analysis of impurities in feedwater and steam

04/07/2023 Power Plant Instrumentation 1

ANALYSIS OF IMPURITIES IN

FEEDWATER AND STEAM

By ,Sukeerthi M

09U242


Introduction

• Safe, reliable operation depends upon the establishment of proper chemical conditions

• Treatments and Instrumentation depend upon – Details of plant type– Circuit design– Physical parameters – Intended operational mode of the plant

• Plants have to measure the key chemical parameters and take action on the basis of these measurements


Water and its impurities

• Good drinking water does not necessarily make good boiler feed water.

• Only 0.65 % of total water is available for domestic and industrial use.

• The purity requirements depend on:– how much feed water is used – particular boiler design


Feedwater

• Economic sense to return as much condensate as possible for reuse.

• Make-up water(chemically treated) will therefore have to be introduced to the system to maintain the correct working levels


Common Impurities

• Dissolved solids: Scales: carbonates and sulphates of calcium and magnesium

• Suspended solids: Sludges : solid mineral or organic particles carried in suspension

• Dissolved gases: Corrosive impurities: oxygen and carbon di oxide

• Scum forming substances: Soda in form of carbonate, chloride or sulphate


Contd…

• Any scale forming salts within the boiler should be chemically controlled so that they produce suspended solids or sludge rather than scale.

• The amount of impurities is expressed in– parts per million (ppm), by weight – in milligrams per litre (mg/l).


Water treatment

• The operating aims for steam boiler plant are as follows:– Long working life.– Maximum efficiency.– Safe operation.– Minimum maintenance.


Treatment

• Chemical Treatment• External water treatment• Conditioning treatment


Identification

• Qualitative identification– Turbidity (clarity)– Taste– Colour– Odour

• Quantitative identification– pH– Conductivity– Total solids


Boiler Water Tests

The following boiler water test shall be made:1 Boiler water pH 2 Boiler water conductivity 3 Boiler water sulphite or hydrazine excess 4 Excess of phosphate 5 Condensate pH (every 24 hours) 6 Hardness of feedwater 7 Excess of oxygen in condensate


Lab Instrument


Instruments used for Analysis• Online monitoring instruments:– Conductivity meter– pH meter– Dissolved Oxygen– Sodium– Phosphate– Oxidation/Reduction Potential– Silica

• Offline batch(grab) and confirmatory analyses– Iron– Copper – Ionic contaminants


Components Employed in Water Treatment


Contd...


Conductivity Meter

• Conductivity: ionic solutes present• Simple indicator of the trends in

chemical conditions• Due to presence of ammonium and

hydroxide• Temperature maintenance must be

done• Its used to monitor condensate ,

feed water and condensed steam


pH Meter

• The pH of a solution is a key parameter in the control of circuit chemistry.

• Potentiometrically using a pH-sensitive glass membrane electrode and a nominally pH-independent reference electrode

• During periods of severe deviation from normal chemistry, the potentiometric methods of pH measurement remain accurate whereas the conductimetric method may introduce significant errors.


Dissolved Oxygen• 2 reasons:

– High oxygen concentration-acidic corrosion– Very low concentration-iron transport and flow accelerated

corrosion

• Instrument: Membrane polarographs-Oxygen-sensitive membrane electrodes of the polarographic or galvanic type are composed of two solid metal electrodes in contact with supporting electrolyte separated from the test solution by a selective membrane- they detect current

• Difficulties faced:– Presence of reducing agents– Walls of sample lines exhibit oxygen demand

• Methods to overcome:– Cooling sample– Minimizing sample length


Sodium

• Glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion

• Capable of reliable measurement up to 0.1μg kg -1

• Maintenance required at regular intervals• Indicates responsiveness and deterioration


Phosphate

• Calorimetric technique where the sample is mixed with acidic solution containing ammonium molybdate together with a redox controlling reagent

• Reagents must be replinished regularly

• Alternate method is:ion chromatography


Oxidation/Reduction Potential• The redox condition of feedwater is given by ORP

measurement.• Working: – The instrument simply measures electrode potential of an

inert surface (Pt) against a reference electrode. – The measured potential becomes more positive as the

balance of species present in the water becomes more oxidizing.

• Thus the measurement is used to alert operators to the need to address issues such as air ingress, deficiency in dosing of reducing agents, etc.


Silica

• Calorimetric technique: The sample is mixed with ammonium molybdate at a controlled low pH to generate a blue compound that can be detected spectro-photometrically.

• Deployed as multichannel instruments, with the possibility to switch between multiple samples.

• Maintenance schedules are required to ensure replenishment of reagents, to check operation of pumping and mixing systems and to recalibrate.


Iron• Monitoring total iron concentrations assures

the rate of transport of corrosion products into the boilers and onto steam turbine blading is kept within acceptable limits.

• It also demonstrates whether the steam/water circuit is potentially suffering from flow-accelerated corrosion

• Its capable of discrimination of iron at concentrations of 1 to 5 μg·kg -1


Copper

• Many older plants have copper alloys in condensers and in the feedwater heaters.

• Copper can readily be oxidized into a mobile form and can subsequently deposit within evaporator tubes or, in high pressure plant, on steam turbines.

• Batch analysis of feedwater can be used to establish for effective control over copper transport is being achieved


Ionic contaminants

• Ion chromatography can be used to identify the source of contaminants in the steam-water circuit that increase conductivity

• By choice of analysis columns it is possible to discriminate and quantify a wide range of cations and anions(organic and inorganic)


Steam Impurities

• The coal ash corrosion attack results form the deposition of the liquid phase of complex alkali iron sulphates.

• The severity of attack varies with temperature. • Design approaches,

– burying the tubes are a continuing maintenance item. – superheater tubing alloys vary widely in resistance to coal ash

corrosion. – Bimetallic tubing is the most likely means of dealing with coal ash

corrosion in the advanced plants. • An interesting approach to the liquid ash corrosion problem

is to operate the superheater tubes above the bell-shaped curve


Effects of Contamination of Feedwater

1 Formation of scale on generating and superheater heating surfaces, which results in: A. Reduction in the boiler efficiency because of the decreased rate of heat transfer, and B. Overheating and burning of tubes resulting in tube failure.

2 Corrosion of all interior surfaces of the boiler by certain salts and air (oxygen);

3 Foaming and priming, which result in moisture carry-over in the steam from the saturated steam drums to the superheaters (where these are installed and in use) or direct to the machinery in plants operating on saturated steam


Drawing Samples For Boiler Water Test

• The sample must be truly representative of the water in the boiler.

• The sampling connection to the boiler shall be shortest possible and well cooled.

• Sampling lines should be flushed thoroughly by allowing the water to run trough for at last half a minute.

• All equipment to be kept clean and rinsed four times with sample water before the sample is measured out


List of Problems Caused by Impurities in Water and their Treatment


References

• “Instrumentation for monitoring and control of cycle chemistry for the steam-water circuits of fossil-fired and combined-cycle power plants,”by Dr. D. G. Friend

• Reverse osmosis technical data• Materials for Steam Superheaters, Jyoti

Swaroop Repaka, February 11, 2010


THANK YOU

analysis of impurities in feedwater and steam

Documents

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