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Periodic blowdown of boilers is also known as intermittent blowdown or bottom blowdown. It is so named because regular sewage discharge occurs intermittently at the lowest points of the boiler water system, such as at the bottom of the boiler’s mud drum and lower header. The purpose of regular sewage discharge is to remove viscous substances, sludge, sediment, and corrosion products that form inside the boiler. The duration for opening the doors for regular sewage discharge should not be too long; according to the regulations, the time taken for discharging sewage from one outlet at a time must not exceed 30 seconds. Prolonged sewage discharge and excessive amounts of sewage not only remove good water and waste thermal energy, but in severe cases can also disrupt the boiler’s water circulation. Especially, backflow of water in the water-cooled wall risers can be very dangerous. In systems equipped with good water treatment facilities, wastewater is generally discharged once per shift, or it can also be discharged once a day. In boilers that employ in-boiler water treatment, such as standard vertical or horizontal boilers, the frequency of blowdown should be increased appropriately; 1–2 times per shift may be sufficient, depending on the quality of the boiler water. Waste discharge should be carried out when the boiler is operating at low load, as deposits tend to form at the bottom of the boiler. It is prohibited to discharge waste from two boilers simultaneously. It is also not permitted for a boiler to have two blowdown points at the same time. Each discharge point should be handled one by one. It is prohibited to use any leverage to increase the diameter of the handwheel or the length of the handle in order to open the drain valve. When abnormal water levels occur in the furnace or an accident happens to the boiler, the blowdown operation must be stopped immediately. On the regular sewage discharge pipeline, two valves connected in series should be installed; one is used for adjustment, while the other ensures the safe operation of the equipment by preventing leaks. The operating procedure for two series-connected valves should follow these rules: First, fully open the second drain valve (counting from the boiler). Then slightly open the first drain valve to preheat the drain pipe system. After the pipeline system has been preheated, slowly open the first drain valve to discharge waste. When closing the discharge valve after discharge is complete, the operation procedure is carried out in the reverse order of the aforementioned procedure. This method is called the method of protecting the second gate. In actual work. Some practices are the opposite of this. As a result, it easily causes leaks in the drainage valves, allowing large amounts of saturated boiler water to be lost, thereby wasting a great deal of thermal energy. The methods for protecting the second valve have the following advantages: 1. Since the first valve is intact, during periods when no drainage is carried out, the second valve remains in a cold state, thereby being protected from exposure to boiler water, corrosion, and sediment deposition. During the sewage discharge operation, the second valve is also not subjected to the impact of high-pressure boiler water as it opens. It is easy to ensure safety and prevent leaks. When the gate is opened to a small degree, it is subject to the strongest scouring from the boiler water. The first valve is adjustable, while the second valve opens fully when opened and closes fully when closed. It opens first and closes last without any adjustment, to ensure it is not affected by the high-pressure boiler water. On the contrary, if the approach of protecting the first gate is adopted, the first gate is already in an environment exposed to boiler water, leading to corrosion. It was washed by sewage during the process of opening the first shut-off valve. When the second valve is opened after the first valve has been fully opened, the second valve is also subjected to the impact of the wastewater; as a result, both valves are affected by the wastewater, failing to achieve the goal of ensuring that one valve remains airtight. 2. First, open the second valve that is not exposed to pressurized boiler water; this valve is easy to open. Conversely, if the first valve that is subjected to pressure immersion is opened first, it will not be easy to open. When the first valve is opened, the second valve is then under the influence of the pressurized boiler water; as a result, neither valve can be easily opened, which is an undesirable operating practice. During boiler operation, the boiler operator must strictly follow the procedures for operating the second control valve. Otherwise, due to improper operation, both shut-off valves will leak, resulting in the loss of hot water and a waste of thermal energy and coal. It should also be emphasized that when the alkalinity and chloride levels of the boiler water exceed the specified limits, large-scale regular (bottom) blowdown should not be carried out; in such cases, continuous blowdown from the surface of the drum must be utilized. The reduction in chloride and alkalinity levels in the boiler water can only be addressed through continuous blowdown. Periodic blowdown can be used as an auxiliary adjustment method only when there is no continuous blowdown (as in some fire-tube or tube-type boilers), or when continuous blowdown is not sufficient to reduce the concentration of the boiler water. Because, as mentioned earlier, regular sewage discharge is mainly used to remove soft scale and sediment.