What are the reasons for the leakage of high and low pressure heaters?

Teflon heaters are a major auxiliary equipment in power plants. Once the heater malfunctions, it will affect the economy and safety of the power plant.
1. Excessive thermal stress generated during startup
When the high-pressure heater is put into operation, it is at room temperature while the feedwater temperature is high. The main components of the high-pressure heater, such as the shell, tube bundle, and tube plate, suddenly experience heat, uneven expansion, and excessive thermal stress, resulting in leakage of the water chamber tube plate of the heater and leakage of the weld between the steel pipe and the tube plate. Due to the temperature change rate of the high-pressure heater exceeding the allowable value during unit startup and shutdown, the tube bundle and tube plate expand unevenly, resulting in a certain amount of thermal stress. Under the repeated action of this stress, the tube bundle is damaged and destroyed.
The allowable temperature drop rate is generally only 1.7 ℃ -2.0 ℃/min, which is stricter than the allowable temperature rise rate of 2 ℃ -5 ℃/min.
2. High pressure heater vibration during startup
When the high-pressure heater is started, it is under an atmospheric pressure of 0.1MPa, while the water supply pressure of the feedwater pump is around 15MPa. The opening time of the feedwater electric valve is relatively short. When a large amount of high-temperature and high-pressure feedwater flows into the water side of the high-pressure heater, the air cannot be discharged in a timely manner, causing the high-pressure heater to be struck by a water hammer and produce vibration, exacerbating the damage and damage to the high-pressure heater. In addition, when high-temperature and high-pressure steam flows outside the tube, it causes lateral and longitudinal erosion of the tube bundle, resulting in and exacerbating the vibration of the high-pressure heater. The phenomenon of leakage of the high-pressure heater due to vibration is very common.
3. Unstable drain water level of high-pressure heater
When the high-pressure heater operates at low or no water level, the tube bundle is subjected to undue erosion, vibration, and overheating of the tube plate, accelerating the damage to the tube bundle. In addition, due to the inability of the high-pressure heater to maintain a qualified drainage water level, the emergency drainage electric valve often acts or is not tightly closed, causing internal leakage, resulting in long-term steam water erosion and vibration of the tube bundle and overheating of the tube plate.
4. Damage to surrounding pipes caused by water leakage in the pipe bundle
The tube bundles inside the high-pressure heater are tightly and orderly arranged together. Due to the water side pressure (about 15MPa) being higher than the high steam side pressure (about 1.8MPa), when the pipes are damaged and broken, the high-temperature and high-pressure water column continuously washes out the surrounding pipes, forming a large area of leakage. In addition, the tube bundle inside the high-pressure heater is in a free state. When the tube breaks, under the action of high-speed water flow, the tube fracture swings freely, continuously hitting the surrounding tubes, causing certain damage to the surrounding tubes.
Teflon heater
5. The damage and destruction of the tube bundle caused by the working medium
1) Erosion and erosion
The cooling section of Superheated steam and the tube bundle at its outlet are vulnerable to the erosion of wet steam. If the steam contains a certain amount of water, erosion damage will occur within the steam section. The tube bundle near the outlet of the steam cooling section has a greater chance of being corroded by steam and water. It is also common for tube bundles near the inlet of the drainage cooling section to be corroded by steam and water.

2) Erosion at the inlet end of pipe water supply
The damaged part usually occurs within a range of approximately 200~at the water supply inlet end of the tube bundle. The erosion at the inlet pipe end is a combined process of erosion and corrosion. Its principle is that the oxide film formed on the surface of the pipe wall metal is destroyed and carried away by high turbulent water supply. In this continuous process, the metal material is continuously lost, ultimately leading to pipe damage. Sometimes the damage surface can expand to the pipe end weld or even the pipe plate. Low pH value of feedwater (less than 9.6) and high oxygen content (greater than 7 μ When g/L), temperature is low (less than 260 ℃), and turbulence is high, erosion is prone to occur.
3) Corrosion
When the pipe material of the low-pressure heater is copper, the low-pressure heater copper pipe is often forced to be replaced due to severe leakage. When the pH value is between 8.5 and 8.8, the corrosion rate of copper is relatively low, while carbon steel requires a pH value of no less than 9.5. The high pH value of boiler feedwater has led to corrosion of copper pipes.
The main factors affecting the corrosion of carbon steel pipe bundles include oxygen content and pH value of feedwater. When the dissolved oxygen in the feedwater is too high or the pH value is too low, the inner wall of the high-pressure heater pipe will be corroded. Therefore, the concentration of dissolved oxygen in the feedwater should not exceed 7pg/L, and the pH value should be maintained between 9.3 and 9.6.
4) Overpressure explosion tube
The increase in outlet pressure of the water supply pump may cause the tube bundle to exceed the designed water supply pressure and cause tube explosion, which often occurs when the high-pressure heater starts or stops.