Boiler basics Maximize your steam boiler's efficiency and longevity (and reduce costs) by following these rules by Dean Wilson Steam boilers are vital to practically every branch of American industry, since they provide heat for employees and production.  A facility's boiler system is generally the single-biggest energy user, yet boilers remain largely misunderstood. Properly cared for, steam boilers provide safe, efficient heat transfer for an enormous variety of processes.  However, poorly maintained boilers gobble up energy dollars at an astounding rate and result in production reliability problems, safety concerns and maintenance headaches. Energy is almost always the single-largest expense associated with steam production.   For example, consider a typical "package" boiler installation in a medium-sized plant.  Generally, this might consist of three or four boilers providing average steam production of 25,000 to 40,000 pounds per hour. If natural gas is used for fuel, the fuel bill for the boilers alone would conservatively average about $60,000 to $80,000 per month.  Incredibly, people who operate and maintain the boiler systems often have no idea of the immensity of the fuel bill.   Conversely, people responsible for tracking and paying the fuel bills often have no idea how the boiler systems are operated and maintained. When you understand both, it becomes clear that neglecting the boiler system is unacceptable. Ironically, the practices that contribute most to boiler efficiency, longevity and reduced maintenance costs are easy and inexpensive.  Training in the fundamental concepts of boiler operation will reduce all costs associated with labor, materials and, most of all, energy. A steam boiler system essentially consists of converting fuel to heat energy, followed by a series of heat exchange processes.  Anything that interferes with these steps results in an energy loss.  To minimize the costs, follow these seven steps to lessen the degree that outside influences interfere with the exchange processes. Tune burners The burner's job (whether gas or liquid fuel) is to mix the fuel with oxygen for combustion.  However, remember that atmospheric air is only about 21 percent oxygen.  Of the remainder, 78 percent is nitrogen and about 1 percent is miscellaneous gases. Only oxygen contributes to combustion.  The remainder does not contribute, but does absorb heat as it passes through the furnace area.  Thus, excessive amounts of air will carry much heat energy out the stack.  On the other hand, insufficient combustion air in the air/fuel mixture results in the generation of carbon monoxide as a byproduct of the combustion process. The combustion process that yields carbon monoxide (a toxic pollutant) produces one-third as much heat as a correctly tuned process that yields carbon dioxide.  For these reasons, tune burners to provide the right amount of combustion air throughout the burner's range.  No more, no less. This requires a flue gas analyzer to measure the amount of carbon dioxide, carbon monoxide and oxygen that exit the stack. Flame appearance and color are not suitable indicators of burner tuning.  The job of burner tuning should be left with an experienced technician. Poor burner tuning also contributes to the formation of carbon soot on the furnace side of the heat exchange surfaces.  Soot has a very high insulating value, and substantially inhibits heat transfer to the water. The presence of any soot in a natural gas-fired boiler is an indicator of poor burner tuning. Eliminate scale Mineral scale accumulation in boilers results when makeup water is not softened.   Scale generally occurs much sooner and to a greater extent in steam boilers providing process steam than in boilers providing steam only for heating since heating boilers generally utilize the same water repeatedly in a continuous, low-pressure closed loop. If part of the steam or condensate is lost through leaks, condensate receiver vents, etc., this system water must be made up with city water or well water.  This "makeup water" contains impurities in the form of calcium, magnesium, iron, bicarbonate and oxygen.  Calcium compounds are by far the largest contributor to scale formation. Scale accumulation dramatically decreases a boiler's heat transfer efficiency.  This is because the scale acts as an insulator on the heat exchange surfaces.  Thus, more fuel must be burned in order to produce the same amount of steam. A quarter-inch thick accumulation of calcium scale will reduce a boiler's efficiency almost 40 percent.  In addition, scale keeps the boiler water from conducting the intense heat away from the metal surfaces, and metal deterioration will occur more quickly.  The scale-forming impurities also plug piping connections to level switches, gauge glasses and other safety devices.  In the worst case, this results in a boiler meltdown or explosion when the water level control devices can't sense the true water level. To minimize scale, minimize the use of makeup water.  This means returning as much clean condensate to the boiler as possible, minimizing blowdown to only the required amount, repairing leaking steam traps and fixing leaks.  Makeup water must be softened in order to remove scale-forming impurities.  Use a small amount of scale-control chemical agent as an insurance against any small quantity of scale-forming hardness entering the boiler. Minimize corrosion Oxygen and carbon dioxide cause corrosion in steam boiler systems. Oxygen is introduced into the boiler in a dissolved form in the makeup water.   Pitting is a sign of oxygen corrosion.  In many cases, the pitting is covered by a bumpy coating of iron and calcium scale. If you scrape this bumpy crust, you can see the pitting underneath.  Reduce or eliminate oxygen pitting by heating the makeup water to the boiling point before pumping it into the boiler.  This heating drives the oxygen out of the water, and it is vented out. The use of a small amount of chemical oxygen scavenger (usually sodium sulfite) acts as insurance against pitting by chemically removing any oxygen that finds its way into the boilers. Carbon dioxide also enters the system via the makeup water.  The makeup water will contain some amount of bicarbonate alkalinity (products of dissolved limestone).   This impurity breaks down under the heat and pressure conditions in the boiler and releases carbon dioxide.  The CO2 exits the boiler with the steam. When the steam condenses (i.e., in a heat exchanger), the CO2 chemically reacts with the cooling condensate to form carbonic acid.  The carbonic acid corrodes the condensate piping network. Several steps address the problem. Minimizing the quantity of makeup water reduces the source of CO2.  The makeup water may be run through a dealkalizer if the water contains large quantities of bicarbonate alkalinity.  Finally, using one or more amine chemicals helps protect the condensate piping from the carbonic acid. Note that amines should be avoided when part of the steam is used for humidification.   It is harmful to breath amine vapors. Install and maintain insulation Consider the boiler room as a warehouse.  The material stored is heat energy.   The piping network in the plant is essentially a conveyor belt that carries this material to points of use in the facility. If there was indeed a conveyor system running throughout the facility and material kept falling off, you would no doubt do something about it.  In our piping "conveyor," the material falling off is expensive heat energy.  This results in wasted fuel dollars, wet steam and safety hazards from hot piping.   Additionally, once this material falls off, you can't put it back on the conveyor. Therefore, you must keep it from falling off.  This requires insulation for steam and condensate piping.  A typical steam piping insulation job will result in a payback of several hundred percent in conserved energy in the first year. Eliminate air in steam Air, in varying amounts, will always be present in steam systems.  Air is one of the best insulators known to man.  In fact, the fiberglass used in fiberglass insulation has practically no R-value.  It's the air films trapped by the spun fiberglass threads that give insulation its insulating value. Air in steam systems creates blankets, or films, of air inside heat exchange equipment.  This results in significant reductions in heat exchange efficiency.  In fact, as little as one-tenth of 1 percent air in the steam inside a heat exchanger can result in a heat transfer efficiency loss of as much as 50 percent. Air is introduced into steam systems in a few ways. First, air is present in a dissolved form in the makeup water.  This air is released when the water is heated in the boilers, and flows out with the steam.  To minimize this source of air, the makeup water should be preheated to near the boiling point to release the air.  The preheated feedwater is then pumped to the boiler.  Again, using hot condensate rather than cool makeup water minimizes the problem. Air also leaks into steam systems when the steam condenses and collapses, forming a vacuum.  This occurs in modulated-control unit heaters and in any other equipment in which steam may be shut off for a period of time.  As the steam collapses into condensate, and no additional steam is available to take its place, the steam volume is reduced nearly 1,600 times.  This results in the formation of a vacuum in the steam space. Air leaks into this space through packing glands, leaking pipe threads, gauge glass washers, etc. You can minimize energy loss by placing automatic air vents at appropriate locations in the steam distribution system and heat exchange equipment.  These vents are usually thermostatic in operation.  They detect the presence of air by its comparatively cool temperature and open to vent the air.  As steam flows to the vent, its temperature causes the vent to close. Correctly apply steam traps Steam traps are often misapplied.  In order to provide satisfactory service, a trap must be a type suitable for the application, sized properly and installed properly. Incorrect steam trap application will result in a number of potential problems, including backup of the condensate into the heat exchange equipment.  This results in heat exchange efficiency loss, increased potential for corrosion and water hammer. Often, an incorrectly sized or installed trap may not work at all.  There is no "one size fits all" steam trap suitable for all applications.  Those who claim otherwise probably have only one trap to sell. Correct selection of traps is made easier by consulting a reputable trap provider for engineering assistance.  A steam trap survey is often appropriate to identify failed or misapplied traps.  The repair or reinstallation of these traps will result in savings many times the cost of the survey. A trap survey also provides maintenance management with a recordkeeping system to more easily monitor the plant's trap population in the future. Maximize condensate return Condensate is almost distilled water quality.  It also contains a substantial amount of heat, which was added by the boilers.  This heat comes from the fuel, which is expensive.  For these reasons, all possible condensate should be returned to the boiler room for reuse. Increasing the percentage of return has a large effect on the cost of boiler operation.   In the boiler room example given at the start of this article, an increase of 10 percent in condensate return would amount to approximately $20,000 per year in fuel savings. In addition, clean, hot condensate contains practically no scale-forming impurities and only small quantities of oxygen.  It is the ideal feedwater. Using condensate instead of cool makeup water reduces energy consumption, scale accumulation, corrosion and the extensive maintenance that results from these problems. Dean Wilson is a boiler systems specialist for Steam Economies Co. Inc., and the author of "Boiler Operator's Workbook" (American Technical Publishers). This article appeared in the February/March 1999 issue of MRO Today magazine.  Copyright, 1999. Back to top Back to Uptime archives