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Regular maintenance of a boiler is crucial to keep it running efficiently and in good condition. One important part of boiler maintenance is blowdown. As steam is formed in the boiler from the feed water, the impurities are left behind. These have to be removed through the blowdown process. Excessive blowdown is costly because it wastes chemicals, water and heat, while inadequate blowdown may lead to deposits or carryover. Blowdown is critical to boiler maintenance because all feed water includes some insoluble impurities. The amount of blowdown is dependent upon the quantity of solids in the feed water and the amount of solids a particular boiler is able to handle. After boiler water solid limits have been set, boiler water tests can be used to control the solids level and the amount of blowdown needed to keep the boiler at peek efficiency. There are several water tests used for this purpose. One test measures the total dissolved solids. This is the measurement of the specific conductance of a neutralized sample of water. Another test uses silica to determine blowdown requirements where its concentration is limited to the potential for selective silica carryover or silica scale. This test is commonly used in boilers operating over 900 psi with turbines. Blowdown control by alkalinity is used where high alkali content is present in the water. Automatic blowdown controllers are also used to regulate blowdown. These are regulated by a probe which measures the specific conductance of the boiler water. When the conductivity reaches a preset limit, the water is discharged. This allows the maximum solids to be carried at all times, minimizing the loss of heat, water, and chemicals through blowdown. All of these combine for greater energy efficiency and maximum production. The two basic methods used for boiler blowdown are continuous or automatic and manual or intermittent control. Automatic is generally preferred because it keeps a more uniform concentration of boiler water solids and can save energy by allowing for heat recovery from the blowdown water. Manual blowdown control consists of opening the main blowoff valve periodically the same way the valve is opened to remove collected sludge. The most effective way of keeping dissolved solids from building up in the boiler incorporates both continuous and manual blowdown. By using both, automatic blowdown will deconcentrate the total solids and manual blowdown will remove the suspended solids which have collected in the lower areas of the boiler. If an automatic blowdown system is in use, the heat in the blowdown effluent can be captured and reused before the water is sewered. Boiler drum pressure, quantity of blowdown and how the recovered heat will be used are the determining factors in the type of heat recovery system required. A typical heat recovery system incorporates at least one flash tank and heat exchanger. The pressure of the flash tank depends on the end use of the steam. After the water goes through the flash tank, it passes through a heat exchanger before going into the sewer. Boiler blowdown helps maintain water solids at acceptable levels, minimizing carryover or deposits, and when maintained properly helps the boiler operate economically and trouble free. |
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