Practical Analytical Instrumentation in On-line Applications

Practical Analytical
Instrumentation in On-Line
Applications
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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

# Item Description

• 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

• 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

• 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

# 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 Specifications
1) 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

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

8) 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

13) 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

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