The CDP or Continuous Blowdown method of boiler layup consists of piping continuous surface blowdown from the operating boiler to the bottom of the stand-by boiler. The overflow from the vent goes to heat recovery or to the drain. This method is typically not used on boilers with superheaters. Advantages of this method are as follows:
- Good corrosion control due to proper sulfite and alkalinity readings
- Boiler contains proper chemical residuals when you return it to service
- Boiler remains hot, so less time and thermal energy is needed to get the unit up to pressure
- Refractory stays hot and fireside corrosion is eliminated
- No need to maintain high chemical residuals. No need to fire the boiler to promote circulation
Disadvantages of this method are:
- Possible suspended solids deposition if you have poor (>1 pm) feedwater quality on operating boiler (1 ppm = 1# sludge/100,000 gallons feedwater)
- Installation costPossible short circuiting
- May not work on multiple boiler operations
- May negate blowdown heat recovery