BOILER CORROSION&

FEEDWATER TREATMENTPRESENTED BY

MOHANKUMAR.M 709BMASANA MUTHU.M 702BNARESH KUMAR.S 716BPRASANTH.M 737B

BOILER FEEDWATERBoiler feedwater is water used to supply ("feed") a boiler to generate steam or hot water. The feedwater is usually stored, pre-heated and conditioned in a feedwater tank and forwarded into the boiler by a boiler feedwater pump.

GENERAL INFORMATION ON FEEDWATER

Suspended solids represent the undissolved matter in water, including dirt, silt, biological growth, vegetation, and insoluble organic matter.Insoluble iron can clog valves and strainers and can cause excessive sludge build up in low lying areas of a water system. It also leads to boiler deposits that can cause tube failure.

Unless your boiler receives water of proper quality, the boiler's life will be needlessly shortened.

Each water supply source requires a specific analysis. Depending upon this analysis, various pretreatment methods may be employed to prepare makeup water for your boiler feedwater system.

WHY PROPER FEEDWATER TREATMENT IS AN ABSOLUTE NECESSITY?

Scale and sludge formation

Boiler Corrosion

Caustic Embrittlement

Priming and foaming

CONDITIONING

The water required for boiler feed purposes i.e for steam generation should be of very high quality and thus requires a lot of treatment.

Untreated waters, containing impurities may lead to the following problems in boilers:

Alkalinity is a measure of the capacity of water to neutralize strong acid.

In natural waters, the capacity is attributable to bases, such as bicarbonates, carbonates, and hydroxides; as well as silicates, borates, ammonia, phosphates, and organic bases. These bases, especially bicarbonates and carbonates, break down to form carbon dioxide in steam, which is a major factor in the corrosion of condensate lines.

Alkalinity also contributes to foaming and carryover in boilers.

WATER HARDNESS Water Hardness is the measure of calcium and magnesium content as calcium carbonate equivalents. Water Hardness is the primary source of scale in boiler equipment.Silica in boiler feedwater can also cause hard dense scale with a high resistance to heat transfer.

ALKALINITY

A well-implemented program will ensure -

Increased heat transfer

Lower fuel expenditures

Lower chemical consumption

Except for steam trap maintenance, water treatment has the most potential for reducing annual operating costs.

CORROSION

Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings.

This means electrochemical oxidation of metals in reaction with an oxidant such as oxygen.

Formation of an oxide of iron due to oxidation of the iron atoms in solid solution is a well-known example of electrochemical corrosion, commonly known as rusting.

Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface.

FACTORS AFFECTING CORROSIONMany factors influence the corrosion rate such as

temperature

pH

oxygen concentration

flow rate

ammonia concentration

TYPES OF CORROSION IN BOILERS

Galvanic corrosion

Pitting corrosion

Caustic corrosion

Acidic corrosion

Hydrogen embrittlement

Oxygen attack

Carbon dioxide attack

GALVANIC CORROSION

Galvanic corrosion occurs when a metal or alloy is electrically coupled to a different metal or alloy.

The most common type of galvanic corrosion in a boiler system is caused by the contact of dissimilar metals, such as iron and copper.

Galvanic corrosion can occur at welds due to stresses in heat-affected zones or the use of different alloys in the welds.

Control & Prevention

Causes include:

•scratches in a metal surface

•differential stresses in a metal

•differences in temperature

Some protection from galvanic corrosion can be achieved if the electrolyte is not present.Without the availability of water molecules the corrosion reaction stops because the electrons cannot find a host to complete the chemical reaction. Where dissimilar metals must be used, protect them from contact with water. If water must be used in contact with dissimilar metals ensure it is deionised and oxygen free.

PITTING CORROSION

Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities.

Pitting is one of the most damaging forms of corrosion.

Pitting factor is the ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss.

PITS FORMED ON A PIPE

Proper selection of materials with known resistance to the service environment. Use higher alloys for increased resistance to pitting corrosion.Control pH, chloride concentration and temperature.

Control & Prevention

Causes

•For a defect-free "perfect" material, pitting corrosion is caused by the environment (chemistry) that may contain aggressive chemical species such as chloride. Chloride is particularly damaging to the passive film (oxide) so pitting can initiate at oxide breaks.

•For a homogeneous environment, pitting is caused by the material that may contain inclusions (MnS is the major culprit for the initiation of pitting in steels) or defects. In most cases, both the environment and the material contribute to pit initiation.

TOOLS USED TO IMPROVE WATER QUALITY

TYPES OF WATER TREATMENT

1. External treatment2. Internal treatment

•Water Treatment – conditioning the boiler water to pre-determined levels by using

variety of chemicals. •Demineralization

– the replacement of specific inorganic salts by ion exchange. •Deaeration

– the removal of dissolved oxygen and carbon dioxide by heating and atomizing the water with steam.

EXTERNAL TREATMENT

i. Clarification

ii. Deaeration

iii. Oil removal

iv. Suspended solids removal

Treatment of water that are done outside of the boiler is called pre-boiler or external treatment.The main physical methods for improving quality of water for boiler include

The purpose of external treatment is to

Coagulation with chemicals

Reverse osmosis

Electrodialysis

This purpose can be achieved by

Reduce suspended solids

Demineralize the feed water

Remove silica.

INTERNAL TREATMENT

The purpose of an internal treatment is toreact with any feed-water hardness and prevent it from precipitating on the boiler metal as scale;provide anti-foam protection and prevents foam carry-over;eliminate oxygen from the water and provide enough alkalinity to prevent boiler corrosion.

In addition, as supplementary measures an internal treatment should prevent corrosion and scaling of the feed-water system and protect against corrosion in the steam condensate systems.

In spite of various external treatments, it is not possible to attain an absolute perfect quality of boiler feed water. Chemical treatment or internal treatment of water inside the boiler is essential to take care of various impurities entering into the boiler such as hardness, dissolved solids, oxygen, and silica.

CHEMICAL ADDITION

During the conditioning process, which is an essential complement to the water treatment program, specific doses of conditioning products are added to the water. The commonly used products include:

Phosphates-dispersants, polyphosphates-dispersants (softening chemicals): • React with the alkalinity of boiler water and neutralize the hardness of

water by forming tricalcium phosphate.• And this insoluble compound can be disposed and blow down on a

continuous basis or periodically through the bottom of the boiler.

Natural and synthetic dispersants (Anti-scaling agents): • increase the dispersive properties of the conditioning products. • They can be:

1. Natural polymers: lignosulphonates, tannins2. Synthetic polymers: polyacrilates, maleic acrylate copolymer,

maleic styrene copolymer, polystyrene sulphonates etc.

Oxygen scavengers• sodium sulphite, tannis, hydrazine, hydroquinone/pyrogallol-based

derivatives, hydroxylamine derivatives, ascorbic acid derivatives, etc. • reduce the oxides and dissolved oxygen.• also passivate metal surfaces.

Anti-foaming or anti-priming agents• mixture of surface-active agents that modify the surface tension of a

liquid.• remove foam and prevent the carry over of fine water particles in

the steam.

OBJECTIVES OF WATER TREATMENT

Prevention of scaling in boiler.

Prevention of stress corrosion cracking.

Prevention of steam contamination.

Other Objectives

The treatment and conditioning of Boiler feedwatermust satisfy three main objectives:

Continuous heat exchange

Corrosion protection

Production of high quality steam

Benefits to You

              BENEFITS TO YOU!!!!!!

Maximum process productivity

Lower cost of operation

Long service life for your critical assets

Ability to focus on your core business…

THANK YOU