6 Feed and Boiler Water

ENMAS GB POWER SYSTEMS (A Division of GB Engineering Enterprises Pvt. LTD)

Chennai – 600 018 INDIA

CUSTOMER : M/S VARDHMAN FABRICS, PROJECT: 1 x 135 TPH AFBC BOILER BUDHNI, M.P. CONT.JOB CODE : WB 017M

OPERATION & MAINTENANCE MANUAL Page.No. 45

CHAPTER – 6 FEED AND BOILER WATER





Chapter – 6

FEED AND BOILER WATER

SI.No DESCRIPTION

6.1 Water Conditioning

6.2 De-aeration

6.3 Alkalinity – Caustic Soda, Ammonia, Amines

6.4 Supplementary Water Treatment

6.5 Significant Salt solutions Properties





6.1 WATER CONDITIONING

DESCRIPTION

a) General

b) Raw Water and Chemical Dosing

c) Water and Steam Sampling

d) Hardness Phosphate Conditioning

e) Phosphate Dosing

f) Phosphate Hide Out

g) Co-ordinated Phosphate – pH Control

h) Congruent Control





6.1 WATER CONDITIONING

a) General

The steam generated in the boiler is supplied to the plant for its various

requirements as well as to the auxiliaries.

There are many water treatment specialists whose services are available to boiler

users. Unless the installation has adequate water testing facilities and personnel

skilled in the interpretation of the results, it is recommended that the care and control

of water be entrusted to such specialists.

The development of highly rated water tube boilers has resulted in the need for

close control of feed and boiler water quality. The successful use of such boilers has

been so dependent upon the proper water conditioning that added impetus has, in

turn, been given to water treatment and to the necessity for accuracy in its control.

While it is beyond the scope of these instructions to cover the subject fully, the

following is submitted as a brief review of some of the outstanding factors, in order to

assist those taking charge of new boiler equipment.

In order to minimise deposits, scale formation and corrosion, in the boiler but also

in the feed line, feed heaters, economisers, and prime movers, it is necessary to

maintain chemical conditions in the boiler water by injecting treatment chemicals into

the steam drum and by chemical dosing of the feed water before it enters the system.

b) Raw water and Chemical Dosing The selection of the chemical depends on the type of the water treatment. Raw

water gets demineralised in the DM plant and then passes through the deaerator to get

degasified. The DM water is dosed with chemicals to attain proper alkalinity and

deaeration and finally fed to boiler by boiler feed pump. To maintain the contents and

property of water under control, HP dosing to drum is often used. Periodic sampling

and testing of water from different parts of the boiler is conducted for record and

control purpose.

Oxygen scavenger should be injected to eliminate dissolved oxygen even if the

feed water is deaerated mechanically.





The amount of the chemical must be determined on the basis of the quality of feed

water actually used, the load and the chemical analysis of the boiler water.

For the injection of the chemical by the chemical dosing pump, most adequate quantity

shall be determined by adjusting the pump stroke considering the content. Chemicals

should be injected continuously during boiler operation so as to avoid erratic boiler

water conditions.

c) Water and Steam Sampling Samples are drawn from feed water, Drum water, Saturated steam, Superheated

steam and turbine inlet steam for measurement of pH, conductivity. In addition to that,

Dissolved oxygen analyzer is provided for feed water. These samples are passed

through SWAS panels where all sample coolers, Rotometers, Pressure and

temperature relieving valve are provided for protection of analyzing instrument.

d) Hardness – Phosphate Conditioning Internal deposits of calcium and magnesium salts, and iron oxides will be

minimised provided the maximum concentrations of hardness salts and iron given in

table for Feed Water Conditions are not exceeded.

a) BOILER WATER QUALITY

Total dissolved solids : 50 ppm

Specific electrical conductivity at 25°C (max) : 100 micro siemen /cm

Phosphate residual : 5 – 10 ppm

Ph : 9.4 – 9.7 ppm

Silica (max) : 1.2 ppm

b) FEED WATER QUALITY

Appearance : Colorless and Clear

Conductivity : 0.15 micro siemen /cm

Ph : 8.5 – 9.2





Oxygen : 0.02 ppm

Hardness : Not detectable

Oil + Organics + Chlorine : Nil

TDS : 0.1

Silica : 0.02 ppm

It is necessary to maintain a phosphate reserve in the boiler water to guard against

scale formation on the internal heating surface of the boiler This type of scale

formation results either from residual hardness in the softened feed water or

contamination of steam condense from the plant due to cooling water infiltration. To

resist this phenomenon, phosphates of sodium are dosed to the steam drum.

The phosphates usually employed are tri-sodium phosphate, disodium hydrogen

phosphate, and monosodium dihydrogen phosphate or sodium polyphosphates. The

choice of phosphate depends on the alkalinity of the feed water, which in many

instances is determined by the preliminary softening treatment of the make-up water.

Acid phosphate e.g., monosodium dihydrogen phosphate is used when it is desired to

reduce the alkalinity in the boiler water, whereas trisodium phosphate is used when an

increase in boiler water alkalinity is desired. Mainly tri-sodium phosphate is dosed.

In the water, phosphates react with calcium and magnesium salts to form

insoluble calcium and magnesium phosphates, which are precipitated as sludge.

The maximum concentration of hardness (in the feed water) that can be

tolerated is indicated in 6.1d. Greater quantities may lead to loss of phosphate

reserves and to the formation of scale on tubes at areas of high heat input, which may

result in tube overheating and failure. Hardness should be absent from feed water to

boilers operating in pressure ranges above 42.12 kg /cm². The small phosphate

reserve normally recommended is not intended to deal with more than slight

contamination arising from condenser leakage, evaporator priming or other unforeseen

irregularities.





e) Phosphate Dosing The recommended method of introducing phosphate is by means of a high-

pressure pump injecting direct to the drum. This avoids precipitating hardness sludge

in feed lines or Economiser.

f) Phosphate Hide - Out A phenomenon associated with high-pressure plant is that known as “Phosphate

hide – out “ where the phosphate concentration apparently disappears or is reduced at

full load, and reappears on reducing load. The advice of a water treatment specialist

should be sought. Phosphate conditioning is not recommended for boilers of 64 kg /

cm² and above which exhibit hide – out.

g) Co-ordinated Phosphate - pH Control Based on earlier work by eminent chemists and demonstrated by others that,

adjustment of the boiler water chemistry to assure that phosphate – alkali balances

approximate the stoichiometric hydrolysis of tri-sodium phosphate will prevent caustic

corrosion.

This system, which is called co – ordinated phosphate – pH control, depends on

the feed of mixtures of orthophosphate or a mixture of caustic and disodium phosphate

to control pH in an otherwise unbuffered water. Monosodium, disodium and trisodium

phosphates can accomplish this purpose as a result of their ability to hydrolyze to (H2

PO4)-, (H P O4) –2 and (PO4)–3, and to establish an equilibrium among these forms, in

conjunction with (O H)-1 concentration or, by interference, the pH.

The reactions of concern are as follows:

(P O4)– 3 + H 2 O = (H P O 4)– 2 + (O H ) –1

(H P O 4)- 2 + H 2 O = (H 2 P O 4) -1 + (O H) -1

(H 2 P O 4)-1 + H 2 O = 3 H + + (P O 4) -3 + (O H)–1





In water containing a low concentration of dissolved solids other than sodium

phosphate, the equilibrium between the various forms of the phosphate ion can be

utilized to control the acidity or alkalinity of the whole system. The ratio of one

phosphate to the other can be controlled to produce any desired sodium to phosphate

(Na / PO4) ratio. If all the phosphate is present in the trisodium form, the (Na / PO4)

ratio is obviously 3.0. At or just below this level, there will be no free hydroxide present.

The term “ free “ sodium hydroxide or “ free caustic “ defines the amount of sodium

hydroxide in solution in excess of that derived from the hydrolysis of trisodium

phosphate according to the following equation

Na 3 P O 4 + H 2 O = Na 2 H P O 4 + Na O H

Drawing No 1.4.1 D 1 shows the relationship between (P O4)–3 concentration and

pH at 25° C for a pure Na3PO4 solution. This curve is the basis for the system of co-

coordinated phosphate – pH control. Phosphate is usually controlled at 5 – 10 ppm to

avoid carryover at the higher pressure and 15 – 25 ppm in the 42 – 84 kg/cm² range.

h) Congruent Control Originally, it was thought that adjustment to assure that phosphate – pH co –

ordinates followed the trisodium phosphate hydrolysis curve would eliminate free

caustic. Cases of caustic corrosion continued to occur, however, in boilers in which the

chemistry had been adjusted following this principle. Various explanations have been

advanced, the most probable being that the solids which precipitate from a super –

saturated Na3PO4 solution are not pure trisodium phosphate, i.e., that as a result of the

concentrating effect at the tube surface, both trisodium and disodium phosphate

deposit on the tube, leaving an excess of sodium hydroxide in the supernatant liquid.

The exact composition of the precipitating phosphate depends on temperature, but has

a Na / PO4 ratio in the range of 2.85 to 2.6 for pressure from 84 to 211 kg / cm². A

ratio of 2.6 is considered safe for all drum type boilers.





The temporary deposition of usually water – soluble chemicals during normal

operation is referred to as “ hideout “. When the steaming rate is appreciably reduced,

or the boiler taken off line, the deposited chemicals redissolve in the boiler water.

Although a number of substances exhibit this phenomenon, sodium phosphate is of

most interest in boiler operation. At temperatures above 117°C, trisodium phosphate

exhibits a retrograde solubility. It is this property that makes hideout possible

Phosphate hideout is also undesirable because it may raise tube wall temperatures

and cause failures similar to those brought about by other deposits.

Phosphate precipitation and the formation of free caustic can also occur if boiler

water is totally evaporated in the steam blanketed area or is highly concentrated within

or under a porous surface deposit.

Congruent control, used when phosphate hideout may occur, is similar to

coordinated phosphate – pH control, except that the Na / PO4 ratio is intentionally

controlled at a specific value, usually 2.6 to 2.8 rather than “ just below “ 3.0. The

concept of maintaining a specific ratio at 2.6 assures that no concentrated caustic

solution can be formed in the event of phosphate precipitation.

In this situation, the sodium and phosphate will invariably leave the solution in such

proportions that the remaining solution has a Na / PO4 ratio lower than the original

ratio, therefore without free hydroxide or caustic.





6.2 DE - AERATION

DESCRIPTION

a) Introduction

b) Thermal De – Aeration

c) Chemical De – Aeration

d) Use of Sodium Sulphite

e) Use of Catalysed Form of Sodium Sulphite

f) Sodium Sulphite Injection Point

g) Use of Hydrazine

h) Ammonia from Excess Hydrazine

i) Hydrazine Reserve

j) Health Regulation

k) Injecting Chemicals





6.2 DE - AERATION

a) Introduction

Dissolved gases are normally present in feed water, being present in the make –

up water or due to condensate coming into contact with the atmosphere {in open feed

tanks, for example}. The principal gases are oxygen and carbon dioxide, the presence

of which can cause corrosion of the feed system, boiler or condensate system. It is,

therefore, advisable to reduce the concentration of dissolved gases in the feed water

to the lowest practicable value.

b) Thermal De – aeration

In order to reduce the gas content of the feed water, it is normal practice to employ

an independent de-aerating plant in which the gases are removed from solution by

steam heating either under pressure or under vacuum, and are then ejected to

atmosphere. The need or otherwise for the installation of a mechanical / thermal de-

aerator is dependent upon various factors, such as plant loading, percentage

condensate return etc. Generally speaking, however, plant with a high evaporation

rate, irrespective of boiler pressure, should be provided with full flow mechanical de-

aeration. The dissolved oxygen content of the water leaving the de-aerator should be

as low as possible Oxygen in the feed water and / or the iron oxide produced by its

reaction with feed pipe work and economiser become particularly damaging in the

higher pressure ranges and de-aeration to the highest attainable degree then becomes

essential, requiring a guaranteed performance of the order 0.007 ppm oxygen

maximum. It is important that regular testing be carried out to check the efficiency of

physical de-aeration. A slight unbalance in temperature / vacuum in a vacuum type

de-aerator can result in high oxygen figures and an oxygen recorder in the de-aerator

discharge line is recommended.





c) Chemical De-Aeration The trace quantities of dissolved oxygen remaining after mechanical de-aeration

should be removed by chemical treatment. In those cases where a de-aerator is not

fitted, full oxygen removal by chemical treatment should be employed.

d) Use of Sodium Sulphite Sodium sulphite is widely used as an oxygen scavenger and has been found

satisfactory at pressures up to about 70 kg /sq. cm. Above this pressure there is

uncertainty as to its stability, but available information suggests that in general it is

satisfactory for pressures up to 95 kg /sq. cm. where alkalinity due to sodium salts is

maintained consistently in the boiler water. The preferred method of application is to

dose the feed water continuously to maintain the concentration in the boiler water

within a specific range.

d) Use of Catalysed Form of Sodium Sulphite Where corrosion in the feed system is experienced, the use of a catalyzed form of

sodium sulphite is an advantage. The effect of the catalyst is to speed up the oxygen –

sulphite reaction, so that it is completed well before the feed water reaches the boiler,

thus giving a greater degree of protection to feed lines and economiser.

f) Sodium Sulphite Injection Point

Sodium sulphite is a solid and this is charged in the feed water system. g) Use of Hydrazine

Hydrazine (N2H4) is a liquid and does not increase the dissolved solid contents in

the boiler water. This is recommended for boilers operating above 42 kg / sq. cm and

is widely used as the oxygen scavenger. Hydrazine is very volatile and it is difficult to

maintain a useful concentration in the boiler water; this demands that the reaction with

oxygen be largely effected in the feed system. Therefore, hydrazine should be injected

at the furthest possible point in the feed system remote from the boiler before feed

pump.





h) Ammonia from Excess Hydrazine

Apart from scavenging of oxygen, the ammonia produced by decomposition of

excess hydrazine adds to the alkalinity of feed water and can provide suitable alkaline

conditions in the steam and in the condensate system. Provided the excess is

controlled to avoid undue rise in the ammonia in the steam, there is little danger of

copper corrosion in the condensing plant but good de-aeration is essential.

e) Hydrazine Reserve

Hydrazine is a volatile chemical, and hence the possible reserves, which can be

maintained in the boiler water, decrease generally as the temperature and pressure

rise. Since the possible reserve also depends on the individual boiler characteristics,

some variation can be found. The important point is to keep whatever reserve proves

practical without excessive chemical consumption. A reserve of about 0.05-ppm is

maintained.

f) Health Regulation

Hydrazine is not normally permitted by “ health “ regulations where the steam

comes into contact with foodstuff.

k) Injecting Chemicals A low-pressure pump is recommended for injecting chemicals for feed water

alkalinity control and oxygen scavenging. This pump should have an adjustable output

over the range 0 to 25 litres per hour.





6.3 ALKALINITY – CAUSTIC SODA, AMMONIA, AMINES

DESCRIPTION

a) Alkalinity Maintained by Caustic Soda and Ammonia

b) Caustic Soda Concentration

c) Amines

d) Volatile Treatment





6.3 ALKALINITY – CAUSTIC SODA, AMMONIA, AMINES a) Alkalinity Maintained by Caustic Soda and Ammonia

Whatever the operating pressure, the feed and boiler water should be alkaline to

safeguard against corrosion not only in the boiler itself but in the feed lines, feed

heaters and economizers.

The chemicals used for this purpose are either non-volatile, e.g. caustic soda,

or volatile e.g. ammonia, cyclohexylamine. For all pressures, the addition of non-

volatile alkali depends upon such factors as residual alkalinity of the make-up water,

the percentage of make-up, and the amount of alkali required in the boiler water.

When the make up is distilled or de-mineralized, this alkalinity should be

provided by continuous feed line dosing with caustic soda or a volatile amine e.g.

cyclohexylamine In the pressure range over 53 kg / sq. cm. On – load corrosion

presents an important risk and so boiler water alkalinity (and consequently feed water

alkalinity) is adjusted to “optimize” the circumstances of the individual installation.

b) Caustic Soda Concentration The caustic soda concentration in the boiler water becomes of increasing

importance in the higher-pressure range. A high caustic soda concentration may allow

very high localized concentrations at the steam / water / metal interface and cause

continuous growth of thick magnetite at the surface which accelerates the reaction

further and which temperature changes may cause to crack and thus allow further

corrosion.

The mechanisms of such corrosion can be quite complicated – they are not fully

understood – but experience has shown that reducing the boiler water concentration of

caustic soda can reduce corrosion, however, inadequate caustic soda concentration

where the water salts are largely chlorides can be just as damaging.

Thus where it is desired to reduce the risk of caustic attack by reducing the caustic

concentration, it is necessary to limit the boiler water concentration of dissolved solids

as well, particularly where chlorides form an important proportion.





Caustic soda is mainly added to maintain the boiler water reserve, the alkalinity

being taken care of mainly by ammonia or cyclohexamine. These observations concerning caustic soda apply to a limited extent to sodium

phosphate and partly explain the reduced concentrations recommended elsewhere for

the high-pressure ranges. c) Amines

Condensate lines may be protected from corrosion by the use of a filming amine,

preferably injected into the steam main. A typical filming amine is “Octadecylamine. ‘

d) Volatile Treatment

The ultimate internal treatment to prevent free hydroxide or caustic alkalinity is the

system called volatile treatment, or “ zero solids “ treatment. The sodium is eliminated

from the feed water by evaporation and/or demineralization of the make – up water

and full flow, mixed bed demineralization of return condensate. No chemical containing

sodium is added for internal treatment. The chemicals used are all volatile materials:

ammonia and/or morphine or cyclohexylamine for p H control, and hydrazine or

catalyzed hydrazine as an oxygen scavenger and corrosion inhibitor.

Such treatment is limited to very high-pressure boilers. It is the only system

possible for once through or monotube boilers and most manufacturers of PWR type

nuclear reactors favour its use in the secondary steam generating loop to overcome

unique crevice corrosion problems. It is occasionally employed in drum type boilers

operating at 197 kg / cm2 and above but provides less safety and corrosion resistance

against a temporary disturbance, particularly that of a condenser leak, than does the

phosphate – pH treatment system. In actual operation, most systems using volatile

chemistry provide a phosphate feeding facility for emergency use during a period when

feedwater is contaminated with dissolved salts.





6.4 SUPPLEMENTARY WATER TREATMENT

DESCRIPTION

a) General

b) Chalets

c) Polymer Sludge Conditioning

d) Anti – Foams

e) Oils Organic Matter and Suspended Solids in Feed Water

f) Iron Copper and Nickel in Feed Water

g) Silica and Aluminium in Feed Water

h) Concentration of Dissolved Solids – Carry Over, Steam Purity

i) Turbine Deposits due to Silica

j) Intermittent Blow Down / I. B. D.

k) Continuous Blow Down / C. B. D.

l) Water Sampling

m) Measurements of Steam Purity

n) Recommendations on boiler water, its sampling & Testing





6.4 SUPPLEMENTARY WATER TREATMENT

a) General

There are commercially available numerous water treatment chemicals to

supplement or in some cases to replace the conventional treatments described

previously.

Notes on some of these chemicals follow, but it is strongly advised that before

using any of these treatments the services of specialist water Treatment Company

should be sought.

b) Chelants

By their chemical nature cheating agents such as the tetra sodium salt of ethylene-

diamine tetra-acetic acid (Na4 EDTA) form soluble complex salts with calcium,

magnesium, soluble iron and soluble copper. Particulate iron e.g. corrosion products

brought into the boiler by condensate corrosion will not be chelated, at the pH level in

the boiler water.

Chelant treatments require a high standard of control and the absence of oxygen

from the feed and boiler water, following chemical scavenging, via a stainless steel

injection fitting.

c) Polymer Sludge Conditioning Polymer sludge conditioning is normally used with phosphate conditioning, the

purpose being to render the phosphate sludge less adherent. Polymers are also used

in conjunction with chelants affording benefits in certain applications. In these cases

the control of residual concentrations in boiler water is less critical than where chelants

are used alone.

d) Anti-Foams Numerous proprietary anti-foams are available. They are used to reduce foaming

(for example of highly alkaline boiler water) thus reducing carry-over.





e) Oils, Organic Matter and Suspended Solids in Feed Water

Oil contamination in boiler feed water can cause tube failures by two mechanisms.

A layer of oil on the inside surface of the tube can have the same effect as other

scales, causing the tube to overheat. The second mechanism is that oil tends to make

phosphate sludge coalesce, again causing a scale to form on the parts of the tubes

subject to higher fluxes, leading to eventual tube failure.

Organic matter in the boiler breaks down, giving rise to corrosive agents such as

carbon dioxide and organic acids.

Suspended solids in the boiler can promote priming and carry-over. Suspended

solids can also bake onto evaporating surfaces again causing tube overheating and

subsequent failure.

Oil, organic matter and suspended solids can also lead to foaming and

consequently, carryover of solids

f) Iron, Copper and Nickel in Feed Water

Above 42 kg / cm 2, iron, copper and nickel produced by corrosion in the feed

system, or iron produced more directly by corrosion in the boiler circuit, are often

associated with particularly damaging corrosion mechanisms such as “hydrogen

embrittlement“ and every effort should therefore be made to minimise pick – up in the

condensate system. Suggested limit is 0.01 ppm at higher pressures .If values exceed

this limit and / or immobile deposits in drums, headers etc. are encountered, advice

should be sought from a water treatment specialist. g) Silica and Aluminium in Feed Water

The hazard of silica is mainly that it is a constituent of some of the most

undesirable scales formed in tubes in the water circuit, notably for their very low

conductivities. An eggshell scale thickness can cause tube failure. It is very difficult to

remove these scales either mechanically or chemically. It is recommended that caustic

soda concentration in the boiler water be maintained at 150 – 200 ppm above the silica

concentration. In order to suppress the tendency for silica to react with calcium, iron or

magnesium etc. in the formation of such scales.





The presence of aluminium, together with silica in the feed and boiler water can

often give rise to analcite (hydrated silicate of aluminium and sodium) type scales,

resulting in tube failures. It is, therefore, important to eliminate aluminium entirely from

the feed water.

Where aluminium salts are used for coagulation of the raw water, strict control of

the process is essential to prevent the transference of either soluble or suspended

aluminium salts to the feed system. h) Concentration of Dissolved Solids – Carryover, Steam Purity

The concentration of solids in the feed dictates the rate of blow down of boiler

water and / or the boiler water concentration. Boiler water concentrations must be

maintained below the values at, which contamination of the steam becomes important,

in order to avoid carryover and the accompanying dangers of deposits in the main

steam valves and prime movers.

The occurrence of carryover is influenced by a number of factors other than the

concentration of salts in the boiler water, such as fluctuations in water level and load,

incorrectly refitted drum internals, and the presence of oil, grease and other foam

inducing materials in the feed water.

Operational experience will be the best guide as to the maximum concentration of

suspended solids, dissolved solids and alkalinity which can be tolerated without giving

rise to carryover. It is advised to adopt lower values of suspended solids to reduce

carryover and the risk of corrosion

i) Turbine Deposits due to Silica All the salts normally found in boiler water are volatile in steam to some degree,

insignificant at the boiler pressures under consideration except for silica which

becomes important above 42 kg / cm2 silica in steam tends to condense and deposit in

medium and low pressure stages of turbines. It is generally believed that such

deposition should remain within acceptable limits provided silica in steam does not

exceed 0.02 ppm as SiO2. For this, it is necessary to limit the silica concentration in the

boiler water .

The boiler water concentrations should be regarded as maxima and lower

concentrations may be preferred since less deposition would be expected on the





turbine. The recommended silica content as given in Table 6.1d is 0 mg / l or ppm.

Silica in the feed water thus should be limited to 0.02 ppm SiO2.

Where blowdown, given to reduce the concentration of dissolved solids, does not

reduce the boiler silica content, extra blowdown must be carried out to achieve this, if

turbine fouling is to be avoided

j) Intermittent Blowdown / IBD Reduction of boiler water concentration of suspended solids, dissolved solids or

blowing down effects silica. Blowing down should never be done via economiser and

water wall drains when on load. Intermittent blowdown / IBD / main blowdown line is

drawn from the water drum for the above purpose. To carry out blow down through the

IBD line , first open the stop valve near the water drum, and then open the stop valve

distant the water drum. After blow down, close the stop valve distant the water drum

first and then close the stop valve near the water drum. Close the valves tight and

ascertain no leakage. Normally not more than 25 mm in the gauge glass should be

blown at one blow down. For sludge removal, short and sharp bursts are most effective.

It is recommended that during normal service the isolating valves on IBD line for control

of suspended solids, dissolved solids or silica concentration or for lowering the water

level in the steam drum should be padlocked open and that drains from furnace wall

bottom headers should be padlocked shut since operation of these latter valves at full

load may disturb circulation and cause very severe damage.

The virtual exclusion of hardness salts in the feed water for high pressure boilers

eliminates the production of a hardness sludge but it is none the less prudent to operate

the blowdown from the lower drum regularly everyday to reduce dissolved iron

concentration and blowing down is therefore solely for concentration control as it

becomes necessary. Deposits that normally occur are iron oxide and copper These

may be partially removed, as opportunity arises, by emptying the boiler at 7 kg/cm2.with

economiser and furnace front wall bottom header drain wide open.

k) Continuous Blowdown / CBD Continuous blowdown offers a means of economic control of the water

concentrations, total dissolved solids or other impurities according to the standard

practice.

This blowdown is extracted at the discharge from cyclone separators within the

drum, at which points the boiler water is at its maximum concentration.





In case that any chemical is used to treat the boiler water in the drum, the amount

of the chemicals shall be considered as a part of the total dissolved solids in feed water.

Sometimes the amount of blowdown must be determined by some content of boiler

water other than the above total dissolved solids.

l) Water Sampling Water sample coolers are provided for boiler water and for the saturated

steam. The boiler water sampling line is drawn from continuous blow down CBD line

while the saturated steam sampling line is drawn from the main steam line before the

steam flowmeter connecting lines. Cooling water inlet and outlet headers are

connected to steam and boiler water sample coolers.

m) Measurements of Steam Purity Amongst the methods available for measuring steam purity are flame

photometries, conductivity of condensed steam, evaporation of condensate to dryness

and sodium ion electrode techniques.

Sodium flame photometry is capable of indicating concentrations of less than 0.03

ppm sodium. Conductivity of a degassed sample should normally be of the order of 2

micro-mhos or less. Evaporation to dryness normally indicates 0.5 to 1.0 ppm.

It has been reported that the sodium specification electrode is not reliable below

0.002-ppm sodium, due to ammonia interference. The sodium ion electrode is,

however, the most commercially useful and accurate method for the determination of

steam purity ‘

Sodium flame photometry and sodium ion electrodes indicate only sodium, not

silica or gases derived from organic matter in the feed water, CO2, ammonia, hydrazine

etc., which tend to “flash off “. in the drum and pass to the plant . These gases may

not be completely removed in a degasser and often give rise to a higher conductivity

than would be expected by sodium analysis. Evaporation to dryness may indicate (for

example) iron which may form a scum on the surface of the water in the drum and be

carried direct to the steam line, being indicated neither by sodium flame photometry nor

by conductivity

There is no absolute measure of impurities in steam suitable for simple

correlation against a rate of turbine fouling but the foregoing methods offer a fairly high

degree of surety against excessive, uncontrolled deposition.





n) RECOMMENDATIONS ON BOILER WATER, ITS SAMPLING & TESTING

BOILER FEED WATER LIMITS

S / No DESCRIPTION LIMIT TESTING

FREQUENCY

1. pH (copper alloy pre-

boiler system) 8.8 – 9.2

Twice a shift – manually

2. Conductivity (Before

cation)

Less than 3.0 micros

mhos/cm Twice a shift - Manually

3. Conductivity (After

cation)

Less than 0.3 micro

mhos/cm (indicating TDS

less than 50 ppb)

4. Dissolved oxygen Less than 7 ppb Once a day – manually

(Indigo carmine method)

5. Total silica Less than 20 ppb Once a shift – manually

6. Residual hydrazine 10 – 20ppb Once a shift - Manually

7. Total Iron Less than 10 ppb Once a day – manually

8. Total copper Less than 5 ppb Once a day – manually

9. Total hardness BDL Once a shift Manually

10. Chlorides BDL Once a shift Manually

11. Permanganate

Number (Organics) BDL Once a day – manually

12. Total CO2 BDL Once a day - manually

13. Oil and grease BDL Once a week - Manually

On line pH, conductivity and D/O analyzer must be calibrated every week.

On line pH, conductivity and D/O analyzer must be periodically being cleaned as per

supplier’s instructions.





BOILER DOWN COMER WATER LIMITS

S/ No DESCRIPTION LIMIT TESTING

FREQUENCY

1. pH 9.4 – 9.7 Twice a shift – manually

2. Conductivity Less than 100.0 micro

mhos / cm Twice a shift – manually

3. TDS Less than 50 ppm

(50 % of conductivity)

4. Residual phosphate 5- 10 ppm Once a shift - manually

5.

2P-M ( P for P

Alkalinity and M for M

Alkalinity

Nil Twice a shift – manually

6. Total silica

Less than 2ppm (to have

less than 20 ppb of silica in

saturated steam.)

Once a shift – manually

7. Total Hardness BDL Once a shift – manually

8. Chlorides (Mercury

Thio cyanate method) BDL Once a shift – manually

On line pH and conductivity analyzer must be calibrated every week.

On line pH and conductivity analyzer must be periodically be cleaned as per suppliers

instruction.





SATURATED STEAM AND MAIN STEAM

S/ No DESCRIPTION LIMIT TESTING

FREQUENCY

1. pH 8.8-9.2 Twice a shift – Manually

2. Conductivity (before

cation)

Less than 3.0 micro mhos /

cm Twice a shift – Manually

3. Conductivity (after

cation

Less than 3.0 micro mhos /

cm (indicating TDS less

than 50 ppb)

4. Ammonia Less than 0.3 ppm Once a shift – manually

5. Total silica Less than 20 ppb Once a shift – Manually

6. Total Iron Less than 10 ppb Once a day – manually

7. Total copper (TG

Condensate) Less than 5 ppb Once a day – manually

8. Total hardness (TG

Condensate) BDL Once a shift Manually

9. Chlorides (TG

Condensate) BDL Once a shift Manually

10. Sodium (saturated

steam)

Less than 20 ppb Once a week –

Manually

On line pH and conductivity analyzer must be calibrated every week.

On line pH and conductivity analyzer must be periodically be cleaned as per suppliers

instruction.





Instrumentation:

1. On line pH, conductivity and DO analyzer must be calibrated every week.

2. On line pH, conductivity and DO analyzer must be cleaned periodically as per

suppliers instruction.

3. Date of calibration and due date must be displayed on the equipment.

4. Cationic resin from cationic column in SWAS panel must be removed every week and

filled with regenerated resin.

5. On line Cationic conductivity meters for boiler feed water, saturated steam, main

steam and TG condensate must be fitted with conductivity cell with a cell constant of

0.01 cm-1 and on line boiler down comer water must be fitted with a conductivity cell

with a cell constant 0.1 cm-1.

As per ASTM D 1125m conductivity cell with a cell constant 0.01cm-1 must measure

the conductivity from 0.05 micro mhos cm-1 to 20 micro mhos cm-1and conductivity

cell with a cell constant 0.1 cm-1 must measure the conductivity from 1.0 micro mhos

cm-1 to 200 micro mhos cm-1

Operation: CBD line must be kept open continuously. Warning: Any deviation from the above said parameters should be taken seriously and boiler must be shut in case of any deviation to prevent permanent damage to the equipment





6.5 SIGNIFICANT SALT SOLUTIONS PROPERTIES

DESCRIPTION

a) Basic Chemistry

b) Measurement Units

c) Hardness

d) Caustic, Carbonate and Bi-Carbonate Alkalinity

e) Titration with Acids

f) Equivalent Mineral Acidity E. M. A.





6.5 SIGNIFICANT SALT SOLUTIONS PROPERTIES a) Basic Chemistry

A raw water comprises rainwater that has dissolved mineral matter from strata

through which it has passed. These mineral salts are almost completely dissociated in

solution and effectively the raw water contains mainly calcium, magnesium, sodium

and potassium cations and bicarbonate, carbonate, chloride, sulphate and nitrate

anions. Softening processes often convert bicarbonate to carbonate and introduce the

hydroxide anion. Sodium and potassium salts are highly soluble but some calcium and

magnesium salts are less soluble and may easily be precipitated e.g. calcium

carbonate, magnesium hydroxide. Calcium sulphate has a particularly undesirable

characteristic in that its solubility decreases with increasing temperature so that whilst

it remains in solution i.e. dissociated as calcium and sulphate ions, in the bulk of a

boiler water, it may be precipitated by high temperature at the steam / water / tube

surface interface and form a scale.

Raw waters contain other mineral and organic matter normally not of direct

importance to boiler operation but often affecting softening processes.

b) Measurement Units

Concentrations are usually expressed in “ppm” i.e. parts solute per million parts

solution by weight; this is the same as “mg / litre “.

Oxygen has traditionally been expressed in “ cc / litre “ i.e. “ ml / litre “ at standard

temperature and pressure but the attached recommendation in Table 6d for Feed

Water Conditions follow current trend and are in ppm. For oxygen,1 ppm = 0.7cc / litre.

Alkalinity and Hardness concentrations are normally expressed in “ ppm as Ca C O 3.

” The obsolescent “ degrees “ (British) may still be encountered -.10 = 1 grain

Ca C O 3 per gallon = 14.3 ppm as Ca C O 3.

A term common in Continental Europe is “ m Val / litre “, this is the same as “

epm “, equivalent per million or the number of equivalents per million or the number of

equivalent weights solute per million parts weight of solution. 1 m val / litre = 1 epm

= 50 ppm as Ca C O 3.





The term pH is used to express the acidity or alkalinity of aqueous solutions due to

ionisation. The acidity or alkalinity is a function of the hydrogen ion concentration

which may be measured electrometrically or calorimetrically. A solution is acid if the

pH value is below 7, alkaline if the pH value is between 7 and 14 and neutral if the pH

value is exactly 7 c) Hardness

The hardness of a water is the calcium and magnesium content and it is usual

to express concentrations of hardness in terms of the chemically equivalent

concentration of calcium carbonate (irrespective of whether it is hardness due to

magnesium or whether carbonate in fact is present). Thus a raw water may have a

total hardness of 80 ppm as Ca C O 3 , of which calcium hardness may be 60 ppm as

Ca C O 3 and the remainder, 20 ppm, is the magnesium hardness expressed as

Ca CO 3. d) Caustic, Carbonate and Bicarbonate Alkalinity

Hydroxide, carbonate and bicarbonate are alkaline and may be measured by

titration against an acid. The term “ alkaline “ is used for these anions or for the salts

they form. Bicarbonate and hydroxide can not (generally) co – exists but react to form

carbonates with the evolution of CO2. Caustic alkalinity, carbonate alkalinity and

bicarbonate alkalinity refers respectively to the concentration of hydroxide, carbonate

& bicarbonate. Concentrations are expressed in terms of the chemically equivalent

concentration of Ca CO 3.

e) Titration with Acid

Titration with acid to pH 9.3, corresponding to the end-point of phenolphthalein,

neutralises hydroxide (if present) and half the carbonate. Titration to pH 4.5,

corresponding to the end point of methyl orange neutralises the remaining carbonate

and bicarbonate (if present). i.e., titration to pH 4.5 neutralises the total alkalinity. By

titration with acid using phenolphthalein as indicator and in the presence of barium

chloride, only the caustic soda is neutralised ; this test is employed where phosphate is

present.





The following relations as given in Table below apply where P, M, and P (Ba Cl 2 )

are respectively the alkalinities measured as above, in ppm .

P, M AND P ( BaCl 2 ) RELATIONSHIP

Titration Hydroxide Carbonate Bicarbonate Total Alkalinity

P = Nil Nil Nil M M

2P < M Nil 2 P M-2P M

2P = M Nil 2 P Nil M

2P >M 2P-M 2 (M-P) Nil M

P = M P Nil Nil M

P>P(Ba Cl 2) P(Ba Cl 2) 2{P-P(Ba Cl 2)} Nil 2P–P(Ba Cl 2)

P(Ba Cl2)= Nil Nil 2 P M-2P M

Alkaline hardness (approximating to the traditional “ temporary hardness“) refers to

the calcium and magnesium hardness that may theoretically be associated with

alkalinity i.e. carbonate, or hydroxide anions. The remaining “ non – alkaline “

(“permanent”) hardness, if present, is the calcium and magnesium hardness

associated with non – alkaline anions, sulphate, chloride and nitrate.

Thus where a water has a total hardness of 80 ppm as Ca C O 3 and its total

alkalinity is 50 ppm Ca C O 3 , it may be said to have an alkaline hardness of 50 ppm

as Ca C O 3 and a non – alkaline hardness of 30 ppm as Ca C O 3 . If the total

hardness were 80 ppm but the total alkalinity were 100 ppm, then the alkaline

hardness would be 80 ppm and there is said to be no non- alkaline hardness (the

remaining 20 ppm alkalinity being associated with sodium and / or potassium)

Hardness and alkalinity concentrations are insufficient by themselves to fully describe

the mineral matter of a water since they alone give no indication of sodium, potassium,

sulphate, chloride and nitrate concentrations.





f) Equivalent Mineral Acidity E. M. A.

The term “ E. M. A. “ equivalent mineral acidity, is often used to express the total of

the sulphate, chloride and nitrate concentrations, again as .C a C O 3 Calcium

carbonate and magnesium hydroxide are relatively insoluble at the temperature of

boiler water and the maintenance of a concentration of free sodium carbonate in the

water of a boiler may be enough to precipitate sufficient calcium (as a free – flowing

sludge of calcium carbonate). and so prevent the formation of calcium sulphate on the

steam producing tube.

Surfaces. Unfortunately sodium carbonate breaks down at high temperature so

that at 14 kg / cm 2, for example, about 90 % of the carbonate may be broken down to

the hydroxide and carbon dioxide (which passes off with the steam) and it becomes

impossible to maintain the necessary 300 ppm carbonate without unacceptably high

hydroxide concentrations (by priming and carryover considerations)

6 Feed and Boiler Water

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