on August 20, 2019 in Boiler Tube Cleaning
Armed with long rods and brushes, early boiler cleaners toiled for hours scraping off combustion debris from a boiler’s fireside and scooping out waterside sediment exposed to all sorts of deadly ash and contaminants. Fortunately, for modern day technicians the days of long sticks and brushes are gone and new inventions keep soot and debris out of their lungs and off their faces. However, the basic problems of boiler soot and scale haven’t gone away.
Fouling affects every boiler of every size from small natural gas home boilers to giant coal-fired power plants. When a fuel like coal, oil, or wood are burned the soot and combustion products collect on the internal components of the fireside creating an insulating layer of ash preventing heat from getting to the water. A study on boiler fouling in power plants by the Queensland Australia College of Engineering and Built Environment stated that “[o]ne of the main effects of the formation of slagging and fouling deposits is that they reduce the heat transfer process between the fireside and the water-steam side and resulting in a sensible increase of the flue gas temperature, reducing the efficiency of the system and increasing corrosion problems in boilers.” The study goes on to note that fireside fouling is responsible for “increasing overall maintenance cost.”
Rods and backbreaking labor have been replaced with modern fire tube cleaning systems that require little effort from the technician and clean tubes and keep ash and debris from escaping into the mechanical room and onto the workers. Straight tube punching machines work by unwinding a tape and cleaning brush down the length of the tube scraping off debris as it travels. The operator then reverses the tape and it self-rewinds back into the cleaning machine. For fire tubes with tougher deposits, cleaning systems with a motorized flexible rotary shaft push back and forth down the length of the tube. A strong cleaning tool at the end of the shaft spins and removes even the most baked-on deposits from the tube’s inner surface. Both cleaning systems are connected to specialized soot vacuum cleaners that suck out loose debris keeping it contained within the cleaning machine.
Boilers constructed in such a way that water flows through the tubes instead of fire are called water tube boilers. The fouling in water tubes is an accumulation of sediment, biological contaminants, or scaling. These tubes can be cleaned similarly to fire tubes using rotary cleaners, but with softer brushes to remove the lighter deposits. Sometimes though, heavy scale forms which requires heavier duty scraping tools, or the use of descaling chemicals like Goodway’s ScaleBreak®. Water tubes may only be accessible by the technician climbing into the boiler mud drum so it’s important to select a smaller and portable cleaning machine that won’t get in the way of the work being performed.
On the other side of the fire is the waterside where water is heated or turned to steam. During the heating process, minerals in the water accumulate on the inside surfaces reducing heat transfer just as soot does on the fireside. In the past, hand scrapers were used to break away these deposits, but advances in chemical engineering have produced descaling chemicals that dissolve scale on contact eliminating the need for scraping. These recirculation cleaning systems continuously pump descaling chemicals through the boiler or heat exchanger dissolving solids as the chemical passes over the scale. The role of maintenance staff is limited only to monitoring the pH of the descaling solution as it returns back to the cleaning system. When descaling is complete the pH of the descaling solution will be neutral indicating that there is no more chemical reaction taking place between the scale and the chemicals. The result though is the complete removal of scale deposits, leaving pristine, cleaned tubes.
For boilers to operate efficiently and without frequent breakdowns, they need a regular schedule for maintenance and cleaning, as well as monitoring during operation. Monitoring stack temperature is a proactive method for identifying the presence of heavy boiler fouling. As discussed, when the inside of the boiler is coated with scale and soot, heat from combustion cannot transfer through the tube wall and into the water. The unused combustion heat is pushed out of the flue and dispersed to the atmosphere. By tracking the flue gas temperature, facility managers can check if flue gas temperature is increasing indicating it is time to clean the boiler. A consistent flue temperature is evidence of a clean boiler.
Boiler cleaning has come a long way from the era of scrub brushes and soot-filled rooms. Advancements in cleaning tools, descaling chemicals, and flue temperature monitoring mean boilers are easier to maintain and can remain efficient throughout their operational lives.
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