Scale formed by hard water on the insides of boilers and of water tubes causes enormous loss of efficiency, sometimes culminating in disaster. Various kinds of water softening apparatus have been devised to overcome this problem and to improve industrial and domestic water supplies
RAILWAY WATER TROUGHS at Castlethorpe, Buckinghamshire, on the L.M.S. (Western Division) main line, are supplied with treated water from the softening plant alongside the line. The plant is of the Lassen-Hjort lime-soda type and has a capacity of 600,000 gallons of untreated hard water a day. A Royal Scot locomotive, at the head of an express train, is picking up water from the troughs at speed.
WATER in its natural state is seldom really soft, unless it is rainwater. Ordinary spring water contains salts and other minerals in solution, and most of us have at one time or another been obliged to use hard water for the homely purpose of washing, and have experienced difficulty in getting the soap to lather. When rainwater is used there is no such trouble.
For drinking purposes, hardness in water is sometimes an advantage, it is true; there is no flatter drink in the world than rainwater. In industrial and transport engineering, hard water is a bugbear indeed. For considerably more than half a century, steam engineers have striven to develop apparatus for the treatment of boiler feedwater on stationary plants and locomotives. Marine engineers in the early days of steam navigation experienced a great deal of trouble through using salt water for their
boilers. This trouble was overcome as far back as the eighteen-thirties by the introduction of surface condensers, which enabled the same fresh water to be used over and over again. Condensation, however, does not solve all the problems of the boiler, as the steam engineer knows only too well.
Hard water will play havoc with the tubes or plates of a boiler, whatever its design. Every one has seen the condition into which a household kettle gets after having been used for some time with hard water. The same thing happens in a large steam boiler. More and more scale forms on the plates and in the water tubes. Not only does scale cause loss of efficiency, as it transmits heat badly, but it can also be exceedingly dangerous if it is neglected. Sulphate of lime and carbonate of lime, automatically cemented by magnesia, which also forms part of the deposit, will form a scale of steadily increasing thickness, and so hard that it is possible to cut it and polish it in the same way as a diamond. With such a scale, the tubes and plates become cut off from contact with the water in the boiler altogether, and the surfaces, no longer cooled by the water, are overheated and the strength of the metal is diminished.
In the history of steam engineering, numerous instances occur of boilers having failed, more or less violently, from this cause. Sooner or later, tubes and plates will be burned so thin that they can no longer withstand the pressure of the steam on the water contained by them. Then a disastrous explosion may occur. In days gone by the science of water treatment was somewhat neglected. Where locomotive boilers were concerned, an interesting early effort was that of Wagner, engineer of the former Bergisch-Markische Railway Company in Germany.
Wagner originated the arrangement known as top feed, injecting his feedwater above a series of trays inside a dome on the top of the boiler barrel. The water trickled down through these trays before joining that already in the boiler, a certain proportion of the deposit being left in the trays, which could be removed and cleaned at intervals.
Wagner first applied top feed to a locomotive in 1863. To-day it is widely used, sometimes with a dome, as on the original engines in Germany, and sometimes without, the trays being placed in the steam space. Domeless top feed is almost universal on locomotives of the Great Western Railway. The Southern Railway favours the topfeed dome on many of its locomotives. The L.M.S. has numerous examples of domeless top feed and of top feed with domes.
A top-feed arrangement does not, however, comprise a water softener in the strict sense of the term, though it serves a similar purpose. One method of dealing with badly scaled boilers was to introduce to them certain chemicals which broke up the scale and caused it to flake off the insides of the plates, removal being effected through mud traps. This has had its disadvantages. In a Yarrow boiler, for instance, scale flaked off in this manner from the upper parts will fall down the tubes before it can be removed, clogging them to a certain extent, and even penetrating and accumulating in the hot water drums below. It is better to prevent the scale from forming than to have to adopt drastic and not entirely efficient methods to remove it later.
Mud in Feedwater
Once the scale-forming constituents have been removed from the feedwater before it passes into the boiler, neither stony deposit nor mud will form inside it. The boiler will need much less attention and its life will be much prolonged. Even a slight deposit, such as 1/64-in., will cause considerable loss of heat. Mud, too, is another form of deposit that can be sometimes nearly as troublesome as scale. Certain chemicals introduced into the boiler feedwater to prevent scale will offer an unwelcome compensation by producing mud or sludge instead. Where the circulation inside the boiler is imperfect, this mud accumulates in heaps in the more sluggish parts, causing considerable trouble as it increases. Other disadvantages beside the deposit of scale
result from the use of hard and dirty water. Certain boilers are admittedly more prone to priming — the passage of water into the steam pipe — than others, but priming is always aggravated by the presence of oil or greasy matter, as well as salts, in the boiler feed. In the chemical treatment, that is, the ‘‘softening” of boiler feedwater, the content of the water fed to a particular boiler must be studied carefully before measures are taken. A form of purification which is ideal in one place may be all but useless in another.
FIRST FULLY AUTOMATIC ZEOLITE INSTALLATION to be laid down for a public water undertaking in Europe. Opened in October 1937 for the West Cheshire Water Board at Mouldsworth, Cheshire, the plant is composed of five softener units, each with a diameter of 8 ft. 6 in. and a length of 12 ft. 6 in. Automatic operation is secured by a patent multiport valve working in connexion with an electric contacting meter fitted on the soft water outlet main. The units are built to withstand a normal working pressure of 150 lb. per square inch.
A water softener can be a quite simple apparatus. An excellent type, which is used to a considerable extent for boiler feed treatment, is the “Permutit” zeolite softener. The zeolite is an insoluble granular material which, as the. water passes through it, exchanges its own sodium salts, which are harmless in boilers, for the harmful matter held in solution in the raw feedwater. The zeolite is contained in a cylindrical steel vessel, into which the feedwater is injected under pressure. As it passes through the bed of zeolite, purification takes place automatically, after which the water continues its flow to the feedwater heater. Each container has its specified capacity and, when the limit of this has been reached and recorded by a meter attached to the apparatus, the water softener is regenerated by having a solution of salt passed through it. The surplus salt solution is then run off into a sewer or otherwise disposed of. No sludge is formed in this apparatus. The zeolite softener belongs to a group working on what is known as the base exchange process.
Another widely used form of treatment is the cold lime-soda process. In this the raw water is charged with lime and soda ash to certain carefully determined proportions. It is then allowed to stand for upwards of four hours, while the sludge is precipitated and allowed to settle. It is then passed through a filter, which arrests the impurities originally contained in solution — chiefly consisting of calcium carbonate and magnesium hydroxide, with the lime that was added in the precipitation process.
Hot Lime-Soda Process
A system giving a softer effluent is the hot lime-soda process, which has the additional advantage of more compact apparatus. In this, the water enters the softener at or near boiling point. The chemical treatment resembles that of the cold process, the same materials being added with the same results, namely, that they reduce calcium and magnesium to a precipitate of calcium carbonate and magnesium hydroxide. Precipitates formed in the hot process are larger and settle more quickly than those formed in the cold process, and it is this fact that makes the use of smaller settling tanks possible.
For the treatment of boiler feed in an ordinary stationary plant, all three main processes may be used. The relative advantages of the hot lime-soda and of the base exchange processes depend on various considerations revealed by a chemical analysis of the raw water. Water with a comparatively high bicarbonate (temporary) hardness is best dealt with by the hot lime-soda process.
The great variety of problems set up by hard water has effected a corresponding variety in the design and arrangement of water-softening installations. The cold lime-soda system may be combined with base exchange. A remarkable combined installation was installed recently at the Hams Hall Power Station, in Birmingham. The feedwater to be treated is used in a series of water
tube boilers carrying a working pressure of 375 lb. per sq. in. The ordinary base exchange plant does not deal with hot water.
In this installation the raw water passes first to a Lassen-Hjort lime-soda water softener, 36 feet high and of 20 feet diameter, incorporating a special measuring and mixing apparatus. This governs to a nicety the passage of the necessary chemicals from the mixing tank and regulates the proportion of chemicals to water. A reciprocating pump raises the solution from the mixing tank into the reservoir of the measuring apparatus.
From the lime-soda plant the half-treated water passes into two sand filters, each with a diameter of 8 feet, through which it is forced under pressure by an electric centrifugal pump situated in the pump house of the installation. Having left these sand filters, the water is subjected to the base exchange process, passing into three “Permutit” pressure water softeners with a diameter of 9 feet. These are capable of treating 432,000 gallons of partly softened water from the lime-soda plant. The building housing them contains also a reinforced concrete tank in which the brine for regeneration purposes is prepared, the salt store being adjacent to this. The whole plant is as compact as it is complete.
CONTROL VALVES and flow indicator panel of a softener unit. This unit is one of a set, installed in April 1937 at Bognor Regis, Sussex, for softening the local water supply.
A zeolite, or base exchange material, is a chemical compound containing the elements of sodium, alumina and silica. It was the English chemist Thomas Way who discovered the base exchange properties of zeolites as long ago as 1850, but it was left for a German chemist, Robert Gans, to discover how to apply them to water softening, and to invent methods of manufacturing such a compound for industrial use. A zeolite is insoluble in water, in contrast to the chemicals used in the two lime-soda
processes, and it is therefore impossible for zeolite-treated water to receive an “overdose”, such as sometimes occurs with the other methods. Base exchange with zeolites forms the commonest method of water softening in use, and it is the perfection of this method that has brought the small water softener into houses for the treatment of domestic water supply.
The regeneration of the base exchange apparatus needs explanation. The first stage in regeneration consists of what is called “backwashing”. This involves the release of a powerful current of water up through the gravel bed which supports the zeolite layer, and through the zeolite itself. The current loosens and cleans the zeolite, and washes out the impurities thus released. The second stage, the salting, is accomplished by a hydraulic ejector forming an integral part of the apparatus. The salts are distributed evenly on the top of the layer of the zeolite and spread correspondingly evenly down through it. A slow rinse, to carry away the salt and the matter extracted from the hard water, completes the process of regeneration. The whole process is exceedingly simple and is virtually foolproof.
In the upflow variety of base exchange water softener, the principle is the same as that already described, though here the raw water enters at the bottom of the shell and is forced upwards, first through the supporting bed of gravel and then through the overlying layer of zeolite, maintaining the zeolite in a state of semi-suspension. This type of softener is generally used in conjunction with an overhead soft-water tank. The regeneration process starts with the salting, this being followed by a down-rinse, and is completed by an up-rinse.
There are innumerable purposes to which the water softener can be applied with advantage. During the past twenty-five years or so it has been more and more used on railways. Large water softeners may be seen attached to many locomotive depots on British main-line railways, as well as to the track water trough installations at such places as Bushey (Hertfordshire), Castlethorpe (Buckinghamshire), and Newbold, near Rugby (Warwickshire), on the L.M.S. Railway. The L.M.S. installation at Castlethorpe, with its companion installations, is of the Lassen-Hjort lime-soda type, and has a capacity of 600,000 gallons of untreated hard water a day.
In textile works water softening has proved invaluable, above all in the manufacture of woollen and worsted garments. A Lassen-Hjort hot limesoda installation at a factory at Brentford (Middlesex), 40 feet high and 20 feet in diameter, has a capacity of 480,000 gallons a day. An enormous installation at the works of Unilever Limited, Port Sunlight (Cheshire), can deal with 2,000,000 gallons a day.
The largest zeolite base exchange installation in Great Britain is at work in the Prenton (Birkenhead) pumping station of the West Cheshire Water Board, and has a capacity of 3,000,000 gallons a day. As the water here is intended for domestic and therefore for drinking purposes, it is not reduced to the zero degree of hardness, as in many industrial installations. The similar, though smaller plant belonging to the same water authority at its Hooton station, in the Wirral Peninsula, has a capacity of 1,250,000 gallons a day.
In October 1937 the first fully automatic zeolite installation to be laid down in Europe by a public water undertaking was installed at Mouldsworth (Cheshire), by the West Cheshire Water Board. There are five softener units, with a diameter of 8 ft. 6 in. and a length of 12 ft. 6 in., built to withstand a normal working pressure of 150 lb. per sq. in. Automatic operation is obtained by means of a patent multiport valve, which works in connexion with a special electric contacting meter fitted on the soft water outlet main.
THE LARGEST ZEOLITE WATER SOFTENING PLANT in Great Britain is installed in the West Cheshire Water Board’s pumping station at Prenton, Birkenhead. The plant has a capacity of 3,000,000 gallons a day.
