COMMON WATER IMPURITIESImpurity SourceEffect
Algae organic growth fouling
Calcium mineral deposits scale
Carbon dioxide dissolved gases corrosion
Chloride mineral deposits corrosion
Free acids Indus. Wastes corrosion
Hardness mineral deposits scale
Magnesium mineral deposits scale
Oxygen dissolved gases corrosion
Silica mineral deposits scale
Suspended solids undissolved matter fouling/scale
There are five major problems directly associated with water quality that will effect boiler performance. These are:
SCALEis a very hard substance that adheres directly to heatingsurfaces forming a layer of insulation. This layer of insulationwill decrease heat transfer efficiency. Scale also results inmetal fatigue/failure from overheating, energy waste, highmaintenance costs and unnecessary safety risks. A one-sixteenthinch thickness of scale in a firetube boiler can result in a12.5% increase in fuel consumption.
CORROSION is defined as the destruction of a metal bychemical or electromechanical reaction with its environment. Themetal is eaten away in much the same manner as fender rusts on acar. Corrosion dramatically increases maintenance costs and cancause unnecessary safety risks. It will occur when levels ofoxygen or carbon dioxide are high, where pH values are low, wherecontact occurs between dissimilar metals and in damp environmentor corrosive atmospheres. Corrosion is anelectrochemical process in which electricity flows through asolution of ions between areas of metal. Deterioration occurswhen the current leaves the negatively charged metal or anode andtravels through the solution to the positively charged metal orcathode, completing an electrical circuit in much the same manneras a battery cell. The anode and the cathode can be differentmetals or areas of the same metal. Corrosion occurs when there isa difference in the electrical potential between them.
FOULING occurs when a restriction develops in piping andequipment passageways and results in inefficient water flow. Thefouling of boiler room equipment directly impacts energyefficiencies and cost of operations.
FOAMING is a condition in which concentrations of soluble salts, aggravated by grease, suspended solids or organicmatter, create frothy bubbles or foam in the steam space of aboiler. When these bubbles collapse it creates a liquid that iscarried over into the steam system. Foaming degrades steamquality and in some cases can create a water slug that isdischarged into the steam lines.
CAUSTIC EMBRITTLEMENT will occur when there is a highconcentration of alkaline salts (a pH value of 11 or greater)that will liberate hydrogen absorbed by the iron in the steel.Caustic embrittlement will be more evident in high temperatureareas of the boiler's waterside and manifests itself in the formof hairline cracks.