What is Boiler blowdown?
Boiler blowdown is simply the process of control the concentration of the dissolved solids in the feedwater.
Many dissolved solids present in the water remain in the boiler when water is boiled and steam is created. If the feed water contains more solids, they can accumulate and gradually reach a level where their water solubility is surpassed and collected from the solution.
Therefore, the level of concentration of the solids must be controlled and this is accomplished by the’ blowing down’ cycle. Once the steam is produced, the boiler discharges essentially pure H2O vapour, leaving the solids deposited into the feed water to remain in the boiler circuits. The net result of constantly adding impurities and eliminating pure water vapour is a steady increase in the boiler water level of dissolved solids.
A certain volume of water is blown off and replaced by feed water automatically, thereby preserving the optimum level of total dissolved solids (TDS) in boiler water.
Calculation of Blowdown Rate:
The amount of blowdown needed to control the concentration of boiler water solids is determined using the following formula:
Blow down (%) = (Feed water TDS x % Make up water) / (Permissible TDS in Boiler water)
The actual concentration of dissolved solids at which foaming will start varies from boiler to boiler. Usually, traditional shell boilers are run with TDS in the range of 2000 ppm for very small boilers and up to 3500 ppm for larger boilers. Provided the:
- Boiler is operating near to its design pressure.
- Steam load conditions are not too severe.
- Other boiler water conditions are correctly controlled
The intermittent blown down is accomplished by manually operating a valve fitted to the discharge pipe at the lowest point of the boiler shell to minimize parameters (TDS or conductivity, pH, the concentration of silica and phosphates) within the specified limits so that the steam output is not likely to be affected.
There is a steady and constant dispatch of a small stream of concentrated boiler water, and replacement by the steady and constant inflow of feed water. At a given steam load, this ensures constant TDS and steam purity. There is no need for frequent operator control once the blow-down valve is adjusted for a given condition.
Though large amounts of energy are lost, there is a chance to recover this heat by blowing into a flash tank and creating flash steam. This flash vapour can be used for boiler water preheating or for any other reason.
Benefits of Blowdown:
Good boiler blow-down control can reduce treatment and operating costs substantially, including:
Lower pretreatment costs
Less make-up water consumption
Reduced maintenance downtime
Increased boiler life
Lower consumption of treatment chemicals