boiler blowdown
TRANSCRIPT
Boiler Blowdown
Engr. Sahadat HussainSr. Consultant, Environmental & Process
Engineering Resources International (ERI)
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.
Water quality needs to be such that weoperate under the following criteria:-
• Freedom from scale
• Freedom from corrosion
• Good quality steam
• Safety
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
Total Dissolved Solids
NonHardn
ess
TotalDissolv
edTotal
Hardness Salts Solids
Keeping the Steam Clean.
Pure steamto plant
Build up ofFeed waterimpurities inwith
impurities
the boiler
Impurities‘blown down’ to
waste
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.
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
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
Automatic TDS Control
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
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
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
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.
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
Boiler Bottom Blowdown Rate
FlowRate
(kg/s)
Pressure (bar g)