Shell And Tube Heat Exchanger is currently the most widely used a heat transfer equipment. Compared with several other inter-wall
Heat Exchangers, the unit volume of equipment can provide a much larger heat transfer area, the heat transfer effect is also better. Due to the compact structure of the equipment, solid, and can choose a variety of materials to manufacture, it is more adaptable, especially in large installations and high temperature, high pressure is widely used.
一、the introduction of tubular heat exchanger
Over the years, the plant to the water heat exchanger in a variety of failures, the largest proportion of tube system leakage. Surface heat exchanger water-side pressure is greater than the pressure on the steam side, once the tube system leaks, the water will be rushed into the shell, causing the steam side full of water. Water will likely be backed up along the pumping pipeline to the turbine, resulting in deformation of the turbine cylinder, expansion of the difference between changes in the unit vibration, and even blade fracture and other accidents.
This type of heat exchanger leakage caused by the entire unit to stop and the turbine into the water accident in the plant occurred in many cases. Therefore, analysis of heat exchanger leakage causes, to find countermeasures to reduce leakage as far as possible is very important.
二、Leakage cause analysis
Tube heat exchanger internal tube system leakage is mainly divided into tube leakage and port leakage.
1 tube port leakage causes
1.1 Excessive thermal stress
Shell and tube heat exchanger in operation, due to the cold, hot fluid temperature differences, so that the shell and tube wall temperature differences between each other. This difference makes the shell and tube thermal expansion is different, when the temperature difference between the two may twist and bend the tube, or make the tube from the flower plate to pull loose, or even destroy the entire heat exchanger. In this regard, it is necessary to structure to consider the impact of thermal expansion, the use of various methods of compensation.
Heat exchanger in the process of starting and stopping the temperature rise rate, temperature drop rate more than specified, so that high plus the tube and tube plate by the larger thermal stress, so that the tube and tube plate connected to the weld or expansion joint damage, causing port leakage: transfer peak when the load changes too quickly as well as the host or heat exchanger failure and sudden stop heat exchanger, if the steam side to stop supplying steam too quickly, or steam side to stop supplying steam, the water side still continue to enter the water, because Pipe wall thin, shrinkage is fast, pipe plate thick, shrinkage is slow, often leading to the pipe and pipe plate weld or expansion joint damage. This is the specified temperature drop rate allowed only 1.7 ℃ / min - 2.0 ℃ / min, than the temperature rise rate allowed 2 ℃ / min - 5 ℃ / min to strict reasons
1.2 Tube plate deformation
Mainly the tube plate processing deformation and processing deformation, the tube is connected to the tube plate, the tube plate deformation will make the port of the tube leakage. High plus the water side of the tube plate pressure is high, low temperature, the steam side is low pressure, high temperature, especially those with built-in hydrophobic cooling section, the temperature difference is greater. If the tube plate is not thick enough, there will be some deformation of the tube plate. The centre of the tube plate will bulge towards the steam side where the pressure is low and the temperature is high. On the water side, the centre of the tube plate becomes depressed when the load on the main engine changes and the pressure and temperature on the steam side changes accordingly. Particularly in the case of large peak regulation, too fast peak regulation or sudden load changes, under the conditions of using a fixed speed feed pump, the pressure on the water side will also change significantly and may even exceed the rated pressure of the high plus feed water: these changes will cause the tube plate to deform leading to tube port leakage or permanent deformation of the tube plate. If the steam inlet door of the Gauga is leaking internally, then after stopping the Gauga in main engine operation, it will cause the water side of the Gauga to be heated up depending on the capacity boost, and if there is no safety valve on the water side or if the safety valve fails, the pressure may rise very high and also deform the tube plate.
1.3 Improper pipe blocking process
Generally used conical plug welding plug. Hit the conical plug when the force should be moderate; pounding force is too large, causing deformation of the pipe hole, affecting the adjacent pipe and the pipe plate even, will cause damage and make it appear new leakage. Welding process, such as preheating, welding seam location and size is not appropriate, will cause damage to the adjacent tube and tube plate connection. The use of other pipe plugging methods, such as pipe expansion plugging and explosion plugging, can also cause leaks in the adjacent orifice if the process is not appropriate. Therefore a strict plugging process should be followed.
2 The cause of leakage of the tube itself
2.1 Scouring and erosion
A cause is when the steam flow rate is high and the steam flow contains large droplets of water, the outer wall of the tube by steam, water two-phase flow scouring, thinning, perforation occurs or by the pressure of the water and drum break. The main reasons for the production of steam and water two-phase flow inside the heat exchanger: one is the superheated steam cooling section inside and its export steam does not reach the design requirements of superheat caused by; two is the hydrophobic water level of the heat exchanger to maintain too low or no water level or hydrophobic temperature is much higher than the design value or hydrophobic flow resistance or pumping pressure suddenly lowered factors such as hydrophobic flash steam, hydrophobic into the next level of the heat exchanger with steam, scouring the heat exchanger tube caused by damage; three is when high plus within a tube damage leakage, high pressure water from the leak at a great speed will be adjacent to the tube or bulkhead scouring damage. Another cause is the direct impact of steam or hydrophobic water. Due to unreasonable anti-scouring plate material and fixing method. In the operation of broken or off, lose anti-scouring protection; anti-scouring plate area is not large enough, water droplets with high-speed airflow movement, hit the anti-scouring plate outside the tube bundle; shell and the distance between the tube bundle is too small, so that the inlet of the steam flow velocity is very high. Stress corrosion rupture is the rupture of a metal or alloy caused by the combined action of tensile stress and a specific corrosive medium. It is characterised by the fact that most of the surface is not damaged and only a portion of the fine crack penetrates the inside of the metal or alloy. Stress corrosion rupture can occur within the commonly used design stress range and therefore has serious consequences. Important factors that can cause stress corrosion rupture are temperature, solution composition, metal or alloy composition, stress and metal structure.
2.2 Tube vibration
Feed water temperature is too low or unit overload, etc., through the heat exchanger tube between the steam flow and flow rate more than the design value, with a certain elasticity of the tube bundle in the shell side of the fluid disturbance force will produce vibration, when the frequency of the excitation force and the natural vibration frequency of the tube bundle or its multiples coincide, will cause the tube bundle resonance, so that the amplitude greatly increased, resulting in the tube and tube plate connection by repeated forces caused by Tube bundle damage, the mechanism of tube bundle vibration damage generally has.
(1) the tube or the tube and the tube plate connection due to vibration and the stress exceeds the fatigue endurance limit of the material, so that the tube fatigue fracture.
②Vibrating tubes rub against the spacer metal in the tube bore supporting the spacer, causing the tube wall to thin and eventually lead to rupture.
③When the vibration amplitude is large, adjacent tubes in the middle of the span will rub against each other, causing the tubes to wear or fatigue fracture.
2.3 Erosion at the inlet end of the tube feedwater
Erosion damage at the inlet end of the tube only occurs in carbon steel heat exchangers, is a damage process of erosion and corrosion together: the mechanism is that the tube wall metal formed on the surface of the oxide film is destroyed by the high turbulence of the feed water and take away, the metal material is constantly lost. This eventually leads to the breakage of the tube. Sometimes the damage surface can be extended to the weld at the end of the tube or even the tube plate: erosion is likely to occur when the pH of the feed water is low (less than 9.6), the oxygen content is high (greater than 7μg/L), the temperature is low (less than 260°C) and the turbulence is high.
2.4 Corrosion
When the low-pressure heat exchanger tube for copper, low plus copper tubes are often forced to replace because of serious leaks. pH 8.5 ~ 8.8 when the copper corrosion rate is the lowest. And carbon steel requires a pH value of not less than 9.5. boiler feed water pH value is too high, resulting in the corrosion of copper tubes. The main factors affecting the corrosion of carbon steel tube bundle are: oxygen content and feed water pH: when the dissolved oxygen in the feed water is too high or pH value is too low, it will make the inner wall of the high plus tube corroded, so the concentration of dissolved oxygen in the feed water shall not exceed 7pg / L, pH value to maintain between 9.3 ~ 9.6. If oxygen is present on the shell side, it will cause oxygen corrosion of the outer wall of the tube bundle. Copper deposits: will cause pitting corrosion, forming pitting pits. Temperature affects the formation of Fe3O4 oxide film on the surface of carbon steel: it is generally believed that above 260°C%, Fe3O4 oxide film is more stable. Below this temperature, the degree of protection of the Fe3O4 oxide film depends on the pH value of the feed water and other environmental factors. pH value greater than 9.6, safe.
2.5 Poor material and process
Poor tube material, uneven tube wall thickness, defects in the tube before assembly, over-expansion at the expansion port, strain damage marks on the outside of the tube, etc., can lead to massive damage to the tube when the heat exchanger encounters abnormal operating conditions
三、Treatment measures
1 Leakage occurs after the processing measures
Leakage occurs when the feed water pressure is reduced and the amount of feed water sent to the boiler is reduced. Therefore, when a heat exchanger tube system leak is found, the heat exchanger should be immediately shut down to reduce the number of damaged tubes and mitigate the degree of damage. When the unit is shut down, it should check whether the high plus is leaking, and find ways to eliminate.
For port leaks, the original weld metal should be scraped and then welded, and appropriate heat treatment to eliminate thermal stress: for the tube itself leaks, should first find out the form and location of the tube bundle leaks, and the choice of suitable plugging process, plugging the two ports of the tube. Regardless of the plugging process used, to ensure the quality of the plugged tube, the end part of the plugged tube must be well treated so that the tube plate and the tube hole are rounded, clean and have a good contact surface with the plug. In the case of cracks or erosion at the joint between the tube and the tube plate, the original tube material and the weld metal at the end must be removed so that the plug is in close contact with the tube plate.
2 Preventive measures
2.1 Preventive measures for port leakage
Heat exchanger manufacturing should be sufficient thickness of the tube plate, there is a good tube hole processing, stacking welding, pipe expansion, welding process outside the operation of the heat exchanger in the start and stop when the temperature rise rate, temperature drop rate does not exceed the provisions of the water side to have a safety valve to prevent overpressure, maintenance to have the correct plugging process.
2.2 Preventive measures for leakage of the tube itself
(1) Scouring erosion preventive measures
Restrict the flow rate of the shell side steam or hydrophobic and prevent flash steam within the evacuation section; steam cooling section of the export steam to have sufficient residual superheat; anti-scouring plate should be firmly fixed, sufficient area, good material; keep the shell side of the water level is normal, prohibit low water level or no water level operation
2） Preventive measures for tube vibration
Install steam-side safety gates on the high steam-adding side; limit the flow rate of steam or hydrophobic on the shell side; the tube spacing should be large enough, which on the one hand reduces the shell-side flow rate, and on the other hand reduces the possibility of the tubes colliding with each other and being damaged by friction: limit the length of the free section of the tube bundle
(3) Erosion prevention measures at the inlet end of the tube feedwater
The flow rate of the fluid in the tube or pipe course affects not only the value of the convective heat transfer coefficient, but also the thermal resistance to fouling and thus the size of the total heat transfer coefficient. Especially for the fluid containing sediment and other more easily deposited particles, the flow rate is too low and may even lead to pipeline gambling plug, seriously affecting the use of equipment. But increasing the flow rate will make the pressure loss increase significantly. Therefore, it is important to choose the appropriate flow rate. Limit the feed water flow rate, stop using a heat exchanger or heat exchanger blocking the number of tubes, will make the tube flow rate increased significantly, then should let part of the feed water through the bypass into the boiler or reduce the unit load; control the feed water oxygen content of small 7μg / L, control the feed water pH value in 9.2- 9.6.
(4) Corrosion prevention measures
Stress relief, stress can have various sources, such as applied stress, residual stress, welding stress and stress generated by corrosion products. Material selection, so that the unit becomes a copper-free system, which is beneficial to the entire unit corrosion and steam crystal quality control; to have a perfect air release system, in the pipeline connection is generally recommended not to use the way of series connection by stage, to prevent the non-condensable gas in the pressure of the lower heat exchanger accumulation; to ensure the normal work of the air release system, in the start-up, the water side, steam side should be clean air, feed water quality to pass; factory to There are good anti-corrosion measures to prevent corrosion in the storage and transportation process, for carbon steel tube heat exchanger, usually on the steam side and water side are taken to fill the nitrogen anti-corrosion methods; heat exchanger out of use, usually according to the length of time out of use, respectively, the use of water, steam or nitrogen filled anti-corrosion measures, in the water side to properly adjust the pH value of deoxygenated water to play a protective role.
(5) Preventive measures for tube leakage caused by poor materials and workmanship
The wall of the tube should be at least 2.0mm to improve the resistance to scouring. Before assembly, each tube should be inspected for flaw detection, hydraulic test, etc.; the tube bundle should be heat treated and free from visual defects; the tube plate and tube holes should maintain a certain degree of roughness, tolerance and concentricity, and the chamfering or rounding of the tube holes should be smooth and burr-free.
(6) Preventive pipe plugging
Preventive plugging is carried out. It is recommended to plug a part of the tube at the same time in the tube plate to open a certain size of the bypass hole, in order to reduce the feed water flow rate, reduce corrosion, this method has been used in a number of power plants at home and abroad, proved to be appropriate to extend the life of the heat exchanger, reduce the number of leaks.
(7) the choice of process
In the heat exchanger, which kind of fluid flow through the pipe, which flow through the shell process, the following points can be considered as a general principle of choice:.
(a) unclean or easy to break down the scaling of the material should flow through the side of easy cleaning. For straight tube bundle, the above materials should generally go inside the tube, but when the tube bundle can be removed for cleaning, can also go outside the tube.
(b) need to improve the flow rate to increase its convective heat transfer coefficient of the fluid should go inside the tube, because the cross-sectional area of the tube is usually smaller than the cross-sectional area between the tubes, and easy to use multiple tube process to increase the flow rate.
(c) corrosive materials should go inside the tube, so that you can use ordinary materials to manufacture the shell, only the tube, tube plate and head to use corrosion-resistant materials.
(d) High pressure materials go inside the tube, so that the shell can not bear high pressure.
e) Materials with very high or low temperatures should go inside the tube to reduce heat dissipation. Of course, if for better heat dissipation, you can also let the high temperature materials go to the shell process.
f) Steam is generally passed into the shell process, because it is easy to discharge condensate, and the steam is cleaner, and its convective heat transfer coefficient is small in relation to the flow rate.
(g) viscosity of the fluid, generally in the shell process space flow through, because in the shell process with a baffle flow, the flow channel cross-section and flow direction are constantly changing, in the low Re number (Re more than 100) can reach the wheezing, conducive to improving the convective heat transfer coefficient of the fluid outside the tube.
All of the above points cannot be met at the same time and sometimes contradict each other, so the main aspects should be grasped and a suitable decision made according to the specific situation
Heat exchanger is an important unit of chemical production process equipment, widely used in petroleum, chemical, light industry, pharmaceutical, energy and other industrial production.
Disclaimer: This article is a reproduction of the network, the copyright belongs to the original author. If it involves copyright issues, please contact us, we will be the first to remove the content.
Source: Chemical Exchange
March 20, 2023
