Practical Analytical Instrumentation in On-line Applications

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Practical Analytical Instrumentation in On-Line

Applications


Silica Analyser# Objectives• Reasons for silica content in water

• Forms of silica presence in water

• Method of Analysis

• Principle of Operation

• Industrial Silica Analyser

• Industrial Applications

• Benefits

• Troubleshooting

• Control of Silica in feed water


Introduction

# Optimum Turbine Performance• Continuous monitoring of silica in super heated steam,

boiler water and feed water is of utmost importance

# Silica (SiO2) has soluble and insoluble forms

• More soluble at higher pH, removed by strong base anion ion exchange

• Insoluble form removed by filtration


Reasons for silica content in water# External contamination• Raw water ingress

• Use of silicon based lubricants and oils

• Feed water system or chemical dosing or reagent problems

# Internal contamination• Condenser dust

• Oil spill absorbent materials

• Fly ash contamination

• Blasting materials


Silica Forms

# Colloidal silica –SiO2-O-SiO2-O-SiO2-• More prevalent in water from surface sources, pH>7

• Insoluble

• Removed by UF, Nanofiltration, Coagulation

# Reactive silica (OH)3Si-O-

• Soluble

• Removed by EDI ( Electro De Ionization), RO ( Reverse Osmosis)


Effect of silica in boiler feedwater• 1 ppm of silica in feed water for a 500W boiler

evaporating 1,500 tonnes of water/hr results in 1 ton of silica being deposited in boiler ( within one month)

• Steam containing silica causes reduction of turbine efficiency by deposition on the turbine blades, nozzles etc

• Silica scale is mostly responsible for bulging and bursting of Wall Water Tubes and super heater tubes


Effect of silica in boiler feedwater• Silica in boiler feed water alters the steam

velocities and the pressure drops, reducing the capacity and efficiency of turbine

• Any minor deviations of silica concentrations in a power plant can have serious and expensive consequences in relation to performance, reliability, efficiency and safety

• It is logical that silica concentration should be monitored closely


Method of Analysis# Molybdenum Blue Reaction• Heteropoly blue method is used to measure

molybdate-reactive silica

• Molybdate 3 Reagent, an acidic molybdate solution is added to the sample to react with any silica and phosphate present to form molybdosilicic and molybdophosphoric acids

• Citric Acid is added, which masks any molybdophosphoric acid present and reacts with excess molybdate. This prevents molybdate from producing an interfering blue-coloured compound


Method of Analysis# Molybdenum Blue Reaction• Colour formed at this point is identical to the final

colour of a 0 µg/L silica sample. This provides a zero reference and compensates for any background turbidity and colour inherent in the sample

• Amino Acid F Reagent is added to reduce molybdosilicic acid to a blue coloured solutions

• The amount of colour formed is directly proportional to the silica concentration of the sample


Principle of Operation# Flow diagram


Principle of Operation# Item Description


Principle of Operation• The soluble silica of the sample, now reacts with the

molybdate and forms silicomolybdate. Since the rate of reaction is slow, it is necessary for the sample + molybdate to be mixed and kept in a reactor for 5 minutes in order to ensure completion of reaction

• For silica concentrations at low ppb level, silicomolybdate complex is reduced with ferrous ions


Principle of Operation• One sample, appropriately conditioned (temperature,

pressure) circulates at a relatively high linear velocity through its respective over sampling cup

• A rate of 90 ml/hr for sample solution and 4.5 ml/hr of each of the 3 reagents is continuously being aspirated by means of a peristaltic pump and sequentially added into the analytical flow circuit

• The high sample-to-reagent ratio minimizes errors as a result of inaccuracies of the reagent delivery rate by the pump


Principle of Operation• To prevent interference by phosphates and to intensify

the colour, the sample mix is reacted with oxalic acid

• The reducing agent (ferrous sulphate) is then added to the sample mix

• The mix now enters the photometer flow cell where the optical density of the solutions is measured by absorption of Infrared Light at a wavelength of 820 nm


Silica Analyser Modules

# Control Module• alphanumeric LCD

• programming Keyboard

• alarm system relays

• Power supply

Source : HACH, Series 5000 Analyser


Silica Analyser Modules# Reagent Supply System• Supplied to analysis module by pressurizing the reagent

containers

• Flow volume and timing are regulated by solenoid valves

• Safety interlock on the compartment door requires reagent depressurization before opening

• Reagent system pressure is supplied from an external source



# Analysis Module• Contains solenoid

valves

• Sample cell

• Measure light at 820 nm






Silica Analyser Specifications1) Range:

0.00 to 5000 µg/L as SiO2

2) Accuracy:

0.00-5.00 µg/L : ±1.0 µg/L or ± 5% of reading, whichever is greater; 500-5000 µg/L; ±7% of reading

3) Minimum Detection Limit :

Less than 0.5 µg/L

4) Precision:

±0.5 µg/L or ±1.0% of reading, whichever is greater


Silica Analyser Specifications

5) Step Response Time :

8.8 minutes for 30 to 50 ºC, 15 minutes for 5 to 40 ºC (field adjustable)

6) Ambient Operating Conditions :

10 to 45 ºC, 5 to 95% non-condensing humidity. Suitable for general purpose, clean, indoor environments

7) Analyzer Sample Requirements :

Regulated to 5 ± 3 psig ( 34.5 ± 20.7 kPa). Flow rate from 100 to 300 mL/minute. Sample temperature between 5 and 50 ºC. A sample pressure control kit is provided


Silica Analyser Specifications8) Sample Inlet Fitting :

¼-inc OD stainless steel compression tubing fitting

9) Recorder Outputs :

Selectable for 0-0.01 V, 0-0.1 V, 0-1 V, or 4-20 mA. Output span programmable over any portion of 0-5000 µg/L range

10) Serial I/O:

RS232 and 20 mA current loop

11) Alarms:

Four programmable relays, two sample concentration alarms, analyzer system warning and analyzer system shutdown alarms each equipped with an SPDT relay, two with contacts rated for 1A resistive load at 30 VAC and 42 VDC and with two contacts rated for 5 A resistive load at 240 VAC

12) Power Requirements :

115/230 VAC, 50/60 Hz, switch selectable; 52 VA, 32 W maximum


Silica Analyser Specifications13) Reagent Pressure Source :

20 to 60 psig regulated ( 137.9 to 413.7 kPa); nitrogen, instrument quality air or compressed air. Filter and regulator are supplied with analyzer

14) Reagent Pressure Inlet Fitting :

¼-inch OD stainless steel compression tube fitting

Sample Drain Fitting : ¾-inch NPT PVC Female

15) Air Purge ( Optional ) :

5–scfh ( standard cubic feet per hour ) instrument quality air, ¼-inch OD stainless steel compression tube fitting

16) Reagents :

Reagents : 2.9 L Molubdate 3 , 2.9 L Citric Acid/Surfactant, 2.9 L Amino Acid F, 2.9 L Silica Standard Solution, SiO2, 500 µg/L ( 250 mL required for standardization)


Industrial Applications# Water Treatment Plant

• Silica analysers are used to measure the efficiency of anion and mixed bed outlets

• Detects the depletion of beds and measure final water quality to ensure it is suitable to enter steam production cycle

# Boiler drum

• Silica build-up is monitored inside the boiler drum

• If the level of silica gets too high, then a ‘blow down’ is initiated to remove contaminated water from the boiler

• Close control of silica levels will help minimize the frequency of boiler blow down, which can be expensive and inefficient if performed too often.


Industrial Applications# Boiler Feed Water

• Measuring the levels of silica in boiler feed water will provide a final check on the quality and acceptability

• Ensure the maximum permissible level of silica in the boiler is not exceeded

# Steam Line

• Silica monitoring within the steam line provides a good indication of the overall steam purity level provided by the boiler drum

• Ensures the protection of the super heater and turbines


Benefits# Reduces demineralization water plant costs

• With its features of lowest detection limit of 0.5 ppb, silica analyser detect early stages of resin saturation, substantially reducing resin generation costs

• The built in sequencer optimizes plant investments and favours implementation of best practices in resin monitoring


Benefits# Determines amount of silica deposits on turbine

segments

• Exceptionally low silica levels can be measured

• Works with automatic 2 point calibration• “absolute zero” silica background determination

• “slope” calibration, results in accurate measurements that are greater then ±0.5%ppb


Benefits# Reduces downtime

• If unchecked, silica forms difficult-to-remove scale deposits on turbine blades, resulting in excessive maintenance and downtime costs

• The silica analyser alerts users to changes silica levels in time for corrective action to be taken before significant downtime is incurred


Troubleshooting# Consistency and Accuracy at Low Concentrations

• Problems with consistent readings at lower concentrations may be caused by humidity in the environment

• Humidity can condense on the sample cell wall in the light path, if sample temperature is below the dew point of the air next to the sample cell in the colorimeter

# Actions to reduce potential humidity and temperature issues

• Make sure the sample cell cover is tight

• Seal any fittings that might leak fluids into the instrument

• Purge the instrument with dry instrument air or dry nitrogen to prevent excess humidity build up inside the instrument enclosure

• Place the instrument in an environmentally controlled (temperature and humidity) building


Control of Silica in feed water# Concentration of silica in boiler feed water should be

controlled strictly to maintain minimum silica level in steam

• Carry over of silica in steam due to faulty operation should be avoided by maintaining accurate boiler drum level

• Load should be increased gradually avoiding overloading, steam separator should be efficient

• Periodical and continuous blow down should be controlled strictly to maintain minimum level of TDS (Total Dissolved Solids) in boiler water


Conclusion# Silica is a major culprit behind the build up of hard and dense

scale inside the boilers and turbines of power generation plants

# Silica deposits can impair the performance of equipment to such an extent that it is imperative to keep it under tight control

# The only way to effectively control silica build-up is through effective online silica monitoring analysers

# Silica Analysers provides early warning of equipment problems, before actual failure occurs, thereby ensuring the plant operates at best possible efficiency


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Practical Analytical Instrumentation in On-line Applications