In the order that they are presented on the Water Treatment Technology certificate of analysis, the items of importance in reviewing raw and process water quality are :-
- In the vast majority of raw waters, calcium is the main source of hardness. Magnesium, which also causes hardness, is usually present in smaller quantities than calcium. For convenience calcium & magnesium are usually combined and shown as one figure ie Total hardness. In boiler and cooling water systems calcium salts are the main cause of scale. Magnesium will form hydroxides or silicates in boiler water. These can be dispersed readily. If the boiler alkalinity is low magnesium phosphate scales can form. In cooling water systems magnesium precipites will only form at pH greater than 10. Although calcium is primarily known for its scale forming tendencies, it also acts as a cathodic corrosion inhibitor. In other words, all other factors being equal, a water containing calcium hardness is not as corrosive as a soft water.
P - ALK
- This parameter is used to cover all OH (hydroxide) alkalinity plus one half the carbonate alkalinity (see below). It is rarely present in raw water. The measurement corresponds to an end point of pH 8.3. As cooling water 'concentrates up' this parameter will increase.
- This parameter is important since it represents a potential source of scale. It is not a measure of the concentration of a specific ion in the water but rather the amount of acid required to neutralise the water (End point pH 4.3). Primarily it covers bicarbonate, carbonate and hydroxide. Also included but of lesser importance, are phosphate and silicates. In raw water total alkalinity is usually a measure of the bicarbonate present, a small amount of carbonate might also be present while little or no hydroxide is available.
- Also referred to as 'free caustic' or 'caustic reserve', results from the decomposition of boiler water carbonates & bicarbonates to produce the hydroxide ion. This usually leads to a boiler water pH in the range 11 - 12. If this is used as a form of alkalinity control it is important to keep internal surfaces clean & free from deposits and to limit the total alkalinity to 10 - 20% of TDS. This will prevent promotion of caustic gouging, embrittlement & carryover.
- A measure (on a scale of 0 - 14) of the degree of acidity or alkalinity of water. Most often raw water will be in the range of 6.5 - 8.0. Above or below this range would indicate contamination. A low pH can result in corrosion of metals while a high pH can result in scale formation.
- This parameter is an all inclusive figure covering dissolved salts. It is a general indicator of the 'corrosiveness' of raw water and also serves to indicate cycles of concentration in both cooling and boiler water systems. TDS measurement also serves to indicate if 'carryover' takes place in boiler water systems. TDS is generally determined by measuring Conductivity. Conductivity measurement is a convenient way to detect change in raw or process water composition. Any major change is likely to be accompanied by a change in TDS/conductivity.
- Scale is not a problem where chloride salts are concerned, but chloride salts are very corrosive in an oxidising environment. Chlorides are present in virtually all raw waters. In analysing corrosion problems the chloride content should be noted. Chlorides are prominent in crevice corrosion and pitting. Austenitic stainless steels are subject to pitting, crevice corrosion, stress corrosion cracking and intergranular corrosion from chlorides. In all cases of design or operation, steps should be taken to prevent or mitigate the concentration of chlorides.
CONCENTRATION FACTOR & CALCIUM BALANCE
- Cycles of concentration are generally indicated by the conductivity measurement. More precisely they can be calculated by the ratio of calcium and chloride ions in raw and process water. Where the feed water has not been softened the ratio between the calcium and chloride concentration factors can be used to calculate calcium balance. The latter is an important indicator for the scaling condition of the water.
- This is one "index" among others of the scaling tendency of raw and cooling waters. It is calculated from TDS, Water Temperature, Calcium Hardness, Total Alkalinity, and pH. The tendency to form scale is indicated, but the amount is not. The index is not effecient in indicating corrosion tendencies.
COOLING WATER INHIBITORS
The concentration of dissolved solids in open recirculating water systems can lead to calcium carbonate scale formation. Two approaches can be taken to the problem. In the first acid can be added to lower both the pH and alkalinity of the cooling water. This increases the tendency of the water to corrode metal systems. The second and preferred method is to continuously add a scale inhibitor. These materials (commonly polyphosphates or phosphonates) interfere with the crystalline growth of calcium carbonate (the scale).
Polyphosphates present no toxicity problems and are commonly used in municipal and other potable water systems. The reversion of polyphosphate to orthophosphate prevents its wider use in open recirculating systems. Where they (the polyphosphates) are used in open recirculating systems it is for their corrosion inhibiting properties rather than their scale inhibiting properties.
Phosphonates are widely used in open recirculating systems to control scaling. They are more effective than polyphosphates and much more stable. Low molecular mass polyacrylates and sulphonated styrenes have also been used to control calcium carbonate deposits. In all cases the inhibitor selected must be evaluated in the light of individual plant conditions.
- Ground or surface waters seldom contain large amounts of phosphates. If present, phosphate generally indicates fertiliser runoff or pollution. Phosphate from raw water can be the cause of scale problems in open recirculating cooling water systems after the water is concentrated. Polyphosphates are used as scale inhibitors. In solution they revert to ortho phosphates and this tends to limit their use. The end product, ortho phosphate tends to form scale with calcium.
- Phosphonates are primarily known as scale inhibitors. When used alone they have only mild corrosion inhibiting properties. Combined with zinc or polyphosphates the corrosion inhibition is excellent. High phosphonate (>20ppm) dosages should not be employed because they attack copper and will lead to problems where there are mixed metal systems particularly those involving aluminium. Aminoethylene phosphonate (AMP) should not be used where continuous chlorination is used since it decomposes in the presence of the latter. Hydroxyethylidene Diphosphonate (HEDP) is a better alternative since it is relatively stable in the presence of chlorine.
- The chief attraction of molybdates is that they are non toxic. When used alone they are not as effective as other inhibitors. Where used they are used in low concentrations with other inhibitors. They are ideally suited for use in cooling water treatment.
- For closed systems nitrites are effective inhibitors. Their main disadvantage is their susceptibility to microbiological attack. Nitrites in closed systems should be operated at alkaline conditions. They therefore are not suitable if aluminium is present.
BOILER WATER TREATMENTS
- The addition of phosphate is probably the most widely used type of chemical treatment for industrial boilers. Formation of calcium phosphate as a non-adherent free flowing 'sludge' is an effective method of handling calcium hardness. It is often used together with 'caustic alk', an oxygen scavenger (eg. tannin or sodium sulphite) and an organic sludge conditioner.
- These can make excellent boiler water inhibitors. They are water soluble vegetable products (usually derived from trees eg Quebracho). They act as oxygen scavengers, metal passivators and sludge conditioners. This multi-function capability makes them very usefull in the treatment of hot water heating and low pressure steam boilers ( below 450psi).
- Sodium sulphite is a commonly used oxygen 'scavenger'. The reaction with oxygen is slow at low temperatures so the use of a catalyst to speed up this reaction is important. At high boiler pressures sodium sulphite decomposes and so tends to be limited to boilers below 900psi.
- These are used to control the corrosion in steam condensate lines. The purpose is to prevent the combination of carbon dioxide, water and oxygen from attacking the steam return lines. This takes the form of either neutralising the acidic solution of carbon dioxide in water or forming a film to prevent its contact with the metal surface below.