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11TH FEBRUARY 2019 Dear students, Excuse me for the long delay in continuing the lessons on boilers. Ihad been laid up with cough which prevented me from sitting on the computer. We will now continue with the boilers lessons.

1. BOILER WATER TREATMENT

Goal Objectives :- The objectives of boiler water treatment are twofold:

(1) To prevent formation of scale, since scale is an insulating medium which leads to overheating of the heating surfaces and subsequent failure.

(2) To prevent corrosion or wastage of the internal structural components of the pressure vessel.

Sources of water:- The main sources of fresh water for the boiler are (1) fresh

water from seawater made by the F/W generator, (2) shore supply of fresh water.

Quality of fresh water :- The quality of F/W made from the F/W generator is normally very good varies on an average from 2 to 10 ppm depending on age of the vessel and is even better than shore supply F/W A question which comes to our reasoning is why

treatment of boiler water is necessary when the source of water is so good, The answer is very obvious in so far that we are discussing the quality of water in the boiler which is different from the water stored in the feed tank. The water in the boiler is in continuous evaporation mode from where the steam generated is supplied to the various utilities. In a boiler plant leakages of steam do occur and that leakage is made good by make up or extra feed water. Now imagine that the average make up feed per day is 5 tons of water. If the quality of the water in the feed tank as made by F/W generator is say 2 ppm, then on a daily basis 10 grams of salt is introduced into the boiler which does not come out with the steam and is accumulated within the boiler making the boiler water denser with advance of time . If no action is being taken for a considerable period of time this water will become as

dense as sea water. Hence this is the reason for the need for boiler water treatment.

This reasoning also leads us to the first functional objective of boiler water treatment which suggests good housekeeping ie reducing the steam leakages in the plant so that consumption of make up feed is reduced, thereby reducing the use of treatment chemicals.Explanation of PPM :- The various measures of the chemical strengths stated in this subject are in terms of PPM and hence a proper understanding or meaning of this term is necessary. The expansion of this term is parts per million but that does not give us a clear or determinate answer.

Imagine a Sintex water tank in the form of a cube of internal dimensions 1M*1M*1M. This tank can contain pure or distilled water of mass 1tonne=

1000litres weighing 1000kg or 1000000 grams. If we have a table spoon of 1 gram measure and we put two tablespoons of salt in this water the strength of the salt in the water is given by 2/1000000 or 2 PPM. Hence PPM is a mass/mass ratio describing the strength of the mixture. It need not be a solution as can be seen when we express the limitation of oil pollution in sea water as per MARPOL ANNEXE1 ,the limitation is expressed as i5 PPM , which means 15 gms of oil per tonne of sea water. In this case oil does not dissolve in sea water, hence it is the strength of the mixture. Another way of expressing it is milligrams/kilograms

The theoretical density of sea water as stated in naval architecture text books is given as 1025 grams/litre. This expression can be rewritten in terms of strength as 25gms of salt/ 1000gms

of water. By multiplying the numerator and the denominator by 1000 each the value does not change and the same becomes 25000/1000000 or 25000 PPM .Hence the theoretical density of seawater is converted in terms of strength of solution as 25000PPM.Salts found in water and their classification:- The salts naturally found in water that affect the boiler are inorganic salts of calcium, magnesium and sodium. Their behaviour and properties are based on the negative radical forming the salt.(1) Alkaline salts :- These are the carbonates of calcium and magnesium, Caco3, and Mgco3. They dissolve very slightly in water and exhibit alkaline qualities. They have a PH value between 7 and 14 .These salts do not exhibit hardness because they dissolve very slightly in water.(2) Scale forming hardness salts:- These are the sulphates of calcium and magnesium’Caso4, and Mgso4. They precipitate early during evaporation of water as hard scale adhering strongly on the heating surface.

(3) Temporary hardness salts:- These are the bicarbonates of calcium and magnesium, Ca(HCO3)2 , Mg(Hco3). On heating they get transformed to carbonates with liberation of carbon-di-oxide.(1) Ca(Hco3)= Caco3 + Co2 + H2O(2) Mg(Hco3=MgCo3+ Co2 + H2o

(4) Acidic hardness salts :- These are the chlorides of magnesium and calcium , and the nitrate of magnesium. Under certain conditions within the boiler these salts get converted to hydroxides with the evolution of the respective acids Hydrochloric and nitric.(5) Salts of Sodium:- These salts are harmless. They precipitate as soft scale easily removable and they are not hard.

The above classification suggests a clue which is the second functional objective and that is to convert all hardness salts of calcium and magnesium to equivalent soft salts of Sodium. This is the basis of chemical treatment.

ExplanatIon of Hardness :- In the classification of salts we have used the word

hardness very freely. Hence this word needs explanation.

Hardness with reference to water defines a degree of lather formation when soap is dissolved in water. It is a comparative quality and plays a very important role in boiler water treatment.

Water is said to be hard when it reacts with soap to form a white scum without producing a lather easily with soap. Hard water contains much matter especially salts of calcium and magnesium which are present mainly as the bicarbonates, sulphates and chlorides.

Soap consists of sodium and potassium salts of certain organic acids derived from fatty acids such as stearic, palmitic and olaeic acids. these salts are soluble in water and lower its

surface tension so that the solution possesses the power to emulsify fats and loosen dirty particles. Soap solution solution reacts with soluble calcium and magnesium salts of the above mentioned fatty acids by double decomposition

It is the insoluble salts which form the scum when soap is added to water and stop it lathering. No soap is available to form lather until all the calcium and magnesium salts have been removed in the scum..Hard water therefore causes a considerable waste of soap . It should be noted that calcium carbonate forms negligible ions of calcium since it dissolves very slightly in water and therefore does not exhibit hardness.Analysis of salts found in sea water and natural fresh water as found in rivers and lakes:- The analysis of salts found in seawater and natural fresh water needs to be assessed as it helps in

determining the amount of treatment required depending on the source of water.

Analysis of salts in sea water:-

FIG 4

The above table is an average representation of salt content in the sea and it does vary from region to region.

From the above table we observe that the largest constituent is common salt (Nacl), which is about 80% of the total salt content. The total salt content is 31.4 kgs /ton of sea water.

Analysis of salts found in natural fresh water:- Table 5From the above table we observe that the largest constituent of the total salt content is calcium carbonate which is about 50% of the total salt content. The total salt content is 405 grams per ton of fresh water and it is about 80 times less than salt content of sea water. The

reason why calcium carbonate is the largest ingredient is that the beds of almost all rivers and lakes are covered by some form of lime stone which is calcium carbonate. The flow of water in rivers causes the erosion of lime stone deposits aiding in the process of dissolving. The presence of calcium carbonate in the water gives an alkaline taste to the water which makes it palatable and hence pure lake or river water is called as aqua –minerale or mineral water. On board ships, the water distilled from sea water cannot be consumed until it is passed over a bed of calcium carbonate (chalk) in the distiller where it is also treated with chlorine to disinfect the water.Water spectrum:- water as it is found in nature exits in at least five states which

are:

(1) At one end of the spectrum we have the sea water with maximum concentration of dissolved salts.(2) By the heat of the sun the sea water evaporates rises , forms clouds of pure distilled water, which falls down as rain. This rain water is at the other end of the spectrum and if it falls in a location where the atmosphere is not polluted, it comes down as pure distilled water. At sea therein water trapped in clean vessels is good distilled water and can be used to top up batteries

The rain water falling down on mountains forms glaciers and gives perennial supply to rivers, water caught in lakes and rivers is the second in order after pure rain water.

(3) The rain water falling on dry land goes down into the earth and collects as ground water which can be pumped out from bore wells.(4) The water which collects in the delta regions of the rivers is the grade slightly better than sea water. These waters can be named Estuary waters.

The salts found in all these waters is the same but the concentration increases from 0 ppm to a maximum of 31400 ppm as the grade changes. The source of water if known aids in determining the treatment required.Mechanism of scale formation:- (Fig 43 & 44) Consider the water stored in a vessel which is being heated. as shown in the figure. Since the water has some impurities it forms a bubble at the heating surface caused by reduction of surface tension due to the impurity. As the temperature rises the bubble filled with water vapour increases in pressure and gets free from the surface, and rises up. At the fringe of the bubble the salts dissolved in the water deposit on the fringe in the shape of a ring of scale. When the bubble gets free it rises up and the space made vacant gets filled up by water from around the bubble. This coating of rings initially develops into a fully scale coated surface as the entire heating surface gets deposited with scale rings. This

phenomenon can be observed in vessels which are used only for boiling water. Their internal surfaces are

covered with a whitish layer of scale.

Effect of temperature in the dissolving of salts:- Most of the existing salts which are crystalline in nature and dissolve in water easily. Their concentration increases with increase of temperature as can be seen from the graph showing the increase in concentration of the salts with increase of temperature. It will be observed that all the curves show a tendency to rise with increase of temperature.(FIG 46) However the salts calcium sulphate and magnesium sulphate behave in a different manner. From 320 F to 1000 F the concentration increases, and from 1000 F the concentration reduces to zero at boiling point which is 2120 F. The salts of calcium and magnesium sulphates are entirely deposited as scale

which is hard and adheres strongly with the heating surface(Fig 45)1. Evaporation of sea water:- During the evaporation of sea water the salts which deposit first are the sulphates of calcium and magnesium, the next are the chlorides of calcium and magnesium, next the carbonate of calcium and magnesium and the last to come out as soft scale is common salt (NaCl). Common salt deposits when the concentration reaches 7 times its original concentration ie when the density reaches 1175grams/litre of sea water. This property is used to advantage when operating the boiler with seawater feed in an emergency situation

FIG 44

Fig 45

Fig 46

Corrosion:- Corrosion or wastage is of three types as

encountered in ships :

(1) Air corrosion or rusting which is continuous is caused by oxygen forming ferrous or ferric oxide.

(2) Acidic corrosion caused by formation of chlorides which liberate HCl causing corrosion of the steel internal parts.

(3) Galvanic corrosion caused by the passage of electric current in an acidic medium such as sea water when two dissimilar metals are immersed in it.

Of the above three mentioned types the first and second types are commonly encountered in boilers, whereas the third type is encountered in coolers and condensers.Acidic corrosion :- The process of acidic corrosion occurs in a boiler when the steady state condition, whilst the boiler is steaming, is caused when sea water enters the boiler and disturbs the balance. The entry could be caused by leaky condenser tubes or by a priming F/W generator. The chemical process involved is as described as under.

(1) Excess of Nacl along with other impurities enter the boiler along with the condensed feed water and given by the following equation:2Nacl + Mgso4 = Mgcl2 + Na2SO4 (1)

(2) Magnesium chloride is soluble in water and on heating in the boiler forms Mg(OH)2 and hydrochloric acid asMgCl2 + 2H2O = Mg(OH)2 + 2HCl (2)

(3) Hydrochloric acid reacts with the iron in the boiler to form ferrous chloride.Fe + 2 HCl = Fecl2 + H2 (3)FeCl2 + 2 H2O = Fe (OH)2 + 2Hcl (4) (on heating )

It can be seen that the result of the attack of acid upon iron is to produce a chloride of iron which breaks down to form an iron hydroxide with regeneration of hydrochloric acid. This is the first stage of corrosion and the spots where ferrous hydroxide is formed is pitted. This ferrous hydroxide then gets converted to ferric Oxide by combining with oxygen which enters the boiler as dissolved oxygen in the feed water.

This gives rise to the third functional objective .

The boiler water should be moderately alkaline to prevent any stray acidic attack by sea water leakage.Caustic embrittlement:- The alkalinity in the water should be moderate. If it is excessive it leads to caustic embrittlement. The phenomenon of caustic embrittlement or inter-crystalline fracture is believed to be caused by high concentration of caustic soda

and the material under stress (boiler under pressure). The stress corrosion cracks the grain or crystal boundaries of the material and failure of the affected part could result. Concentrations of sodium hydroxide required for embrittlement to occur vary with operating conditions.

A guide to the amount of concentration of sodium hydroxide required for embrittlement to occur vary with operating conditions and roughly about 86000PPM is required. Normally such concentrations would never be found in a boiler, but leakages at tube ends, boiler mountings, manhole doors, whereby water is flashed off to steam, leaving behind the solids locally, can cause the high concentrations required. Caustic embrittlement defects are mainly exhibited as radial cracks at tube ends on the tube plate. The material of the tube plate between the cracks is brittle and breaks off when hit with a hammer.

Effect of temperature in the dissolving of salts:- Most of the existing salts which are crystalline in nature and dissolve in water easily. Their concentration increases with increase of temperature as can be seen from the graph showing the increase in concentration of the salts with increase of temperature. It will be observed that all the curves show a tendency to rise with increase of temperature.(FIG 46) However the salts calcium sulphate and magnesium sulphate behave in a different manner.

From 320 F to 1000 F the concentration increases, and from 1000 F the concentration reduces to zero at boiling point which is 2120 F. The salts of calcium and magnesium sulphates are entirely deposited as scale which is hard and adheres strongly with the heating surface(Fig 45)

Carbon dioxide :- Carbon dioxide exists in small amounts in the atmosphere. Being heavier than air it remains at the lowest level when the atmosphere is stagnant. In feed tanks as well as ballast tanks, the atmosphere above the water level being stagnant splits up into three layers. The higher strata is occupied by nitrogen being the lightest, the middle strata is occupied by oxygen being heavier than oxygen, and the lower strata is occupied by carbon dioxide being the heaviest. This strata of carbon dioxide is in contact with the water as found in a freshwater feed tank. If the water contains dissolved carbon dioxide, carbonic acid may be formed, which can cause corrosion.

The carbon dioxide may have been absorbed into the feed water due to contact with the atmosphere, it can also be formed due to breakdown of bicarbonates and carbonates present in the feed. Carbonic acid partially dissociates into hydrogen ions and bicarbonate ions, hence the hydrogen ion content of the water is increased. The bicarbonate ions can combine with the ferrous metal to form ferrous bicarbonate which dissociates into ferrous carbonate and carbonic acid, which is re-dissolved into the water. If there is a supply of dissolved oxygen in the water the ferrous carbonate is converted into ferric hydroxide with regeneration of the carbon dioxide. Thus the process may be a continuous one providing there is a continuous supply of dissolved oxygen in the water. This reaction due to carbon dioxide is represented below in a simplified form. CO2 + H2 O =H2CO3 (1) (reversible reaction)

Fe + H2 CO3 =FeCO3 + H2 (2) 4FeCO3 +O2 + 6H2O = 4Fe(OH)2 + 4CO2 (3)Fe(OH)2 = 2Fe2O3 +6H2O (4)

This is how the ferrous hydroxide converts to the final rust ferric oxide. The moderate alkalinity prevents the formation of carbonic acid which is weak and unstable. Chemical treatment :- Before the advent of phosphate treatment the medium and low pressure boilers were treated with either (1) caustic soda or(2) a mixture of soda and lime. Both these chemicals which are strong alkalies known as hydroxides achieved the required functional objective of converting the hardness salts to equivalent sodium salts, but with the converted salts all being alkaline substances which resulted in high alkaline levels, thereby resulting in caustic embrittlement of the boilers. The repairs took a long period to complete since these boilers were of the riveted type and so the delays. In some cases the repairs were considered uneconomical and hence some

ships were scraped even before their due date. From the chemical equation reproduced here for caustic soda and soda - lime treatment we can visualise the excess alkalinity. Caustic soda treatment:- MgSO4 +2 Na OH=Mg(OH)2 +Na2SO4 ( The magnesium hydroxide is a strong alkali and dissolves in water and hence causes increase in residual alkalinity. Soda lime treatment.:- MgSO4 + Ca (OH)2 =Mg(OH)2 + CaSO4 Ca SO4 + Na2 CO3 = CaCO3 + Na2SO4 (The underlined salts are alkaline salts which increases the alkalinity level.) The use of sodium phosphates for the chemical treatment of boiler water was observed to be harmless and hence this treatment was recommended by the British standards institution as early as 1957. Phosphate treatment:- Phosphate treatment is now widely used for the conversion of all hard salts to equivalent salts of sodium. It has no side effects like increase in alkalinity levels which was a feature of the caustic soda and soda-lime treatment. However in this treatment there is a minor side effect of sludge formation, which is well taken care of by the use of coagulants which have no chemical side effects. The phosphate used is disodium or tri sodium phosphate. The phosphate radical of the sodium phosphate combines with the calcium and magnesium radical to become calcium phosphate and magnesium phosphate. Both these salts do not dissolve in water and hence cannot react chemically with any other salt in the water and therefore stable and has no side effects. Being insoluble in water, these salts settle down in the bottom on heating surfaces and hence retard the heat transfer. Hence the need for coagulants to keep the sludge in suspension in water as colloids. The relevant equations

of conversion using tri sodium phosphate are given below. (Table.6) Table.6

From above equations we note that the conversion of calcium and magnesium salts to equivalent salts of sodium is accomplished. It will also be noticed that the bicarbonates of calcium and magnesium are omitted. The reason is that the bicarbonates are temporary hardness salts and they convert to carbonates on heating and hence do not need any treatment. The coagulants as listed above do not chemically react with the converted phosphates but convert the sludge by coagulation. The suspended converted phosphates are removed in due course when flash blows are given to reduce density levels in the boiler water.

So far mention is made of the conversion of hardness salts to equivalent salts of sodium. No mention has been made of the entry of air into the boiler and the treatment of oxygen which is a major constituent of air. In open feed systems air can enter the boiler freely and hence no treatment is recommended for oxygen treatment in boilers having open feed system. Boilers operating with open feed systems work at low pressures and hence the risk of air corrosion is low and hence not dealt with. Where boilers operate with closed feed systems, air entry into the boiler is prevented completely by the use of de-aerators and air ejectors. Dissolved oxygen which enters the boiler with the make up feed is dealt by the following chemicals: BOILERS FOR MOTOR SHIPS 71

(1) Sodium sulphite

(2) Hydrazine

Sodium sulphite acts as a scavenger for oxygen having affinity for oxygen and works chemically as follows forming sodium sulphate which is harmless. It only contributes to the density of the boiler water which is not desirable. 2Na2SO3 + O2 =2Na2So4 Hydrazine is a gas denoted by H2N4 and has to be injected continuously into the boiler. It reacts with oxygen as follows: N2H4 + O2 =N2 + 2H2O The nitrogen liberated is harmless and cannot combine with oxygen at low temperatures as encountered in the boiler. Hydrazine if used in excess is bad because it liberates ammonia which corrodes brass fittings in contact with the water. Moreover ammonia is a poisonous gas. The reactions of excess ammonia are shown as under 3 N2H4 = 4NH3 + N2 2N2H4 = 2NH3+ N2 + H2 Hence hydrazine is used in boilers working above 42 bar working pressure and the excess hydrazine is controlled and monitored and should be maintained between 0.1 and 0.3 PPM . WATER TREATMENT CHEMICALS RECOMMENDED AS PER BS1170 OF 1968

Table.7

Testing of boiler water:- The testing of boiler water is complementary to treatment and must be done on a regular basis so that suitable corrective action or changes in treatment are done to suit the correction required. The various tests and their purpose is given as under. (1) TDS or density of total dissolved salts as the name suggests is the density of the water with dissolved salts. The water may contain un-dissolved salts such as phosphates of calcium and magnesium which does not matter. The density is therefore found by a hydrometer (salinometer) or a dipping type electronic density meter. The density is measured as soon as the water is collected in the sample jar and when the temperature reaches 940C. The density should be converted to ppm. The sample water is filtered and collected in a sample bottle for further tests.

The purpose of measuring TDS is to ensure that the density of boiler water is within the recommended level and if not to take suitable corrective action. (2) Hardness Test :- Take 100 ml of boiler water sample in a bottle with a glass stopper. Add 0.2 ml of standard soap solution (reagent) at a time shake vigorously after each addition of soap until a lather persists for a least five minutes. The bottle is laid horizontally so that a large area of surface is obtained and the thickness of the persisting lather should be at least 3mm.

The calculation in ml of soap solution used x X 10 = P.P.M. of CaCO3 equivalent hardness. Standard soap solution is a soap which is capable of reacting with one milligram ofequivalent CaCO3 to form a persistent lather for 5 minutes, 100 ml of boiler water sample used has x mgs of CaCo3. But x mgs of CaCo3 is contained in 100 ml of the boiler water sample.

Hence the concentration is (x/105) i.e. [(x / 1000) / (1/100)] or [(x X 10) / (1000 X 100X 10)] or 10 x PPM In all calculations, the result is given in P.P.M. of CaCO3 equivalent because the molecular weight of CaCo3 is 100 and it simplifies the calculations. The purpose of hardness test is to monitor the conversion of hardness salts to equivalent salts of sodium, and ensures that hardness does not exceed a predetermined maximum value. Alkalinity Test:-

Test for alkalinity to phenolphthalein: Take 100 ml of sample of boiler water. Add 10 drops of phenolphthalein – result a pink coloration. Titrate with (N/50) H2SO4 so as to clear the sample. Calculation ml. Of N/50 acid used X 10 = P.P.M. of CaCO3 equivalent Phenolphthalein is less alkaline than hydroxides or carbonates and when it is added in a sample containing hydroxides and or carbonates it will turn pink in colour. The acid used after this coloration will first neutralise the hydroxides, forming salts. It will then react with the carbonate molecules present forming bicarbonate molecules. Bicarbonate molecular are less alkaline than phenolphthalein. Hence, the pink coloration will disappear once all the hydroxides and carbonates have been dealt with by the acid one bicarbonate molecule is formed form two carbonate molecules. Hence, in this test the quantity of acid used is a measure of the alkalinity by the hydroxides present and half the carbonates. Total alkalinity: Continue with the same sample used initially for alkalinity to phenolphthalein add 10 drops of Methyl-

orange -- result. Yellow coloration. Titrate with N/50 Sulphuric acid until the colour becomes pink. Calculation: Ml. Of N/50 acid used for both tests x 10 = P.P.M. OF CaCO3 equivalent for total alkalinity. Methyl orange indicator is less alkaline than phenolphthalein and Bicarbonates. It can be used initially in place of phenolphthalein or as in more normal, as a continuation after the alkalinity to phenolphthalein test. If no yellow coloration results when the Methyl orange is added to the sample no Bicarbonates are present. Hence, no carbonates must have been present. Therefore the Alkalinity as determined in the Alkalinity to phenolphthalein test has been due to Hydroxides alone. Hydroxides and Carbonates can co-exist together in a soluation but Hydroxides and Bicarbonates cannot. For all practical purpose the Alkalinity to phenolphthalein can be considered as caustic Alkalinity (The difference is not much) and the residual Alkalinity, i.e. total Alkalinity minus. Alkalinity to phenolphthalein as Alkalinity due to carbonates. General Explanations for the Calculation: For complete chemical reaction we have:

CaCo3 +H2SO4 = CaSo4 + CO2 i.e. 100 ml. Of CaCo3 will combine with 98 mol. Of H2SO4 for complete chemical reaction. But both these substances are dissolved in water and hence we must relate them to their concentrations to obtain the correct mass. One normal solution is defined as the Gram equivalent weight of a chemical dissolved in water to form a litre of its solution. Equivalent Wt. = Molecular wt. / valency.

The valency of H2SO4 is 2, hence equivalent wt. Of H2SO4 = 98/2 =49.

Therefore one normal solution of H2SO4 is made by dissolving 49 gms of H2SO4 in 1000 ml. of solution. If “a” ccs of H2SO4 is used in the reaction, then the mass of H2SO4 used will be: a x (1/50) x (49/1000) gms of H2SO4 Let the mass of CaCO3 in the sample be denoted by x. Then x = a x [49/ (50 X1000) x (100/ 98)] = ( a /1000) gms but x is dissolved in 100 ml. of boiler water sample and hence in terms of “a” it will be (a/1000) X (1/100) which is [(a X 10) / 106] in P.P.M. Hence the quantity of (N/50) H2SO4 used X 10 = P.P.M. OF CaCo3 equivalent. CHLORIDE TEST: Continue with the same sample add 2ml. of Sulphuric Acid, add 20 drops of potassium chromate indicator. Titrate with N/35 Silver Nitrate solution until a brown coloration results. Calculation Ml. of M/35 solution used x 10 = P.P.M. of Cl. Or ml. of N/50 Silver Nitrate solution used x 10 = P.P.M. of CaCO3 equivalent for chlorides. Purpose:- Chlorides may be present in the boiler water sample. It is essential that they be measured as they would be an indication of salt water leakage from a leaky condenser or result of priming of fresh water generator. The Alkalinity to phenolphthalein sample taken has had the hydroxides and Carbonates dealt with and they will play no further part therefore in the test conducted for Chlorides. The sample is made definitely acidic by the addition of a further small quantity of acid. This is to speed up the chemical reactions which next take place. Silver Nitrate has affinity for potassium chromate and chlorides. Its principal preference however is for the chlorides. When it has reacted with the chlorides present in the sample, it is then free to react with the potassium chromate. In doing so, it produces a reddish brown coloration. It is therefore apparent that the amount of silver nitrate

solution used is direct measure of the chloride content of the boiler water sample. PHOSPHATE TEST (RESERVES) This test is conducted for boiler to determine the phosphate reserve in the boiler which is necessary to deal with the extra feed being taken in. Take 50m of hot boiler water sample, add 4 grains of potassium nitrate crystals, put 25ml. of the above into a test tube. Stand test tube in a water bath at 38 degree to 45 degrees C. Add 5ml, of ammonium molybdate reagent. A sample of boiler water which contains sodium phosphates and dissolved potassium nitrate will turn cloudy when ammonium molybdate is added. This reaction to produce a cloudiness in the sample is quicker the larger the quantity of phosphates present. If the cloudiness does not result until after five minutes have elapsed, the phosphate reserve in the boiler water is too low (below 20 P.P.M.). If the cloudiness results before two minutes have elapsed, the phosphate reserve is too high (above 70 P.P.M.). It will be appreciated that the sample of boiler water must be hot in order to dissolve effectively the crystals. SULPHITE TEST (RESERVE):- This is also a test to determine the sulphite reserve in the boiler to tackle the small quantities of dissolved oxygen. Take 100ml. of boiler water sample add 2 ml. of sulphuric Acid. Add 1 ml. of starch solution Titrate with potassium iodide–iodate solution until sample is blue in colour. Calculation:- M 1 of Iodide-Iodate solution used x 12.5 = P.P.M. of Na2SO3 in sample. The boiler water sample is made slightly acidic to speed up to chemical reactions which are to take place. Potassium Iodide-Iodate produces a blue coloration through reaction with starch, but it has a preferential chemical reaction with sulphite if it is

present in the sample. Hence when the potassium Iodide-Iodate solution has dealt with all the sulphite present, it is then free to react with the starch present in the sample, producing a blue coloration. It is therefore apparent that the amount of potassium Iodide-Iodate solution used is a direct measure of the sulphite content in the boiler water sample. As far as possible air should be excluded in the tes,. otherwise, an incorrect result may occur. If the test indicates that adequate reserve of sodium sulphite is present in the boiler water, there is no need to conduct a test for dissolved oxygen.

DESIRABLE TEST RESULTS FOR BOILER WATER Table.8 CORRECTIVE ACTION CHART Table

CORRECTIVE ACTION CHART

*1 The hardness test is a fairly accurate test and that determines the true state of conversion of all hardness salts. The phosphate reserve is a reserve quantity to deal with the extra feed flowing in continuously. (MAX) denotes the maximum permissible limit. *2 All reserve quantities have a higher and lower limit. *3 There are occasions when water taken from ashore is of unknown quality and in all probability it may be from bore well supply or estuary river water supply which has a higher hardness than natural fresh water. Hence the need to test the water for hardness. Naturally found f/w has an average hardness of 120 to 150 ppm, whereas tube well water may have a hardness of 300 to 400ppm and water at the river mouth may have a hardness of about 500 to 600 ppm. *4 Air leaks through drains and low pressure areas if the steam system. The air leaking in absorbs the sulphite reserve and hence the excess consumption of sulphites. Contamination of Boiler water by oil:- Fuel oil enters the boiler water space, when the heating coils in storage tanks, service tanks or daily service tanks are holed and oil finds its way into the observation tank, especially when the heating steam to the defective coil is shut, and from there into the boiler especially when the towel filters in the cascade tank are saturated with oil and filtering becomes inefficient. The oil will not reveal until the boiler is shut down and cold, when oil separates and rises above the water and can be seen in the gauge glass above the water level. All this will happen if the following activities are unattended: (1) Level in observation tank not seen/ cannot be seen. Reason water is in agitated condition caused by any or some of the steam traps not working and allowing steam entry into the observation tank.

(2) Oil level seen in observation tank but no action taken to find out which steam heating coil is leaking, and not taking suitable corrective action. (3)Filter towelling on filter cages not being changed for long time rendering the filtration inefficient and ineffective by the filtering medium getting saturated with oil.

Cleaning method :- The oil if allowed to remain inside can cause the scale formation on the heating surface hard and tenacious difficult for removal. The method adopted is chemical washing or cleaning. The water in the boiler is to be boiled with caustic soda or washing soda in the proportion of 25 kgs of caustic soda or washing soda per ton of water in the boiler. The boiling should be carried on for at least 18 to 24 hours. By keeping the vent cock open and not allow the pressure to build up in the boiler. The boiler warming up procedure should be followed. After the required period of boiling the boiler may be cooled down and contents drained into the machinery space bilges. On opening up the boiler and cleaning up the remaining water , it will be seen that the boiler surfaces are clean and free of oil. Washing soda converts easily into caustic soda when mixed in water and caustic soda has an affinity for oil especially vegetable oil. With mineral oil the affinity is not so much but it does form an emulsion with the oil which is an ancient derivative of organic decay. The equation for conversion of washing soda to caustic soda is given under: Na2CO3 + 2H2O = 2Na OH =CO2 +H2O References:- (1) Reed’s volume 8 (2)Milton’s Boilers(latest edition), (3) BS 1170 of 1968 Chemical treatment of marine boilers.

FUNCTION 4 ORAL QUESTIONS BASED ON BWT

(1) What are the objectives of BWT.(2) What are the sources of fresh water for

boilers in the ship.(3) What is the need for treatment when

the freshwater generator is producuing ample water with purity of about 2-3 ppm.

(4) Explain the term ppm as applied to boiler water.

(5) Name the hardness salts.(6) What are the soft salts.(7) QQWhat are the objectives of chemical

treatment .(8) How chloride assists in corrossion. show

by suitable chemical equations.(9) What is caustic embrittlement.(10) What are the bad effects of carbon di

oxide in water.(11) If the consumption of sodium hydroxide

treatment chemical is increasing , what does it indicate.

(12) If phosphates consumption increases what does it indicate.

(13) How does oil enter the boiler .(14) If it has entered how is it to be

removed.