Analysis of heater leakage causes and countermeasures

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This is a good article; it presents a clear analysis of the issues and offers insightful perspectives. It’s worth reading! Analysis of the causes of heater leaks and corresponding countermeasures – Heaters are one of the main auxiliary devices in power plants. Once a heater fails, it not only affects the economic efficiency of the power plant but also often poses a direct threat to the safe operation of the main engine or other equipment, potentially leading to serious equipment damage. Failures of heaters, especially those in the high-pressure heating system, occur frequently; they are second only to boiler tube ruptures and rank as the second most common type of failure in power plants. Statistics show that among the various faults of water heaters, pipe system leaks account for the largest proportion. In a surface-type regenerative heater, the pressure on the water side is higher than that on the steam side; once a leak occurs in the piping system, feedwater will flow into the shell, resulting in water filling the steam side. Water may flow back into the turbine along the extraction pipes, causing issues such as deformation of the turbine cylinder, changes in expansion differences, unit vibration, friction between moving and stationary parts, bending of the main shaft, and even blade breakage. There have been many such accidents involving water ingress into the turbine due to heater leaks, both domestically and internationally. Therefore, it is of great importance to analyze the causes of heater leakage and identify countermeasures to minimize leaks as much as possible. 　　1 Analysis of the causes of heater leakage Leakage in the internal piping of U-tube heaters can be primarily categorized into leakage within the tubes themselves and leakage at the connections (leaks at the joints where the tubes are joined to the tube sheet by expansion joints or welding): 1.1 Causes of leakage at the tube ends include: 1.1.1 Excessive thermal stress During startup and shutdown, if the rate of temperature increase or decrease exceeds specified limits, the tubes and tube sheet of the high-pressure heater are subjected to significant thermal stress. This can cause damage to the welds or expansion joint connections between the tubes and the tube sheet, leading to leakage at those points. When there are rapid changes in load during peak demand periods, or when the main engine or heater fails and stops operating suddenly, if steam supply is stopped too quickly on the steam side, or if water continues to flow into the heater after steam supply is halted, the thin walls of the tubes contract rapidly while the thicker tube sheet contracts more slowly. This often results in damage to the welds or expansion joint connections between the tubes and the tube sheet. This is why the permitted temperature drop rate specified is only 1.7°C–2.0°C/min, which is stricter than the permitted temperature rise rate of 2°C–5°C/min. 　　1.1.2 Tube sheet deformation The tubes are connected to the tube sheet, and deformation of the tube sheet can cause leakage at the ends of the tubes. The water side of the high-pressure tube sheet has high pressure and low temperature, while the steam side has low pressure and high temperature; especially in cases with an internal steam trap cooling section, the temperature difference is even greater. If the thickness of the tube sheet is insufficient, the tube sheet will deform to some extent. The center of the tube sheet bulges toward the steam side, where the pressure is lower and the temperature is higher. On the water side, the tube sheet develops a central depression. When the load on the main unit changes, the pressure and temperature on the steam side of the high-pressure heater change accordingly. Especially when the peak shaving amplitude is large, the rate of peak shaving is too fast, or there are sudden changes in load, the pressure on the water side can experience significant variations when fixed-speed feedwater pumps are used; these variations can even cause the pressure to exceed the rated value for the feedwater to the high-pressure heaters. Such changes can lead to deformation of the tube sheet, resulting in leaks at the pipe ends or permanent deformation of the tube sheet. If there is internal leakage in the steam inlet valve of the HP heater, shutting down the HP heater while the main turbine is in operation will cause the water side of the heater to be heated, resulting in a constant-volume pressure increase. If there is no safety valve on the water side or if the safety valve fails, the pressure may rise to very high levels, which can also cause deformation of the tube sheet. 　　1.1.3 Improper pipe plugging technique Generally, conical plugs are welded to plug the pipes. Apply moderate force when driving in the tapered plug ； The impact force is too great, causing deformation of the tube holes; this affects the connections between adjacent tubes and the tube sheet, leading to damage and new leaks. During welding, factors such as improper preheating, as well as inappropriate weld position and size, can cause damage at the joints between adjacent tubes and the tube sheet. Other pipe plugging methods, such as expansion plugging and explosive plugging, can also cause leakage at adjacent pipe openings if the process is not proper. Therefore, a strict pipe plugging process should be followed. 　　1.1.4 Poor manufacturing quality
The tube sheet of the high-pressure heater is made of alloy steel, while its tubes are made of low-carbon steel. Before welding, a layer of low-carbon steel must be surfacing-welded onto the tube sheet ； Welding defects often remain due to inadequate surfacing welding techniques. 　　1.2 Reasons for leakage in the pipes themselves 　1.2.1 Erosion by scouring 　One reason is that when the flow velocity of steam is high and the steam stream contains large-diameter water droplets, the outer wall of the pipe is subjected to erosion by the two-phase flow of steam and water, which causes it to thin out, leading to perforations or rupture due to the pressure of the feed water. The main reason for the formation of a two-phase flow of steam and water inside the heater is that the steam in the superheating steam cooling section and at its outlet does not reach the required degree of superheating ； Secondly, factors such as the hydrophobic level in the heater remaining too low or being absent, the hydrophobic temperature being much higher than the design value, high flow resistance of the hydrophobic fluid, or a sudden drop in the extraction pressure can cause the hydrophobic fluid to flash; as a result, the hydrophobic fluid enters the next stage of heater already containing steam, which erodes the heater tubes and causes damage ； Third, when a pipe in the high-pressure heater is damaged and leaks, high-pressure feed water rushes out at great speed from the leak site, thereby eroding and damaging adjacent pipes or partitions. 　　Another reason is direct exposure to steam or hydrophobic substances. Due to inappropriate material and fixing method of the impact shield. Breaks or falls off during operation, thus losing its anti-erosion protective function ； The area of the impact shield is not large enough; water droplets, carried by fast-moving air currents, strike the tube bundle outside the impact shield ； The distance between the shell and the tube bundle is too small, resulting in a very high steam velocity at the inlet. 　　1.2.2 Tube vibration When the feed water temperature is too low or the unit is overloaded, and the steam flow rate and velocity between the heater tubes exceed the design values by a significant amount, the tubular bundle, which has a certain degree of elasticity, will vibrate under the influence of the perturbative forces from the fluid on the shell side. When the frequency of these forcing forces matches the natural vibration frequency of the tubular bundle or a multiple thereof, resonance occurs, resulting in an increase in the amplitude of vibration. This repeated stress on the connections between the tubes and the tube sheet can lead to damage to the tubular bundle. The mechanisms behind tubular bundle vibration-induced damage include: ① Vibration causes the stress on the tubes or at their connections with the tube sheet to exceed the material’s fatigue limit, leading to fatigue fracture of the tubes ； ②The vibrating tube rubs against the metal of the support partition within the tube holes of that partition, causing the tube wall to thin out and eventually leading to rupture ； ③When the vibration amplitude is high, the pipes located adjacent to each other in the middle of the span rub against one another, causing the pipes to wear out or suffer fatigue fractures. 1.2.3 Erosion at the water inlet end of the pipes Erosional damage at the inlet pipe end occurs only in carbon steel heaters; it is a damage process resulting from both erosion and corrosion. The mechanism involves the oxide layer formed on the pipe wall surface being destroyed and carried away by the highly turbulent water flow, leading to a continuous loss of metal material. This ultimately leads to the damage of the pipe. Sometimes, the area of damage can extend to the pipe end welds or even the tube sheet: erosion is likely to occur when the feedwater pH is low (below 9.6), the oxygen content is high (above 7 μg/L), the temperature is low (below 260°C), and turbulence is high. 　　1.2.4 Corrosion When the tubes of low-pressure heaters are made of copper, these copper tubes often have to be replaced due to severe leakage. The corrosion rate of copper is lowest at a pH value of 8.5 to 8.8. Carbon steel, on the other hand, requires a pH value of not less than 9.5. The excessively high pH value of the boiler feedwater led to the corrosion of copper tubes. The main factors affecting the corrosion of carbon steel tube bundles include oxygen content and feedwater pH: high dissolved oxygen levels or low pH values in the feedwater can cause corrosion of the inner walls of the superheater tubes; therefore, the dissolved oxygen concentration in the feedwater should not exceed 7 pg/L, with the pH value maintained between 9.3 and 9.6. If oxygen is present on the shell side (for example, when the air release valve of the superheater is not in use or is not functioning properly, resulting in incomplete air discharge, or when the steam-side drain is not emptied properly during shutdown), it will cause oxygen corrosion on the outer wall of the tube bundle. Copper deposition: It can cause pitting corrosion, resulting in pitting pits. Temperature affects the formation of the Fe3O4 oxide film on the surface of carbon steel: It is generally believed that above 260°C, the Fe3O4 oxide film is relatively stable. Below this temperature, the degree of protection provided by the Fe3O4 oxide film depends on the pH value of the feed water and other environmental factors. It is safe when the pH value is greater than 9.6. 　　1.2.5 Overpressure-induced tube rupture The factors that cause excessive pressure on the water side with high water addition include the following: In systems equipped with constant-speed drive feedwater pumps, if the design pressure on the water side of the heater is determined solely based on the feedwater pressure under normal operating conditions, then during startup or at low load levels, due to the smaller opening of the boiler feedwater control valve, the feedwater flow rate decreases and the pressure at the outlet of the feedwater pump increases. This can result in the tube bundle being subjected to a feedwater pressure exceeding the design value, leading to tube rupture. This often occurs after a sudden drop in load during operation or an emergency shutdown, following the shutdown of the furnace. Secondly, during unit operation, when the high-pressure heater has to be shut down for some reason, if the feedwater inlet and outlet valves are tightly closed but the steam inlet valve is leaking, the feedwater trapped on the side of the heater tubes will be heated by the leaking steam, causing the feedwater pressure in the tube bundle to rise significantly. 　　When the pressure on the water side is too high and no safety valve is installed on that side, the excessive pressure can cause the pipes to swell, become thicker, and crack. 　　1.2.6 Poor material quality and manufacturing processes: Poor quality of the pipe material, uneven wall thickness, defects in the pipes prior to assembly, excessive expansion at the flanged areas, and pull marks on the outside of the pipes – all of these can lead to severe damage to the pipes when the heater is subjected to abnormal operating conditions. The excessively thin wall of the heat exchange U-tube is the fundamental structural cause of leakage. 2 Countermeasures 　　2.1 Measures to take after a leakage occurs 　　When a leakage takes place, the feedwater pressure drops, resulting in a reduction in the amount of feedwater supplied to the boiler. Therefore, when a leak is detected in the heater piping system, the heater should be shut down immediately to reduce the number of damaged pipes and mitigate the extent of the damage. 　　When the unit is shut down, it is necessary to check for leaks in the high-pressure heater and find ways to eliminate them. In the case of port leakage, the existing weld metal should be scraped off and then rewelded, along with appropriate heat treatment to eliminate thermal stress. For leaks in the tubes themselves, it is necessary first to determine the type and location of the leakage in the tube bundle, and then select an appropriate method to seal both ends of the tubes. Regardless of the pipe plugging process used, to ensure the quality of plugging, the ends of the pipes to be plugged must be properly treated, so that the tube sheet and tube holes are smooth and clean, providing a good contact surface with the plug. In the event of cracks or erosion at the junction of the tube and the tube sheet, it is necessary to remove the original tube material and weld metal from the ends so that the plug can make tight contact with the tube sheet. 　　2.2 Preventive Measures 　　2.2.1 Measures to Prevent Port Leaks 　　The heater should be equipped with tube sheets of sufficient thickness; in addition to proper tube hole processing, cladding, tube expansion joining, and welding techniques, it is necessary to ensure that the temperature rise and drop rates during the start-up and shutdown of the heater do not exceed specified limits. A safety valve should be installed on the water side to prevent overpressure, and correct methods for blocking tubes must be employed during maintenance. 　　2.2.2 Measures to prevent leakage from the pipes themselves 　　① Measures to prevent erosion by scouring 　　Limiting the flow rate of steam or drain water on the shell side, and preventing flashing in the condensation section ； The steam at the outlet of the steam cooling section must have sufficient residual superheat ； The impact shield must be securely fixed, have sufficient area, and be made of high-quality material ； Maintain the water level on the shell side at normal levels; operation at low water levels or without water is prohibited. 　　②Vibration prevention measures for pipes: Install a steam-side safety valve on the high-pressure heater steam side ； Limit the flow rate of steam or hydrophobic substances on the shell side ； The spacing between the tubes must be sufficient; this reduces the flow velocity on the shell side and, at the same time, decreases the likelihood of damage due to collisions and friction between the tubes: it limits the length of the free section of the tube bundle (i.e., its span). 　　③Preventive measures against erosion at the water inlet end of pipes: Limiting the flow rate of the feed water, or shutting down a group of heaters, or when there are many blocked pipes, all of these factors can lead to a significant increase in the flow velocity inside the pipes. In such cases, it is necessary to allow some of the feed water to enter the boiler via a bypass route, or to reduce the load on the unit ； Keep the oxygen content in the feed water below 7 μg/L, and maintain the pH value of the feed water between 9.2 and 9.6. 　　④Corrosion prevention measures: When selecting materials, it is advisable to make the unit a copper-free system, which is beneficial for both the corrosion protection of the entire unit and the control of steam quality ； A proper venting system is necessary; it is generally not recommended to use a series connection of heaters in sequence for pipe connections, in order to prevent non-condensable gases from accumulating in the heaters with lower pressure ； Ensure the proper operation of the air release system; at startup, air must be removed from both the water side and the steam side, and the quality of the feedwater must be satisfactory ； Good anti-corrosion measures must be in place at the time of manufacture to prevent corrosion during storage and transportation. For carbon steel tube heaters, nitrogen filling is typically used on both the steam side and the water side as a means of anti-corrosion ； When the heater is not in use, anti-corrosion measures such as filling it with water, steam, or nitrogen are typically applied depending on the duration of inactivity; additionally, the pH value of the deoxygenated water on the water side is adjusted appropriately to provide protection. 　　⑤Precautions against overpressure pipe rupture: Safety valves should be installed on the water side. 　　⑥Preventive measures against pipe leakage caused by poor material quality and manufacturing processes. The pipe wall thickness should be at least 2.0 mm to enhance its resistance to erosion. Before assembly, each tube must undergo inspections such as flaw detection and hydrostatic testing ； The U-tube should be heat-treated and free of visible defects ； The tube holes in the tube sheet should maintain a certain level of roughness, tolerance, and concentricity, and the chamfers or rounded edges of these holes should be smooth without any burrs. 　　⑦Preventive tube plugging 　　Perform preventive tube plugging. It is recommended to create bypass holes of appropriate size on the tube sheet while blocking a portion of the tubes. This helps to reduce the flow rate of the feed water and thereby lessen corrosion. This method has been used in various power plants around the world, and it has proven effective in extending the lifespan of heaters and reducing the frequency of leaks. Last edited by At forty, one is free from doubts on 2006-12-8 15:19