POWER PLANT CHEMISTRY

FEB 21/2013

FEED WATER TREATMENT & CORROSION CONTROL

CONTENTS OF THE PRESENTATION

Feed water treatment

FEB 24/2004

Insoluble impurities Insoluble impurities are very largely metal metal oxides oxides derived from corrosion of the plant itself, particularly the materials of construction of the condenser and feed systems. Such corrosion is minimised by maintaining the feed water at an alkaline pH and by removing dissolved oxygen down to very low concentrations.

Soluble impurities Soluble impurities in feed water most frequently arise from in leakage of cooling water from condensers and other coolers, but can also results from poor quality of make-up water.

Drum Pr.

Kg / sq.cm

61- 100 101-165 166 - 205 Once-thro

ugh units

Hardness, ppm, max nil nil nil nil

pH at 25oC 8.8 - 9.2 8.8 – 9.2 8.8 – 9.2 8.8 – 9.2

Diss.Oxygen,ppm,max 0.007 0.007 0.007 0.007

Cond. ( H+ ), µS/cm, max 0.5 0.3 0.3 0.2

Silica, ppm, max 0.02 0.02 0.01 0.01

Iron , ppm , max 0.01 0.01 0.005 0.005

Copper , ppm , max 0.01 0.005 0.003 0.003

Hydrazine , ppm , residual 0.01– 0.02 0.01-0.02 0.01-0.02 0.01-0.02

o Feed water pH- the use of volatile alkalis (NH3

)

o By making the pH of feed water slightly alkaline (9.0-9.2) the corrosion rate of ferrous alloys can be minimised. Where the system is also contain copper based alloys similar benefits can be obtained

o Oxygen Controlo Physical de-aerationo Chemical de-aeration

MAGNETITE LAYER

HEMATITE LAYER

There is no excessive accumulation of alkali in the boiler water, the volatile alkali passing out the boiler with the steam.As condensate are formed in the lower pressure section of the turbine, in the condensers and on the steam side of the feed heaters, the alkali immediately available in these parts of the steam/water system, raising the ph of the condensates and hence suppressing corrosion.

Ammonia (NH3) is the most widely used volatile alkali because of its low cost, ready available and its stability at high temperatures

The objectives of any chemical treatment for a modern, safe and efficient thermal power plant are:

To reduce corrosion of metals and equipments

To avoid scale formation

No turbine deposits/corrosion

No BTF due to cycle chemistry problem Optimize feed water treatment / no FAC Eliminate the need to chemical cleaning

Optimize procedures for shutdown/lit up

Bench mark in the world class

Corrosion is a natural process and is a result of the inherent tendency of metals to revert to their more stable compounds, usually oxides. Most metals are found in nature in the form of various chemical compounds called ores. In the refining process, energy is added to the ore, to produce the metal. It is this same energy that provides the driving force causing the metal to revert back to the more stable compound.

IRON OXIDE REFINING MILLING

STEEL CORROSION IRON OXIDE

bacterial rods unicellular green algae filamentous fungi (molds)

Boiler system tube shows high pH gouging

Economizer tube severely damaged by oxygen

Deposit accumulation restricted heat transfer, leading to long-term overheating

NN22HH44 + O + O22 NN22 + H + H22OOPhysical de-aeration by De-aerator

Reduce Flow Accelerated Corrosion by Feed Water Treatment

Thin-lipped burst caused by rapid overheating

Caustic gauging

Caustic reacts with the magnetite and forms sodium ferrite (NaFeO2) and sodium Ferroate (Na2FeO2).

Maintain boiler water Phosphate to avoid caustic gauging

Boiler system failure

Boiler tube shows effect of acid attack

Violent rupture caused by hydrogen embrittlement

Acidic condition generated on tube surface beneath the deposits. Presence of chloride was confirmed

Boiler system failure

Liquid metal embrittlement of boiler tube caused by copper deposits and high temperature (greater than 1600 °F).

Steam blanketing caused metal wastage on top of sloped tube

Steam purity

. Overheating of this superheater tube was caused by deposits that resulted from boiler carryover into the steam

Chemical de-aerationThe most widely used agent is Hydrazine

(N2H4).It is steam volatile, slightly basic and reacting

readily to remove even traces of dissolve oxygen at temperature above 1500C producing only volatile or gaseous products, i.e.,

NN22HH44 + O + O22 NN22 + H + H22OOAt higher temperature hydrazine itself decomposes

to produce ammonia and nitrogen, i.e.,

NN22HH44 4NH4NH33 + N + N22

To maintain boilers and turbines at a high level of availability and efficiency, the chemical control of water and steam purity is aimed at the prevention of:1.Corrosion of feed, boiler and steam systems.2.Scale and deposit formation on heat transfer surfaces.3.Deposition and corrosion of turbines.

Drum Pr.

Kg / sq.cm

61-125 126-165 166-205 166-205

Treatment

Type

phosphate phosphate phosphate AVT

pH at 250C 9.1-10.1 9.1-9.8 9.1-9.6 > 8.5

Silica, ppm , max 7.0 – 0.9 0.9 – 0.2 0.10 0.10

PO4 residual , ppm 5 - 20 5 - 10 2 - 6 ---

TDS , ppm , max 100 50 15 ---

Cond. µS / cm , max 200 100 30 ---

Cond. ( H+ ), µS/cm, max

--- --- --- 0.2

Trisodium phosphate (Na3PO4) and disodium phosphate (Na2HPO4) can both effectively produce alkalinity by hydrolysis in water:

Na3PO4 + H2O Na2HPO4 + NaOH

Na2HPO4 + 2 H2O NaH2PO4 + NaOHThe important difference between this source of alkalinity and that produced by free caustic soda lies in the reversibility of the above reactions. If acid species appear in the boiler water they will be neutralised by the sodium hydroxide:

HCl + NaOH NaCl + H2O

Recommended parameters of Super heated Steam Purity GuidelinesParameters 210 MW 500 MW

pH at 25°C 8.8-9.0 9.0-9.2

ACC Cond. µm/cm, 0.2 0.2

Silica, ppb, (max) 20 10

Sodium, ppb, (max) 5.0 5.0

NH3, ppm, (max) 0.5 1.0

Total Fe, ppb, (max.) 10 10

Total Cu, ppb, (max.) 5 3

Ref: Code of Practice on Power Plant chemistry, by (OS) COS-ISO-00-OGN-OPS-CHEM/015, Oct.2003

o Solubility of impurities in steam decreases as steam expands in turbine

o NaCl and NaOH most corrosive chemicals

o Solubility of caustic exceeds 100 ppb at HP turbine pressure and temp.

o In LP turbine, caustic solubility in steam decreases and deposit concentration goes up to 90%.

Drum Pressure Restriction Reactive Silica in steam (ppm)60 8.280 3.8

100 1.9120 0.9130 0.7140 0.52145 0.48150 0.40155 0.37160 0.30165 0.27170 0.25175 0.22180 0.18185 0.10

A potential major source of ingress of impurity into the boiler water system is from leakage of cooling water into the main condenser steam space (because, steam space is maintained by vacuum) which is called condenser leakage

Left side hot well conductivity increased more rapidly than right side indicates condenser tube leak in left pass of condenser

Boiler and condensate silica increased and cannot be controlled

without CBD and/or CPU.

Boiler and condensate silica increased and cannot be controlled

without CBD and/or CPU.

o Online sodium increases (normal <2ppb).o After Cation Conductivity increases

( normal value <0.2 µS/cm for 500MW plant)

o Total hardness of condensate will be high.

o Boiler and condensate silica will go on increasing and cannot be controlled without CBD and/or CPU.

o Hot well makeup will be low as well as level will be high.

o De-aerator level will go high.

1. Open CBD 100%, makeup will go high. (Heat as well as DM water loss)

2. Increase the concentration of phosphate and free alkalinity in boiler.

3. Limit as for as possible the boiler de-superheater spray water to prevent contamination of the system.

4. Isolate one path of condenser, and see the results and vice versa.

5. Put CPU into service if available.

If the leak is in minor in nature:If the leak is in minor in nature:o In running units, isolate one half the

condenser at a time and minor the chemical parameters and find out which half is leaking.

o Isolate the leaky half portion.o Plug or repair the leak.o Normalise and comeback to full load.

First is path detection Flood test Candle Test Dye test Foam Test Bubbler Method

Condensate polishing is employed to purify the return steam condensate in order to meet the quality requirements of high pressure thermal cycles and minimize consumption of make-up water.

Improvement in the quality of condensate and "cycle" clean up

Reduced blow down & make up requirements

Improvement in boiler water quality for drum type boilers

Quick start up and as a result, full load conditions are reached early giving economics

S.NO. PARAMETER CONTROL LIMITS FREQUENCY OF TESTING

WITHOUTALKALIZER

WITHALKALIZER

1. Conductivity at 25oC, S/cm

<1.5(preferably below 0.5)

<2.2 Continuous

2. Dissolved oxygen, g/l

<100 <10 Continuous

3. Total copper, g/l

<20 <20 Once in 2-months

4. Total iron, g/l <20 <20 Once in 2-months

5. pH at 25oC 6.0-8.0 8.5-9.0 Once in 2-months

Conductivity is the basic criteria of quality for the stator cooling water and a low level must be maintained to eliminate the possibility of flashover. Dissolved oxygen in stator water is the main reason for corrosion. Dissolved oxygen in stator cooling water is generally removed by purging with nitrogen gas of high purity (99.99%, min.). Removal of dissolved oxygen depends on the nitrogen pressure which is normally maintained at 0.2 kg/cm2 with alarm for nitrogen pressure in primary water tank set at 0.4 kg/cm2 for better efficiency.

A bypass ion exchange polishing plant is commonly installed to control the conductivity of the stator water and can also trap some of the suspended matter in water.

Polishing plant is generally designed to treat about 2-5% of total stator water flow. The polisher contains a bed of mixed resins consisting of strong cation resin in h-form and strong anion resin in oh-form. Analysis of the exhausted resin can be used to know how much corrosion products are removed by the mixed bed.

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