The Management of Motor-Wagon Boilers.
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The Question of Water Supply.
No harm can be done in washing out even more often than weekly, and cleanliness is thereby assured. A strong force of water should be directed through the boiler from a washout hose so as to sweep all mud off the tire-box crown, and from between tubes, and the ledge at the side of the fire-box should be scraped all round with a mud rake. All cocks on the boiler should be cleaned each time it is washed out; the plugs of cocks should be taken out and tallowed, and a wire passed through the waterway to make sure that the passage is clear. It has not infrequently happened that motor wagons have been hung up on the road through failure of both pump and injector, and on examination it has been found that the passage way in the clack boxes was filled up solid by caked mud, and there was no possibility of the water getting through. Similarly, many boilers have been burnt because the gauge glass has shown " false water." If the top cock is blocked, water will be drawn up to a false height, by the vacuum produced above it as the steam in the glass con' denses, and will even be held in the glass after the water has sunk below the bottom cock, If the bottom cock is choked up, the water gauge may show anything. Too much importance cannot be made of the duty of cleaning out all the waterways in boiler fittings at least once a week.
To all owners of steam motor lorries the question of the purity of the water used is of greater importance than is usually supposed. It should be hardly necessary for us to refer to contamination due to dirt, straw, etc., were it not that water is drawn by many lorries from wayside sources such as brooks, ponds, etc. We have had recently under our notice a case of the late delivery of an important consignment of goods arising front neglect in seeing that such substances were not drawn into the tank in spite of the perforated ball on the end of the sucker. In the case we refer to, the straw, etc„ blocked the supplementary strainer in the tank itself, causing the pump to fail, which in turn led to the fire having to be dropped. In this particular instance the load had to be removed a:nd the body of the lorry taken off before the only hand hole in the tank could be got at. It would seem, for several reasons, advisable to always have a hand hole in the side of water tanks.
It is, however, with the effect of the chemical impurities in water that we are concerned, as they are of more importance in the case of a motor-lorry boiler than in almost any other type, owing to the small size and yet large amount of work these boilers have to perform. It is not at all unusual for a lorry boiler to hold no more than 40 gallons of water when at half-glass, whilst it may evaporate .too gallons during the day. The result of this is that although the impurities may only be, proportionately, very small in the morning, they will be considerably concentrated after the day's work. This, however, is a point we will return to later.
The chief of the chemical substances which occur as impurities in water, and which render it hard, are carbonates and sulphates of lime and magnesium, all of which tend to form scale, etc., on the plates of the boiler if allowed to concentrate. This scale may be deposited, as stated on p. zos of our issue of May 18th, in the form of mud, or may, as is often the case where care is not taken, set as a hard scale on all the plates below the water level. This. coating,
if allowed to get too thick, will materially decrease the efficiency of the boiler, as well as leading to the risk of the plates becoming overheated and burnt. The reason for this latter wilt readily be seen, when it is remembered that iron and steel arc good conductors, and quickly lead away the heat communicated to their surface, owing to the fact that they are kept comparatively cold throughout most of their thickness by the water inside the boiler. The scale, or mud formed on a plate by water is a very bad conductor of heat, and, therefore, although a thin layer does not seem to do much harm, yet if allowed to thicken it may cause the plates Lo reach a high temperature and "become burnt " and permanently damaged. If scale forms, too, in the tubes of a water-tube boiler, the heating surface, and, consequently, the efficiency of the boiler, may become materially reduced, especially with the small tubes employed in motorcar work. As an example of this, if a tin. tube has an ;;tii coating of scale in it, its heating surface is reduced 25 per cent.
A few words on the properties of the impurities already mentioned will allow us to more readily realise their action in the boiler itself.
Carbonate of lime (calcium carbonate, Ca Co3).—This is the chief cause of hardness, as although it is by itself almost insoluble in water, which, in fact, is only able to take up about two grains per gallon, yet when carbonic acid (CO2) is present, as it is in all natural water, the carbonate of lime dissolves freely in it. Upon any water being heated up to
i boiling point, the carbonic acid s driven oil as a gas, and the carbonate of lime is therefore thrown down as a solid, and this is what occurs in a boiler. If the action is slow, the particles formed are large and hard, hut if quick, as in a lorry boiler, a loose mud results, which if dealt with at once may be washed out, but if allowed to remain may become cemented into a hard scale. It will be noticed that by simply heating the water this impurity may be got rid of, but this is usually too expensive a matter to be carried out in a separate vessel before the water is put into a boiler.
By adding lime water, or milk of lime Ca (OTT), to water, the calcium carbonate may be thrown down, the action being shown thus :—
Ca CO, +CO, +Ca (OH) 2 =2 Ca CO2 +H5 0.
Carbonate of Magnesium (Mg CO3) is very much like carbonate of lime in its behaviour. It is, however, more soluble in water free from carbonic acid than the carbonate of lime, and so, if lime is used to remove it, sufficient quantity should be added to the water to allow of magnesia Mg (01.1)2 being formed.
These two substances, the carbonates of lime and magnesium, form what is known as the " temporary " hand. 'less of water, as, when they are precipitated by lime, no solids are inserted in the water in their place.
Sulphate of lime (calcium sulphate, Ca SO4) is not re.. flawed by ordinary boiling, and, therefore, if present at all, will, if not removed, go on concentrating until its saturation point is reached, when a very hard scale will be deposited on the plates, etc. This substance is far more soluble in cold water than in hot; it can, however, be removed by adding sodium carbonate, Naz CO3, to the water, the reaction being : Ca SO4+Na3 CO3=Ca CO3÷Na2 SO4.
Insoluble. Soluble.
As the sodium sulphate remains in solution, the solids in the water are not reduced, but as it is extremely soluble it does not form scale.
Magnesium sulphate (Mg SO4) does not form scale itselt, but, owing to its acting on carbonate of lime and, with it, forming sulphate of lime and carbonate of magnesium, its presence is objectionable. Its action is :- Mg SO4 +Ca CO3 =Ca SO4 +Mg CO3.
The sulphate of magnesium can be removed from water by adding caustic soda, Na0II, or lime and sodium carbonate.
The above are not, of course, the sole impurities in water, but only the chief of the harmful ones met with in the majority of cases. The methods given of getting rid of the impurities mentioned could not be applied in the boiler itself and would necessitate the use of a properly-constructed water softener, except where it was only desired to free the water from the temporary hardness, due to carbonates of lime and magnesiutn, by the use of lime. In this case, even, settling tacks of considerable size would be required, owing to the slowness with which the precipitate settles and the water clears. Where a large number of steam lorries are in use and able always to draw water from one supply, if the water is hard it is probable that the saving in boiler repairs and stoppages would pay for the installation of a small water softener. In the majority of cases, however, only a few lorries are kept at one point, and these draw their water
supply from various sources, which may vary in composition. This use of several kinds of water is an advantage, as it is generally found that the scale f ormed under these conditions is much softer and more easily got rid of than if the water is all from one source.
We are aware that, with ordinary stationary and locomotive boilers, certain disincrustants, such as caustic soda, quebracho, etc., are used with success to prevent scale, but with a small motor lorry boiler, if not used with very great care, they would lead to excessive priming.
Returning now to the question of concentration of impurities owing to evaporation. We assumed above that a boiler having a capacity of 40 gallons evaporates 400 gallons of water per day. If the water contains only to grains per gallon of dissolved solids originally, there will be 4,400 grams left at the end of the day in, or with, the 40 gallons of water. Probably a considerable quantity of this amount will be thrown down in the form of scale or mud, but other substances will remain in the water itself in a concentrated state. This, as well as filling a boiler too full, is a common cause of priming, which not only leads to mechanical difficulties, but, as the hot water carried over does no work, heat is lost and there is a diminution of the fuel efficiency. The simplest remedy for bad water is to blow off the saltimpregnated water, if necessary, every night, as, not only will the dissolved salts be then got rid of, but the scale already formed, will be loosened. We have known cases where boilers, which previously gave trouble and had to be periodically taken to pieces to clean, were maintained in constant service for considerable periods by resorting to this simple method. It is, however, a matter for regret that all makers do not make allowance for the quick refilling of boilers which have been emptied, for if this is at all a tedious matter, drivers will not pay the attention to it that they should, especially as it has to be done, as a rule, at the end of a hard day's work. Of course, a partial removal of the water is better than leaving the whole of it in the boiler, and this can more effectually be done by filling the boiler as full as possible and blowing off as low as allowable. If the water is bad it is better to empty completely.
As stated in our article on the management of boilers, in addition to this blowing off, the boiler should be well washed out at least once a week, as this prevents accumulation and hardening of mud and scale. It should be seen that washout plugs are provided, so that the water jet may be made to play on all horizontal or inclined surfaces which are below the water level, as these are the points at which scale accumulates most. With the locomotive type of boiler at present coming into use on so many types of lorries, every care should be taken to prevent an accumulation of scale or mud oh the top of the fire-box, as this is one of the points most exposed to the fire gases and of maximum evaporation.
Before leaving this subject of water, it is perhaps necessary to speak of the highly injurious effect of the slightest amount of grease introduced into a boiler. A thin film of this substance on a plate offers a very great resistance to the passage of heat, and there is nothing which leads so quickly not only to burnt plates and leaky tubes, but also to priming. As, however, condensed water is not often used for lorry boilers, this danger is not so great as with the earlier types of vehicles, which had condensers which were liable to pass forward into the boiler some of the oils used for lubricating the cylinders.