Water treatment-Steam Boilers Chandran Udumbasseri, Technical consultant

cudumbasseri@yahoo.co.in.

Steam boilers Types

There are two types of steam boilers

1. Fire tube boiler (hot combustion gas passes through tube with water

surrounding this tube)

2. Water tube boiler (hot combustion gas passes outside the tube and water

through the tube)

Classification Boilers are classified based on pressure

1. High pressure boilers operate above 15 psig

2. Low pressure boilers operate at 15 psig or below.

Boiler capacity

Boiler capacity is expressed in the following units.

1. Horse Power.

2. Pounds of Steam per Hour

3. BTU per Hour

1HP means energy to convert 34.5 pounds of water to steam from (and at) 212oF at

sea level.

High pressure boiler treatment

The treatments for high pressure boilers are:

1. Scale control

2. Sludge conditioning

3. Corrosion control(O2 & CO2)

4. Foaming control(Steam Drum)

Low pressure boiler treatment

Low pressure boilers need treatments for controlling corrosion.

Steam Boiler components

External water treatment:

1. Make up water treatment

2. Pre-heater for make up water

3. Make up water storage section

4. Feed water deaerator

5. Feed water pre-heater economizer

6. Feed water treatment with oxygen scavenger

Boiler

1. Steam drum (to produce steam)

2. Mud drum (to collect sediments in the boiler water)

3. Boiler tube (connecting steam drum and mud drum)

4. Super heater (super heating steam to higher temperature)

5. Steam header (distributing steam to the process equipments)

6. Blow down – drum

7. Flash drum (to collect low pressure steam and utilize for deaerator heating)

Flow diagram of water side steam boiler

Make up Water

Preheater

Steam Header

Deaerator

Condensate Meter

return

Meter

Super Heater

Meter

Continuous Blow

Drain Down

Preheater

Steam

Water return

Boiler tube

Flash

Drum

Mud Drum

Intermittent blow down

Make up

water Storage

tank

Steam

Drum

Storage section

Make up water

Make up water is from external source and should be treated and stored.

The condensed steam returned from distillation system (condensate return) is

combined with make up water to make boiler feed water.

Deaerator

The feed water is contacted with steam to strip dissolved oxygen.

The oxygen scavenger is added before the feed water goes to storage system.

Inside steam boilers

In water tube boilers a mud drum is provided at the lowest point of water tubes to

remove any sludge accumulation.

In the steam drum steam and water are separated.

This steam is further heated to generate super heated steam.

Usually feed water is added to steam drum.

Chemical feed for internal boiler treatment may be added to steam drum.

The blow down is discharged to flash drum to recover low pressure steam and use the

steam for deaerator.

The continuous blow down water may be used to preheat make up water by heat

exchanger.

Steam Header

The steam from boiler is discharged to a header for distribution to the process system.

The condensate from steam is returned to deaerator.

Metering

These are fitted to know the quantities of water, steam, etc.

TDS and Alkalinity

TDS in boiler: TDS comes from Make up water and chemicals added and get

concentrated as water is converted to steam. The control of TDS level is critical in

boiler operation. High TDS level gives high boiler efficiency. Too high TDS interfere

in the boiler operation. Blow down is necessary to control TDS. Water is removed

continuously from steam drum. It is advisable to keep the TDS in the boiler water just

below the maximum allowed in order to save fuel, water and treatment chemicals.

TDS is calculated from conductivity. For boiler with polymer dispersant, the TDS =

0.7 times conductivity. For boilers with tannin the factor was 0.7 – 1.0.

Alkalinity

The alkalinity comes from hydroxyl and carbonate ions. Hydroxyl alkalinity

(causticity) is necessary to protect the boilers against corrosion. The high causticity

causes foaming. This excess caustic will attack boiler causing “embrittlement”.

Phosphate program removes calcium as calcium phosphate. This is precipitated and

can be removed by blow down.

The pH should be kept above 11.0 to prevent the formation of sticky sludge that

adheres to boiler surface.

Causticity can be caused by the formation of sodium hydroxide from bicarbonate.

Bicarbonate forms carbonate, carbon dioxide and water at high temperature. The

carbonate hydrolyzes to give hydroxide and carbon dioxide. The carbon dioxide

produced increases the acidity of steam.

Blow down calculation

F = E + B

F = Feed water (lb/hr)

E = Steam generated (lb/hr)

B = Blow down (lb/hr)

Blow down is related to feed water and cycle of concentration as follows:

C = F/B or F = BC

C = COC

F = Feed Water (lb/hr)

B = Blow down (lb/hr)

Blow down as % is expressed as follows:

%B = 100/C

F = E + B

F = BC

Then,

BC = E + B

B(C-1) = E

B = E/(C-1)

Determination of Feed water requirement

Feed water = Make up water + Condensate Return

Example

A boiler with 450 psig operates at 5000 μmhos conductivity. The feed water

conductivity is 250 μmhos. It produces 40,000 lbs of steam/hr

Then C = 5000/250 = 20

%B = 100/C = 100/20 = 5

The blow down is B = E/(C-1) = 40000/(20-1) = 40000/19 = 2105

The feed water is

F = E + B = 40000 + 2105 = 42105 lbs/hr or 42105/8.33 = 5054gal/hr

Determination of Make water requirement

Make up water is the water from the external water treatment system provided to the

deaerator. The amount of make up is affected by blow down, steam leaks,

consumption of steam and condensate leaks.

In systems where steam is not used for process about 5 to 10 % make up is required.

M = Feed – condensate return

Usually condensate has negligible conductivity, then

% M = Feed water conductivity x 100

Make up conductivity

Example 2

Make up conductivity is 900 μmhos, for the above boiler, then

% M = 275 x 100/900 = 30%

So make up water is 30% of feed water

%R = 100 - %M = 100 – 30 = 70%

Make up quantity = Feed x %M = 42105 lbs/hr x 0.3 = 12632 lbs/hr

Condensate return = 42105 – 12632 = 29473lb/hr

The difference between steam produced and condensate return represents both steam

and condensate loses.

Loss = E-R = (steam generated – condensate return) = 40000 – 29473 = 10527 lb/hr

This is the loss

Regular calculation will show the system performance. It indicates steam loss through

leaks and the development of leaks through running.

Chemical treatment

If the above boiler has to be operated with phosphate level of 60ppm (as PO4),

Phosphate loss due to blow down can be calculated:

= Blow down (lb/hr) x 60 ppm x 24 hr/day

1000,000

= 2105 x60 x24/1000, 000 = 3.03 lbs/day

If sodium hexa meta phosphate is selected for treatment (contains 90.5% phosphate),

then 1lb chemical contains 0.9105 lb phosphate.

Then chemical required = 3.03/0.905 = 3.33lb/hr

Boiler water treatment and control

Deposit formation

The dissolved solids form sludge as water gets concentrated during steam generation.

So solids like Calcium and magnesium salts become less soluble as temperature

increases (CaCO3, CaSO4, MgCO3, Mg(OH)2).

They form scale in the hotter areas of the boiler.

When a steam bubble forms on a heater surface a thin film of water become

concentrated with dissolved materials and the temperature may be 30OF above the

average boiler temperature. This causes precipitation of solids.

Problem due to scaling

The steam boilers use an external heat source much hotter than the boiler water. The

metal tubes in the boiler are kept cooled by the boiler water. The scale acts as

insulating material and the metal tube operates at high temperature. The tube can loss

tensile strength and rupture at high temperature.

Common scales in boilers

The common scales are calcium and magnesium salts and iron salts.

Calcium carbonate is from the break down of calcium bicarbonate.

Calcium sulfate – this is found in boilers when proper treatment is not taking place –

high hardness and low alkalinity.

Calcium phosphate – this will form sludge which can be removed by blow down (if

pH is below 11.0, then it deposits).

Magnesium phosphate – this will form when the hydroxide content and silica content

are low.

Magnesium silicate – if pH is above 11.0, the silica limit is more than half the

phosphate level, and then this deposit is formed.

Iron oxide – it is formed from dissolved iron in feed water and hydroxide in boiler

water. The dissolved iron comes from condensate return. This is removed by blow

down. it will deposit if a sludge conditioner is not present.

Boiler water internal treatment

The hardness that enter boiler should be treated to form sludge rather than becoming

scale deposits.

1. Low pressure boilers (15 psig and below)

In LPB, with no make up water and blow down, sodium hydroxide is added to

maintain a pH of 10.5 to 11.5. This prevents corrosion.

Total Alkalinity (M-alkalinity) greater than 50 ppm (as CaCO3) in the feed

water will provide sufficient causticity in the boiler.

2. High pressure boilers (above 15 psig)

Phosphate forms sludge with calcium and magnesium. Sodium hydroxide may

be required to maintain hydroxyl alkalinity.

Sodium hexa meta phosphate is used as it is inexpensive and has high

phosphate level (90.5%).

Sodium tri poly phosphate is also economical

Sodium hexa meta phosphate and tetra pyro phosphate may be added to feed

water or directly to steam drum

Di-sodium phosphate must be added to steam drum. This should not be added

to feed water as it forms calcium phosphate scale readily.

Prevention of sludge deposits

Synthetic organic compounds are added to avoid sludge deposits. The organic added

disturbs the crystal structure of the scale and prevent from scale formation.

In LPB – Tannin is used. It also absorbs a small amount of dissolved oxygen.

Poly acrylates, copolymer of methacrylate and sulfonated styrene are preferred

dispersants. The co polymer is best for feed water with iron concentration exceeding

0.1 ppm.

Water carryover in steam

The steam should not contain any contaminants or water.

This mist formed during boiling and bubbling of water is carried away by steam, this

mist will have the same level of dissolved solids as in boiler water.

Foaming carrier – alkalinity, TDS and suspended solids interact and create foam in

boilers.

Heavy foam causes carry over of liquid with steam.

So maintain total alkalinity less than 20 % of the TDS and total suspended solids 8 %

of TDS and these levels can control foaming.

Priming carryover

Liquid surge in to the steam drum can cause water to be carried by steam. This

priming is caused by mechanical problems. There is no chemical to control this.

Silica carryover

Silica can volatilize and enter the steam. It forms a deposit when condensed. This is

controlled by maintaining low silica in the boiler water.

Determination of carryover

If steam condensate conductivity is less than 25 μmhos, then carryover will be

insignificant.

High conductivity shows either carryover or leakage into steam condensate system

Hardness must be checked. If hardness is found in condensate then, there is leakage of

raw water into the condensate.

Silica level in boilers

Boiler pressure Silica level

0 -15 150 ppm as SiO2

16-149 150 ppm

150 – 299 150 ppm

300 – 449 90 ppm

450 – 599 40 ppm

600 – 749 30 ppm

750 20 ppm

Silica deposit in the steam condensate system should be analyzed.

The deposits that are mostly magnetic oxide are due to corrosion of iron. This is

controlled by using neutralizing amines.

If the deposit is silica, this can be reduced by blow down or by removing silica from

make up water. If the deposits are mostly sodium salts (NaOH, Na2CO3, Na2SO4,

Na3PO4) they came into steam by carryover.

Additional anti- foam should be added and the equipment should be inspected for any

mechanical causes.

Removal of oxygen from feed water

Mixture of oxygen and water is very corrosive. It doubles with every 18oF increase in

temperature.

Oxygen corrosion is recognized by pits in the top of the steam drum or at the water

line. Oxygen can be removed from feed water by mechanical or chemical deaeration.

Mechanical – the make up and condensate return is contacted with live steam, using

trays, sprays or both. The oxygen is vented along with a small amount of steam.

Check that a plume of steam is always flowing through the deaerator vent. The

pressure of the deaerator and temperature of the outlet water must be controlled.

Deaerator must operate at + 3 psig or more. The temperature of water should be

within 2oF of the temperature given below.

Deaerator water outlet temperature for boiler systems at various pressure at sea

level

Deaerator

pressure, psig

Deaerator outlet water

temperature oF

Deaerator

pressure,psig

Deaerator outlet water

temperature, oF

0 212 11 241.6

1 212.3 12 244.4

2 218.5 13 246.4

3 221.5 14 248.4

4 224.4 15 250.3

5 227.1 16 252.2

6 229.8 17 254.1

7 232.2 18 255.3

8 234.8 19 257.0

9 237.1 20 258.8

10 237.4

Level of sulfite to be carried in boiler water

Boiler pressure, psig Sulfite residual (ppm SO3)

0 -15 20-40

16 -149 20-40

150 - 299 20-40

300 – 449 20-40

450 – 599 20-40

600 - 749 15-30

750 15-30

Condensate corrosion

Oxygen and carbon dioxide promote corrosion in condensate line. Oxygen corrosion

is prevented by removing it in the boiler and preventing leaks into the condensate.

Estimating CO2 in steam

1. CO2 in fed water is removed by deaerator. Bicarbonates and carbonates in

boiler under the influence of heat liberate CO2 which goes to steam. CO2 level

in steam can be estimated from bicarbonate and carbonate concentration in

feed water.

CO2 = Bicarbonate x 0.79 + Carbonate x 0.35

= (M-2P) x 0.79 + 2P x 0.35

M is methyl alkalinity and P is phenolphthalein alkalinity

Example

If P = 2ppm and M= 15ppm

Then

Bicarbonate CO2 = [15-2x2] x 0.79 = 8.7

Carbonate CO2 = [2x2x0.35 = 1.4

CO2 = 8.7 + 1.4 = 10.1 ppm CO2 in steam.

The CO2 corrosion causes the thinning of condensate line – mainly at the bottom of

the pipe and at pipe threads.

Control by neutralizing amine

Control limits: use volatile amines. Add amine to keep pH in condensate between 7.5

and 8.0. it should not fall below 7.5.the chemicals are fed to the boiler steam drum.

Continuous feeding is required.

Morpholine, diethyl amino ethanol, and cyclo hexyl amine are the amines used.

Morpholine: It has a Low Vapor Liquid Distribution Ratio (VLDR) and will drop out

of steam very early. It can be used short to moderate length condensate line. It can be

used in high pressure system only because of its high boiling point. Very little

Morpholine is lost during deaeration in the deaerator from condensate return.

DEAE: it has a VLDR between that of Morpholine and cyclo hexyl amine. It is a

good choice for moderate length condensate line.

The low boiling point of DEAE + water mixture makes DEAE suitable for low

pressure boilers as well as high pressure boilers.

Cyclo hexyl amine: it has a high VLDR. It protects extremely long condensate system

is necessary to treat long systems with Morpholine to protect the early part of the

system where steam starts condensing. Cyclo hexyl amine can also be used in low

pressure boilers.

Cyclo hexyl amine should not be used when feed water alkalinity exceeds 75 ppm

because of the low solubility of cyclo hexyl amine bicarbonate. This bicarbonate

deposits in flow areas at the far end of the condensate return. This can be avoided by

reducing feed water alkalinity or by using DEAE.

Physical and chemical properties of neutralizing amines

Morpholine DEAE Cyclo hexyl amine

BP oF (100%) 264 325 273

BP oF

(amine-water azeotrope)

- 210 205

Decomposition temp oF 644 794 626

VLDR 0.4 1.7 4.7

Sp Gr (100% amine) 1.002 0.88 0.86

pH, 100 ppm solution 9.7 10.3 10.7

Amount of amine

required to maintain pH

= 8.0 in water containing

10 ppm CO2

37 22 15

Neutralizing amine select chart

Amine Low pressure High pressure (above 15 psig)

Short system

(< 800ft)

Medium

system

(<1mile)

Long

system

(>1 mile)

Morpholine X

DEAE X X X

Cyclo hexyl

amine

X

(Cyclo

hexyl amine

+

Morpholine)

X X

A mixture of Morpholine and cyclo hexyl amine can be used to provide full protection

in medium and large system. The optimum blend is determined by measuring actual

pH at various locations in the condensate return system.

If sample from far section have a low pH than other samples, increase cyclo hexyl

amine in the mixture and vice versa.

Another pH survey should be made whenever the ratio is changed; an initial blend of

1 part of CHA and 3 parts of Morpholine is a good starting point.

Control of filming amine

0.7 to 1 ppm filming amine – octadecyl amine- is used. They will coat the condensate

pipe and prevent CO2 in the water from coming in to contact with the pipe wall.

If there is high in leakage of air, then filming amine are better to use.

But these amines are not recommended for system that have had corrosion problem in

the past. They must be added directly to steam header through a quill instead of to the

steam drum. Inadequate dosage can result accelerated pitting type corrosion due to

incomplete surface coverage.

Steam Boiler – chemical treatment calculations

Steam boiler classification

1. High Pressure (HP) steam boiler = Pressure 40 bar; Temperature = 320oC

2. Medium Pressure (MP) steam boiler = Pressure 10 bar, Temperature = 220oC

3. Low Pressure (LP) steam boiler = Pressure 5 bar, Temperature = 160oC

Note: Temperature corresponding to pressure can be found out from standard steam

tables

Conversion

Kg/cm2g can be converted to psig by multiplying with 14.223

Example

A boiler has normal pressure 12 kg/cm2g. To convert this to psig,

= 12 x 14. 223 = 170.68 psig

Capacity

Boilers are expressed in hp, lbs steam/hr, kgs steam/hr, BTU/hr or MBTU/hr

Conversion

Unit Convert to By multiplying with

hp Lbs steam/hr 34.5

Lbs/hr Kgs/hr 0.453592

Kgs/hr Lbs/hr 2.205

Kgs/hr Tons/hr 0.001

hp BTU/hr 33472

BTU/hr kBTU/hr 0.001

Note: 1Boiler Horse Power (BHP) = 13.1547 times a normal horse power

Example

100hp = 100 x 34.5 = 13450 lbs of steam/hr

100hp = 100 x 33475 = 3347200 BTU/hr = 3347.2 kBTU/hr

So 1lbs of steam/hr = 0.24886 kBTU/hr = 248.86 BTU/hr

Boiler water limits

Limits on Boiler water conditions for an effective treatment program

Boiler

pressure

psig

TDS max

ppm

Silica max

ppm

Sulfite

range

ppm SO3

Orthophosphate

range

ppm PO4

Hydroxyl

alkalinity

ppm OH

0-15 3500 150 20-40 30-60 20-200

16-149 3500 150 20-40 30-60 20-200

150-299 3500 150 20-40 30-60 20-200

300-449 3000 90 20-40 30-60 15-200

450-599 2500 40 20-40 30-60 13-180

600-749 2000 30 15-30 30-60 13-180

750 2000 20 15-30 30-60 13-100

Example

A MP steam boiler of class 12kg/cm2 has design capacity 4000 kg/hr. There are two

such boilers operating simultaneously. The following data are given for the boiler

operation.

Condensate return, % = 25%

Cycle factor = 40

Feed water specification:

Dissolved oxygen = 0.2ppm

Calcium hardness = 4ppm

Feed water alkalinity = 17.5ppm

Monitoring

Sulfite residual = 20ppm

Ortho phosphate = 14ppm

Demineralized water is used as make up water.

Recommended DM specification:

Conductivity = 10μS/cm max

Silica = 0.5ppm max

Chloride = 5.0mg/l max

Hardness = nil

TDS = 25ppm max

Recommend the dosage of sulfite and phosphate

Calculation

1. Pressure

The data shows the boiler is 12kg/cm2

pressure

Converting to psig

12 x 14.223 = 170.68psig

For such boilers the limits are

TDS = 3500ppm

Silica = 150ppm

Sulfite – 20-40ppm

Phosphate = 30-60ppm

Hydroxyl Alkalinity = 20-200ppm

2. Capacity

The capacity of the boiler is 4000kg/hr.

Converting to Tons/day

4000 x 24/1000 = 96Tons/day

There are 2 boilers, then total

= 96 x 2 = 192Tons/day

Example calculation

So total steam generated is 192 Tons/day

Data shows condensate return as 25% of this value

So condensate return = 192 x 0.25 = 48 Tons/day

3. Blow down

Cycle of Concentration is given as 40

% Blow down = 100/C= 100/40 = 2.5

Steam generated = 192 Tons/day

So blow down = 192 x 2.5/100 = 4.8 Tons/day

4. Feed water required

F = E + B = 192 +4.8 = 196.8 Tons/day

5. Make up water required

M = F-R

R is the condensate return which is given as 25% of Steam generated and equals to

48 Tons/day

M = 196.8 – 48 = 148.8 Tons/day

6. Steam used for process

L = E – R = 192 – 48 = 144 Tons/day

Summary

48 T/d 144 T/d Condensate Return (R) Steam Lost (L)/used 192 T/d Steam generated (E)

148.8 T/d 196.8 T/d Make up (M) Feed water (F)

4.8 T/d

Blow Down (B)

Deaerator

Steam generated: 192 T/day

Condensate Return: 25 % = 48 T/day

Cycle Factor: 40

% Blow Down: 100/40 = 2.5% = 192x2.5/100 = 4.8 T/day

Feed Water: E + B = 192 + 4.8 = 196.8 T/day

Make up Water F – R = 196.8 – 48 = 148.8 T/day

Steam Lost: E – R = 192 – 48 = 144 T/day

Chemical requirement:

Feed water Oxygen 0.2 ppm

Feed water Ca Hardness 4 ppm

Feed water total Alkalinity 17.5 ppm

Sulfite level 20 ppm

Phosphate level 14 ppm

1. Phosphate Requirement

Phosphate level, 14ppm

1. Blow down loss

Blow Down: 4.8 T/day

Phosphate loss: 4.8x14ppm/1000, 000 = 4.8x14x1000/1000, 000 kg/day

0.0672 kg/day = 0.148 lbs/day

If Sodium Hexa meta phosphate is used then it contains 90.5 % PO4

Phosphate loss 0.148x100/90.5 = 0.1637 lbs/day

Steam Boiler

2. Phosphate for Hardness

Feed water 196.8 Tons/day = 196.8x1000 kg/3.785 = 51995 gallons/day

For sodium hexa meta phosphate 1ppm Hardness require 0.0056 lbs of phosphate

per gallon feed water.

So for 4ppm (given Data) hardness the required phosphate is:

4x0.0056lbs/1000 gallons = 0.0224 lbs /1000 gallons

For 51995 gallons of feed water:

0.0224x51995/1000 = 1.1647 lbs/day

Total sodium hexa meta phosphate required:

0.1637 + 1.1647 lbs/day = 1.3284 lb/day

= 1.3284/2.205 = 0.6024kg/day

Phosphate, Caustic & Polymer requirements for boilers.

Chemical Formula %PO4 PO4 required/1000

gallons feed water

per ppm hardness

(lbs)

Lbs of NaOH

required/100lbs

phosphate treatment

Lbs 100%

active

polymer per

pound

phosphate

Di-sodium

phosphate Na3PO4

12H2O

26.0 0.02 11 0.03

Di sodium

phosphate Na3PO4 65.7 0.0082 28 0.07

Tri sodium

phosphate Na3PO4

12H2O

25.1 0.021 0 0.03

Tri sodium

phosphate Na3PO4

H2O

52.0 0.01 0 0.06

Sodium tri

polyphosphate Na5P3O10

6H2O

61.1 0.0088 33.6 0.07

Sodium tri

polyphosphate Na5P3O10 76.4 0.0068 43.5 0.09

Tetra sodium

pyrophosphate Na4P2O7 71.0 0.0072 30.08 0.08

Tetra sodium

pyrophosphate Na4P2O7

10H2O

42.7 0.012 17.9 0.05

Sodium hexa

meta

phosphate

(NaPO3)6 90.5 0.0056 78.4 0.10

2. Sodium Hydroxide requirement

For 100 lbs of phosphate 78.4 lbs of NaOH is required.

NaOH needed =1.3284x78.4/100 lbs of NaOH = 1.04 lbs /day

3. Synthetic polymer requirement

For each lb of PO4 require 0.1 lb of 100% synthetic polymer

= 0.1 x 1.3284 = 0.1328 lb/day of 100% synthetic polymer

If polymer is 50 % then 50x 0.1328/100= 0.0664lb/day

4. Sodium sulfite requirement

Sulfite is at 20 ppm level

1. Lost by blow down

4.8x1000x20/1000, 000 = 0.096 kg/day = 0.21168 lb/day

1 lb of sulfite for every 1.575 lb of sodium sulfite

So sodium sulfite required: 0.21168x1.575 lb/day = 0.333lb/day

Requirements per day:

Chemical Feed water 196.8T/day

Full capacity (100%)

Lbs Kgs

Sodium hexa meta phosphate 1.328lbs 0.602kg

Sodium Hydroxide 1.04 lbs 0.472kg

Synthetic polymer (50%) 0.0664 lbs 0.0301kg

Sodium sulfite 0.333 lbs 0.151kg

Phosphate calculation is based on sodium hexa meta phosphate and sulfite on sodium

sulfite just for convenience.

Chemical treatment

If Nalco 1742 is used for phosphate treatment then dosage is calculated as follows.

100 ml of Nalco 1742 contains 3.728gms of PO4. Nalco 1742 contains sodium tri

polyphosphate. Tri polyphosphate contains 76.4% of PO4 (refer the table given

above).

Phosphate loss by blow down and its compensation

Calculation shows that 0.148lbs/day phosphate is lost through blow down.

This corresponds to 0.148 x 100/76.4 = 0.1937 lbs/day (as tri polyphosphate)

Phosphate loss for hardness

= 4 x 0.0068 x 51995/1000 = 1.41426 lbs/day

(Note: 0.0068 is from the above table)

So total sodium tri polyphosphate required

0.1937 + 1.41426 = 1.60796 lbs/day = 1.60796/2.205 kgs/day = 0.7292 kgs/day.

Nalco 1742 is containing 3.728% PO4

So 3.728kg in 100 Lt (specific gravity is taken as 1.00 for convenience)

0.7292kg require 100 x 0.7292/3.728 = 19.56 Lt/day of Nalco 1742

So hourly dosage is 19.56/24 = 0.815 Lt/hr

Dosage may be adjusted to 0.815 Lt/hr

The dosing pump shows number of stroke per minute. The calibration chart of a

metering pump is shown below. The chart shows relation between pump strokes per

minute and the amount of liquid pumped as dosing rate, liters of liquid per hour.

DOSING CURVE OF METERING PUMPS

0

2

4

6

8

10

0 20 40 60 80 100

PUMP STROKES

DO

SIN

G R

AT

ES

, L

t/H

r

In the above example the dosing pump should be set at a stroke rate of 7strokes per

minute for the pumping of 0.815Lt per hour.

Sulfite treatment to remove dissolved oxygen

Sulfite is at 20 ppm level: Blow down loss

Lost by blow down = 4.8x1000x20/1000, 000 = 0.096 kg/day = 0.21168 lb/day

Nalco product N780 contains sodium meta bisulfite. This on dilution by 100 times

will give a solution containing 0.357% sodium bisulfite.

1lb of sulfite needs 1.1875 lbs of sodium meta bisulfite

So sodium meta bisulfite needed = 0.096 x 1.1875 kgs/day = 0.114 kgs/day

100 ml of 100 times diluted Nalco 780 contains 0.357% meta bi sulfite

0.114kg/day needed 100 x 0.114/0.357 = 31.93 Lt/day = 1.33 Lt/hr

The dozing pump scale is in number of stroke per minute. The calibration chart of an

oxygen scavenger pump is given below. The chart shows relation between pump

strokes per minute and the amount of liquid pumped as dosing rate, liters of liquid per

hour.

CALIBRATION OF OYGEN SCAVANGER DOZING

PUMP

0

1

2

3

4

5

6

7

8

9

0 20 40 60 80 100

PUMP STROKE PER MINUTE

DO

ZIN

G R

AT

ES

, L

t/H

r

From the calibration curve, for 1.33lt/hr dozing, the pump should be set at 11.5

strokes per minute.

General information on boiler treatment - summary

A complete program for boiler water may need the following treatments

1. Softening of Make up water.

2. Internal treatment

2.1. Scale formation

2.2. Sludge conditioning

2.3. Corrosion control (O2 & CO2)

2.4. Foam control

2.5. Blow down control

The treatment methods authorized for meeting these need include the following steps.

1. Softening of Make up water – removal of Ca & Mg.

2. Phosphate addition – to prevent formation of CaCO3 and CaSO4 scales

3. Synthetic polymer addition – formation of sludge with proper characteristics.

4. Alkalinity addition – (sodium hydroxide) to prevent magnesium scale

formation

5. Sulfite addition – to prevent oxygen corrosion

6. Amine addition – to prevent CO2 corrosion.

7. Anti-foam agent addition – control foam in steam drum

8. Blow adown adjustment – to ensure a silica content in low to prevent

carryover.

9. TDS level in low – to minimize foaming

10. The causticity and suspended solids low enough to prevent foaming or

carryover.

Limits on Boiler water conditions for an effective treatment program

Boiler

pressure

psig

TDS max

ppm

Silica max

ppm

Sulfite

range

ppm SO3

Orthophosphate

range

ppm PO4

Hydroxyl

alkalinity

ppm OH

0-15 3500 150 20-40 30-60 20-200

16-149 3500 150 20-40 30-60 20-200

150-299 3500 150 20-40 30-60 20-200

300-449 3000 90 20-40 30-60 15-200

450-599 2500 40 20-40 30-60 13-180

600-749 2000 30 15-30 30-60 13-180

750 2000 20 15-30 30-60 13-100

Determination of blow down required

Example 1

A boiler operates at 150 psig. The feed water contains 4 ppm silica and 225 ppm TDS.

What is the parameter that control COC?

By the table for 150psig boiler, the limits are TDS = 3500 ppm and Silica = 150 ppm

Maximum COC

Silica COC = B/F = 150/4 = 37.5 COC

TDS COC = 3500/225 = 15.6COC

COC based on TDS is the lower one. So it will be the controlling parameter for this

boiler. So maintain TDS below 3500 to keep silica below 150 ppm.

Example 2

A boiler operates at 450 psig. Feed water has silica 4ppm and TDS 225 ppm.

What is the parameter that control COC?

By the table for 450psig boiler, the limits are TDS = 2500 ppm and Silica = 40 ppm

Maximum COC

Silica COC = 40/4 = 10

TDS COC = 2500.225 = 11.1

COC based on silica is lower. Maintain the boiler blow down to keep silica below 40

ppm.

But it is easier to measure TDS than silica. So it will be more convenient to control

TDS. So keep TDS to occur below 10COC.

For 10COC, TDS = 225 x 10 = 2250 pm

If conductivity is checked, then 2250/0.7 = 3214 μmhos

Example 3

A 150psig boiler produces 40000 lbs of steam /hr. the blow down is calculated based

on maximum COC of 15.6

B = E/(C-1) = 40000/(15.6-1) = 2740lbs/hr

Blow per day = 2740 x 24 = 65760

Blow down gal/day = 65760/8.33 = 7894 gal/day

Example 4

A boiler of 450psig and with 40000 lbs of steam /hr, has maximum COC = 10.

Calculate blow down.

B= E/(C-1) = 40000/10-1 = 4444 lbs/hr

Blow down per day = 4444 x 24 = 106656 lbs/day

Blow down in gal/day = 106656/8.33 = 12804 gal/day.

Determination of phosphate required to replace blow down loss

Example 5

A boiler of 150 psig used sodium hexa meta phosphate and kept at 60 ppm phosphate

level. The blow down calculated is 65760 lbs/day. Calculate phosphate loss.

Phosphate loss = 65760 x 60/1000000 = 3.95lbs/day

Sodium hexa meta phosphate contains 90.5% phosphate

Required chemical = 3.95/0.905 = 4.36 lbs/day

If di sodium phosphate is used it contains 65.7 % of phosphate,

Required chemical = 3.95/0.657 = 6.01 lbs/day.

Example 6

A boiler 450 psig make a blow down of 106656lbs/day and phosphate level is 60

ppm. Calculate phosphate loss.

Loss = 106656 x 60/1000000 = 6.4 lbs/day

Chemical required (sodium hexa meta phosphate) = 6.4/0.905 = 7.07 lbs/day.

Di sodium phosphate = 6.4/.657 = 9.74lbs/day

Determination of phosphate for hardness

Any calcium or magnesium hardness reacts with phosphate added to form precipitate.

Additional phosphate required to react with the hardness is given below as pounds of

phosphate chemical required per 1000 gallons feed water per ppm hardness.

Phosphate, Caustic & Polymer requirements for boilers.

Chemical Formula %PO4 PO4 required/1000

gallons feed water

per ppm hardness

(lbs)

Lbs of NaOH

required/100lbs

phosphate treatment

Lbs 100%

active

polymer per

pound

phosphate

Di-sodium

phosphate Na3PO4

12H2O

26.0 0.02 11 0.03

Di sodium

phosphate Na3PO4 65.7 0.0082 28 0.07

Tri sodium

phosphate Na3PO4

12H2O

25.1 0.021 0 0.03

Tri sodium

phosphate Na3PO4

H2O

52.0 0.01 0 0.06

Sodium tri

polyphosphate Na5P3O10

6H2O

61.1 0.0088 33.6 0.07

Sodium tri

polyphosphate Na5P3O10 76.4 0.0068 43.5 0.09

Tetra sodium

pyrophosphate Na4P2O7 71.0 0.0072 30.08 0.08

Tetra sodium

pyrophosphate Na4P2O7

10H2O

42.7 0.012 17.9 0.05

Sodium hexa

meta

phosphate

(NaPO3)6 90.5 0.0056 78.4 0.10

The total dose of feed water is the sum of the requirement for maintaining 30 – 60

ppm phosphate level and the hardness reaction.

The chemical dosage should be gradually increased or decreased as necessary (about

10% per day) to maintain the desired level.

Example 7

A boiler 150psig has capacity 40,000 lbs steam/hr and the blow down is 2740 lbs/hr.

calculate total feed water. Feed water has hardness of 5 ppm. Calculate phosphate

required.

F = E + B = 40000 + 2740 = 42740 lbs/hr = 42740 x 24 = 1025760 lbs/ day

= 1025760/8.33 = 123140 gal/day.

Feed hardness is 5 ppm.

From above table phosphate required for 1000 gallons of feed water/ppm hardness is,

0.0056 lbs for sodium hexa meta phosphate.

So chemical requirement is 5 x 0.0056 = 0.028 lbs/1000 gallons

For 123140 gal/day, 0.028 x 123140/1000 = 3.45lbs/day.

To maintain a phosphate level of 60 ppm, the required sodium hexa meta phosphate is

4.36 lbs/day (Refer example 6)

So total hexa meta phosphate required = 3.45 + 4.36 = 7.81 lbs/day

Determination of Caustic Soda required

The boiler water hydroxyl alkalinity should be maintained between 20 – 200 ppm to

avoid corrosion and magnesium phosphate sludge formation.

Example 8

Calculate sodium hydroxide needed in the example 7.

The amount of sodium hydroxide required for 100 pounds of phosphate is 78.4 lbs

(for sodium hexa meta phosphate)

So sodium hydroxide needed for 7.81 lbs on phosphate is 7.81 x 78.4/100 = 6.12 lbs

of sodium hydroxide.

Example 9

A boiler 450psig requires 9.74lbs per day of anhydrous disodium phosphate to replace

loss in blow down. Calculate sodium hydroxide needed.

From the above table, 28 ponds of anhydrous disodium phosphate is required per 100

pounds of phosphate chemical.

For 9.74 lbs of phosphate, NaOH needed is = 9.74 x 28/100 = 2.73 lbs/day.

Determination of synthetic polymer dosage

There is no easy method to determine the concentration of polymer in boiler water. So

to ensure adequate polymer level, 0.1 pounds of polymer per pound of available PO4

in the phosphate chemical is used.

Example 10

A boiler 150psig requires 7.81 lbs hexa meta phosphate to maintain 60 ppm PO4 level.

Calculate polymer requirement (use above table).

Table shows 0.1 pounds of polymer per pound of sodium hexa meta phosphate.

For 7.81 lbs of phosphate the required polymer is = 7.81 x 0.1 = 0.781lbs/day of

100% polymer.

If the polymer used is 50%, then the requirement is = 7.81 x 0.1 x 100/50 =

1.56lbs/day.

Determination of sulfite required

The sulfite levels to be maintained in different types of boilers are:

0 – 599 psig require 20 – 40 ppm of sulfite as SO3.

600-750 psig require 15 – 30 ppm of sulfite as SO3.

If there is no deaeration, the sulfite increases. In such cases, the initial dosage is

calculated using the middle of the range given above and then adjusted to maintain the

desired level.

Example 11

For the boiler, 150psig, in example 3, the required blow down is 65760 lbs/day.

Calculate the sulfite required.

For boilers of class 150psig, the required sulfite range is 20 to 40 ppm as SO3. if we

take middle value, then it is 30ppm.

Sulfite required is = 30 x 65760/1000000 = 1.97 lbs/day.

For each pound sulfite (SO3), 1.575 pounds of sodium sulfite is required.

Then = 1.97 x 1.575 = 3.10lbs/day.

Example 12

For boiler, 450psig, in example 4, the blow down rate is 106656 lbs/day. Calculate the

required sulfite.

The required sulfite level is 20-40ppm SO3.

Sulfite = 106656 x 30/1000000 = 3.20lb/day.

Sodium sulfite = 3.20 x 1.575 = 5.04lbs/day.

Determination of Amine dosage

These chemicals are consumed based on the feed water rate rather than blow down.

Summary of required Calculation

For a boiler that uses make up water and blow down the calculation steps are as given

below:

1. Estimate the blow down rate based on the COC and steam rate.

2. Determine phosphate required to maintain a level 30-60ppm of PO4 in the

boiler based on the chemical loss due to blow down.

3. Determine the phosphate required to react with the hardness in the feed water.

4. Determine the sodium hydroxide required, based on the total phosphate added

to the system (blow down loss + hardness reaction).

5. Determine the synthetic polymer required.

6. Determine the sulfite dose based on the blow down loss

7. Determine amine dose based on feed water rate and amine level

Example 13

Boiler spec:

Pressure = 150psig

Steam generation = 40,000 lbs/hr

Feed water spec:

TDS = 225 ppm

SiO2 = 4 ppm

Hardness = 5ppm

1. COC is 15.6 based on TDS (example 1)

2. Blow down = 65760 lb/day (example 3)

3. Feed water rate = 123,140 lb/day (example 7)

4. Phosphate dosage = 7.81lb/day (example 7)

5. Sodium hydroxide dosage = 6.12 (example 8)

6. Polymer dosage = 1.56 lb/day (example = 10)

7. Sodium sulfite dosage = 3.10 lb/day (example 11)

Mixing

The proper mixture for this boiler include, sodium meta phosphate, polymer and

sodium sulfite. The sodium hydroxide is not included in the mixture as it varies. And

it should be added separately.

The mixture proportion is:

Phosphate = lb of chemical x 100/lb total = 7.81 x 100/(7.81 + 1.56 + 3.10) =

781/12.47 = 762.6%

Polymer = 1.56 x 100/12.47 = 12.5%

Sodium sulfite = 3.1 x 100/12.47 = 24.9%

Feeding

1. The initial feed rate of this mixture is 12.47lbs/day

The allowed variation is within 10% of this target value

Conclusion

Most of the topics are covered in the above presentation. Any further doubt in the

subject matter will be cleared in the next session.