Boiler Water Scale Prevention: How Does a Water Quality Analyzer Monitor Hardness and Silica to Extend Equipment Service Life?
Introduction
The formation of scale in boilers can have negative economical and equipment-related implications. The formation of the hard layer caused by the presence of hardness and silica in water can lead to a reduction in thermal heat transfer. Carbon steel, which is the common material in boilers, has a thermal conductivity of approximately 50W/m.K. Whereas, boiler scale like calcium sulfate can have 0.6W/m.K conductivity. It means that the heater has to increase its power massively to pass heat through the scale.What is Boiler Water Scale and Why is it a Problem?
Boilers are devices that produce steam through phase change of water. Typically they will use natural gas, propane, oil, coal or electricity as the heating source. When water with high hardness or silica enters the boiler it leaves behind scales that can deposit on the heating elements. Moreover, sludge is formed at the bottom that is blown out of the boiler to maintain chemistry.Reasons for Scale Formation
Now that we know that scale formation is a problem for boiler operation, we can move to the reason why scale formation occurs:
Water Hardness
Hardness in water is indicated by the presence of calcium and magnesium ions. Under the presence of heat, the ions precipitate as carbonates and sulfates. In alkaline conditions, they become calcium carbonate and magnesium hydroxide. These form a hard layer over the heater surface. Even when their concentration is <0.1ppm, high pressure boilers with high temperatures can accelerate their crystalline deposition. If left uncontrolled, it can lead to reduction of pipe internal diameters.Presence of Silica in Water
In comparison to hardness, silica forms a more glass-like layer. It is much more dense than the lime scale. The conductivity of silica is much worse than lime scale. Even with the microscopically thin layer the required heat transfer can cause the heating elements to fail or overheat. The deposition is near permanent. Their removal may require mechanical drilling or hazardous hydrofluoric acid. Moreover, at 400 psig pressure silica converts to vapor which can carry over to equipment like turbines and damage their blades.Recommended Water Quality Standards for Boilers
The water quality parameters are usually determined by the boiler manufacturer. It is recommended to maintain the water chemistry within the defined range. However, these are typically the same as mentioned in the ASME (American Society of Mechanical Engineers) code CRTD-Vol. 34. The title of the code is “Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry”. It is the industry's benchmark standard for watertube boilers. The general guideline provides a table that highlights the need for tightening chemistry requirement with operating pressure:|
Parameter |
0-300 psig |
301-600 psig |
901-1,000 psig |
|
Total Hardness (ppm) |
<0.3 |
<0.2 |
None Detected |
|
Silica (ppm) |
<150 |
<40 |
<8 |
|
Total Alkalinity (ppm) |
<700 |
<500 |
<200 |
|
Specific Conductance |
7000 S/cm |
5000 S/cm |
2000 S/cm |
How Water Quality Analyzers Work
Water quality analyzers are the eyes that keep the boiler's process in check. The presence of calcium and magnesium ions with silica is undetectable to the human eye. Water quality analyzers use sophisticated technology to detect these impurities in the context of boiler water scale prevention. Here is how they do it:Monitoring Hardness
The most modern and rapid water quality analysis technique for hardness is ion-selective electrodes (ISE). The sensor is designed to react with calcium and magnesium ions.It works on the principle of Nernst equation:Monitoring Silica
In comparison to hardness the monitoring of silica requires much more sophisticated design in water quality analyzers especially to prevent boiler water scale. The most common method is the Molybdenum Blue method. We convert the water containing silica into a deep blue color for detection.Silica reacts with molybdate in acidic conditions. It results in the formation of silicon molybdate blue. The intensity of the blue color represents the presence of silica in water. An emitter with 810nm frequency shines light through the sample water. The absorber on the other end detects how much light actually passed through. Generation of signal is directly representative of the silica present in water. The detection capability of silica water quality analyzer is 0 to 200 ppb with ±1% accuracy.Real-Time Monitoring and Early Warning for Scale Prevention
Monitoring the presence of magnesium calcium ions and silica in real-time comes with massive advantages for boiler scale prevention.
Automated Control System
Lab analysis reports may take hours to diagnose a problem which are then followed by manual operation. It causes formation of scale during the process. Modern real-time monitoring of water quality analyzers provide real-time values with direct control actions through its integration with Distributed Control System (DCS).
The system can initiate blowdown of the boiler to purge concentrated minerals. Moreover, chemical pumps can inject scale inhibitors the moment hardness increases to protect heating elements in boilers.
Preserving Heat Transfer Capability
Through the early detection we can prevent the formation of scale. As we mentioned earlier, a 1/16” can reduce the boiler's operating efficiency by 5%. The system can generate alarms which means avoiding the localized hot spots that lead to tube bulging or hydrogen embrittlement.
Forensic Analysis
Modern water quality analyzers can detect and record their data for 30-days or higher. Through their integration with DCS operators can detect an upward trend over the course of the day. Which may be an indicator of the demineralizers degradation or exhaustion for boiler operations.
Reducing Energy Consumption and Maintenance Costs
By preventing the formation of scale, there is less overheating and corrosion fatigue. It can extend the life of boilers and associated equipment by 20 to 30%. Moreover, the need for aggressive chemical cleaning is greatly reduced, preventing thinning of the boiler metal with time. Here is a table that shows efficiency impact of scale in boilers which is conveniently avoided by using of water quality analyzers:
|
Scale Thickness (inches) |
Efficiency Loss (%) |
Energy Cost Increase (%) |
|
0.016 |
5 |
5-10 |
|
0.043 |
10 |
10-15 |
|
0.094 |
15 |
15-20 |
|
0.177 |
20 |
20-25 |
Conclusion
Boilers are an expensive equipment that requires constant supervision during operation to prevent long shutdowns for maintenance. One of the leading causes of boiler efficiency degradation and equipment damage is the presence of hardness and silica in boiler feedwater. Advanced water quality analyzers that work on the principle of Ion-Selective Electrodes (ISE) and Molybdenum Blue method can detect minute changes in the levels of magnesium/calcium ions and silica respectively. The result is better boiler control, equipment longevity, reduction in maintenance, and improved efficiency.
If you are looking for an experienced water quality analyzer manufacturer then consider Shanghai BOQU Instrument Co., Ltd., founded in 2007 in Shanghai, China. They provide high-end water quality monitoring solutions. With over 100 dedicated professionals, BOQU specializes in advanced analyzers and sensors for parameters like hardness and silica. Explore all their technologically advanced sensors at https://www.boquinstrument.com/.
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