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Six signs that your auxiliary boiler needs maintenance


As well as examining early signs that auxiliary boilers may be in need of maintenance or repair, this article also discusses preventative maintenance practices to ensure they are operating at peak efficiency


Michael W. Valentino PE USA*


Power plant auxiliary boiler sizes vary widely, from under 100 000 lb/h for smaller power generating stations to hundreds of thousands of pounds steam per hour for larger scale power plants. Maintaining a power generation station auxiliary boiler (Figure 1) in top operating condition can yield significant dividends. Often, there are early warning signs that a boiler is in need of attention. So, what are some of these early warning signs?


Elevated stack temperatures


One of the first signs that your auxiliary boiler is possibly in need of maintenance are elevated stack temperatures for a given firing rate and operating steam pressure. Figure 2 shows the relationship between stack temperature, excess air (oxygen or O2


translates to 0.4


oxygen (which leads to pitting and corrosion); pH; hardness; dissolved and suspended solids; alkalinity; iron and copper; and organic compounds. Waterside surfaces should be inspected periodically and regularly for scale buildup and signs of corrosion or pitting. Other system components of interest include water softeners, feed tanks and deaerators, pumps, valves, and chemical feed systems. These systems should be inspected and maintained on a regular basis. A regular boiler blowdown schedule covering boiler water, water columns, and sight gauge glasses should be conducted.


Fireside soot in the flue gas), and gross


combustion efficiency. Broadly speaking, each additional 1% O2 points efficiency decline.


Figure 1. Maintenance underway on a firetube boiler.


C increase in boiler stack temperature, boiler efficiency is reduced by approximately 1%. While this may not sound like much, a 66.7o


A good “rule of thumb” guideline is that for every 22.2o


C increase in stack temperature can result in an efficiency loss of 3 percentage points. This amount of efficiency change can add up to significant additional fuel usage and cost over the course of a year.


For reference, gross combustion efficiency uses the fuel’s gross calorific value (GCV), which includes the latent heat released when water vapour from combustion condenses. Net combustion efficiency uses the fuel’s net calorific value (NCV), which excludes the latent heat of the water vapour, representing only the heat available above condensation temperature. The NCV yields calculated efficiencies 5 – 10 percentage points higher than GCV calculations, depending on what fuel is fired. Countries such as the UK and other European countries often base combustion efficiencies on NCV whereas the USA bases combustion efficiencies on the GCV, which will always yield lower calculated results. In either case, since auxiliary boilers operate in non-condensing mode, the efficiency changes based on changes in stack temperature changes will yield similar percentage changes in efficiency whether GCV or NCV is used.


When first commissioned, an auxiliary boiler is in a clean condition and free of appreciable soot and scale. The service technician that set combustion tuned the auxiliary boiler to provide optimum efficiency while also providing safe and reliable operating conditions. Over time, however, boiler stack temperatures for a given load


and operating conditions can change: ● scale can build up on waterside surfaces due to poor water quality;


● soot can accumulate on fireside surfaces due to improper combustion, the causes of which we will touch upon below; and


● burner components can fail or fuel and air settings can be altered.


Waterside scaling


Good boiler water quality begins with proper water chemistry, both in the feedwater makeup and boiler water. Figure 3 shows scale buildup inside a boiler tube. The associated fuel loss with this scale buildup is depicted in Table 1. Boiler manufacturers and water quality experts can provide recommended maximum concentration levels of contaminants in the feedwater makeup and in the boiler water itself. The list of contaminants and other key water properties often include: dissolved


18 | March 2026 | www.modernpowersystems.com


Sooting of auxiliary boiler fireside surfaces is often the result of poor combustion or flame impingement, leading to flame quenching. In addition, elevated levels of carbon monoxide and other unburned


hydrocarbons can result.


Whether due to changes in environmental conditions (seasonal or otherwise), changes in fuel–air mixing from originally set values, or other reasons, the sooting of fireside surfaces will result in poorer heat transfer and lower boiler efficiency.


Figure 4 depicts soot buildup in a firetube style auxiliary boiler, which requires cleaning to maintain good heat transfer. Similar to scale buildup on waterside surfaces, boiler efficiency loss in the range of 3% to 6% and even greater can be expected with a sooted up boiler. The same is true with watertube style boilers and auxiliary boilers, where soot buildup occurs on the tube exterior surfaces, and scale deposits will occur on the inside of tubes carrying the water to be heated.


Table 1. Fuel loss as a function of scale Fuel loss, % of total use


Scale thickness Normal scale High iron scale 1/64 inch


1.0% 1/32 inch 3/64 inch 1/16 inch 2.0% 3.0% 3.9% 1.6% 3.1% 4.7% 6.2%


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