ASME SECTION VII-2008 pdf download

ASME SECTION VII-2008 pdf download

Electrode-type boilers generate saturated steam by con- ducting current through the water itself. Boiler water con- ductivity must be monitored and controlled. If the conductivity is too low, the boiler will not reach full operating capacity. When the conductivity is too high, overcurrent protection will shut off the power. High boiler water solids may cause foaming and shorts to ground. It may be necessary to increase blowdown or add antifoam agents to control foaming. Very pure water will limit steam output and will require addition of addi- tives to increase the boilerwaterconductivity. The operator is advised to consult the boiler manufacturer and a water treatment consultant for specific advice. Solids from the saturated steam tend to accumulate slowly on the insulators supporting the electrodes from the grounded shell. The unit must be shut down periodically so the insulators can be washed off to prevent arcing. High voltages up to 16,000 V may be used. Protection is needed for ground faults, overcurrent, and, for three phase systems, loss of phase. The main electrical discon- nect switch must be locked out before performing mainte- nance on the boiler. C1.300 PACKAGE BOILERS Small sized power boilers can be completely assembled in the boiler manufacturer’s shop, including burners, fans, controls, sootblowers, etc. These boilers are shipped to the operating site by barge, rail, or over the road vehicles. They are called package boilers and can be designed to operate at high pressures and capacities. Operating pres- sures of1,000 psig (7 MPa) and capacities of300,000 lb/hr (140 000 kg/hr) and steam temperatures of 950°F (510°C) can be achieved. C1.400 FIELD-ASSEMBLED BOILERS Boilers that are too large to ship completely assembled must be field erected. Some are capable of producing up to several million pounds per hour of steam superheated to 1,100°F (595°C). Natural circulation boilers may be designed for pressures up to about 2,900 psig (20 MPa).
Furnace implosions can occur in very large boilers with balanced draft furnaces if the unit is subjected to the full ID fan suction capability at low flow conditions. Sudden loss offuelcancauserapiddecreaseinfurnacetemperature, which can also contribute to a sudden reduction in furnace pressure. Boilers with positive pressure furnaces have forced draft fans only. Furnace pressure varies from 5 in. to 25 in. of water (1.3 kPa to 6.2 kPa) as the boiler operates from minimum to maximum. Design pressure of furnaces rarely exceeds 28 in. of water [about 1 psig (7 kPa)] because of the cost of reinforcing the furnace wall support system. With pressurized furnaces, the use of portable hand torches for ignition is impractical; therefore, electrically actuated ignitors should be provided. Compressed air should be available to actuate the shielding or aspirating air jets that permit safe opening of the observation doors and the openings through which oil burner guns are inserted. Mechanical interlocks are recommended to prevent opening observation doors or burner openings unless compressed air is available at the aspirating jets. Fly ash should notbe allowedto accumulate on observation door sills. Access doors should be bolted or locked closed to prevent accidental opening when under pressure. Inner casing leakage may cause rapid overheating and failure of the outer casing; therefore, the operators should make periodic inspections for hot spots and be instructed to shut down the unit if overheating of the outer casing is detected. Coal fired pressurized furnaces introduce the problem ofsealing offthe pressurizedfeeders fromthecoal bunkers. This is usually done by a column of coal between each bunker outlet and the feeders. No-coal-flow alarms should be provided to guard against loss of the seal, which could result in a fire in the bunker.
Several organizations publish valuable operational guidelines. Some of these guides are general in nature and others go into great detail on specific topics or equipment related to power boileroperation. Mostpower boileropera- tional problems have already been discovered and can be avoided by learning from the combined knowledge and experience of others, such as the following: (a) National Fire Protection Association (1) 8501 Single Burner Boiler Operation (2) 8502 Prevention of Furnace Explosions/Implo- sions in Multiple Burner Boilers (3) 8503 Pulverized Fuel Systems (4) 8504 Automatic Fluidized-Bed Boiler Operation (5) 8505 Stoker Operation (6) 8506 Heat Recovery Steam Generator Systems (b) American Society of Mechanical Engineers (1) ASME Boiler and Pressure Vessel Code — Sec- tion I, Rules for Construction of Power Boilers (2) ASME Consensus on Operating Practices for the Control of Feedwater Chemistry in Modern Industrial Boilers (3) ASME PTC 4.1, Steam-Generating Units — Efficiency testing of boilers by the heat loss and input– output methods is fully described (c) boiler manufacturers (1) equipment operation and maintenance manuals (2) feedwater and boiler water quality requirements (d) fuel equipment manufacturers (1) equipment operation and maintenance manuals (2) fuel handling, preparation, and burning (3) combustion safeguard systems (e) boiler insurance companies (1) Boiler Code compliance questions (2) inspection service (3) operation and inspection guide publications (4) accident or failure statistics (f) state and local agencies: rules and regulations for design or operation

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