What procedures are included in boiler water treatment?
02 Jan '18, 12:08
Main requirement safe and efficient operation of boiler
The boiler water must be sufficiently free of deposit forming solids to allow rapid and efficient heat transfer and it must not be corrosive to the boiler metal.
Scale deposits and corrosion result in efficiency losses and may cause boiler tube failures and inability to produce steam. The predominant cost factor for producing steam is fuel costs. These costs will increase if there are scale deposits. Deposits in boilers may result from hardness contamination of feedwater.
The primary minerals in the water that make "hard" water are Calcium (Ca++) and Magnesium (Mg++). They form a scale in piping, water heaters and whatever the hard water contacts. Hardness contamination of the feedwater may result from either deficient softener systems or raw water in leakage of the condensate. Deposits act as insulators and slow heat transfer. The insulating effect of deposits causes the boiler metal temperature to rise and may lead to tube-failure by overheating. Large amounts of deposits throughout the boiler could reduce the heat transfer enough to reduce the boiler efficiency.
Water treatment for scale deposit control
Internal chemical treatment for deposit control is achieved either by adding a treatment to prevent the contaminants from depositing or by adding a treatment chemical that will allow for easy removal by blowdown.
With phosphate treatment, the boiler water is conditioned to form a fluid sludge which can be removed by bottom blowdown. Formation of this sludge requires that alkalinity from caustic be present in the boiler water. If sufficient alkalinity is not maintained in the boiler water, a sticky precipitate will form and reduce heat transfer.
Sludge conditioners enhance the removal of precipitates from boilers. Sludge conditioners are organic polymers, which combine with the precipitates to permit the particles to be dispersed. This makes removal by blowdown easier.
Boiler Corrosion with dissolved oxygen
One of the most common reasons for boiler corrosion remains the action of dissolved oxygen in the make-up water and feedwater.
Heating of Feedwater to remove dissolved oxygen
Good boiler water treatment is dependent not just on the chemicals but also on the daily maintenance of the condensate and feedwater systems, as well as the boiler itself. It is important that the condensate returns in its full amount and that, if possible, the hot well temperature is kept at 85-95°C for deoxygenizing and that the boiler is regularly skimmed and blown down. The crew must still ensure skilled and efficient monitoring and maintenance of the complete system.
Treatment for Corrosion control
Although thermal deaeration removes most of the dissolved oxygen in the water, chemical oxygen scavengers are used to scavenge remaining oxygen. These are sodium sulfite and Hydrazine.
Contamination by seawater and oil promote heat transfer resistance on the waterside of the boiler, causing the steel structure facing the flame and flue gas to be heated beyond its design point. Structural damage/rupture will occur when the contamination becomes sufficiently high, resulting in a leaking boiler.
02 Jan '18, 12:27