1. STEAM & WATER ANALYSIS SYSTEM
AN OVERVIEW
PRESENTED BY :
DEPARTMENT OF IPC & CHEM. LAB - SANTALDIH TPS - WBPDCL
10-Jan-2011
A K SARKAR
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2. Most of the Power generation in India is Thermal Power. That means
WHY SWAS ?
Most of the Power generation in India is Thermal Power. That means
Coal or Gas is used as fuel to heat water in the boiler. This water
becomes steam and this steam runs the turbine to produce electricity
by means of Generator connected with the turbine.
The turbine is a masterpiece of engineering. Its design is very compli-
-cated and intricate. The turbine is very costly and is very carefully
manufactured and installed in a power station. Boiler, various tubes
and pipes that make the Power Plant are also important. All this is
dependent on good quality of steam. If the steam produced is of bad
quality, it starts damaging this equipment by means of corrosion. The
turbine/boiler and various tubes/pipes in the power plant start getting
damaged and eventually fail.
It is therefore essential to arrest all the elements that cause the
corrosion effects of various kinds. It calls for modern methods that
keep a 24-hours-a-day vigil on these elements, however small these
may be. This can be precisely done by an On-line Steam and Water
Analysis System, popularly termed as SWAS.
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3. THE IMPORTANCE OF MONITORING BOILER PARAMETERS
The power plants using steam need some water source. As all of us
know, water is no more a free resource. Further, the quality of water
available from rivers, dams or underground sources is deteriorating
every day.
If we use such contaminated water for generating steam, it will have an
immediate impact on the complete plant and machinery in the power
plant, as there will be a lot of erosion and corrosion that will take place.
In any power plant running on steam, the purity of boiler feed water
and steam is absolutely crucial; especially to steam turbine, steam
boiler, super heater, condenser and other steam equipment. To prevent
damage of steam turbine, steam boiler and other apparatus due to
scaling and corrosion, on line steam and water analysis of critical
parameters is inevitable. A well-engineered SWAS can measure all
these parameters accurately and reliably. As we know, anything that
can be measured accurately can be controlled accurately.
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4. WHAT ARE THESE CRITICAL PARAMETERS ?pH
CONDUCTIVITY
HYDRAZINE
SILICA
DISSOLVED OXYGEN
SODIUM
OTHER PARAMETERS
Monitoring of other parameters such as alkalinity, hardness, calcium, chloride,
phosphate, dissolved ozone is also required, depending on the size of the
plant and the quality of water / steam equipment.
Let us see the significance of each of these parameters : pH
In a soln. pH approximates but is not equal to p[H], the –ive logarithm (base10)
of the molar concentration of dissolved hydronium ions(H3O+); a low pH indicates
high concentration of hydronium ions& a high pH indicates low concentration.It is
a measure of the acidity or basicity of a soln. The pH scale ranges from 0 to 14.
Ultra pure water has a neutral pH value of 7. A pH less than 7 is acidic and
greater than 7 is basic or alkaline. In the steam circuit the normal practice is
to keep the pH value of feed water at slightly alkaline levels. Accurate pH
analysis can therefore help in preventing the corrosion of pipe work and
other equipment.
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5. CONDUCTIVITY
Conductivity (G), the inverse of Resistivity (R) is the ability of a material to
conduct electric current. Since the charge on ions in solution facilitates the
conductance of electrical current, the conductivity of a solution is proportional
to its ion concentration.
Conductivity in water is affected by the presence of inorganic dissolved solids
such as chloride, nitrate, sulfate and phosphate anions (ions that carry a -ive
charge) or sodium, magnesium, calcium, iron and aluminium cations (ions that
carry a +ive charge). Organic compounds like oil, phenol, alcohol and sugar do
not conduct electrical current very well and therefore have a low conductivity
when in water.
Conductivity is also affected by temperature: the warmer the water, the higher
the conductivity. For this reason, conductivity is reported as Conductivity at 25
Degrees Celsius (250C).
The basic unit of measurement of conductivity is the mho(Ω) or siemens(s).
Conductivity is measured in micromhos per centimeter(µΩ/cm) or microsiemens
per centimeter(µs/cm). Distilled water has a conductivity in the range of 0.5 to
3.0µs/cm. Industrial waters can range as high as 10000µs/cm.
Ultra-pure water or steam is almost a bad conductor of electricity. It becomes
conductive only when there is some contamination. Therefore conductivity
measurement can give a quick indication of steam contamination.
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6. HYDRAZINE (N2H4)
Hydrazine is an inorganic chemical compound with the formula N2H4. It is a
colourless liquid with an ammonia-like odour H
However, hydrazine has physical properties
that are closer to those of water. Hydrazine is N N
highly toxic and dangerously unstable, and is H
usually handled while in solution for safety reasons H H
The use of hydrazine as an oxygen scavenger and a source of Feed water
alkalinity has advantages that are well known.
a) It prevents frothing in the boiler.
b) It minimizes deposits on metal surfaces.
c) It removes the dissolved oxygen in the steam by chemical action.
Hydrazine does not produce corrosive gases at high temperatures and pressures,
and in application, reacts with oxygen to form nitrogen and water:
N2H4 + O2 → 2 H2O + N2
Hydrazine does’nt contribute solids to the system, so boiler blowdown is reduced.
Hydrazine helps to maintain a protective magnetite(Fe3O4) layer over steel surface,
and maintain feed water alkalinity to prevent acidic corrosion. It is essential
to doze hydrazine correctly, which can be done with the help of an on-line
Hydrazine analyzer.
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7. In order that the turbines are operated at maximum performance,
SILICA (SiO2)
Most boilers making steam for turbines rarely have excessive carryover of
boiler water in the steam. While, silica deposits in turbine can occur even when
boiler water carryover is negligible. The reason is: steam selectively “picks up”
silica from the boiler water, dissolves it, and carries it to the turbines, where it
redeposit. The key to minimizing silica carryover is in keeping the boiler water
silica content below certain levels, the concentration depending on operating
pressures.
Steam is a solvent for silica. The maximum solubility of silica in steam is a direct
function of both the steam density and temperature. As steam temperature or
density decreases, the silica solubility also decreases. Since pressure affects
steam density and has a bearing on steam temperature, the solubility of silica in
steam increases with pressure.
The presence of silica in steam can lead to deposition in superheater tubes and
on the turbine blades. Small deposits on the turbine blades can result in a loss of
efficiency, whilst larger deposits can cause permanent mechanical damage.
In order that the turbines are operated at maximum performance,
continuous monitoring of silica in steam, boiler water and feed water
is highly recommended. It is also essential to check silica levels at the
source of water input to the plant.
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8. At high temperatures dissolved oxygen attacks and causes corrosion
DISSOLVED OXYGEN ( DO2 )
At high temperatures dissolved oxygen attacks and causes corrosion
of components and piping. The result is pitting, which may eventually
causes puncturing and failures.
Dissolved oxygen is also responsible for leakage at joints and gaskets.
Hence it is essential to measure and control it precisely.
Do you Know that 10 ppb of DO2 in a boiler of 450TPH creates 90 Kg.
of rust per year !
SODIUM (Na)
The presence of sodium signals contamination with potentially corrosive
anions, e.g. chlorides, sulphates etc.
Under conditions of high pressure and temperature, sodium salts are
responsible for stress corrosion – cracking of boiler and superheater
tubes.
Sodium is available in abundance in the environment and it enters even
from small openings. Sodium measurement can thus reveal possible
leakage conditions anywhere in the steam circuit, including condenser
sections.
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9. Phosphate hideout is a phenomenon that causes boiler water control
PHOSPHATE (PO4)
Phosphate hideout is a phenomenon that causes boiler water control
difficulties. It is defined as the disappearance, by precipitation or absorp-
tion, of the ionic phosphate (PO4) species, in the boiler water under high
heat transfer or high load conditions.
The best way to recognise phosphate hideout is to graph load changes,
pH, and phosphate over a period of time. The variations of the graphs
will clearly indicate that phosphate increases with decreasing load, and
that the pH decreases with decreasing load.
Events which may lead to phosphate hideout include:
1) Start-up after chemical cleaning.
2) Changes in unit load.
3) Changes in burner configuration.
& 4) Metal oxide ingress and deposition on heat transfer surfaces.
Phosphate hideout causes confusion, frustration over uncontrollable water
chemistry, and corrosion. Some phosphate-related corrosion may be
severe.
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10. SWAS ON-LINE ANALYSERS
ON-LINE ANALYSERS (AT SANTALDIH TPS)
Line
Nos.
Samples taken from
SWAS ON-LINE ANALYSERS pH Ks Kc
N2H4
SiO2 DO Na
PO4 1.
FW at Economiser I/L 2.
CBD from Boiler Drum 3.
Saturated Steam 4.
Superheated Steam 5.
Condensate bef. LP Htr. 6.
CEP Discharge 7.
DM Make-up Water 8.
Deaerator Feed Water 9.
DMCW Aux. Header O/L
10.
DMCW (B) HF O/L
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11. All the analyzers as mentioned, work efficiently if the temperature,
THE NEED FOR SAMPLING
All the analyzers as mentioned, work efficiently if the temperature,
pressure and flow conditions of sample are maintained properly.
Further, the sensors are capable of handling the water / steam
samples at particular pressure and temperature only.
This necessitates the use of the sample conditioning system.
THE SAMPLE CONDITIONING SYSTEM
The sample conditioning systems have several functions. The sample
must be:
1. Withdrawn from the process.
2. Transported.
3. Conditioned.
4. Introduced into the analyzer.
& Disposed off.
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12. Position of Sample Extract for Conductivity measurement
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13. Position of Sample Extraction Probes for pH measurement
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14. Position of Sample Extraction for Silica measurement
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15. Position of Sample Extraction Probes for DO measurement
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16. Position of Sample Extraction for Sodium measurement
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17. Position of Sample Extraction for Hydrazine & PO4
measurement
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18. SAMPLE EXTRACTION PROBES – For Withdrawal of Sample
Studies by CEGB(Central Electricity Generation Board, UK) suggest
a more important factor in obtaining representative particulate samples
may be the maintenance of a sufficiently high transport velocity in the
sample line to prevent hideout of the suspended species. Thus proper
sample extraction probe selection is the first step for ensuring accurate
analysis of steam / water.
SAMPLE PIPE WORK – For Transport of Sample
Sample lines should be kept as short as possible. Use of unnecessary
large line diameters should be avoided so as to avoid delayed response
and the damping-out of transients.
Sample pipework must be of a material, which will not react chemically
with any species in the sample. Stainless steel AISI 316 is the standard.
The number of bends and joints should be kept to a minimum.
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19. SAMPLE COOLERS – For Conditioning of sample
The Sample coolers form the heart of the sampling system.
These should preferably be of coil-in-shell type design. This
should make use of counter flow type of heat exchange
principle.
The coolers with Double helix type coil design give better
approach temperature than conventional single coil designs,
apart from being compact in size.
A built-in shell relief valve takes care of possible mishap due
to high pressures caused by coil rupture etc.
FILTRATION :
For the majority of on-line analyzers, the presence of unwanted
particulate matter can be damaging. To arrest this damage, a small
High-pressure Filter with a sintered stainless steel element is necessary
to permit regular cleaning.
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20. PRESSURE REDUCTION & REGULATION
Traditional Pressure Reducing devices do not offer pressure ‘regulation’.
In such cases, any pressure fluctuations in the inlet conditions can get
transmitted to analysers and this is not a healthy condition for analysis.
Pressure Reduction as well as ‘regulation’ is essential to ensure accurate,
reliable and stable performance of on-line analysers.
CHILLED WATER
Use of chilled water becomes necessary when the cooling water available
on site is not capable of cooling the sample to the temperatures required
by the analysers.
If available cooling water is upto 400C, a well-designed sampling system
can ensure sample outlet temperatures of 450C. Most of the modern
analysers can work comfortably with these sample temperatures.
However, if cooling water temperature exceeds 400C, use of chilled water
becomes necessary. Of course for some analysers, samples are
always required to be conditioned at 250C by means of Chillers.
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21. SANTALDIH THERMAL POWER STATION, WBPDCL BOILER WATER CHEMISTRY REPORT
O & M (CHEMISTRY)
Date_________ Shift _____________ Time ________
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