CHAPTER XV

PUMPING STATION PRACTICE

Boilers

In the average steam-operated pumping station, large preventable waste of money often occurs in the boiler room. This comes from overlooking the fact that the boiler room is a manufacturing plant, producing from fuel, air and water a product, steam, by sensitive chemical and physical changes. As in every manufacturing plant, low cost of production can only be attained by preventing needless waste, and the search for and prevention of such wastes has been found by many water works managers to be an interesting as well as profitable undertaking.

Money is saved in a boiler plant by handling fuel and ashes at minimum expense, by burning the fuel as completely as practicable with a minimum quantity of air, because heating air not needed for combustion reduces the heat available for making steam, and by transferring the heat from the gases of combustion to the water (and to the steam in the case of superheaters) with minimum losses during transmission.

In carrying on these operations unremitting attention to safety is imperative, for any boiler in operation is potentially dangerous. Fortunately, by requiring boilers to be built in accordance with the Boiler Code of the American Society of Mechanical Engineers, as well as the boiler code of the State where they are to be used, and having them inspected by a responsible boiler insurance company, the good construction and safety of new boilers is readily assured. After they go into operation, it is desirable to have them similarly inspected internally at least once a year and externally twice a year. Types of boilers. The type of the pressure parts of a boiler is the first feature about a boiler room to be considered in an investigation to ascertain the way to operate it most economically. In every type of boiler used in pumping stations a considerable proportion of the metal walls through which the heat of the hot gases is transmitted to the water is provided by tubes. If the hot gases pass through

the tubes the boiler is called "fire tube" and if water passes through the tubes the boiler is called "water tube."

Fire tube boilers. In fire tube boilers, the tubes are within a large drum or shell. With very few exceptions the fire tube boilers used in pumping stations are set with this drum horizontal, with the furnace at one end, the gases passing backward outside the shell to the rear of the shell where they enter the fire tubes and pass forward to the front, whence they are taken through breeching to the chimney. On account of this arrangement of the boiler in its setting, it is termed a "horizontal return tubular boiler."

Increase in the diameter of the shell or in the steam pressure requires proportional increase in the thickness of the shell plates and, as it is generally held that the danger of overheating the plates directly over the fire increases with the thickness of the plates, the size and working pressure of fire tube boilers are much below those of the average water tube boiler now built, except for locomotive and marine service. Today fire tube boilers for stationary use are rarely built for a working pressure exceeding 150 pounds or larger than 200 horse power in rating. In their proper field, which includes many small pumping stations, they are economical, if properly installed and operated, and a good, small set of such boilers costs much less than a comparable set of water tube boilers.

Water tube boilers. Water tube boilers are classified according to the position of the water tubes into vertical, inclined and horizontal types. In the last the tubes are not horizontal, but inclined at an angle of about 15° with the horizontal. A few water tubes boilers combine features of two types. By varying the arrangement of the tubes in many types of these boilers, without changing the surface area of the tubes, it is possible to provide either a boiler of the highest efficiency in absorbing heat, such as is desirable for operation under a steady load, or one of somewhat lower efficiency, but capable of quickly responding to a heavy increase in the load, such as is suitable for a station operating under fluctuating loads. While the two boilers will have the same horse power rating, the resistance to draft in the former will be greater than in the latter.

Nearly all the heating surface of water tube boilers is in the tubes, and it is practicable to use drums of much smaller diameter than those of fire tube boilers of equal rating. The tubes and drums are so connected that the entire assemblage of pressure parts can be

supported in a way leaving the parts free to expand and contract without placing any weight or strain on the masonry of the setting, which is very desirable. Fire tube boilers can be similarly supported, but quite often they are supported by the walls of the setting, in order to save the expense of independent steel supports. Water content of boilers. Water tube boilers contain less water per square foot of heating surface than do fire tube boilers, and consequently it is easier to maintain the correct water level in the latter than in the former. Practically, this is unimportant if good firemen are employed, and it results in greater ease in forcing the boilers when a sudden peak load comes on the station.

The quantity of water per square foot of heating surface in boilers varies considerably with their design and size. A 250 horse power vertical water tube boiler will probably average about 1.4 cubic feet of water per square foot of heating surface, a horizontal longitudinal drum water tube boiler about 1.3 cubic feet, and an inclined water tube boiler about 1.25 cubic feet. Horizontal return tubular boilers of this size are rarely built, but two 125 horse power boilers will hold about 1.9 cubic feet of water per square foot of heating surface.

Efficiency of pressure parts. The purpose of the pressure parts of a boiler is to transmit to the water the heat radiated from the fire and carried by the furnace gases on their way to the chimney. Such a small proportion of the steam is generated by radiant heat in most boilers that it is usually omitted from any investigation of efficiency, although it should not be forgotten, and only the absorption of heat from the gases is considered. The heat absorption is the more effective as the opportunity for contact between all portions of the furnace gases and the heating surfaces of the boiler is increased. In the horizontal return tubular boiler, the hot gases pass from the furnace and combustion chamber over a large part of the shell of the boiler to the back of the setting, and it is usually believed that only a small part of them comes into actual contact with the shell. At the back of the boiler, the heat of the gases tends to force a large part of them through the upper rows of fire tubes. This unequal distribution of the gases over the heating surface is responsible for a part of the difficulty encountered in attempting to operate such boilers much above rating. Various methods of improving these conditions have been tried and some of them have been found meritorious on test, but none has come into general use.

The circulation of the furnace gases over the heating surfaces of water tube boilers is so controlled by baffles that the contact of the gases with the tubes is intimate from the point where the gases first enter the banks of tubes. As the gas passes over the tubes, heat is abstracted from it and its volume per pound decreases. In a quite approximate way, the baffling is arranged to decrease the area of the gas passages through the banks of tubes, and thus keep the velocity of the gases approximately constant. This, in turn, tends to fill the passages completely with the gases. The baffling of boilers carrying up to about 200 pounds steam pressure is practically standardized for different fuels and for operation up to about 150 per cent of rating. For higher pressures and higher rates of operation, the baffling for any given set of conditions becomes a special problem in designing the boiler. It is sometimes considered best in such cases to expose more of the tube surface of the lower rows of tubes to the radiant heat of the fire, since this increases the heat absorption and the boiler efficiency and the relatively cool tubes prevent the furnace temperature rising to a point where the maintenance of the furnace brickwork becomes too costly.

In case a considerable change is made in the quality of the fuel, or it is desired to add a boiler to others served by a chimney with little surplus draft capacity, a rearrangement of the baffling of the boilers often enables the changes to be made at minimum expense and with little if any decrease in efficiency.

The efficiency of the pressure parts in transmitting heat when they are new is by no means so important to the boiler user as their efficiency after they have been in service for a few years. The relative ease of cleaning boiler heating surfaces, both wet and dry, is important, because experience proves that surfaces difficult to clean are rarely kept clean.

The efficiency of a plate in transmitting heat from hot gases to water is reduced by deposits of soot and dust on the dry surface and layers of scale on the wet surface. A rough rule formerly often used in estimating the reduction in efficiency in this way is that a layer of soot 2 inch thick causes a reduction in efficiency of about 9 per cent and a layer inch thick about 25 per cent. A layer of scale 32 inch thick according to this rule, will reduce the efficiency about 8 per cent, one 16 inch thick about 12 per cent, and one inch thick about 20 per cent. These approximate figures show how

important it is to keep boiler surfaces clean, and explain why some. boiler plants with equipment of rather low efficiency are actually furnishing steam at less cost than plants with equipment of higher efficiency which are not kept as clean as the first plants. In addition to the loss in efficiency due to scale, its presence causes most of the bent tubes, sagging plates and explosions recorded by the inspectors of the boiler insurance companies. Scale can be removed from water tube boilers more easily than from fire tube boilers. The dry side of the heating surfaces of fire tube and small water tube boilers is easily cleaned with steam lances attached to steam hose. The larger water tube boilers are generally provided with built-in mechanical soot blowers.

The operation and maintenance of all types of steam boilers are so important that the Boiler Code Committee of the American Society of Mechanical Engineers published in Mechanical Engineering of May, 1925, elaborate "Rules for the Care of Power Boilers." These rules will be revised from time to time and the latest edition can be obtained from the Society at 29 West 39th Street, New York.

Effect of boiler type on operation. In properly designed water tube boilers, the water circulates freely at any rate at which furnaces have yet been fired. When furnaces burning oil and pulverized coal are fired at such a rate that the furnace brickwork lasts a very brief period, no trouble is experienced from inferior circulation of water in such boilers. In the case of fire tube boilers, where the gases must pass through, instead of around, the tubes, the circulation of the water becomes turbulent when an attempt is made to push the boilers much above rating. At the same time the resistance to draft offered by the limited area of the tubes for the return passage of the gases of combustion keeps down the rate at which coal can be burned per square foot of grate area, at least, with efficiency. As a result. of these conditions water tube boilers are regularly operated for long periods with high efficiency with an output of steam per square foot of heating surface which is practically unattainable with fire tube boilers.

In buying boilers, except large high-pressure stationary units and marine boilers, it is customary to give the size of the boiler in horse power, 1 horse power being 10 square feet of heating surface. This method of rating is quite misleading because a square foot of heating surface in one boiler may give much more steam at a given pressure