C

C. GENERAL MAINTENANCE

FEED WATER AND BOILER WATER TREATMENT

CARE OF BOILER OUT OF SERVICE • • • • • • • • • • • -HYDROSTATIC TESTS • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • -

BOILlNGOUT ••••••••••••••'•""""""""""""""" WATER WASHlNG FIRESIDES • • • • • • • • • • • • • • • • • • • • • • • -ACID CLEANlNG • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •• • • • • • • • • • -

C-1

C-2

C-3

C-4

C-5

C-6

29

~) MANUAL 0-0

c- 1

(1)

C- I _ TREATMENT OF FEEDWATER AND BOILER WATER

Mitsubishi Heavy Industries' Nagasaki Shipyard & Machinery Works has a wealth of experience and excellent industrial research laboratory facilities to bank on as regards the treatrnent of feedwater and boiler water. It therefore is recommended that any questions or difficulties experienced as regards the treatment of feedwater and boiler water be referred to the Company for advice.

Control of Feedwater and Boiler-water Impurities With boilers in service, it is recommended that at least once everyday feedwater and boiler water be sampled for analysis and appropriate measures taken for qualitative control. When sampling boiler water, care is to be exercised so as to take the sample that truly represents boiler water, by for example cleaning the sampling vessel twice to three times with boiler water beforehand. Feedwater and boiler water sampled are to be thoroughly and carefully analyzed using an appropriate analysis equipment, it accordance with instruction given on the use of the equipment. Every effort is to be made to control oxygen, pH, salt, dissolved solids, phosphoric acid, etc. in feedwater and boiler water to within the specified l imi ts.

Requirements specified for the ship' s boiler as regards the control of feedwater and boiler-water impurities follow.

Feedwater (1) Oxygen :The oxygen content of feedwater is to be controlled

to below O. 5 ppm. (2) pH : The recommended pH value is 7. O to 9. O. For the pH

control purpose, however, it is recommended that the target be set at 8. 5 to 9. O.

Boiler Water (1) pH : pH of boiler water requires to be controlled

to 10.8 to 11. 3.

(2) Salt : The salt content of boiler water (as C1~) requires to be controlled to 300 ppm as far below as possible.

(3) Dissolved solids : Dissolved solids in boiler water require to be controlled to 2000 ppm and as far below as possible.

(4) Phosphoric acid : Phosphoric acid of boiler water (as P043~) requires to be controlled to 20 to 40 ppm.

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MANUAL 0-0 30

C-1 (2)

1. FEED WATER AND BOILER WATER TREATMENT

Limits of Chemical Concentration

Item Water B oiler Water Feed Water to Boiler

pH (at 25 []) 10.8 - 11 .3 7.0 9.0

Phenolphthalein Alkalinity (CaC03)

Max.500 ppm

Total Alkalinity (CaC03) Max.600

ppm Chloride (C I ~) ppm Max.300

Total Solid ppm Max.2000

Excess Phosphate (P043~) 20 - 40

ppm Hardness (CaC03) ppm Max. I .O

Oxygen ppm Max.0.5

Note : - (1) Feed water in this table means the mixture of condensate and distilled water to supply into the boiler.

(3) Estimate the pH value from alkalinity tends to give pH readings varying with silica, Ca, Mg, and other salt contents of boiler water and hence is not necessarily

deemed appropriate : resort to this method only as a means to obtain a rough guide (alkalinity serves merely as an auxiliary means in determining the pH level) .

Also, be sure to control pH to the target value while, on the other hand, keeping

alkalinity at the minimum necessary level.

Limiting the P alkalinity to within a certain range would make it possible to inhibit the alkali corrosion even if boiler water concentration should take place

on the heating surface, etc.

31 MANUAL 0-0

32

BOILER ANTISCALES AND THEIR INJECTION UANTITIES EX.

C-1 (3)

l . Boiler Antiscale

The following chemicals of Ameroid Co. manufacture are to be used. pH enchanting agent (GC) or KALGEN 459 P04 enchanting agent (ADJUNCT B) or ALCON 401

(1) GC Containing sodium hydroxide as its principal ingredient, this chemical is used for controlling pH of boiler water.

(2) ADJUNCT B This chemical, the alkaline antiscale with tri-sodium phosphate as its principal ingredient, is used for controlling P04 content of boiler water.

It serves to prevent the accumulation of scale deposits as well as to inhibit the boiler steel corrosion.

2. Injection Quantity (1 ) Initial injection

Antiscale Initial injection qt' t (g/ton) pH & P04 values

ADJUNCT B Approx. 80 Resulting pH & P04 values :

P04 : 20 ppm GC Approx. 25 PH : 10.8ppm

Notes : 1) The injection quantities indicated are calculated with

feedwater hardness at zero. 2) In the initial phase of operation, reaction with iron

content of boiler steel produces iron phosphate film on

the steel surfaces causing phosphoric acid to remain below the specified limit.

In such an instance, inject additional doses of ADJUNCT B .

(2) Makeup injection Accordin*' to the results of boiler-water analysis, makeup does of antiscales are to be injected with the values indicated in the following table as reference targets.

Antiscale Qt' y required to raise Qt'y required to raise

P alkalinity Phosphoric acid radical

(as CaCO ppm) by (as P043~ ppm) by

1 Oppm, g/ton 1 Oppm, glton

ADJUNCT B Approx.40

GC Approx.8 .

Notes : 1) The injection quantities indicated are calculated assuming distilled water to

be used as boiler feedwater.

2) The values given are mere theoretical targets and hence require to be controlled as appropriate for actual boiler load and feedwater quality so

that the desired boiler- water quality can be ensured in each particular application.

33

MANUAL 0-0 32

c- 1

(5)

3. Deoxidizing Agent For removal and deactivation of residual oxygen in the feedwater system, hydrazine (N2H4) is to be injected into the cascade tank outlet (or feed pump suction inlet) by means of drip-injection device.

In'ection uantit In case of I 0~~;o concentration of hydrazine solution.

Initial injection : Approx. 70 g/ton (at 30deg.C and atm. press.) Makeup injection : Approx. 45 g/ton (at 60deg.C and atm. press.)

Boiler Blowdown Blowing down boiler water by operating the blowdown valve and replenishing the amount of water blown down with fresh water is of vital importance of reducing the concentration of boiler water to satisfactory level for operation. Also, the boiler blowdown is just as important to discharging sludge, oily substance, and other impurities accumulated in the boiler .

The amount of boiler water to blowdown and how often to blowdown are to be decided based of factual data, such as the results of boiler-water analysis, turgidity of sampled boiler water, etc., so that the aforementioned requirements on boiler-water chemistry can be satisfied.

Makeu -water and Boiler-water Treatment The boiler disorders caused by inadequate control of boiler water and feedwater may be broadly defined as follows.

(1) Overheating of heat-transfer surfaces due to accumulation of scale and oil/grease deposits

(2) Corrosion (and caustic embattlement) (3) Turbine and other associated equipment disorders due to carryover.

Some restrictions, therefore, require to be imposed as follows in order to prevent the above disorders.

* To cope with (1) above, the hardness and oil/grease content of feedwater require to be controlled to within allowable limits, as does silicic acid contained in boiler water.

* To deal with (2) above, feedwater pH and boiler-water pH require to be controlled to

allowable levels.

* Salts defy the generalization, some serving to inhibit the corrosion and some promoting

the corrosion. Chlorine (chloride), however, generally promotes the corrosion when

present in a large amount and hence requires to be controlled to as low a level as is

practicable .

* To prevent the problem (3), dissolved solids and oil/grease contained in boiler water

require to be controlled to within allowable limits.

34

MANUAL 0-0 33

c-1 (6)

Restriction of Individual im urities

(1) Hardness The formation of scale deposits on the boiler drum and evaporating tube internal surfaces and the accumulation of sludge within the boiler are attributable, principally, to

hardness-constituting elements carried into the boiler by feedwater.

Leaving the feedwater supply as it is while obviously high in hardness and treating it in

the boiler has potential of having the impurities adhere to heat-absorbing surfaces and also can cause the rise in antiscale consumption and quantity of dissolved solids.

The best practice dictates, therefore, that the hardness of feedwater be restricted to within an allowable limit and what is left of the hardness removed through boiler-water treatment in the boiler.

Specifically, the hardness of feedwater for a I .57 MPa(16 kg/cm2g) boiler is to

becontrolled to I ppm. With makeup water produced by distilling seawater in a desalination plant, assuming impurities carried over through evaporation to be same as those in raw seawater (of total salts, C1=55,~o,Ca=1.2%,and Mg=3.7%),the hardness brought in per I ppm of chlorine (C) is 0.34 ppm. Then, even with the allowable limit of chlorine at 5 ppm, the hardness upon evaporation is 1.7 ppm, which suggests that judging from the rate of makeup feed, restricting the feedwater hardness to within the above mentioned limit is not difficult.

In cases where the distilled-water tank is coated with water cement (definitely not encouraged) or where there is leakage in the condenser, however, Iarge amounts of calcium and tnagnesium can eventually enter the boiler, warranting due precaution.

In some instances the required hardness is specified for boiler water. The hardness values suggested for boiler water are prone to error and often turn out to be unrealistic.

For this reason and also since the boiler-water hardness should remain about I ppm as 10ng as excess phosphoric acid and pH of boiler water are controlled to the values given in Table I , MHI does not make it its practice to specify the hardness requirements.

(2) pH Value (Alkalinity) The boilers are made ahnost entirely of steel, and iron, beside being susceptible to

heavy corrosion in acidic environments, dissolves in neutral pure water to produce iron hydroxide as follows.

Fe + 2H20 = Fe(OH)2+ H2 lron Water lron

hydroxide Hydrogen When iron has dissolved in pure water, pH with Fe(OH)2 at the saturation point is 0.9. The larger the pH value, I.e., the higher the alkalinity, the more sharply the amount of iron

dissolution goes down. It therefore is necessary for the pH value to be maintained constantly at a high level in order for iron being prevented from becoming corroded. A word of caution is deemed in order because pH at too' high a level can backfire in the from

of caustic corrosion or caustic embattlement or carryover.

Also, it is known that boiler water is not uniform in concentration throughout its body.

So, everything considered, the reasonable proposition is that pH of boiler water should be maintained at 10.8 to 1 1 .3.

Alkalinity may be deviled into total alkalinity and active alkalinity.

35

MANUAL 0-0 34

c-1 (7 )

Included in the total alkalinity reading are caustic soda, potassium carbonate, and potassium bicarbonate, all these being put together. The active alkalinity, on the other hand, called phenolphthalein alkalinity (or P alkali, in short), represents the amount of alkali high

enough in stren*'th to turn phenolphthalein indicator in color.

It is the active alkalinity that serves as a rough guide to the pH Ievel.

With all the foregoing in consideration, therefore, the phenolphthalein alkalinity as CaC03

is to be controlled to about 50 to 300 ppm when boiler water is in excess of 1000 ppm in concentration and to about 30 to 400 ppm when boiler water concentration is below 1000

ppm.

(3) Dissolved Oxygen The dissolved oxygen constitutes a single greatest factor in causing corrosion, being responsible for boiler steel corrosion in well more than 50% of the time.

In reaction between water and iron, which is defined as

Fe + 2H20 = Fe(OH) 2 + H2, hydrogen thereby produced is absorbed in metal surface to serve as a negative factor in the reaction. When oxygen is present, however, it causes hydrogen to disappear by oxidation, so the reaction progresses toward the right term of the formula with the dissolution of iron taking place continuously.

In addition, iron hydroxide turns into ferric hydroxide by being oxidized by oxygen, ferric hydroxide settling down to produce rust, thereby causing the corrosion of iron to

progress intermittently.

2Fe(OH)2 + 1/2 02 + H20 = 2Fe(OH)2

hydroxide hydroxide The pitting corrosion, found concentrated under the drum waterline, is attributable to the

dissolved oxygen. lron transported from the feedwater piping into the drum, plus iron oxide produced within the drum, accumulates as deposits or settles down to cause the pitting.

The presence of carbon dioxide gas together with dissolved oxygen in saturated-steam piping or condensate piping can cause a total corrosion.

Carbon dioxide gas is produced by dissolption of bicarbonate in makeup feed as well as by dissolution of sodium carbonate within the boiler.

2NaHC03 - Na2C03 + H20 + C02 S odium Sodium Carbon bicarbonate carbonate dioxide gas

Na2C03 + H20 - 2NaOH + C02 Sodium Caustic Carbon carbonate soda dioxide gas

35 MANUAL 0-0

36

C-1 (8)

The use of sodium carbonate as boiler antiscale, therefore, is not recommendable. Carbon dioxide gas dissolves in water and acts as carbonic acid but does not cause by itself

so heavy corrosion. If there is oxygen together, however, the effect of the dissolved carbon dioxide gas becomes pronounced, as it to play a role of some oxidizing medium as indicated by the following reaction formulas.

C arbonic Ferrous acid bicarbonate

2Fe(HC03)2 + 1/2 O + 5H20 = 2Fe(OH3) + 4H2C03 ... . ... (2) Ferric

hydroxide

The dissolved oxygen in feedwater, therefore, requires to be strictly controlled, preferably to below 0.25 ppm for 1 6 kg/cm2g boiler even though it is specified to be 0.5 ppm.

For removal of dissolved oxygen, it is recommended that volatile hydrazine be continuously added to feedwater. Hydrazine reacts as follows

N2H4 + 02 - N2+2H20

(4) Chlorine (Chloride) Salts, when present in large quantity, raise the electric conductivity of liquid to promote

the corrosion and hence require to be controlled to as low a level in quantity as is practicable.

Feedwater containing too large an amount of magnesium salt can produce the sediment of magnesium hydroxide upon entering the boiler, and this is said to raise the corrosiveness of boiler water by bringing down the boiler water pH value.

All the same pH and oxygen remain the greatest factors in causing the corrosion. and the

measurement of chloride is performed more or less to obtain a rough idea as to the amount of dissolved solids contained.

The relation between the dissolved solids and chlorine ion, though subject to some variations depending on the water quality or performance of the water-making device, can be established by measuring these elements in boiler water of each individual boiler

beforehand. Then, it is possible, as well as does no hanu for practical purposes, to estimate the amount of dissolved solids. It is assumed that the concentration of boiler water, as long as it is controlled using the amount of dissolved solids as a yardstick, can

not rise to such a level as to greatly promote the corrosion except in special instances.

The Feedwater Committee, therefore, sets forth no requirements in particular as to the allowable limit of chlorine.

37

MANUAL 0-0 36

c-1 (9)

(5) Oil/Grease

OiL!grease coming out of auxiliary machinery, especially those started in reciprocating motions, enters feedwater and then adheres to the boiler heat-absorbing surfaces to cause the

overheating trouble there upon exidation.

Also, since boiler water is alkaline, both animal and vegetable oils are liable to be emulsified

in it. The results are foaming of boiler water and degradation in steam purity.

Although the allowable limit is specified for the amount of oil/grease in boiler water in some

instances, it seems highly likely that such an oil/grease will be several-hundreds fold greater

in concentration at the boiler-water surface than the mean oil/grease content. As specifying

such a limit is deemed to make not so much of sense, it is only requested that feedwater and boiler water should be controlled so that the amount of oil/grease can be kept to zero as far

as is practicable.

(6) Silica (Silicic Acid)

The scale that contains silica is the poorest in heat transfer and thus the most harmful to boiler operation.

Also, what is called the silica carryover takes place, in which silica carried in steam enters

the turbine and deposits itself on the turbine blades as hard scale encrustation. This is the

single most nettlesome phenomenon, which warrants a serious consideration in connection with the care of high-pressure boilers. For a I .57 MPa(16 kg/cm2g) boiler, it is recommended that the silica content of boiler water

be controlled to below 50 ppm to cope with both the scale formation and silica carryover.

Silica acid should be no cause of concern so long as distilled water produced from seawater is used as makeup feed in nearly all the instances.

When water is obtained from land, however, necessary precautions are to be taken in this

respect.

(7) Dissolved solids

The larger the amount of dissolved solids in boiler water, the more dampened the steam becomes, to the point where a large amount of solids are carried in steam as carryover to the turbine.

The tendency of such a carryover varies with the steam evaporation rate, boiler type, and performance of the steam separator in the steam drum, as well as with constituents of solids even if the solids are same in quantity.

Although the relation between these factors still defies quantitative definition, at least a

rough standard can be grven for safe and practical operational purposes from the past experience. In normal operation, the amount of dissolved solids is to be kept to below 2000 ppm.

38

MANUAL 0-0 37

C-2. CARE OF BOILER OUT OF SERVICE

C-2 (1)

For the out-of-service boiler, exercise adequate care to minimize the corrosion of its pressure part and also inspect it closely.

In cases where the boiler is put out of service for 24 hours or more, either fill the boiler completely with water (wet storage) or drain the boiler of water and thoroughly dry it (dry storage). Whether it is the wet storage or the dry storage, the purpose is to eliminate air and dissolved gas from within the boiler for prevention of boiler internal surface corrosion.

Also, when placing the boiler in storage, be sure to keep the gas side completely clean. Any residual soot deposits on evaporating tube surfaces could absorb moisture from air and cause the surface corrosion.

Wet La -u Method The wet lay-up is preferable as it requires less preparation, the boiler can quickly be returned to

service, and protection of the waterside is adequate.

This method can be safely used for a lay-up of any length of time, if the fireroom temperature is

not below freezing.

(1) When the boiler is being cooled down after shutting off firing ,boiler compound (Na/P04=2.8 mole ration) of trisodium phosphate (Na3P04) and dosodium phosphate (Na2HP04) ,and hydrazine shall be added to the boiler water by the chemical injection system in such a manner as to make the boiler water of phosphoric acid (P043~) of about 50 ppm and hydrazine (N2H4) of 100 200 ppm (pH is

about 10.5-10.6). The boiler water should be kept in high alkalinity to protect the boiler from corrosion. Since the boiler water density during the wet laid-up period is very high compare with that of ordinary operating condition.

The boiler should be carefully blown down when starting operating to bring the boiler water concentration down to the normal value (with the boiler water treating linrits).

For this ,some amount of make-up water is necessary and the distilled water should be prepared beforehand accodingly.

(2) When the pressure has gone down to nearly zero, open the steam drum air-vent valve. (3) When the pressure is almost off the boiler, fill the boiler with distilled water until, it

issues from the air vent valves, then close the valve. (4) Put a hydrostatic pressure of 0.34(3.5 kg/cm2g) to 0.49 MPa(5 kg/cm2g) on the boiler.

Hold this pressure until the boiler has cooled to fireroom temperature, then bleed the boiler, using the air vent valve, to be sure all air is out. Hold a hydrostatic pressure of about 0.20(2 kg/cm2g)to O:34 MPa(3.5 kg/cm2g) on the boiler.

(5) Maintaining the alkalinity at a uniform level throughout respective boiler parts is an important consideration, so periodically sample boiler water for analysis during the storage and replenish what have been spent of alkalinity and sodium sulfite.

(6) In case the atmospheric temperature threatens to fall below the freezing point, take care to maintain the boiler room temperature at higher than 5 l~::: so as to prevent the boiler

water from freezing.

(7) When interrupting the wet storage to put the boiler back into service, bring down the

steam drum water level.

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MANUAL 0-0 38

c-2 (2)

Dr La -u Method In case the wet lay-up method cannot be performed, resort to the dry lay-up method.

(1) While the boiler still remains warm, drain out boiler water and open up the boiler for ventilation until completely dried internally.

(2) Remove the end plate of the waterwall lower header, to check and make sure that no residual water remains collected inside the header.

(3) If necessary, burn coke or charcoal in a container within the furnace to promote the internal surface drying.

(4) When completely dried, put quick line or calcium chrolide in a shallow dish for placement in the drum and header and then close the end plate and manhole cover. Use 2 to 5 kg of moisture absorbent for I OOO kg of boiler water when quick line is employed and I .8 kg of silica gel for I OOO kg of boiler water, as a matter of standard practice.

(5) Be sure to close securely all the air-inlet openings into the furnace and provide the cover

on the stack.

(6) Check the moisture absorbent every one to two weeks at the beginning and every one to three months thereafter at the circumstances call for and renew deteriorated absorbent.

Other Cares for Protection Do not forget the protection for the gas side, as well as for the boiler-water side. Have the

gas side cleaned of soot or dust while in preparation for the storage.

Soot or slug becoming moistened by moisture of air can cause the corrosion the gas side. Be absolutely sure to close the furnace and F.D.F. inlet vane and cover up the stack to prevent

the ingress of air or moisture.

Periodically open and close the F.D.F. inlet vane to make sure that it can operate successfully.

Maintain the boiler gas side and casing in the dry state as far as is practicable.

39 MANUAL 0-0

40

C-3

(1)

C-3. HYDROSTATIC TESTS

Hydrostatic test pressure should not be higher than necessary to prove the intended test. The maximum allowable hydrostatic test pressure should be carried out only for design pressure.

• See the hydrostatic test planning particulars included in this Manual for the maximum test pressure to employ. The specified pressure is to be applied to ensure the boiler structural integrity, only for inspection by applicable ship classification society surveyor.

(1) To check the boiler and fittings for leaks, a test pressure of about 85 percent of the safety

valve popping pressure is sufficient. Caution should be used to avoid accidentally raising the pressure enough to open a safety valve.

(2) Hydrostatic tests up to normal feed line pressure may be applied with the feed pump if handled carefully to avoid sudden changes in pressure, producing shock or impact stresses

in the boiler. For maximum test pressure use the test pump. (3) Before applying a hydrostatic pressure, it is advisable to cool the boiler to approximately

fireroom temperature. The water used for filling should be warmer than the boiler metal, to avoid moisture condensation on the fireside.

(4) Place test clamps (gags) on all safety valves, if the test pressure is to be higher than 85

percent of the safety valve popping pressure. Safety valves should never be opened by hydrostatic pressure.

(5) The pressure gauge to be used should be checked, before applying the maximum test pressure .

(6) When filling the boiler open the vent valves on the boiler drum, to bleed off all air, close

the valves when water runs out.

(7) Before lowering the pressure, take up the slack on the nuts of the inspection hole and manhole fittings, were new gaskets have been fitted.

The nuts should be pulled just snug with the wrenches supplied for the purpose. Do not use a pipe or other extension on the wrench handle.

(8) When inspection is completed, open the vent valves and lower the pressure slowly by cracking a drain valve.

(9) Remove the safety valve gags, replace the lifting lever and easing gear.

40 MANUAL 0-0

41

c-4

(1)

C-4. BOILING OUT

If the presence of oil is found on the waterside of the boiler, it must be removed by boiling out.

This is necessary after assembly of a new boiler, after completion of repairs requiring extensive replacement of tubes, or if oil has entered the feed water from some other source.

Boiling out is also a quick and efficient method of removing various types of scale. The chemicals to be used and the strength of solution required, depends on the character of the scale.

Consult the boiler water chemist.

Boiling out to remove oil requires the use of a fairly strong caustic solution.

One such solution is 2 kg of caustic soda and 4 kg of tri-sodium phosphate, for each I OOO kg of

cold water required to fill the boiler. This chemical solution is sufficient to remove ordinarily

compounded lubricating oils or the usual protective oil coating applied to tubes before shipment.

Straight mineral lubricating oils used for high temperature engines requires stronger solutions.

If such oil is present in the boiler used 4 kg tri-sodium phosphate and 4 to 5 kg caustic soda per

1000 kg of water. In addition it is advisable to add detergent (wetting agent) amounting to about 0.5 percent of the boiler water. There are other chemical solutions which can be used.

There are many satisfactory compounds for boiling out, they are sold under various trade names by reputable firms. When such compounds are used, follow the manufacture'sinstructions.

41 MANUAL 0-0

42

c-4 (2)

Steamin Method This is a method in which steam is injected by way of a temporary piping externally connected to the air-vent valve, for soda boiling. Water is to be discharged by way of the bottom blow-down valve and through a temporary discharge piping. The discharge piping is to be connected to the boiler water sampling tank, which is requires for inspection or analysis of boiler water.

(1) Inject chemicals into the waterwall tubing as required, using the chemical injection device.

Dissolve the chemicals in clean water, in proportion to the weight of water required to completely fill respective boiler parts, for injection. See the "planning particulars" included in this Manual for the weight of water required to fill each of the boiler parts.

(2) Upon finishing the injection of chemical solution, gradually blow in steam in such a manner as to permit condensed water to collect until the boiler is completely filled with water to the point where water begins to overflow by way of the air-vent valve.

(3) Maintain the boiler pressure at about 0.34 MPa(3.5 kg/cm g) and adjust each air-vent valve so that the quantity of overflowing water from each individual air-vent valve and bottom

blow-down valve will be approximately in proportion to the quantity of water in each corresponding boiler part.

(4) Determine the progress in removal of oil through test of sample water taken from the discharge piping. Analyze the sample water for alkalinity and then add chemicals required to maintain the chemical concentration at the specified level: by so doin*", continue the soda boiling until no trance of oil can be seen in the sampled water. The oil content can be detected by examining the surface of cooled sample water for sign of oil.

If possible, analysis by ether method, etc, should be in order.

42 MANUAL 0-0

43

C-4

(3)

Flfm Method If steam and electric power are available and the auxiliaries are ready for service a boiler can be

boiled out using a light fire. When the firing method can be used, it is much simpler and does not require temporary piping.

The quantity of solution to use should be figures for the weight of cold water required to fill the

boiler to normal steaming level.

(See "Design Data".)

(1) Dissolve the chemicals in water and inject into the boiler with the compound ejector.

(2) Close the boiler and fill to the normal starting level.

(3) In cases where the boiler is of new construction with new refractories in it, avoid the sharp

thermal expansion of the refractories by preheating the furnace internals by burning firewood inside the furnace for 3 to 4 hours or by firing the burner intermittently.

(4) Ignite and shut down burner using the burner tip, repeatedly until the pressure is gradually

raised to the level corresponding to 85 to 88% of normal operating pressure. Maintain the pressure at that level, for soda boiling. If necessary to maintain the pressure,

intermittently fire the burner.

(5) If the refractory is new, alternate the burner at 15 minute intervals.

(6) When the boiler is thoroughly heated, raise the water level slowly' to about 75 to 100 mm

above normal operating level, then give a surface blow followed by several short bottom blows to bring the water level 30 to 50 mm below normal.

(7) Refill the boiler slowly, until the water level is reached to about 75 to 100 mm above normal, and additional chemical based on the approximate amount of water blown down.

(8) Perform the soda boiling continuously for 24 to 48 hours, blowing down boiler water every 6 to 8 hours through the blow-down valve by I OO to 1 50 mm on the level gauge each time.

3.2 Wash and Inspect (1) After boiling out is completed by the firing method, blow down the boiler through the

water drum bottom blow off valve while the boiler is fairly warm. Discharge the water overboard, to avoid damaging paint in the bilges by the strong caustic solution.

(2) Open up the boiler and wash it down with a hi**h pressure water hose, playing the hose into all tubes.

(3) Carefully

process.

inspect the boiler, and if any trace of oil remains, repeat the boiling out

44

MANUAL 0-0 43

C-5 (1)

C-5. WATER WASHlNG FIRESIDES

Slag is a mixture composed of sodium sulphate or a mixture of sodium sulphate and van adium pentoxide, and lesser amounts of the oxides of other impurities.

When burner is kept in proper adjustment, the burner tip kept clean and in good condition, and the soot blowers operated at correct intervals, slag formation may be slowed down. However slag formation will eventually accumulate on the tubes and should be removed before it has bridged over between tubes.

A water washing schedule should be set to coincide with normal fireside cleaning. Operating practice will indicate at what intervals of fireside cleanings water washing is necessary.

Since slag is soluble in hot fresh water, hot fresh water is sprayed on the slag encrusted tubes with a lace, using sufficient force to soften the slag and knock it off the tubes.

There are two methods in water washing. One is to use a hand nozzle and the other is to utilize the soot blower in spraying hot water.

The former permits concentrated washing of important points so that effective washing can be done with relatively small quantity of water resulting in less moisture of the boiler ; but much time and labor is required. With the latt.er, washing can be done easily in a short space of time but it requires relatively much water resulting in larger moisture of the boiler.

Water washing is usually carried out at a dock. A member of the crew can do it quite easily.

44 MANUAL 0-0

45

c-5 (2)

If preparation has been made in advance while the boiler is cooling, water washing will be done in 6 - 8 hours although it depends on the extent of dirtiness.

The following is the order of process of water washing the fireside with a hand nozzle.

(1) Prepare an apparatus to supply adequate quantity of hot water and a hose and nozzle for

spouting hot water. -

(2) Remove casing access doors and dusting panels to facilitate the work.

(3) Provide a means for immediate and constant draining of the waters and the removal of the sludge, resulting from washing down.

(4) Water under 0.49MPa(5 kg/cm2g) and at a temperature of 80 deg. C should be sprayed on

to the tubes, using an armored hose. Work from the top of the boiler down, in cases where slag removal is extremely difficult, secure from washing, and allow the water to soak

into the slag for a period of 30 minutes to one hour. Then continue washing down the tubes.

(5) Attention should be paid not to let washing water penetrate behind the refractory. It will be effective to lay a sheet of canvas on the furnace floor. If small quantity of water is absorbed by refractory and heat-insulating materials, the bad effect will be removed by slowly drying soon after finish of washing.

(6) Preferably complete the washing with warm water m short tlme rt Is not desrrable to continue the washing over 8 hours.

(7) As soon as possible after washing is completed, Iight off the boiler at minum F.O. press. The drying out operation should be done very slowly and should be continued until the boiler is thoroughly dried out. When the boiler has entirely been washed, the drying should be made at least for 12 hours.

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C-6. ACID CLEANING

C-6 (1)

Thanks to development of good inhibitors, it has become possible to effectively remove scale on the waterside of the boiler by acid cleaning.

However, acid cleaning requires a suitable treatment according to actual conditions of the boiler,

otherwise it will not only damage the boiler but also be attended by dangers. So this work should be done by specialized constructors having experienced and competent personal with proper equipment. It is desirable for the work to be done under the leadership of Mitsubishi, Nagasaki Shipyard, if

possible.

Boiler operators should have knowledge of the following general items.

6.1 E ui ment The following devices should be prepared for the acid cleaning.

(1) An acid fill.ing tank of sufficient capacity to hold the prescribed amount of acid and inhibitor.

(2) One centrifugal acid filling and circulating pump with bronze impeller designed to deliver a minimum of from 0.2 to 0.4 m3/min at 35mTH or of such capacity as so fill the

unit in not more than 2 hours.

(3) Suitable temporary piping and fittings to connect both the pump and tank to the boiler.

(4) A typical cleaning solution would contain 28 percent to 34 percent hydrochloric (muriatio) acid, an inhibitor and water. The amount of acid used would be roughly I O to 20 percent by volume of the volume of ~vater necessary to fill the component to be cleaned. See "Design Data".

Selection of the concentration of acid used (one to six percent by weight for any one unit) depends upon the type and amount of scale or rust and other impurities to be removed.

Special attention must be given to ensure that the acid solution is not enriched by ferrio

or cupric ion, which can be caused from the removed scale or deposits containing a large portion of ferrio or cupric oxide.

This will impair the effect of the inhibitor. In such a condition, keep the acid solution

below the maximum allowable concentration of ferrio or cupric ion, by adding renewing the reducing agent.

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c-6 (2)

Procedure (1) If a trace of oil is found in the boiler, it must first be boiled out with an alkaline solution.

A boiling out period of from 6 to 10 hours is adequate when followed by acid cleaning.

(2) Remove all tools, rags and other foreign material from the waterside of the boiler. Secure all header end plates, the manhole plates and valves, that will make the boiler watertight and isolated from all boiler accessories except the water gauges.

(3) In order to ensure safe removal of hydrogen gas generated during the acid cleaning, connect a pipe line to the highest vent of the part to undergo acid cleaning. Never do welding while acid cleaning is being performed.

(4) Temporary piping for pumping the acid liquid into the boiler should be connected from the tank to the pump suction, and from the pump discharge to the nozzle of the bottom blow pipe. Piping should also be connected. From the other steam pipe nozzle of the boiler steam drum to the tank.

(5) Start filling the boiler with water 75 degree C to 90 degree C. While the feed water is being injected into boiler through the main or auxiliary feed system, start pumping the acid and inhibitor into the boiler through the chemical feed pipe. If all the acid has not

been pumped into the boiler by the time the water shows in the gauge glass, secure feeding the boiler, until all the acid has been injected into the boiler.

(6) When all of the acid liquid has been injected into the boiler, continue to fill the boiler

with water until the level in the steam drum is high enough to cover all the tubes to undergo acid cleaning. This is necessary for the circulation of the cleaning solution and for cleaning all the tubes of the unit.

(7) When acid-cleaning the entire steam drum, fill the boiler with water containing the same

proportion of acid. ~

(8) Run the circulating pumps 3 to 5 minutes each hour. If it is found that a wide temperature spread exists between various parts of the boiler or if the scale deposit is

excessive, use a half hour cycle.

(9) Check test samples of the cleaning solution with a standard sodium hydroxide solution

and methyl orange indicator. The value of this depends upon being able to get a true representative sample. The acid concentration should be tested at the end of each circulation period .

When the concentration has leveled off, and remains constant over two successive tests the dissolution can be considered complete.

( I O) Empty the boiler through the skin valve using compressed air,

(1 1) Average acid contact time should be from 6 to 8 hours. If the scale has not been removed

during this period, the probable cause is :

(a) Weak solution ; repeat process. (b) Misidentified scale and wrong solution used, high in silicates, sulphates or oil.

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c-6 (3)

General Precaution (1) Before injecting the acid solution into the boiler it is good practice to check the

effectiveness of the inhibitor. A weak inhibitor will pernrit excessive corrosion of the

boiler metal.

(2) A quick check of inhibition consists of diluting the concentrated acid to approximately 5

percent, then drop in a piece of coldrolled steel in the solution and heat to 65 degree C.

Well mixed inhibited acid produces very few hydfogen bubbles which should be small and difficult to see.

For comparison run a test with inhibited acid.

(3) Check the metal temperatures particularly the steam drum before injecting acid, before and during each pumping operation. This can be down by using thermocouples located at critical points or by contact pyrometer.

It should be noted that THE TEMPERATURE IS A MOST CRITICAL FACTOR in the procedure. Too low a temperature, under 50 degree C, will result in poor dissolution of

most deposits. Too high a temperature, more than 80 degree C for most inhibitors, will increase the corrosion rate appreciably, if not to a damaging degree. Stay within the temperature limits prescribed by the manufacturer of the inhibitor. If there is any doubt,

stay under a temperature of 65 degree C.

(4) After draining out the acid, wash down the boiler with fresh water, using the same washing process as described after boiling out.

(5) The boiler should be boiled out with an alkaline solution (see Boiling Out). This is to both clean the boiler of suspended particles and to return all surfaces to an alkaline base.

(6) After boiling out, wash the boiler with a strong stream of hot fresh water with a hose.

(7) After acid cleaning DO NOT ENTER OR WORK in the boiler until it has been filled at least once with water, and preferably after the boiling out process.

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