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ASME BPVC 2023 Section VII

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101.1.3 Maintaining Proper Water Level. Boiler water level control and furnace pressure control, where required,
are subsystems of combustion control. Every effort should be made to place these on automatic operation as soon as
possible during start-up because otherwise, they require constant operator attention.
CAUTION: Operating without sufficient water to cool pressure parts is the most common way to destroy a boiler. The best,
safest, and only advice is, If in doubt, shut it down
101.1.4 Maintaining Proper Furnace Pressure. Boilers are designed so that furnace pressure is negative or positive
with respect to atmospheric pressure. Negative pressure furnaces are commonly called balanced draft furnaces because
the induced draft ID fan is removing flue gas at the same rate the forced draft FD fan is adding combustion air. With
furnace pressure controlled slightly negative at 0.1 in. to 0.5 in. 3 mm to 13 mm of water, flue gas leaks from the
boiler are eliminated and ambient air infiltration, which can lower boiler efficiency, is minimized.
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 of fuel can cause rapid decrease in furnace temperature, which can
also contribute to a sudden reduction in furnace pressure.
Boilers with positive pressure furnaces have FD 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.
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 not be allowed to accumulate on observation door sills. Access doors should be bolted or locked closed to prevent
accidental opening when under pressure.
Leakage of the inner casing may cause rapid overheating and failure of the outer casing; therefore, the operators should
make periodic examinations for hot spots and be instructed to shut down the unit if overheating of the outer casing is
101.1.5 Maintaining Clearance for Expansion. All boilers expand when they are heated from ambient temperature.
Expansion is provided for within the boiler itself by the Manufacturer. It is necessary to maintain expansion clearance
between the boiler, its associated piping and ductwork, and other parts of the installation.
Generally, gas-fired and oil-fired boilers with capacities of 50,000 lb/hr to 400,000 lb/hr 23 000 kg/h to 180 000 kg/h
are bottom supported, so they expand upward. Larger-capacity boilers hang from structural steel and therefore expand
downward. The support method chosen is based on the most economical means of providing for thermal expansion.
101.1.6 Reference Sources. 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 boiler operation. Most
power boiler operational problems have already been discovered and can be avoided by learning from the combined
knowledge and experience of others; refer to the standards and additional sources listed in and Standards.
ASME Boiler and Pressure Vessel Code Section I, Rules for Construction of Power Boilers
ASME CSD-1, Controls and Safety Devices for Automatically Fired Boilers
ASME PTC 4, Fired Steam Generators
ASME PTC 4.2, Coal Pulverizers
ASME PTC 4.3, Air Heaters
ASME PTC 8.2, Centrifugal Pumps
ASME PTC 11, Fans
ASME PTC 25, Pressure Relief Devices
CRTD-Vol. 34, Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern
Industrial Boilers
Publisher: The American Society of Mechanical Engineers ASME, Two Park Avenue, New York, NY 10016 https://
NFPA 85, Boiler and Combustion Systems Hazards Code
Publisher: National Fire Protection Association NFPA, 1 Batterymarch Park, Quincy, MA 02169
ANSI/ASSE Z10 Occupational Health and Safety Management Systems

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