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Traditional Style Blowoff Tank Sized to National Board
Blowoff tanks are the traditional way of handling boiler intermittent blowdown. These style of tanks are normally recommended for utility and other larger steam process boiler applications. The tank is sized to collect each blowdown while allowing radiant heat to escape. The cooled blowdown in the tank then mixes with each new blowdown. Other applications require additional flows into the tank. This can include continuous blowdown, deaerator overflows, and other waste condensate flows.
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The National Board of boiler and pressure vessel manufacture makes recommendations for the sizing of Blowoff Tanks. Most States use or accept the National Board recommendations for sizing. A few states have their own Blowoff Tanks sizing rules. The location of the installation is important when designing a tank.
For sizing to national board the boiler steam drum size must be know. The calculation 4/12 x Drum "DIA.in FT." x Drum "LENGTH in FT." gives the cubic foot holding capacity of the tank. Tanks are generally available in diameters 18", 20", 24", 30", 36", 42", 48", 54", 60" and 72" outside diameter. Tank overall heights can be 3', 4', 5', 6', 7', 8', 9' and 10'. The diameter and tank height is selected to give the holding capacity based on the boiler drum size. Selection of a drain and vent must also be made. These sizes are determined from the flow rate, velocities, and pressure drops. To make this selection the blowdown valve size and boiler operating pressure must be known. When contacting your representative for sizing please include the boiler steam drum size, blowdown valve size, boiler operating pressure, and location of installation.
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- Sizing to National Board - Catalog selector charts provide sizing to meet the requirement of the National Board recommendations.
- ASME Code Rated - Each unit is stamped and registered in accordance with the requirements of ASME Code Sec. VIII, Div. 1.
- Stainless Steel Wear Plate - As provided standard on all Penn tangential inlets. Welded to the shell interior prevents erosion of vessel side walls.
- Minimize Cooling Water - Each blowdown accumulates in the bottom half of the tank so that the next blowdown mixes with it and is cooled to drain.
- Quiet Release of Steam - Vent sized for low velocity release of steam to atmosphere. Vent noise levels below 90 dB(A).
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For these applications multiple inlets into the tank can be used. A automatic drain after- cooler can also be included to cool condensate to drain.
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