boilers’ feed water treatment/conditioning & رایط آب و... · pdf...

In The Name of God

Boilers’ Feed Water

Treatment/Conditioning & Optimization

By: Arjomand MehrabaniAdjusted for The Second Meeting of Steam Technology

(By Packman Company)Modified: Fall 1394

Contents

• Definitions & Terminology

• Overview & Basic concepts

• Types & Classifications

• Structure & Parts

• Feed Water & Quality

• Water treatment & Conditioning (External/Internal)

• Methods & Materials

2

A closed vessel for heating of water to

make hot water / saturated steam /

superheated steam, or any combinations

by application of heat from combustible

fuels in a self-contained or attached

furnace.

What is a Boiler?

3

STEAM GENERATION REQUIREMENTS

Container

Thermal Energy

Water

4

Basic Units in boilers

regardless of size / usage:

Feed water system

(supply quality water)

Fuel system

(supply energy source)

Draft systems

(provide air for combustion)

Combustion system

Steam system

(collect and control produced steam)

Combusted gases transfer system

(starts: furnace and terminates: flue gas stack)

Other parts (controller, instruments, valves, …)5

Fire - Tube BoilersCombusted gases pass through the tubes

while the water and steam are round the tubes

6

Water - Tube BoilersWater is circulated inside the tubes and combusted gas

passes outside of the tubes.

7

Water-

Tube

Boilers

(Riser

Tubes)

Floor

Ceil

Walls

8

Boiler Classifications

Operational Pressure

• Boilers with pressure under 200psi (~15atm) are

considered as low pressure.

• Boilers with pressure between 200 and 500psi

(~35atm) are considered as medium pressure.

•

• Boilers with operational pressure between 500 and

2000psi (~135atm) are categorized as high pressure.

9

Boiler ClassificationsWater Circulation

Types:

Natural circulation: Water circulation is based on the

density difference between the temperature in riser

and down-comer tubes.

Forced circulation: A pump is used to help the better

circulation of water in tubes and drums.

10

Water

System

11

Water Impurities

• Dissolved Solids

Ionic

Non-Ionic

• Suspended Solids and Materials

Sands, Soils

Microorganisms, Bacteria, Algae

Oils

Emulsions, Collides, Colloidal Silica

• Gases 𝐍𝐇𝟑 , 𝐂𝐇𝟒 , 𝐇𝟐𝐒 , 𝐂𝐥𝟐 , 𝐎𝟐 , 𝐂𝐎𝟐

12

Anions Cations

-HCO3+++Al

--CO3+NH4

-Cl ++Ca

-F H+

-NO3++Fe

-OH +++Fe

PO4--- ++Mg

HPO4- ++Mn

H2PO4- +K

SO4-- +Na

Ions in Natural Water

13

System of Water Quality Maintenance

Boiler water is purified for:

• Reduction of fouling deposits Enhancing of thermal efficiency

Reduction of fuel consumption,

Prevention from overheating of tubes (formation of

hot spots)

Extending of operation period

Enhancing of maintenance, reduce repairments

Increasing of system life

Increasing system safety

… 14

15

16

System of Water Quality Maintenance

Boiler water is purified for:

• Minimizing of boilers’ systems corrosionEnhance maintenance, reduce replacements

Increase system life

Meeting safety regulations.

Enhancing steam quality.

…

17

18

19

System of Water Quality Maintenance

Boiler water is purified for:

• Avoiding of steam carry over

Various forms:

foaming

priming

spray/mist carry overEnhancing steam quality

Preventing deposition of solids over turbine blades,

valves, steam pipes, …

Preventing from thermal shocks

… 20

21

System of Water Quality Maintenance

Boiler water is purified for:

• Prevention from caustic embrittlementAn inter-crystalline cracking of boiler metal

without any prior deformation that results in boiler

explosion.

This is due to:

- Leakage of water in boiler’s body and

concentrating of dissolved solids.

- Presence of stress in metal

22

Results of Poor Water Treatment:

EXPLOSION

23

Deposit control:

• Reducing hardness salts of calcium and magnesium

• Alkaline hardness: removed by boiling

• Non-alkaline: not removed by boiling (special treatment)

• Reduction in silica contents (forms hard silica scales)

Feed Water Treatment

24

External water treatment:

• Removal of suspended/dissolved solids and dissolved gases

• Pre-treatment: sedimentation and settling

• First treatment stage: removal of dissolved solids

• Major Processes

a) Lime-Soda process

b) Ion exchange (Demineralization)

c) Reverse osmoses

d) Electrodialysis

…

Feed Water Treatment

25

Temporary Hardness

Lime Soda Process

External Water Treatment

26

Removal of Temporary Hardness

3 2 2 3 2

3 2 2 3 3 2

5

SP 3

3 2 2 3

14

SP 2

____________________________________________

Ca(HCO ) Ca(OH) 2CaCO H O pH 10 10.5

Mg(HCO ) Ca(OH) MgCO CaCO 2H O

K MgCO 2.6 10

MgCO Ca(OH) Mg(OH) CaCO

K Mg(OH) 4.0 10

:

__________________________________

3 2 2 2 3 2Mg(HCO ) 2Ca(OH) Mg(OH) 2CaCO 2H O

27

4 2 3 3 2 4

2 2 2 3 2 3

4 2 3 2 2 3 2 4

CaSO Na CO CaCO Na SO

MgCl Ca(OH) Na CO Mg(OH) CaCO 2NaCl

MgSO Na CO Ca(OH) Mg(OH) CaCO Na SO

Removal of Permanent Hardness

28

با سپاس از توجه شما

Solids-contact clarifier

29

30

Coagulation/Flocculation

Coagulation of colloidal particles

31

FiltersRemoval of suspended solids

32

Ion-exchange process (softener plant)

• Water passes through bed of natural zeolite of synthetic resin to remove hardness

• Calcium (Ca) and magnesium (Mg) replaced with sodium (Na) ions

• Does not reduce TDS

External Water Treatment

1

Softener Plant

2

DM System

3

4

Osmosis

• Solutions of differing concentrations

• Separated by a semi-permeable membrane

• Water moves to the higher concentration

External Water Treatment

5

Reversed Osmosis (RO)

• Higher concentrated liquid pressurized

• Water moves in reversed direction

External Water Treatment

6

RO System

7

8

Electrodialysis

9

Electrodialysis for Water Treatment

10

External Water Treatment

&

Internal Water Conditioning

11

Prevention of internal scale by forming

deposits in sludge shape

Injection of chemicals

•Different chemicals for different water impurities

•Conditions:

For low amounts of feed water impurities

For low pressure boilers, TDS content has higher

limits

Internal treatment alone not recommended

Internal Water Conditioning

12

Scale control

Injection of chemical additives

• Minerals (Na2CO3, Na3PO4, Na2HPO4,

NaH2PO4, (NaPO3)6 ,…)

• Organics (Chelants, Lignins, Starches,

Tannins, …)

• Minerals + Organics

Chelants (NTA, EDTA, …)

Internal Water Conditioning

13

De-aeration• Removal of various dissolved gases from

water (O2, CO2, …)

• These gases are corrosive.

• De-aerator can decrease amount of dissolved

gases in water.

External Water Treatment

14

External Water Treatment

Steam

Storage Section

De-aerated

Boiler Feed

Water

Scrubber

Section (Trays)

Boiler Feed

Water

Vent

Spray Nozzles

• O2 and CO2 removed by heating

feed water

• Economical treatment process

• Vacuum type can reduce O2 to 0.02

mg/L

• Pressure type can reduce O2 to

0.005 mg/L

Mechanical de-aeration

15

Removal of Dissolved Oxygen TracePitting Corrosion control

• Sodium sulphite:

Increases TDS: increased blow down

• Hydrazine

Does not increase TDS: used in high pressure boilers16

2 3 2 2 42Na SO O 2Na SO

2 4 2 2 2 2 2N H (NH NH ) O 2H O N

Mechanical (Boiler design)

High level of water in drums

Deficiency on separator operation

Sudden increasing boiler load (reduction of pressure)

Chemical

High TDS

High alkalinity

Oil and organic impurities

Causes of Carry Over

17

Corrosion & scale control

Acid (H+) attack

Adjustment of water pH based on

manufacturer recommendation

Water Conditioning

18

Caustic embrittlement control

Adjustment of water NaNO3/NaOH ratio

depending to boiler pressure (According to

manufacturer recommendation)

Water Conditioning

19

• Controls TDS of the boiler water

• Boiler sludge conditioning

• Blows off water and replaces it with feed water

• Conductivity measured as indication of TDS levels

Boiler Blow Down

20

Benefits

• Lower pretreatment costs

• Reduced maintenance downtime

• Increased boiler life

• Lower consumption of chemicals

Boiler Blow Down

21

Types of blow down• Batch (Periodic)

• Manually operated valve reduces TDS

• Large short-term increases in feed water

• Substantial heat loss

• Continuous• Ensures constant TDS and steam purity

• Heat lost can be recovered

• Common in high-pressure boilers

Boiler Blow Down

22

• Application of proper external treatment

• Application of accurate internal conditioning

• Amounts of used chemical

• Amounts of produced wastes/sludge

• Amounts of wasted water (blow down)

• Cost of monitoring & Control

• Economy of water treatment/conditioning

• Recirculation of used steam

Water/Chemical Economics / Optimization

23

THANKS

FOR YOUR

ATTENTION