Sharing a company’s solution for dealing with low vacuum levels in condensing steam turbines

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This post was last edited by LQ198619 on 2017-7-23 21:04. The operating characteristics of a condenser are primarily determined by its vacuum level; this factor is related to whether the turbine can operate safely and efficiently, as well as to the economic efficiency of its operation. A decrease in the vacuum of the condenser leads to a corresponding increase in the steam consumption of the turbine. The vacuum level of the vaporizer is used as an indicator; its performance directly affects whether the turbine can operate safely and efficiently, as well as the economic efficiency of its operation. The reasons for the creation of vacuum in a condensing steam turbine are as follows: First, it is achieved by the rapid condensation of the steam expelled by the turbine into water inside the condenser, resulting in a sharp reduction in volume. Second, it is accomplished by the use of a water jet ejector to continuously remove the non-condensable gases from within the condenser. 　Based on current operating experience with condensers, a decrease in the vacuum level of the condenser leads to an increase in the steam consumption of the turbine. As the vacuum level drops, not only does the temperature of the exhaust cylinder rise, causing the center of the turbine bearings to shift, but an excessively low vacuum level can also result in severe vibrations in the turbine unit, thereby damaging the equipment. Furthermore, when the vacuum in the condenser decreases, it is necessary to increase the steam flow rate to ensure the proper operation of the unit. However, an increase in steam flow rate leads to an increase in axial thrust, which exceeds the bearing capacity and can cause abnormalities in the turbine. Therefore, it is very important to explore measures to improve the vacuum level of the unit, focusing on the factors that affect the vacuum level of condensing steam turbines. I. Main factors affecting vacuum level 1. Vacuum system: When the vacuum system is not airtight and there are leaks, it will result in a lower vacuum level. 2. Circulating water system – Terminal difference of the condenser. The terminal difference of the condenser refers to the difference between the outlet temperature of the cooling water in the condenser and the temperature of saturated steam at the pressure within the condenser. The terminal difference can, to a certain extent, indicate the heat exchange condition of the condenser. Under the same conditions, an increase in the terminal temperature difference of the condenser indicates that the inner surface of its heat transfer tubes is dirty, or that there is excessive air on the steam side of the condenser, which prevents heat exchange in those tubes. Cooling water inlet temperature: The lower the temperature of the cooling water at the inlet, the lower the exhaust temperature of the condenser, and accordingly, the higher the vacuum level. 3. Scaling of condenser steel tubes: When a thick layer of hard scale forms inside the steel tubes, the overall heat transfer coefficient of the condenser steel tubes decreases. When the vacuum in the condenser decreases, the effective enthalpy drop of the turbine is reduced, which ultimately results in the turbine being unable to operate properly and a decrease in its safety factor. 4. Pump malfunction: Insufficient or excessive water level in the pump, as well as too low operating water pressure, can all directly affect the proper operation of the pump, thereby leading to a decrease in the vacuum level of the condenser. This is because for a pump with a certain pumping pressure, if the amount of working water is insufficient, the pressure in the suction chamber will increase; whereas if too much working water is used, blockages in the diffuser tube can occur, leading to an increase in the pressure in the suction chamber. Additionally, the lower the pressure of the working water, the less pumping capacity the pump has, and its ability to function properly is affected, ultimately resulting in a decrease in vacuum level. 5. Turbine shaft seal steam supply system: If the design of the steam supply system for the shaft seals of the turbine units is inadequate, it can severely affect the proper operation of the low-pressure cylinders. This leads to a severe shortage of steam supplied to these shaft seals, and a large amount of air will leak into the condenser through the low-pressure shaft seals, thereby reducing the vacuum level in the system. 6. Other reasons: Other factors include air leakage in the condensate pumps, an unreasonable design of the heat recovery system, operation of the condenser at an elevated water level, as well as improper dosing of chemicals or sewage disposal in the circulating water – all of these can affect the vacuum level of the condenser. Furthermore, abnormalities in the valves connected to the steam side of the condenser or improper sealing of these valves during operation, as well as the low performance of the low-pressure heater system compared to its design specifications, can all result in an excessive thermal load on the condenser, leading to a decrease in its vacuum level. II. Measures to improve vacuum level 1. Conduct thorough inspections of the vacuum system. Air can enter the condenser in several ways: on one hand, it enters along with the steam, and on the other hand, it leaks in through areas where the vacuum system is not properly sealed. Generally, the leakage amount from the former is very small, less than 10% of the total amount. Therefore, it is mainly caused by improper sealing of the vacuum system; the areas where there is a leak may include the low-pressure cylinders, the pipes for venting air, the steam shaft seals, and the condenser itself. Therefore, the key areas to be inspected in a vacuum system include: the welds connecting the condenser throat to the low-pressure cylinder exhaust pipe, the joint surfaces of the low-pressure cylinder as well as the steam seals at both ends, the flanges and valves that operate under negative pressure, and the flange connections of the pipes that link the medium-pressure and low-pressure cylinders. Detection method: When the candle is brought close to the leak, the flame will be stretched out. 2. Economic regulation of circulating water: When other conditions remain unchanged, increasing the amount of cold water used will inevitably result in a lower temperature rise of the cooling water. Whether the amount of cooling water is sufficient can be indicated by the rise in cooling water temperature. If the temperature difference is too large, the amount of cooling water used should be increased accordingly. However, although increasing the amount of cooling water can improve the system’s vacuum level, it also increases the power consumption of the circulation pump; therefore, tests are needed to determine the optimal amount of water to use in order to maintain a satisfactory vacuum level while ensuring economic efficiency. 3. Remove dirt from all areas. Scale can increase the resistance loss in the cooling tubes of the condenser; removing dirt thus enhances heat transfer. 　Determine whether the cooling surface is fouled: Compare it with the operating data when the cooling surface is clean. Removal method: Since the scale in the initial stage is loose and there is a lot of sludge, the dry cleaning method can be used. Taking advantage of the pattern of the turbine being turned on during the day and off at night, hot water from the deaerator is used to fill the steam side of the condenser; fans are then used to dry out the tubes, which causes the scale to crack, after which it can be washed away with cold water. When hard scale is found on the cooling copper tubes of the condenser, acid treatment can be used. The components of scale include carbonates, sulfates, silicates, and sludge. Organic acids such as sulfamic acid (with a volume concentration of around 5%) can be used for cleaning. After cleaning, backwashing is carried out using water from a high-altitude cooling tower at high flow rates; sodium triphosphate is added, and after circulation and neutralization, the liquid is discharged. Cleaning the condenser can effectively improve the heat transfer coefficient, as well as enhancing safety and efficiency. Furthermore, using circulating water with better quality is also a key aspect in preventing scaling in the condenser tubes. When there is an excessive amount of algae and bacteria in the circulating water, bactericides should be added promptly to prevent the accumulation and growth of microorganisms. 　　4. Properly control the operation of the steam ejector: When the nozzle of a steam ejector becomes blocked, a set of spare steam ejectors should be activated, and the nozzle of the ejector that is no longer in use should be inspected. If the pressure of the working steam in the steam ejector is low, this pressure should be increased. At the same time, a reasonable operating water flow rate and pressure for the exhaust pump should be adopted to ensure its proper operation. 5. Others: It is also necessary to check whether the atmospheric safety valve has a water supply issue, as well as whether there are leaks in the condenser level gauge. Adjust the steam supply volume and pressure to the shaft seal in a timely manner to prevent a drop in vacuum when the load decreases. About a month ago, the vacuum level of the condensing turbine in one of our company’s units began to drop abnormally. Through discussions in the workshop, it was suspected that scaling on the cooling tubes of the condenser was causing a reduced cooling effect, which in turn led to an increase in the temperature of the condensate water and thus a decrease in vacuum level. As a solution, we started using desalinated water supplied to the inlet pipeline of the condenser as cooling water; this approach proved effective, but it increased the consumption of desalinated water. Moreover, as the operating cycle of the turbine continued and scaling became more severe, the amount of desalinated water needed would increase further. Since the pumping system available for this purpose has limitations, this method only provided temporary relief rather than a permanent solution. After discussion, we decided to carry out online scaling removal treatments on the condenser. First, it should be noted that our condenser is dual-channel; the established cleaning plan involves shutting down one side for cleaning while the other side continues to operate. On the morning of July 18th, we followed this plan by reducing the device’s processing capacity to its minimum level, thereby decreasing the amount of steam used by the turbine. Once production stabilized, we began to shut down one side of the condenser’s circulating water system. Throughout this process, we paid close attention to changes in vacuum levels. After the shutdown was complete, a specialized team responsible for cleaning the heat exchanger tubes injected the prepared chemical agents into the tubes via the inlet, and then removed them again through the outlet. Once one side had been cleaned, we proceeded to clean the other side. After cleaning both sides, the results over these past few days have been very satisfactory; there is no longer a need to use desalinated water for cooling, and the vacuum level of the condenser has returned to normal. That is roughly the process involved in handling this. Since I am not an employee of the workshop, my explanation might not be very clear; please forgive me for that. Our condenser is similar to the one shown in the diagram below; since I couldn’t bring my phone, and I also forgot to bring a camera to take pictures, there are no photographic records available. http://down.hcbbs.cc/attachment/forum/201706/10/203353z7y06fdo70r6eod0.jpg Petrochemical zone —— “Creative Ideas for Energy Saving”: The second round of this activity is in full swing. http://bbs.hcbbs.com/forum.php?mod=viewthread&tid=1666758 (Source: Haichuan Chemical Forum) 2017: “My Technical Upgrades” in the field of coal chemical industry. http://bbs.hcbbs.com/thread-1786290-1-1.html (Source: Haichuan Chemical Forum Website) First round of voting for the best e-books on Haichuan Forum —— Chance to win prizes by voting (for members of the Management and Technology Group and those in grade 5 or above). http://bbs.hcbbs.com/thread-1799455-1-1.html (Source: Haichuan Chemical Forum Website)