Download - Boiler blowdown
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Boiler Blowdown
Engr. Sahadat HussainSr. Consultant, Environmental & Process
Engineering Resources International (ERI)
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Common Impurities in Raw Water
• Dissolved solids - scale forming substances. The principalones are the carbonates and sulphates of calcium andmagnesium. Not all dissolved solids will form scale.
• Suspended solids - sludge formingorganic
substances which areinusually solid mineral or particles, carried
suspension. Not generally a problem with most waters.
• Dissolved gases - corrosive. Oxygen and carbon dioxide.
• Scum forming substances - mineral scum forming or foamingimpurities usually containing soda in the form of a carbonate,chloride or sulphate.
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Water quality needs to be such that weoperate under the following criteria:-
• Freedom from scale
• Freedom from corrosion
• Good quality steam
• Safety
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ppm = mg/l
Units•
of ConcentrationParts Per Million (ppm)1 kg in 1,000,000 kg (1,000 Tonnes) of water
1 lb. in 100,000 lmp. gallons of water
• Milligramme Per Litre (mg/l)11
1
mg
mg
mg
in
in
in
1,000,000 mg of water1,000 g of water
1 litre of water
100 ppm1,000 ppm
10,000 ppm
= 100 mg/l = 99.99% purity= 1,000 mg/l = 99.9% purity
= 10,000 mg/l = 99% purity
ppm = mg/l
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Total Dissolved Solids
NonHardn
ess
TotalDissolv
edTotal
Hardness Salts Solids
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Keeping the Steam Clean.
Pure steamto plant
Build up ofFeed waterimpurities inwith
impurities
the boiler
Impurities‘blown down’ to
waste
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Deciding on the Required Boiler Water TDS.
Maximum TDS tolerated Maximum TDSby boiler type
Lancashire
(ppm)10,000
4,5002-Pass Economic
3,000-3,500Package and 3-Pass Economic
2000-3000Low Pressure Water Tube
1,500Medium Pressure Water Tube
2,000Coil Boilers and Generators
••
The above figures are offered as a broad guide onlyThe boiler maker should always be consulted for specific recommendations.
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Calculating Blowdown Quantity.
Let:
F = Feed water TDS (ppm)
B = Boiler target TDS (ppm)
S = Boiler steaming rate (kg/h)
F x SAmount to be blown down = B - F
This equation is valid for any consistentunits eg lb/h, μS/cm
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The effect of too high TDS in the boiler.
High TDS and/or Contamination of Fouling of HeatExchangersControl Valvessuspended solids.
Blockage of Steam Traps
Boiler
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Automatic TDS Control
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Project Boiler
10,000 kg/hsteam toplant
Feed waterwith 250
Maximumallowable
ppmimpurit
ies
boiler TDS =2,500 ppm
Boiler operating pressure = 10 baroCSaturation temperature = 184
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Project Boiler
Given Information
F = Feed water TDS (ppm) = 250 ppm
B = Boiler target TDS (ppm) = 2,500 ppm
S = Boiler steaming rate (kg/h) = 10,000 kg/h
F x SAmount to be blown down =B - F250 x 10,000= 2,500 - 250
= 1,111 kg/h
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actual blowdown?How much is the
Ba - F
=
=Amount of excess blowdown = 1282 - 1111
= 171 kg/h
1,282 kg/h
250 x 10,0002,200 - 250
Amount actually blown down = F x S
Given Information
Ba = Actual Boiler TDS (ppm) maintained = 2200 ppm
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How much energy is there in theblowdown?
Energy blown down (kW) = 1111kg/h x 782 kJ/kg3600 seconds
= 241 kW
A typical house central heating system is ratedat 13 kW, so this is enough heat for
24113
= nearly 19 houses.
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Net saving
Saving in blowdown x Sensible heat lost x Cost of fuelNet Savings =GCV of fuel x Efficiency of boiler x Sp. gravity of fuel (if oil)
=
= Rs. 695 per day
Total Saving = Rs. 20,850 per month
171 x 24 x (184 - 25) x 6 x 1008000 x 80 x 0.88
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Boiler Bottom Blowdown Rate
FlowRate
(kg/s)
Pressure (bar g)