What are the causes of leakage of high and low pressure heaters?

Teflon heater is a major auxiliary equipment of power plants. Once the heater fails, 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, the high-pressure heater is at room temperature, while the feed water temperature is high. The main components of the high-pressure heater, such as the shell, tube bundle, and tube sheet, are suddenly heated, expand unevenly, and the thermal stress is too large, resulting in leakage of the heater water chamber tube sheet and leakage of the steel pipe and tube sheet weld. Because the temperature change rate of the high-pressure heater exceeds the allowable value when the unit is started and stopped, the tube bundle and the tube sheet expand unevenly, thereby generating a certain amount of thermal stress. Under the repeated action of this stress, the tube bundle is damaged and destroyed.

The generally specified allowable value of the temperature drop rate is only 1.7℃-2.0℃/min, which is stricter than the allowable value of the 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 water pump is about 15MPa. The opening time of the water supply electric door is short. When a large amount of high-temperature and high-pressure water flows into the water side of the high-pressure heater, the air cannot be discharged in time, causing the high-pressure heater to be impacted by water hammer and vibrate, which aggravates the damage and destruction to the high-pressure heater. In addition, when the high-temperature and high-pressure steam flows outside the tube, it produces lateral and longitudinal scouring on the tube bundle, which produces and aggravates 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 the high-pressure heater

When the high-pressure heater is running, it runs at a low water level or no water level, causing the tube bundle to be subjected to undue scouring, vibration and overheating of the tube sheet, accelerating the damage of the tube bundle. In addition, because the high-pressure heater cannot maintain a qualified drain water level, the emergency drain electric door often moves or is not closed tightly, causing internal leakage, causing the tube bundle to be subjected to steam-water scouring vibration and overheating of the tube sheet for a long time.

4. Damage to surrounding pipes caused by leakage of tube bundle

The tube bundle inside the high-pressure heater is arranged tightly and orderly. Since the water side pressure (about 15MPa) is higher than the steam side pressure (about 1.8MPa), when the tube is damaged and broken, the high-temperature and high-pressure water column continuously flushes the surrounding pipes, causing large-scale 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 broken end of the tube swings freely and continuously hits the surrounding pipes, causing certain damage to the surrounding pipes.

5. Damage and destruction of the tube bundle by the working medium

1) Erosion

The tube bundle at the superheated steam cooling section and its outlet is easily corroded by wet steam. If there is a certain amount of water in the steam, erosion damage will occur in the steam section. The tube bundle near the outlet of the steam cooling section has more chances to be corroded by steam and water. It is also common for the tube bundle near the inlet of the hydrophobic cooling section to be corroded by steam and water.

2) Erosion at the water inlet end of the tube

The damaged part generally occurs within the range of about 200~ at the water inlet end of the tube bundle. The erosion of the inlet tube end is a process of erosion and corrosion. The principle is that the oxide film formed on the surface of the tube wall metal is destroyed and carried away by the highly turbulent water. In this continuous process, the metal material is continuously lost, which eventually leads to the damage of the tube. Sometimes the damage surface can be expanded to the tube end weld or even the tube sheet. Erosion is prone to occur when the pH value of the water is low (less than 9.6), the oxygen content is high (greater than 7μg/L), the temperature is low (less than 260℃), and the turbulence is large.

3) Corrosion

When the low-pressure heater tube is made of copper, the low-pressure copper tube is often forced to be replaced due to severe leakage. When the pH value is 8.5~8.8, the corrosion rate of copper is low. Carbon steel requires a pH value of not less than 9.5. The pH value of the boiler feed water is too high, which leads to the corrosion of the copper tube.

The main factors affecting the corrosion of the carbon steel tube bundle are: oxygen content and water pH value. When the dissolved oxygen in the feed water 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 feed water should not exceed 7pg/L, and the pH value should be maintained between 9.3 and 9.6.

4) Overpressure pipe burst

The increase in the outlet pressure of the feed water pump may cause the pipe bundle to exceed the designed feed water pressure and burst. This situation often occurs when the high-pressure heater is started or stopped.