WATER TREATMENT SYSTEM
Reproduced from 'Machinery Lloyd and Electrical Engineering' (Volume 36, No. 23 - 7th November 1964) with kind permission of the publishers.
Three-quarters of the Earth's surface is covered by water but the industry is facing a gr wing problem of water shortage. This is because insufficient water of acceptable quality is available for industrial use. Seawater, due to the high concentration of dissolved salts, is not usable in many applications while river waters can vary considerably in quality, depending upon locality.
River waters are often used for cooling purposes in industrial plants, especially where the cooling system 13 of the once-through type. Treatment of such cooling systems is not usually an economic proposition except for chlorination to prevent biological growths. [he problem of corrosion is tackled by the use of corrosion-resistant materials, and scale formation is rarely a problem.
Recirculating Cooling Systems
More usually, cooling systems are of the recirculating type incorporating a cooling tower whereby a proportion of the circulating water is evaporated to give the cooling effect. The normal method for treating such systems is to use a method of control based on the Langelier equation. Using this method, the rate of bleed-off of circulating water from the cooling system is adjusted in relation to the quantity of water evaporated at the cooling tower to give an optimum number of concentrations of the circulating water, relative to make up water. Conditions are arranged to give circulating water which is slightly encrusting (i.e. non-corrosive). The circulating water is then treated with a tannin/phosphate inhibitor to stabilise the water and prevent scale formation. In this way, corrosion and scale formation is prevented simultaneously. When makeup water of high alkalinity is used the optimum number of concentrations will be low and may be below the lowest number of concentrations tolerable in practical operation. In these cases, the makeup water can be treated with a mineral acid in addition to the tannin/phosphate inhibitor.
Conversely, with the makeup of low alkalinity, the optimum number of concentrations will be high and may exceed the maximum at which the system will be operated in practice. In such cases, auxiliary treatment with alkali (e.g. lime, sodium bicarbonate, sodium carbonate, caustic soda) is used with the tannin/phosphate treatment.
Recently, considerable success has been achieved by the use of a polyelectrolyte in place of the tannin/phosphate inhibitor. For example, the treatment of a heavily scaled cooling system with the polyelectrolyte has resulted in the rapid removal of the scale. There are indications that, when the polyelectrolyte is in use, it may even be possible to run a system at a higher level of concentration than the optimum calculated according to the Langelier equation. This can result in considerable savings of makeup water.
The procedure frequently adopted in small systems (where the above system of concentration control is difficult to apply) is to start off with a suitable dosage of polyelectrolyte, run the system without bleed off, and completely empty and refill the system once or twice per month. This means that control is extremely simple and water losses are kept to a minimum.
Many small, open recirculating systems are subject to corrosion either because the makeup is naturally corrosive or because the concentration of aggressive constituents such as chloride takes place. Pick up of acid gases from the atmosphere can also cause corrosion even where the makeup water is non-aggressive. Corrosion of the iron surfaces by water is associated with oxygen attack at breaks in the normally protective oxide film which is formed on the metal surface. Since water in an open recirculating system is constantly aerated by passage over the cooling towers, it is impracticable to tackle the problem by trying to remove the dissolved oxygen from the water. The system of treatment adopted is based upon repairing weaknesses in the protective oxide film, thus suppressing the dissolution of the metal. Sodium nitrite is used in this connection and has been successful in many and varied applications. A disadvantage formerly experienced with the use of sodium nitrite as a corrosion inhibitor is the bacterial oxidation of nitrite, resulting in a loss of treatment. However, this problem has been solved by using a combination of sodium nitrite and a bactericide; the bactericide prevents the oxidation of the sodium nitrite and also has a sterilising effect on biological growths in cooling systems. This combination has also been applied with considerable success in chilled water systems used for cooling milk in dairies, plattens in plastic presses, and air cooling in air conditioning units.
Sodium nitrite is also used as a corrosion inhibitor in closed systems such as diesel engine cooling systems. Corrosion in the closed system is influenced by the limited supply of dissolved oxygen. Dissolved oxygen entering with the filling water can initiate corrosion when the limited dissolved oxygen which remains in the circulating water may provoke vicious pitting.
The addition of a controlled do age of sodium nitrite maintains the protective oxide film on the metal in good repair. The sodium nitrite inhibitor operates most efficiently in the pH range 7 to 8·5. It is, therefore, necessary to guard against low pH values being obtained in the circulating water due to the pickup of small amounts of exhaust gases.
Sodium nitrite does not protect galvanising and its use is not recommended in systems that are galvanised unless attack on the galvanising has already occurred. In this case, or if galvanised components form only a small part of the system, it is preferable to sacrifice the small amounts of galvanising and use sodium nitrite for protecting the rest of the system. There is also some evidence that sodium nitrite can react with lead and lead-bearing alloys such as solder but experience shows that this is of negligible proportions at the temperature normally experienced and some systems with soldered joints have been running on this treatment for t years without any difficulties.
Dissolved Oxygen Removal
The treatment of water for use in steam boilers is more complex than that just described for cooling systems. Dissolved gases (especially oxygen) in a boiler feed water can cause corrosion in the feed system, economisers and boiler. When the feedwater enters the boiler, the dissolved gases are liberated and pass over into the steam and condensate system, again giving rise to corrosion. Hardness salts in the feed water are precipitated in the boiler to form scale. Soluble salts will concentrate in the boiler and, if the level of concentration is not controlled, will eventually cause carryover of boiler water into the steam space.
The quantity of dissolved oxygen in the feed water can be kept to a minimum by maintaining as high a feedwater temperature as is consistent with efficient boiler feed pump operation, thereby reducing the quantity of anticorrosion chemical treatment which will be required. In high-pressure boilers, mechanical deaeration plant is usually included in the feed system but, even in this case, chemical treatment is necessary to remove the last traces of oxygen. The treatment chemicals (oxygen scavengers) used are sodium sulphite and 35 percent hydrazine, the choice between these two depending upon both technical and economic considerations. The latter is usually used in high-pressure boilers where dissolved solids must be kept to a minimum and where the conditions are such that decomposition of sodium sulphite with the formation of hydrogen sulphite might occur. The decomposition products of hydrazine under boiler conditions are ammonia and nitrogen. Although ammonia normally reacts with copper, oxygen must be present for the reaction to proceed and the ammonia produced by the decomposition will not cause an attack on copper alloys n the system because oxygen is absent.
Hydrazine reduces the ferric oxide film on the boiler surface to magnetite and when first used in a boiler this reaction proceeds until all the ferric oxide has been reduced. Magnetite reacts with dissolved oxygen, being converted back to ferric oxide, and thus may act as a reserve of oxygen scavenger in the boiler.
In low-pressure boiler plants, where a mechanical deaerator is not used, the quantity of hydrazine required to remove all the dissolved oxygen makes its use uneconomic. Since in these plants the critical level of dissolved solids is considerably higher, the use of sodium sulphite is a more practicable proposition.
Complete removal of oxygen by sodium sulphite may take between two and twenty minutes, depending upon the plant conditions. During the reaction time, dissolved oxygen may be participating in corrosion processes and for this reason, it is essential in some plants that the reaction is completed more quickly. An accelerated reaction is obtained by using anhydrous-sodium sulphite with which is blended a powerful catalyst of the sulphite oxygen reaction. The presence of the catalyst enables the complete removal of dissolved oxygen in less than twenty seconds.
It has been known for corrosion to start in new boilers before commissioning (e.g. if the boiler has been tested and then left in a wet condition) and once started such corrosion can be difficult to suppress. One approach to this problem is to treat the new boiler plant with hydrazine during the commissioning and first few weeks of a boiler run. This is usually sufficient to suppress any incipient corrosion and provide a protective layer of magnetite in the boiler.
Treatment for the prevention of scale formation in boiler plant maybe by removing hardness salts before the water is fed to the boiler, or by precipitating the hardness salts inside the boiler as a mobile sludge which may be removed by blowing down. External treatment can be by ion exchange or precipitation methods.
When internal treatment is used, alkaline treatment is added to the feedwater, or directly injected to the boiler shell, to precipitate the hardness salts in a free-flowing form so that they do not adhere to the boiler metal. When sludge conditioning is inadequate, the resultant adherent deposits necessitate longer periods for cleaning and short steaming runs in addition to the increased risk of overheating of the boiler metal. By the use of polyelectrolytes, the physical properties of sludges are so modified that they become non-adherent.
Good control is essential to successful water treatment. Regular control tests must be carried out to ensure that satisfactory levels of treatment are maintained.
In the case of boiler water treatment, the quantity of chemical treatment and the amount of blowdown must be related to the steaming conditions of the plant. With the modern tendency towards automatic control of the boiler plant, there has been the development of automatic proportioning of chemical treatment, although regular control tests must still be carried out to check conditions. The quality of water blown down must be accurately controlled d as excessive blowdown will result in a loss of treatment chemicals and a failure to maintain the required treatment level. Conversely, too little blowdown will result in an excessive concentration of dissolved and suspended solids in the boiler which could lead to deposition and carry-over. A system of automatic intermittent blowdown has been developed to enable accurate control of blowdown to be maintained. If such a system is used in conjunction with automatic chemical dosing equipment and regular checks made to ensure that the required level of treatment is being maintained, this boiler plant can be kept at optimum efficiency with a minimum of attention to water treatment.
The selection of correct water treatment for a particular application involves many factors and consultation with a reputable firm of water treatment specialists is necessary to ensure a sound, up to date approach.