One reason is that when the flow rate of steam is high and there are large drops of water in the steam flow, the outer wall of the pipe is washed away by steam and water two-phase flow, and it is thinned, perforated or under water pressure.
Tubular heat exchanger internal cause of two-phase flow: a steam cooling section of superheated steam and export is not up to the design requirements of the superheat caused; two is a hydrophobic water heat exchanger to keep low or no water or steam temperature is much higher than the water flow resistance is large or sparse or steam pressure suddenly the decrease of the hydrophobic hydrophobic value flash design, enter the next level of heat exchanger with steam, heat exchanger tube erosion damage; the three is when the damage occurred Gaojianeimou pipe leakage, high-pressure water leakage from a great speed will be out of adjacent pipes or baffle erosion. Another reason is the direct impact of steam or hydrophobicity. Because the material and fixing way of the punching plate are not reasonable. In operation, it breaks or falls off and loses the function of scour protection. The area of the scour plate is not big enough, and the droplet is impacting on the tube outside the guard plate with the movement of high-speed air. The distance between the shell and the tube bundle is too small, which makes the inlet speed very high.
The tubular heat exchanger is stress corrosion cracking refers to the co action of pulling stress and specific corrosion and rupture caused by the metal or alloy. It is characterized by the fact that most surfaces have not been destroyed, and only a part of the fine cracks penetrated the metal or the alloy. Stress corrosion cracking can be produced within the commonly used range of design stress, so the result is serious. The important factors that cause stress corrosion cracking are temperature, composition of solution, composition, stress and metal structure of metal or alloy.
Water temperature is too low or the unit overload, through the heat exchanger pipe between the steam flow and flow rate more than the design value is large, has a certain flexibility in the tube shell side fluid exciting force under the action of vibration, when the excitation frequency and the natural frequency of vibration of tube bundle or its multiple match, will which lead to the bundle resonance, the amplitude increases greatly, resulting in tube and tube plate connection force caused by repeated bundle damage, damage to the general mechanism of tube vibration:
(1) the stress at the junction of pipe or tube and tube plate is more than the fatigue endurance limit of the material and makes the pipe fatigue fracture because of the vibration.
2. The tube is rubbed with the clapboard metal in the tube hole supporting the diaphragm, making the tube wall thinning and finally causing the rupture.
(3) when the vibration amplitude is large, the adjacent pipes in the middle position of the span will friction each other, making the pipe wear or fatigue fracture.
Calandria heat exchanger entrance pipe end of the erosion damage occurs only in carbon steel heat exchanger is damaged, the interaction process of a kind of erosion and corrosion of the pipe wall: the mechanism is formed on the surface of the metal oxide film by high water turbulence and destroy away metal material loss. In the end, the pipe was damaged. Sometimes the damage surface can be extended to the pipe end weld or even the tube plate. When the water supply pH value is low (less than 9.6), the oxygen content is high (greater than 7 mu g / L), the temperature is low (less than 260 degrees), and the turbulence intensity is large, it is easy to corrode.
When the tube of the low pressure heat exchanger is copper, the low copper pipe is often changed because of the serious leakage. When the pH value is 8.5~8.8, the corrosion rate of copper is the lowest. The carbon steel requires that the pH value is not less than 9.5. The high pH value of the boiler water supply leads to the corrosion of the copper tube. The main factors affecting the corrosion of carbon steel tubes are: oxygen and water: when the pH value of dissolved oxygen in the water is too high or too low pH value, high pressure pipe wall will be corroded, so the water dissolved oxygen concentration of not more than 7pg / L, pH value is maintained at between 9.3~9.6. Oxygen corrosion on the outer wall of the tube will be caused by the presence of oxygen on the side of the shell. Copper deposition: it causes point corrosion to form pitting pit. Temperature affects the formation of Fe3O4 oxide film on the surface of carbon steel. Generally speaking, the Fe3O4 oxide film is stable when the temperature is above 260 centigrade. When the temperature is lower than that, the degree of protection of Fe3O4 oxide film depends on the pH value of water supply and other environmental factors. When the pH value is greater than 9.6, it is safe.
The material of the pipe is not good, the thickness of the pipe wall is uneven, the tube is defective before assembly, the expansion area is distended, and the outer side of the pipe is damaged. When the heat exchanger meets abnormal conditions, it will cause a lot of damage to the pipe.