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John Ayers
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NFPA 85 Boiler And Combustion Systems Hazards Code.pdf ##BEST##

Complete with a time-saving chapter on common requirements for all boilers, NFPA 85 contains important provisions for boiler design, installation, operation, training, and maintenance. Updated for 2011, this comprehensive document applies to the full range of large boilers and systems, including:

NFPA 85 Boiler and Combustion Systems Hazards Code.pdf

Boiler hazards not only impact operational performance, but they can also cause explosions and jeopardize the lives of facility occupants and maintenance personnel. NFPA 85, Boiler and Combustion Systems Hazards Code, gives everyone involved with large boiler installations and pulverized fuel systems the information they need for fire safety compliance -- from design and installation to inspection.

Boiler hazards not only impact operational performance, they can cause explosions and jeopardize the lives of facility occupants and maintenance personnel. Revised for 2015, NFPA 85, Boiler and Combustion Systems Hazards Code gives everyone involved with large boiler installations and pulverized fuel systems the information they need for fire safety compliance -- from design and installation to inspection.

NFPA 8504: Standard on Atmospheric Fluidized-Bed Boiler Operation contributes to the safe operation of boilers and the prevention of combustion hazards. It covers design; installation; operation; and maintenance of boilers and their fuel-burning, air supply, and combustion products removal systems. (PDF, Approx. 66 pp., 1996; 1993 PDF also available)

Several issues can make boilers dangerous, according to The National Board of Boiler and Pressure Vessel Inspectors. One factor is the lack of education and training on repairing boilers. Another issue is equipment problems. NFPA 85-2019 gives guidelines for boiler operator and maintenance training, as well as guidance for the strength of the structure, combustion and draft control equipment, interlocks, alarms, and other related controls that are essential to the safe operation of boilers and combustion systems.

NFPA 85-2019 addresses minimum guidelines for the installation, operation, and maintenance of boilers. This code covers many types of boilers, such single and multiple burner boilers, atmospheric fluidized bed boilers, heat recovery steam generators, pulverized fuel systems, and stokers. This code also outlines various hazards and considerations that should be paid attention to when working with boilers.

In the early history of boilers, solid fuels were most common, as these were the most readily available. Coal and wood were the most common. Coal has lost its popularity, mostly due to its high sulfur content and the associated pollution created by combustion. Wood has regained some popularity along with other biomass fuels due to its sustainability.

Waste heat from another process such as in internal combustion gas engine or turbine exhaust can be used as the heating source in lieu of burning fuel in a boiler. Another source of energy that could gain popularity is electricity. Hot water generated by an electric boiler produces no greenhouse gases at the site. Where electricity can be generated from 100% renewable sources an electric boiler is a zero-emission heat generator.

Combustion chamber: Combustion requires three components: fuel, oxygen and heat. All three of these must be controlled to efficiently control a boiler. The rate of fuel being supplied is controlled to match the requirements of the process to maintain the boiler temperature. The rate of oxygen, as a component of air, is controlled to provide complete stoichiometric combustion of the fuel. Too little air and not all the fuel is not burned, resulting in pollution, including hazardous carbon monoxide and wasted fuel. When excess air is supplied energy is wasted as the excess air robs heat from the flame.

This combustion process typically occurs in a combustion chamber within the boiler. This section of the boiler is often also referred to as the furnace. The combustion chamber walls are also heat exchanger surfaces for internally fired boilers. Radiation is the primary mode of heat transfer from the flame to the water in this section of the boiler. The heat source can also be from a source outside of the boiler. Heat from a combustion turbine or waste heat from a process can be directed to boiler. These are examples of external heat sources.

A water-tube boiler is one in which the fluid to be heated passes through a series of pipes or tubes and the combustion gases are external to the tubes. Heat from the combustion gases passes through the metallic tube walls. Large water-tube boilers are constructed with ferrous materials.

Fire-tube boilers have large diameter tubes located in a large drum. The fire combustion gases pass though these tubes, heating the water in the large drum. The fire tubes are typically arranged horizontally. The boilers can also be arranged in a vertical configuration to reduce footprint. In condensing application vertical configuration also simplifies drainage of condensed flue gases.

Fire-tube boilers are classified by the number of passes the combustion and combustion products make. The first pass consists of the large diameter cylindrical fire box. The hot gases make a 180-degree turn a second pass through a series of smaller tubes. The gases again turn 180 degrees; typically, the third pass has smaller bore tubes but more tubes than the second pass. Three or four pass boilers are the most common.

Cast iron sectional boilers are made of modular cast iron sections. The combustion chamber is low in the boiler and surrounded with water channels. Hot combustion gases typically turn up and toward the front of the boiler then back toward the back, similar to a three-pass fire-tube boiler, but instead of the gases passing though tubes, the gases pass through extended surface gas channels on the exterior of the castings. The water is contained inside the castings. Each cast section is connected to the adjacent section with tapered connectors that are compressed when the sections are bolted together.

Fuel and air are controlled in concert, but the quantity of fuel delivered is a function of thermal input required to control the boiler temperature. The air is then controlled to optimize the combustion process.

Corrosion of boilers is a major source of boiler failure. Corrosion occurs on both the water side and the fire side. Corrosion on the water side is caused by oxygen in the water. In a tightly closed system, little to no water treatment is required because oxygen is absorbed as piping rusts but stops as soon as all oxygen is consumed. On larger and/or leaky systems where make up water is constantly required, water side corrosion can be controlled by adjusting acidity, alkalinity and oxygen by the addition of chemicals. Biocides and scale inhibitors are often added.

Often overlooked requirements of boiler controls are the requirements for wiring of external devices not provided with the manufacturer provided controls. One is the requirement for interlocking the boilers with the source of combustion air. The intake dampers or combustion air fans are not integral components of the controls often supplied by the burner manufacturer. Another external control element is the remote shutdown switch. This requirement is defined in CSD-1 CE-110 (b). These elements external to the burner/boiler controls require the specifying engineer to include these interface requirements in the design documents.

Hot water boilers have several advantages over larger power boilers in small systems. Condensing boilers have become the most commonly installed boilers in new installations, where the entire system is designed to take advantage of the increased efficiency afforded by lower water temperatures. The lower complexity of hot water boilers and hot water systems have a proven history of safe, reliable and efficient operation when properly installed and maintained and can save costs for the owners over the life of the system.


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