may lead to the burning of the metal of the tubes.

Any formation of scale on the interior surface of a boiler keeps the water from such a surface and increases its tendency to burn. Particles of loose scale that may become detached will lodge at certain points in the tubes and localize this tendency at such points. It is because of the danger of detaching scale and causing loose flakes to be present that the use of a boiler compound is not recommended for the removal of scale that has already formed in a boiler. This question is covered in the treatment of feed waters. If oil is allowed to enter a boiler, its action is the same as that of scale in keeping the water away from the metal surfaces.

[Ill.u.s.tration: Fig. 41]

It has been proven beyond a doubt that a very large percentage of tube losses is due directly to the presence of scale which, in many instances, has been so thin as to be considered of no moment, and the importance of maintaining the boiler heating surfaces in a clean condition cannot be emphasized too strongly.

The internal cleaning can best be accomplished by means of an air or water-driven turbine, the cutter heads of which may be changed to handle various thicknesses of scale. Fig. 41 shows a turbine cleaner with various cutting heads, which has been found to give satisfactory service.

Where a water-driven turbine is used, it should be connected to a pump which will deliver at least 120 gallons per minute per cleaner at 150 pounds pressure. This pressure should never be less than 90 pounds if satisfactory results are desired. Where an air-driven turbine is used, the pressure should be at least 100 pounds, though 150 pounds is preferable, and sufficient water should be introduced into the tube to keep the cutting head cool and a.s.sist in washing down the scale as it is chipped off.

Where scale has been allowed to acc.u.mulate to an excessive thickness, the work of removal is difficult and tedious. Where such a heavy scale is of sulphate formation, its removal may be a.s.sisted by filling the boiler with water to which there has been added a quant.i.ty of soda ash, a bucketful to each drum, starting a low fire and allowing the water to boil for twenty-four hours with no pressure on the boiler. It should be cooled slowly, drained, and the turbine cleaner used immediately, as the scale will tend to harden rapidly under the action of the air.

Where oil has been allowed to get into a boiler, it should be removed before placing the boiler in service, as described previously where reference is made to its removal by boiling out with soda ash.

Where pitting or corrosion is noted, the parts affected should be carefully cleaned and the interior of the drums should be painted with white zinc if the boiler is to remain idle. The cause of such action should be immediately ascertained and steps taken to apply the proper remedy.

When making an internal inspection of a boiler or when cleaning the interior heating surfaces, great care must be taken to guard against the possibility of steam entering the boiler in question from other boilers on the same line either through the careless opening of the boiler stop valve or some auxiliary valve or from an open blow-off. Bad accidents through scalding have resulted from the neglect of this precaution.

Boiler brickwork should be kept pointed up and all cracks filled. The boiler baffles should be kept tight to prevent by-pa.s.sing of any gases through the heating surfaces.

Boilers should be taken out of service at regular intervals for cleaning and repairs. When this is done, the boiler should be cooled slowly, and when possible, be allowed to stand for twenty-four hours after the fire is drawn before opening. The cooling process should not be hurried by allowing cold air to rush through the setting as this will invariably cause trouble with the brickwork. When a boiler is off for cleaning, a careful examination should be made of its condition, both external and internal, and all leaks of steam, water and air through the setting stopped. If water is allowed to come into contact with brickwork that is heated, rapid disintegration will take place. If water is allowed to come into contact with the metal of the boiler when out of service, there is a likelihood of corrosion.

If a boiler is to remain idle for some time, its deterioration may be much more rapid than when in service. If the period for which it is to be laid off is not to exceed three months, it may be filled with water while out of service. The boiler should first be cleaned thoroughly, internally and externally, all soot and ashes being removed from the exterior of the pressure parts and any acc.u.mulation of scale removed from the interior surfaces. It should then be filled with water, to which five or six pails of soda ash have been added, a slow fire started to drive the air from the boiler, the fire drawn and the boiler pumped full. In this condition it may be kept for some time without bad effects.

If the boiler is to be out of service for more than three months, it should be emptied, drained and thoroughly dried after being cleaned. A tray of quick lime should be placed in each drum, the boiler closed, the grates covered and a quant.i.ty of quick lime placed on top of the covering. Special care should be taken to prevent air, steam or water leaks into the boiler or onto the pressure parts to obviate danger of corrosion.

[Ill.u.s.tration: 3000 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers in the Main Power Plant, Chicago & Northwestern Ry. Depot, Chicago, Ill.]

BRICKWORK BOILER SETTINGS

A consideration of the losses in boiler efficiency, due to the effects of excess air, clearly indicates the necessity of maintaining the brick setting of a boiler tight and free from air leaks. In view of the temperatures to which certain portions of such a setting are subjected, the material to be used in its construction must be of the best procurable.

Boiler settings to-day consist almost universally of brickwork--two kinds being used, namely, red brick and fire brick.

The red brick should only be used in such portions of the setting as are well protected from the heat. In such location, their service is not so severe as that of fire brick and ordinarily, if such red brick are sound, hard, well burned and uniform, they will serve their purpose.

The fire brick should be selected with the greatest care, as it is this portion of the setting that has to endure the high temperatures now developed in boiler practice. To a great extent, the life of a boiler setting is dependent upon the quality of the fire brick used and the care exercised in its laying.

The best fire brick are manufactured from the fire clays of Pennsylvania. South and west from this locality the quality of fire clay becomes poorer as the distance increases, some of the southern fire clays containing a considerable percentage of iron oxide.

Until very recently, the important characteristic on which to base a judgment of the suitability of fire brick for use in connection with boiler settings has been considered the melting point, or the temperature at which the brick will liquify and run. Experience has shown, however, that this point is only important within certain limits and that the real basis on which to judge material of this description is, from the boiler man's standpoint, the quality of plasticity under a given load. This tendency of a brick to become plastic occurs at a temperature much below the melting point and to a degree that may cause the brick to become deformed under the stress to which it is subjected.

The allowable plastic or softening temperature will naturally be relative and dependent upon the stress to be endured.

With the plasticity the determining factor, the perfect fire brick is one whose critical point of plasticity lies well above the working temperature of the fire. It is probable that there are but few brick on the market which would not show, if tested, this critical temperature at the stress met with in arch construction at a point less than 2400 degrees. The fact that an arch will stand for a long period under furnace temperatures considerably above this point is due entirely to the fact that its temperature as a whole is far below the furnace temperature and only about 10 per cent of its cross section nearest the fire approaches the furnace temperature. This is borne out by the fact that arches which are heated on both sides to the full temperature of an ordinary furnace will first bow down in the middle and eventually fall.

A method of testing brick for this characteristic is given in the Technologic Paper No. 7 of the Bureau of Standards dealing with "The testing of clay refractories with special reference to their load carrying capacity at furnace temperatures." Referring to the test for this specific characteristic, this publication recommends the following: "When subjected to the load test in a manner substantially as described in this bulletin, at 1350 degrees centigrade (2462 degrees Fahrenheit), and under a load of 50 pounds per square inch, a standard fire brick tested on end should show no serious deformation and should not be compressed more than one inch, referred to the standard length of nine inches."

In the Bureau of Standards test for softening temperature, or critical temperature of plasticity under the specified load, the brick are tested on end. In testing fire brick for boiler purposes such a method might be criticised, because such a test is a compression test and subject to errors from unequal bearing surfaces causing shear. Furthermore, a series of samples, presumably duplicates, will not fail in the same way, due to the mechanical variation in the manufacture of the brick. Arches that fail through plasticity show that the tensile strength of the brick is important, this being evidenced by the fact that the bottom of a wedge brick in an arch that has failed is usually found to be wider than the top and the adjacent bricks are firmly cemented together.

A better method of testing is that of testing the brick as a beam subjected to its own weight and not on end. This method has been used for years in Germany and is recommended by the highest authorities in ceramics. It takes into account the failure by tension in the brick as well as by compression and thus covers the tension element which is important in arch construction.

The plastic point under a unit stress of 100 pounds per square inch, which may be taken as the average maximum arch stress, should be above 2800 degrees to give perfect results and should be above 2400 degrees to enable the brick to be used with any degree of satisfaction.

The other characteristics by which the quality of a fire brick is to be judged are:

Fusion point. In view of the fact that the critical temperature of plasticity is below the fusion point, this is only important as an indication from high fusion point of a high temperature of plasticity.

Hardness. This is a relative quality based on an arbitrary scale of 10 and is an indication of probable cracking and spalling.

Expansion. The lineal expansion per brick in inches. This characteristic in conjunction with hardness is a measure of the physical movement of the brick as affecting a ma.s.s of brickwork, such movement resulting in cracked walls, etc. The expansion will vary between wide limits in different brick and provided such expansion is not in excess of, say, .05 inch in a 9-inch brick, when measured at 2600 degrees, it is not particularly important in a properly designed furnace, though in general the smaller the expansion the better.

Compression. The strength necessary to cause crushing of the brick at the center of the 4 inch face by a steel block one inch square. The compression should ordinarily be low, a suggested standard being that a brick show signs of crushing at 7500 pounds.

Size of Nodules. The average size of flint grains when the brick is carefully crushed. The scale of these sizes may be considered: Small, size of anthracite rice; large, size of anthracite pea.

Ratio of Nodules. The percentage of a given volume occupied by the flint grains. This scale may be considered: High, 90 to 100 per cent; medium, 50 to 90 per cent; low, 10 to 50 per cent.

The statement of characteristics suggested as desirable, are for arch purposes where the hardest service is met. For side wall purposes the compression and hardness limit may be raised considerably and the plastic point lowered.