BOILER WATER TREATMENT

A boiler is a closed vessel where water under pressure is transformed into steam by the application of heat. In the boiler furnace, the chemical energy in the fuel is converted into heat. The function of the boiler is to transfer this heat to the contained water in an efficient manner. As the water boils, its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder. Therefore, boiler equipment is dangerous and must be treated with care.

Controlling pH and conductivity in boiler feed water is critical. Key benefits include:

  • Reduce the potential for carryover into the steam system
  • Eliminate excessive blow down which wastes water, chemical, and energy
  • Eliminate overfeeding chemical to address high solids excursions
  • Reduce time operators spend testing and adjusting system

SENSORS & INSTRUMENTATION FOR BOILER WATER TREATMENT

The most important parameters to monitor in boiler applications are conductivity and pH. These are measured in different streams from boiler feed water to boiler water to condensate return.

pH TDS Conductivity Control for Boiler Water Treatment

pH Control:
In boiler water control, the pH plays a vital role. A slightly alkaline pH must be maintained to prevent corrosion. The alkaline will help to maintain a passivation layer which protects the boiler from corrosion attack. The typical pH level maintained in different points of boiler is included in the table below.

pH Boiler Water

Conductivity Control:
Conductivity is also a key parameter for boiler control, as it directly relates to energy savings. Lack of conductivity control leads to operational problems like scaling inside the boiler, necessitating increased fuel consumption.

Conductivity control is primarily important for minimizing the mineral level in the boiler through replacing process water with fresh water. This is sometimes referred to as “blow down.” While blow down is an important process, excessive blow down should be prevented because it is a waste of water and energy.

Conductivity monitoring also helps improve condensate recovery. Condensate is typically the purest water in boiler system, but without proper boiler control, it may become contaminated. Maintaining high condensate purity is key to optimizing boiler operation costs, as only pure condensate by be recovered.

One more area where conductivity should be considered is the boiler feed water, which can affect the cycles of concentration of the boiler.

TYPES OF BOILERS:

There are two principle types of boilers that are used for industrial applications:

• Fire Tube Boilers: Products of combustion passed through the tubes, which are surrounded by water.

• Water Tube Boilers: Products of combustion passed around the tubes containing water. The tubes are interconnected to common channels or headers and eventually lead to a steam outlet for distribution to the plant system.

Fire Tube Boiler:
Fire tube or “fire in tube” boilers create steam by passing hot gases, created through combustion, through long steel tubes. The tubes are in direct contact with water, so the heat from the gases is transferred to the water via thermal conduction. Fire boiler tubes have low initial costs and are relatively fuel efficient and easy to operate. However, they are generally limited to capacities of 25 tons/hr and pressure of 17.5kg/cm2.

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Fire Tube Boiler Water Treatment

Water Tube Boiler:
The reversed set up, where in water passes through steel tubes and hot gasses circulate outside the tubes, is called a water tube or “water in tube” boiler. These boilers can be of single or multiple-drum types. They can be built to any steam capacities and pressures, and have higher efficiencies than fire tube boilers.

Boiler Water Treatment

Packaged Boilers:
Pacakged boilers are pre-built by a manufacturer, making them easy to deploy. Set up requires steam pipes, water pipes, fuel supply, and electrical connections and can be achieved very quickly. Packaged boilers are ideal for tight spaces due to their compact size; this also makes packaged boilers cheaper to operate. Due to their size and limited efficiency, packaged boilers are not recommended for large scale power plants and operations.

COMMON PROBLEMS WITH BOILER WATER

Completely pure water, which would be ideal for a steam generation system, is generally not achievable. Therefore, impurities in boiler feed water should be monitored and addressed. Impurities in the supplied water for a boiler system may be classified as:

  • Dissolved soilds
  • Dissolved gases
  • Suspended matters

The major problems associated with boiler feed water impurities are the following:

  • Scaling/deposition
  • Corrosion-oxygen attack, caustic corrosion
  • Boiler water carryover

Toroidal conductivity sensors and instruments should be used for scrubber control since these sensors are highly resistant to fouling. These sensors are also recommended due to the high conductance of many scrubbing solutions and by-products.

Scale / Deposition:
Scale is one of the most common deposit related problems. Scale is the buildup of solid material from the reactions between the impurities in water and tube metal on the waterside tube surface. Scale acts as an insulator that reduces heat transfer, causing a decrease in boiler efficiency and excessive fuel consumption. More serious effects are overheating of tubes and potential tube failure (equipment damage). Wasted fuel due to scale may be approximately 2-5 percent depending on the thickness of the scale. Salts that have limited solubility in the boiler water form scale. When concentrated by evaporation, these salts reach the deposit site in a soluble form and precipitate. Sludge refers to the accumulation of solids that precipitate in the bulk boiler water or enter the boiler as suspended solids.

Oxygen Attack:
The most common causes of corrosion occur inside boilers. Dissolved oxygen in feed water can become very aggressive when heated and reacts with the boiler’s internal surface to form corrosive components. Oxygen attack can cause further damage to steam drums, mud drums, boiler headers and condensate piping.

In the absence of oxygen, water reacts with iron to form magnetite. Water reacts with iron to form magnetite only in the absence of oxygen. The presence of oxygen promotes the formation of Hematite or Red Iron Oxide, which is non-protective. Oxygen corrosion takes the form of localized deep pitting and can quickly lead to tube failure.

Preventing oxygen corrosion is quite simple: Keep oxygen out!

Acid Attack:
Another common cause of corrosion is acid attack. Acid attack occurs when the pH of feed water drops below 8.5. The carbonate alkalinity in the water is converted into carbon dioxide gas (CO2) by the heat and pressure of the boilers. CO2 is carried over within the steam. When the steam condenses, CO2 dissolves into water to form carbonic acid (H2CO3) and reduces the pH of the condensate returning to the boilers. Acid attack may also impact condensate return piping throughout the facility.

Boiler Water Carryover:
Boiler water carryover is a contamination of steam with boiler water solids. High concentrations of soluble or insoluble solids in boiler water can help stabilize and strengthen the bubbles’ surface in boiler water, giving rise to foaming. Generated steam can carry the foams into this process. Substances such as oil, alkali, greases, organic matter and suspended solids are known to cause foaming.

A sudden surge of boiler water caused by a rapid change in load causes priming. Priming can cause boiler water carryover by allowing small droplets of water to be released into the steam space. Such carrying over causes contamination in the many processes for which steam is used.

BOILER WATER CONTROL

To assure acceptable steam purity, the American Boiler Manufacturers Association (ABMA) specifies boiler water composition limits (Table 1) by operating pressure. As operating pressures increases, requirements for boiler water composition becomes more stringent.

Water tube boilers experience heat flux rates as high as 250,000 Btu/hr/ft2, far greater than in boilers in service when the ABMA limits were established. Combined with dimensional restrictions on modern units, this has raised the need for new guidelines to replace the outdated ABMA limits. Table 2 from the ASME Research Committee on Water in Thermal Power Systems reflects the need for feed water to be extremely pure. These new guidelines will continue to be refined. Table 1 & 2 mentioned, below.

Boiler Water Composition Limits
Boiler Water Guidelines for Water Tube Boilers

 

CONCLUSION

pH and conductivity control in boiler feed water is critical. Key benefits include:

  • Reduce the potential for carryover into the steam system
  • Eliminate excessive blow down which wastes water, chemical, and energy
  • Eliminate overfeeding chemical to address high solids excursions
  • Reduce time operators spend testing and adjusting system