The cleanliness of steam is specifically reflected by the moisture content carried by steam machinery, the direct dissolving capacity of steam for salts, and the salt concentration in boiler water. Therefore, in addition to strict feedwater treatment, the following specific measures should be taken to enhance steam quality based on the specific causes of steam contamination.
1. Steam-Water Separation
Enhancing the separation efficiency of steam-water separation equipment inside the steam drum reduces the mechanical carryover of moisture by saturated steam and improves steam quality. Steam-water separation equipment operates based on the following principles or functions:
- Gravity Separation: Utilizes the difference in density between steam and water for natural separation.
- Inertial Separation: Changes the flow direction of the steam-water mixture, leveraging inertial force to alter the velocity direction of water droplets and steam, thereby separating them.
- Centrifugal Separation: Uses the centrifugal force generated by the rotational motion of the steam-water mixture to separate water droplets from steam.
- Water Film Separation: Steam carrying water droplets flows along metal surfaces or grids, forming a water film that flows downward, thereby separating water droplets from steam.
Currently, the steam-water separation equipment widely used in China includes cyclone separators, corrugated plate separators, and perforated plates.
2. Steam Washing
Steam-water separation equipment can only reduce the moisture content carried by steam, i.e., it only reduces the salt content from mechanical carryover, but it cannot address the issue of steam dissolving salts. Steam washing not only reduces the salt content from mechanical carryover but also decreases dissolved salts, thereby improving steam quality. Steam washing involves passing mechanically separated steam through a layer of clean water to dissolve some of the salts into the clean water, further enhancing steam quality. Since the salt content of clean water is always lower than that of boiler water, salts dissolved in steam quickly diffuse into the clean water layer upon contact. Simultaneously, salts in water droplets carried by steam also diffuse into the clean water layer. Thus, steam washing is effective in reducing both dissolved and mechanical carryover.
The widely used steam washing device is the through-layer type steam washer, which includes bell-type and perforated plate types. The perforated plate type is simple in structure, has low resistance loss, offers a large washing area, and provides effective cleaning, making it widely adopted.
3. Boiler Blowdown
Boiler blowdown refers to the process of discharging a portion of boiler water with high salt concentration and high slag concentration during operation. Blowdown is necessary because feedwater entering the steam drum contains salts, and during chemical treatment of boiler water, scaling substances transform into slag. Additionally, corrosion of boiler metal by boiler water produces some corrosive products, resulting in various soluble and insoluble impurities in the boiler water. As the boiler water continuously evaporates and concentrates, the content of these impurities gradually increases, leading to higher salt concentration and slag concentration, which can affect steam quality and boiler safety.
Boiler blowdown is divided into intermittent blowdown and continuous blowdown:
- Intermittent Blowdown: Aims to periodically remove sediments (slag) from boiler water. It should be conducted at locations where sediments accumulate the most, such as the lower header of the evaporator. The amount and frequency of intermittent blowdown depend on feedwater quality. For condensing thermal power plants, where feedwater is relatively clean condensate, the amount of intermittent blowdown can be significantly reduced, and the intervals between blowdowns can be extended.
- Continuous Blowdown: Continuously removes dissolved salts from boiler water to maintain salt content and other water quality indicators within specified limits. It should be conducted from areas with the highest salt concentration, typically near the evaporation surface of the steam drum. Continuous blowdown also removes some fine slag particles.
The blowdown rate depends on feedwater quality and boiler water quality. Reducing feedwater salt content and increasing blowdown water (boiler water) salt content can lower the blowdown rate. To ensure steam quality, the blowdown rate cannot be too low. However, increasing the blowdown rate will lead to heat and working fluid losses. Therefore, the blowdown rate must be limited and comply with relevant regulations. For condensing power plants using chemically demineralized water as makeup water, the blowdown rate should not exceed 1%; for those using softened water, it should not exceed 2%.
4. Internal Water Treatment
Although makeup water undergoes external water treatment to remove most impurities, a small amount of scaling substances may remain. Additionally, potential leaks in the system may introduce raw water, increasing feedwater hardness. When boiler water evaporates, scaling can easily form. Therefore, preventive internal water treatment, also known as boiler water correction treatment, is necessary. This involves adding specific chemicals to the boiler to convert scaling substances into non-adhesive, mobile slag (insoluble compounds) that can be discharged through blowdown.
In thermal power plants, the highly effective trisodium phosphate softening method is widely used as a supplementary softening treatment for boiler water. The phosphating method involves continuously pumping a trisodium phosphate solution into the steam drum. Calcium and magnesium ions in the water combine with phosphate ions to form insoluble calcium and magnesium phosphates, which precipitate out of the water. Thus, during phosphate correction treatment, boiler water should maintain appropriate alkalinity. To keep the boiler water in a scale-free state, an adequate residual amount of trisodium phosphate must always be maintained. During operation, phosphate levels in the boiler water must be regularly monitored and controlled to prevent scale formation.
