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Minimizing Blowdown Rates Can Substantially Reduce Energy Losses

Posted by Ritchie Ware on Jun 20, 2017 10:22:34 AM

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The importance of boiler blowdown is often overlooked. Despite the best efforts to pretreat boiler feedwater, it can still contain impurities including suspended and dissolved liquids. These impurities can accumulate in the boiler and cause problems with piping, steam traps and other process equipment. To avoid problems, the water must occasionally be discharged or “blown down” from the boiler.

Minimizing the blowdown rate can substantially reduce energy losses, and improper blowdown can cause increased fuel consumption, requiring additional chemical treatments and heat loss. In addition, since the blowdown water is the same temperature and pressure as the boiler water, it can be recovered and reused in the boiler operations.

Suggestions:

  1. Review blowdown practices to identify opportunities to save energy.
  2. Examine the operating practices for boiler feedwater and blowdown rates developed by the American Society of Mechanical Engineers (ASME), including operating pressure, steam purity and deposition control.  
  3. Consider an automatic blowdown control system.

Types of Blowdowns

Surface blowdowns are used to remove particulates, dissolve materials in the boiler water and to control boiler water chemistry. Surface blowdowns can reduce contamination or boiler water that has been overtreated with chemicals.

Mud or bottom blowdowns are used to control the amount of sludge in the boiler water sand remove suspended solids that settle out of the boiler water and form a heavy sludge. These blowdowns are usually manual processes that last a few seconds, performed in intervals of several hours.

Blowdown rates usually range from 4% to 8% of the boiler feedwater flow rate, but they can be as high as 10% when makeup water has high solids content. The optimum blowdown rate is determined by several factors, including boiler type, operating pressure, water treatment and quality of makeup water.

Automatic Blowdown Control Systems

Conductivity, total dissolved solids (TDS), silica or chloride concentrations and/or alkalinity are reliable indicators of salts and other contaminants dissolved in boiler water. An automatic blowdown control system can monitor the pH and conductivity of the boiler water and allow blowdown only when necessary. Automatic blowdown control systems with continuous blowdown TDS measurements are available on the market, with a probe that provides feedback to a controller driving a modulating blowdown valve.

EXAMPLE*

Assumptions:

  • Installation of an automatic blowdown control system reduces blowdown rate from 8% to 6%
  • Makeup water temperature of 60°F
  • Boiler efficiency at 80%
  • Fuel valued at $8.00 per million Btu ($8.00/MMBtu)
  • Total water, sewage and treatment costs at $0.004 per gallon  

 

Boiler Feedwater

Initial: 100,000/(1 – 0.08) = 108,696 lb/hr

Final: 100,000/(1 – 0.06) = 106,383 lb/hr

 

Makeup Water Savings = 108,695 – 106,383 = 2,312 lb/hr

Enthalpy of Boiler Water = 338.5 –Btu/lb; for makeup water at 60°F = 28 Btu/lb

Thermal Energy Savings = 338.5 – 28 = 310.5 Btu/lb

 

Annual Fuel Savings

2,312 lb/hr x 8,760 hr/year x 310.5 Btu/lb x $8.00/MMBtu/(0.80 x 106 Btu/MMBtu) = $62,886

Annual Water and Chemical Savings

2,312 lb/hr x 8,760 hr/year x $0.004/gal/8.34 lb/gal = $9,714

Annual Cost Savings = $62,886 + $9,714 = $72,600

 

*Adapted from Energy TIPS fact sheet originally published by the Industrial Energy Extension Service of Georgia Tech.

 

Topics: Preventative Maintenance Checklist, Boiler Tuning

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