Boiler blowdown

Energy Saving in boiler blowdown

1. Boiler Blowdown Engr. Sahadat Hussain Sr. Consultant, Environmental & Process Engineering Resources International (ERI)

2. Common Impurities in Raw Water • Dissolved solids - s cale forming substances . The principal ones are the carbonates and sulphates o f calcium and magnesium. Not all dissolved solids will form scale. • Suspended solids - sludge forming organic substances which are in usually solid mineral or particles, carried suspension. Not generally a problem with most waters. • Dissolved gases - c orrosive. Oxygen and carbon dioxide. • Scum forming substances - mineral scum forming or foaming impurities usually containing soda in the form of a carbonate, chloride or sulphate.

3. Water quality needs to be such that we operate under the following criteria:- • Freedom from scale • Freedom from corrosion • Good quality steam • Safety

4. ppm = mg/l Units • of Concentration Parts Per Million (ppm) 1 kg in 1,000,000 kg (1,000 Tonnes) of water 1 lb. in 100,000 lmp. gallons of water • Milligramm e Pe r Litr e (mg/l) 1 1 1 mg mg mg in in in 1,000,000 mg of water 1,000 g of water 1 litre of water 100 ppm 1,000 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

5. Total Dissolved Solids Non Hardness Total Dissolved Total Hardness Salts Solids

6. Keeping the Steam Clean. Pure steam to plant Buil d u p of Feed water impurities in with impurities the boiler Impurities ‘blown down’ t o waste

7. Deciding on the Required Boiler Water TDS. Maximum TDS tolerated Maximum TDS by boiler type Lancashire (ppm) 10,000 4,500 2-Pass Economic 3,000-3,500 Package and 3-Pass Economic 2000-3000 Low Pressure Water Tube 1,500 Medium Pr e ssure Water Tube 2,000 Coil Boilers and Generators • • The above figures are offered as a broad guide only The boiler maker should always be consulted for specific recommendations.

8. Calculating Blowdown Quantity. Let: F = Feed water TDS (ppm) B = Boiler target TDS (ppm) S = Boiler steaming rate (kg/h) F x S Amount to be blown down = B - F This equation is valid for any consistent units eg lb/h, μ S/cm

9. The effect of too high TDS in the boiler. High TDS and/or Contamination of Fouling of Heat Exchangers Control Valves suspended solids. B l ockage of Steam Traps Boiler

10. Automatic TDS Control

11. Project Boiler 10,000 kg/h steam to plant Feed water with 250 Maximum allowable ppm impurities boiler TDS = 2,500 ppm Boiler operating pressure = 10 bar o C Saturation temperature = 184

12. 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 S Amount to be blown down = B - F 250 x 10,000 = 2,500 - 2 50 = 1,111 kg/h

13. actual blowdow n ? How much is the B a - F = = Amount of excess blowdown = 1282 - 1111 = 171 kg/h 1,282 kg/h 250 x 10,000 2,200 - 2 50 Amount actually blown down = F x S Given Information B a = Actual Boiler TDS (ppm) maintained = 2200 ppm

14. How much energy is there in the blowdown ? Energy blown down (kW) = 1111kg/h x 782 kJ/kg 3600 seconds = 241 kW A typical house central heating system is rated at 13 kW, so this is enough heat for 241 13 = nearly 19 houses.

15. Net saving Saving in blowdown x Sensible heat lost x Cost of fuel Net Savings = GCV of fuel x Efficiency of b o iler 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 100 8000 x 80 x 0.88

16. Boiler Bottom Blowdown Rate Flow Rate (kg/s) Pressure (bar g)