power plant chemistry

ROLE OF CHEMISTRY IN POWER PLANT

FEB 24/2004

CONTENTS OF THE PRESENTATION

FEB 24/2004

Feed water treatment

FEB 24/2004

Insoluble impurit ies Insoluble impurit ies are very largely metal oxides metal oxides derived from corrosion of the plant itself, par ticularly 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 .

Parameters 200 MW 500 MW

Boiler Pressure, Kg/cm2 150-170 185-195

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

DO, ppb, (Max) 5.0 5.0

Res. N2H4, ppb 10-20 12-20

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 Feed water pH- the use of volatile alkalis (NH 3

)

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

There is no excessive accumulation of alkali in the boiler water, the volati le 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 par ts of the steam/water system, raising the ph of the condensates and hence suppressing corrosion .Ammonia (NH 3) is the most widely used volatile alkali because of its low cost, ready available and its stabil ity at high temperatures

Physical de-aeration

De-aerator

40 ppb D.O. in Condensate Water

5 ppb D.O

Solubil ity Law & Henry Law

Chemical de-aerationThe most widely used agent is Hydrazine (N 2H 4).

It is steam volatile, slightly basic and reacting readily to remove even traces of dissolve oxygen at temperature above 150 0C producing only volatile or gaseous products, i.e.,

NN 22 HH 44 + O + O 22 NN 22 + H + H 22 OO

At higher temperature hydrazine itself decomposes to produce ammonia and nitrogen, i.e.,

NN 22 HH 44 4NH4NH 33 + N + N 22

It is assumed that the reduction of oxygen by hydrazine probably follows a heterogeneous reaction mechanism. It is common experience to find that two or four weeks elapse after initiating hydrazine treatment before a residual can be detected in the boiler water. This is because of following reactions

6 Fe 2O 3 + N 2H 4 N 2 + 2 H 2O + 4 Fe 3O 4

4 CuO + N 2H 4 N 2 + 2 H 2O + 4 Fe 3O 4

likely that the actual reduction of oxygen takes place in two steps involving the oxidation and reduction of oxides of iron.

4 Fe 3O 4 + O 2 6 Fe 2O 3

6 Fe 2O 3 + N 2H 4 4 Fe 3O 4 + N 2 + 2 H 2O

To maintain boilers and turbines at a high level of availabil ity and ef f iciency, 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 sur faces.3.Deposition and corrosion of turbines.

Parameters 200 MW 500 MW

Boiler Pressure, Kg/cm2 150-170 185-195

pH at 25°C 9.2-9.5 9.1-9.4

Cond. µS/cm, (Max) 30 30

Silica, ppm, (Max) 0.2 0.1

Phosphate, ppm 2-4 1-2

Chloride, ppm, (Max.) 1.0 0.5

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

Trisodium phosphate (Na 3PO 4) and disodium phosphate (Na2HPO4) can both effectively produce alkalinity by hydrolysis in water:

Na 3PO 4 + H 2O Na 2HPO 4 + NaOH

Na 2HPO 4 + 2 H 2O NaH 2PO 4 + NaOHThe impor tant dif ference between this source of alkal inity and that produced by free caustic soda l ies in the reversibi l i ty of the above reactions. If acid species appear in the boiler water they wil l be neutral ised by the sodium hydroxide:

HCl + NaOH NaCl + H 2O

Recommended parameters of Super heated Steam Purity GuidelinesParameters 200 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%.

o 15 Conductivity analyserso 5-6 pH analyserso 1 multi -channel Sil ica analyser (@ 5

channels)o 1 Hydrazine analysero 2-3 D.O. analyserso 1-2 Sodium analyserso 1 Phosphate analyser + some more...

CEP dischargeCEP dischargeDirect and af ter cation conductivity

measurements, supplemented by sodium monitoring, to provide warning of condenser leakage. Dissolved oxygen measurements are also required in order to establish the adequacy of oxygen removal at the condenser .

Condensate Polishing Plant outletCondensate Polishing Plant outletConductivity, ACC, Reactive Sil ica, Na + in

outlet water are required to monitor the per formance of the plant and for need of resin bed regeneration. Additionally, the measurement of Chloride & sulphate in the outlet water has assumed increasing impor tance.

De-aeratorDe-aerator

D.O. measurement at Inlet & Outlet of de-aerator.

For testing purposes it is necessary to have the facil ity to sample and monitor oxygen at both inlet and outlet of the de-aerator to insure satisfactory per formance.

Final Feed WaterFinal Feed Water

Conductivity, ACC, D.O, pH, Na +, Cl -, SO - -4 &

Total Iron.

Comprehensive chemical monitoring of feed water just before it enters the boiler, to provide a f inal check on quality and acceptabil ity .

Boiler WaterBoiler WaterConductivity, pH, Phosphate, Cl -, reactive

si l ica are required to conform the correct boiler water conditions are being maintained.

Steam (SS & MS)Steam (SS & MS)Measurement of pH, reactive si l ica,

ammonia, sodium, conductivity to ensure that criteria based on the need to minimize salt deposition in the superheaters, reheaters and turbines are being met.

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

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

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

o Total hardness of condensate wil l be high.o Boiler and condensate sil ica wil l go on

increasing and cannot be control led without CBD and/or CPU.

o Hot well makeup wil l be low as well as level wil l be high.

o De-aerator level wil l go high.

1. Open CBD 100%, makeup wil l 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.

I f 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 t ime and minor the chemical parameters and f ind out which half is leaking.

o Isolate the leaky half por tion.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

Parameters 200 MW 500 MW

pH at 25°C 8.8-9.0 9.0-9.2

ACC Cond. µm/cm, 0.3 0.2

Silica, ppb, (max) 20 10

DO, ppb, (max) 40 40

NH3, ppm, (max) 0.5 1.0

Total Fe, ppb, (max.) 10 10

Total Cu, ppb, (max.) 5 3

Sodium, ppb, max - 5

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

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 star t up and as a result, ful l load conditions are reached early giving economics

S.NO. PARAMETER CONTROL LIMITS FREQUENCY OF TESTING

WITHOUTALKALIZER

WITHALKALIZER

1. Conductivity at

25oC, µS/cm

<1.5(preferably below 0.5)

<2.2 Continuous

2. Dissolved oxygen,

µg/l

<100 <10 Continuous

3. Total copper, µg/l <20 <20 Once in 2-months

4. Total iron, µg/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 el iminate the possibi l i ty of f lashover. 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/cm 2 with alarm for nitrogen pressure in primary water tank set at 0.4 kg/cm 2 for better ef f iciency.

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 f low. 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.

THANKS !

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