Why is it necessary to blow down a boiler?

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What are the functions of continuous blowdown and periodic blowdown in boilers?       Continuous blowdown, also known as surface blowdown, is a method in which the boiler water with the highest concentration is continuously discharged from the surface layer of the water in the steam drum. Its function is to reduce the salt content and alkalinity in the boiler water, preventing excessively high concentrations from affecting the quality of the steam.       Periodic blowdown, also known as intermittent blowdown or bottom blowdown, serves to remove sludge accumulated at the bottom of the boiler and the soft precipitates formed after phosphate treatment. Regular sewage discharge lasts for a short time, but it has a strong ability to remove the sediments from the boiler. Waste discharge is an important aspect of boiler water quality management. This article explains and discusses various aspects such as the concept of boiler waste discharge, the calculation of waste discharge rates, the proper use of waste discharge devices, and the recovery and utilization of heat generated by waste discharge, with the aim of emphasizing the importance of waste discharge control, ensuring safe operation, reducing consumption, and saving energy.   I. Concept of boiler blowdown
1. To ensure that the quality of boiler water meets specified standards and that impurities in the water remain within certain limits, it is necessary to continuously remove from the boiler the water containing high levels of salts and alkalis, as well as deposited scale, sludge, and loose sediments. This process is known as boiler blowdown.     2. Blowdown methods: Boiler blowdown can be categorized into continuous blowdown and periodic blowdown. Continuous blowdown, also known as surface blowdown, involves the continuous discharge of a portion of boiler water from the area where the concentration of salts and alkalis is highest. This helps to reduce the amounts of salts, alkalis, silicic acid, and suspended impurities in the boiler water. Therefore, the continuous blowdown pipe is installed 80–100 mm below the normal water level. Periodic blowdown, on the other hand, is used primarily to remove deposits such as sludge and sediment from inside the boiler; hence, its blowdown outlets are typically located at the lower part of the boiler drum and at the bottom of the headers. The regular blowdown process is short in duration, and it should be carried out when the boiler is at a high water level, under low load, or when it is shut down. On small boilers, regular blowdown is usually the only type installed. II. Calculation of boiler blowdown The amount of boiler blowdown is directly related to the quality of the feedwater.       The higher the alkalinity and salt content of the feedwater, the greater the amount of blowdown required by the boiler.       1. Calculation of the blowdown rate: The indicator for boiler blowdown is expressed as the blowdown rate, which is the percentage of the volume of blowdown water (Q_blowdown) relative to the boiler’s steam generation rate (Q_steam). It is expressed as follows: K=Q_contaminant/Q_vapor×100%. When the boiler water quality remains stable, according to the principle of mass balance, the amount of a particular substance that enters the boiler with the feedwater is equal to the sum of the amount removed by the wastewater and the amount carried away by the saturated steam. Then, (Q_contaminated + Q_vapor) × S_inlet = Q_vapor × S_vapor + Q_contaminated × S_contaminated. In this equation, S_inlet, S_vapor, and S_contaminated represent the concentration of a certain substance in the inlet water, saturated steam, and wastewater respectively. The value of S can be calculated based on the salt content or on the concentration of a specific component such as alkalinity or chloride ions. Then, K = Q_wastewater / Q_steam = (S_feed – S_steam) / (S_wastewater – S_feed) × 100%. 2. When calculating the blowdown rate, the following three points should be noted: (1) The blowdown rate can be calculated separately based on alkalinity or chloride ions (there is a relatively fixed proportional relationship between chloride ions and total salt content; chloride ions are usually used as a substitute for total salt content). The larger of these two values is taken as the blowdown rate. Generally, for heating boilers, the blowdown rate should be kept below 10%.        (2) For boilers with a larger capacity, their steam-water separation devices work effectively, resulting in very low humidity in the steam. In this way, the salt content in saturated steam is much lower than that in the feedwater; therefore, when calculating the blowdown rate for such boilers, the salt content in the steam can be ignored. That is, K = S_feed / (S Blowdown – S_feed) × 100% (3) For most industrial boilers, especially those with small drum volumes, simple steam-water separation systems, and a high amount of water carried by the saturated steam, the steam humidity is usually around 3%. (Compared to a blowdown rate controlled within the range of 5%–10%, this is already considered a relatively high value.) Under such conditions, the salt content in the steam cannot be ignored when calculating the boiler’s blowdown rate. Because K = (S_in – S_vap)/((S_waste – S_in)) = CL_in – S_in/(CL_waste – CL_in – CL_vap). Here, CL_vap/CL_waste represents the steam humidity; CL_waste equals CL-boiler water, that is, the chloride ion concentration in the wastewater, which is equal to the chloride ion concentration in the boiler water. In this formula, CL_in, CL_waste, CL_vap, and CL-boiler water represent respectively the chloride ion concentrations in the feedwater, wastewater, saturated steam, and boiler water.       Clearly, if the salt content in the steam is ignored, the calculated blowdown rate will be overestimated (the difference is greater than the steam humidity). For industrial boilers, for every 1% increase in the blowdown rate, fuel consumption increases by 0.3%. This wastes fuel and prevents an accurate assessment of the boiler’s energy consumption and overall management level.  III. Boiler blowdown devices The continuous blowdown device refers to the short blowdown pipes within the boiler body, blowdown valves, and internal blowdown conduits in the drum, etc.      The sewage conduit must be of sufficient length and installed horizontally; one end of the conduit must be sealed. Each boiler shall be equipped with an independent blowdown pipe. The sewage pipe should have as few bends as possible to ensure unobstructed drainage and to lead to a safe location. The connections between the blowdown pipe and the boiler drum, headers, and blowdown valves must be secure and free from corrosion.  Gate valves, segment valves, or angle stop valves are recommended for use as drain valves. For boilers with a nominal diameter of the blowdown valve of φ20~65mm, and a rated evaporation capacity of ≥1t/h or an operating pressure of ≥0.7Mpa, two blowdown valves connected in series should be installed on the blowdown pipe. During blowdown, the blowdown valve is subjected to the erosion from high-temperature liquids and wear caused by dirt; after blowdown ceases, it gradually cools down to room temperature. To mitigate the harsh operating conditions faced by blowdown valves—such as frequent exposure to pressure differences (large pressure drops), scaling, corrosion, wear, vibration, and thermal shock—there is a specific operating sequence for the series-connected blowdown valves. The connection order is: boiler drum (or lower header) → Valve 1 (slow-acting valve) → Valve 2 (fast-acting valve). During blowdown, Valve 1 must be opened first, followed by Valve 2; since Valve 2 bears the pressure difference and is prone to damage, this sequence helps protect it ； When stopping wastewater discharge, first close Valve 2, then close Valve 1 (Valve 2 bears the pressure difference and is prone to damage). This ensures that Valve 1 operates under zero pressure difference during opening and closing, resulting in better operating conditions and a longer service life. During major repairs, it is sufficient to inspect or replace Valve 2. Valve 1 is a slow-opening valve; it is commonly a tilting-ball type drain valve or a slow-opening gate-type drain valve, that is, an ordinary gate valve. However, it must have the ability to resist alkaline corrosion from furnace water ； Valve 2 is a quick-opening valve; swing-gate or rack-and-pinion gates are commonly used to meet the requirements regarding its operating action and timing for sewage discharge.  IV. Recovery and utilization of heat from blowdown The blowdown rate of boilers is generally 3–10% of the boiler’s capacity. To recover the considerable amount of heat carried away by this wastewater, expansion tanks for periodic and continuous blowdown are typically installed in the boiler room. Separate the steam and water generated from the boiler water due to pressure reduction, and utilize them separately. Steam is generally introduced into atmospheric thermal deaerators for the deaeration of feedwater, while wastewater is cooled through heat exchangers to utilize its heat before being safely discharged into sewers.  V. Summary By properly and reasonably draining impurities, sludge, and scale from the boiler water, as well as controlling the alkalinity and salt content of the boiler water, the quality of the boiler water can be brought into compliance with **standards. This ensures the cleanliness of the heating surfaces, meets the requirements for qualified steam quality, extends the service life of the boiler, and allows for full utilization of the waste heat generated during blowdown, thereby achieving energy-saving effects.        Therefore, design, manufacturing, installation, and operating units must pay close attention to the issue of boiler blowdown. They should fully understand its significance, determine the appropriate amount of blowdown, operate the blowdown devices correctly, and recycle the waste heat generated during blowdown. This helps to ensure that the boiler operates safely, reliably, and over a long period of time under economic conditions, reduces unnecessary losses, and achieves energy savings.