Condensate and Feedwater Corrosion and Treatment

8/3/2019 Condensate and Feedwater Corrosion and Treatment

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Condensate and Feedwater

Treatment.


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The 3 Main elements associated

with Feedwater treatment arecomponents of Water

H2O Good

O2 Bad

H+

Bad OH- Good (Most of the time)


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Types of Corrosion

General Corrosion - Hematite

Localised Corrosion


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General Corrosion

Hematite Layer


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Localised Corrosion

Pitting O2 plus Cl Often associated withscale deposit

Crevice Corrosion starts in pre-existingcrack or crevice, flanges with porous gaskets

Selective Leaching one element of a multi-element metal

Galvanic Corrosion different metal, differentzones

Stress Corrosion/Stress Cracking


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The principal Contributors to corrosion

in boiler condensate and feedwatersystems are:

Oxygen

Carbon Dioxide

Low pH

Deposits


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Oxygen Corrosion

Oxygen,dissolved in boiler water having

traces of chlorides or solids cancause pitting corrosion of metal

surfaces.

The resulting condition may be

severe, even at low pressure.


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Oxygen and other gases must

be removed from thefeedwater before they can

enter the boiler


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Oxygen Removal

can be accomplishedboth

mechanically and chemically


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A Mechanical means of

removing oxygen is called adeaerator.


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Deaeration

can be eithervacuum or pressure

enhanced.


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The Turbine Condenser


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Deaerating Feedwater Heater


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Boiler Steam/Water Drum

has a role in thedeaeration of the feedwaterat startup.


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Chemical Deaeration

is carried outby the addition of an

oxygen scavenger


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Hydrazine, Carbohydrazide

Theoretical 1mole of Hydrazine will scavenge 1 moleof Oxygen

Usually injected at 3:1 ratio to maintain a residual atentry to the boiler

Hydrazine promotes the formation of a magnetitelayer

Decomposes to form ammonia

Combines with condensate in the condenser to formNaoH

Assists in pH control.

Ammonia is highly corrosive to copper


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CO 2


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Corrosion in steam andcondensate return lines may

be caused by a low pHinduced by carbonic acid

resulting from the associationof CO

2with water.


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A buildup of acidreduces condensate pH

leading to loss of metal andgrooving of the condensate

piping.

( General Corrosion)


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The affect is accelerated in

the presence ofO2


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Volatile, alkaline, neutralising

chemicals are traditionallyused to control low pH.


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Chemicals used to control pH

in the condensate systeminclude:

ammonia

morphaline


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They function only toneutralise

are effective only againstcorrosion initiated by a low

pH


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Neutralising Amines.


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High CO2 levels will demand a greaterquantity of Neutralising Amines

Neutralising Amines are ineffectiveagainst O2

The use of a film forming Amine can be

more effective.


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Filming Amine

A long Chain Molecule having both ahydrophilic and a hydrophobic end.


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The hydrophilic end bonds to themetal forming an adherent, non-

wettable, monomolecular layer ( filmon the metal surface.

Prevents contact between the lowpH condensate and the metalbeneath as well as inhibiting O2attack.

Not really effective where pittinghas occurred

can undercut iron oxide depositscausing rapid removal and possibleproblem associated with steam trapblockage etc.


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Combineduse of the

two aminesprovides amore effective

corrosion control


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By theaddition

of anoxygen scavenger

the regimeis further enhanced.


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Condensate and Feedwater

Polishers

Resin must be suitable for Higher water

temperatures

Pick up metals scale crude etc.

Can be detrimental if they are allowed

to exhaust.

NA and Cl slip.


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Low pHin

Local Areasis the secondmost common

cause of corrosion


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Magnetite

Above 200 Deg C mild steel corrosion results in theformation of magnetite

forms a strongly adherent barrier between the boiler

water and the metal.

Corrosion stops when a uniform layer is formed.

If the layer is disrupted rapid general corrosion canoccur.

Water chemistry must be controlled to maintain thislayer.

pH is maintained > 9.0 , =< 10.5


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Phosphate Control


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Na Hydroxide and Tri Na Phosphate arecommon chemicals used to control pH.

To prevent caustic attack such as

Caustic Gouging the amount of Na hasto be carefully monitored


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Caustic Gouging

Scale is formed by deposit of solid material fromboiler water

This scale is porous

Boiler Water enters the scale and forms a cavity

between the scale and the metal

(Wick Boiling)

continuous operation concentrates the corrosion

product 100ppm TDS - 220,000 ppm Caustic

Corrosion accelerated locally


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Phosphate dosing using either

acoordinated

orcongruent methodis used to control

free Hydrate alkalinity(Caustic)


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Coordinated Method

Boiler Water pH is monitored and kept

below a value responding to a curverelating the pH and Tri Na Phosphate

( Na3Po4 )

Na and PO4 maintained in a ratio of


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Congruent method

Used more commonly in higherpressure boilers

Dosing maintains pH and a ratio of Na :PO4 of 2.6 to 2.8


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Silica


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Silica

High levels of silica form an adherent scale

Above 4Mpa silica begins to volatilise and is

carried over to the after boiler and turbinesections of the plant

Silica can hideout in a boiler at high pressurenot showing up in boiler water analysis andthen redissolving into the boiler water at lowerpressures.


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Maximum silica recommended

2 Mpa 150ppb

4Mpa 40

8Mpa 2


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Hydrogen Embrittlement


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Hydrogen can evolve in a low pHenvironment

If damage to the magnetite layer occurs,It can diffuse into the boiler metal

React with ferrous carbide in the metal

Produce methane

Gas pressure at grain barriers cancause separation

Lowers the carbon content and strengthof the metal


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Stress Cracking

can occur in areas of high tensile stressin the presence of an excess of freealkalinity ( NaOH )

Crack may initiate at an existing flaw orhigh stress point and then begin topropagate as a very fine segregation

line between grain boundaries

Not uncommon above 4Mpa


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Build up of Deposits.

Failure of Heating surfaces.

Deposit Control through Blowdown.


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Equipment Associatedwith Condensate and Feedwater water

Quality Control.

Continuous and Intermittent Blowdown

Hydrazine Dosing System

Amine Dosing System

Phosphate Dosing System

Sampling and Analysis Equipment

Condensate and Feedwater Corrosion and Treatment

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