Technical

Leakage treatment of shell-and-tube heat exchangers

Shell-and-tube heat exchanger is one of the most widely used heat exchange equipment at present. Compared with other inter-wall heat exchangers, the unit volume equipment can provide much larger heat transfer area and better heat transfer effect. Due to the compact structure, strong, and can choose a variety of materials to manufacture, so strong adaptability, especially in large-scale device 4 and high temperature, high pressure are widely used.

First, Introduction of tubular heat exchanger

For many years, the leakage of the piping system accounts for the largest proportion among all kinds of faults of the water supply heat exchanger in the plant. The water side pressure of the surface heat exchanger is greater than the steam side pressure. Once the pipe system leaks, the feed water will rush into the shell, causing the steam side full of water. It is possible that water will flow back into the steam turbine along the extraction pipe, causing the steam turbine cylinder deformation, differential expansion changes, unit vibration, and even the blade fracture and other accidents.

 This type of heat exchanger leakage caused by the whole set of equipment and steam turbine shutdown accidents occurred in the plant. Therefore, it is very important to analyze the reasons of heat exchanger leakage and find out the countermeasures to reduce the leakage as much as possible.

Second, leakage reason analysis

The leakage of the inner pipe system of the tube heat exchanger is mainly divided into the leakage of the tube itself and the leakage of the end.

1.Cause of pipe port leakage

1.1Excessive thermal stress

In the operation of shell-and-tube heat exchangers, due to the different temperatures of cold and hot fluids, the temperatures of shell and tube wall are different from each other. This difference makes the thermal expansion of the shell and tube different, when the temperature difference between the two may be a large tube twisted, or tube from the ceiling to loosen, or even destroy the whole heat exchanger. Therefore, it is necessary to consider the effect of thermal expansion in structure and adopt various compensation methods. During the start and stop of the heat exchanger, the temperature rise rate and drop rate exceeded the regulation, so that Gawga's pipe and tubesheet were subjected to greater thermal stress, and the weld or expansion joint of the pipe and Tubesheet was damaged, cause port leakage: peak load changes too fast and the main engine or heat exchanger failure when the sudden shutdown of the heat exchanger, if the steam side stop steam supply too fast, or steam side stop, water side continue to enter the feed water, because the tube wall is thin, shrinkage fast, tube thickness, shrinkage slow, often lead to tube and tube-plate weld or expansion joint damage. This is the reason why the required temperature drop rate is only 1.7℃/min -2.0℃/min, and the ratio of temperature rise rate is 2℃/min -5℃/min.

1.2 Tube sheet deformation

It is mainly the deformation of tubesheet and the deformation produced during the processing. The tube is connected with the tubesheet. The tubesheet deformation will cause the leakage of the end of the tube. High pressure and low temperature on the water side of the tube sheet, low pressure and high temperature on the steam side, especially the built-in drain cooling section, the temperature difference is greater. If the thickness of tubesheet is not enough, tubesheet will have some deformation. Tubesheet center will be low pressure, high temperature steam side bulging. On the water side, a central depression occurs in the tubesheet. When the main engine load changes, the steam side pressure and temperature changes accordingly. Especially when the peak regulation amplitude is large, the peak regulation speed is too fast or the load is sudden, under the condition of using constant speed feed pump, the water side pressure will also change greatly, it may even exceed the rated pressure of the high feed water: these changes can cause tubesheet deformation leading to leakage at the pipe end or permanent deformation of the tubesheet. If Gawga's inlet valve is leaking, the high pressure on the high water side will be heated after shutdown of the main engine. If there is no safety valve on the water side or the safety valve fails, the pressure may rise very high, it also deforms the tubesheet.

1.3 Improper plugging process

Generally used taper plug welding plug pipe. When the tapered plug is driven into, the force should be moderate; the hammer force is too large, which causes the deformation of the pipe hole, affects the adjacent pipe and tubesheet joint, and will cause damage and new leakage. During the welding process, such as preheating, welding seam location and size is not appropriate, will cause adjacent pipe and tubesheet connection damage. Other pipe plugging methods, such as expansion pipe plugging, explosion pipe plugging, such as improper process, will also cause the leakage of adjacent pipe orifices. Therefore, the strict pipe plugging process should be followed.

2.Cause of leakage of pipe itself

2.1 Erosion

One reason is that when the steam flow speed is high and there are large water droplets in the steam flow, the outer wall of the pipe is scoured and thinned by the steam-water two-phase flow. The main reasons of the steam-water two-phase flow in the heat exchanger are as follows: firstly, the superheated steam in the superheated steam cooling section and its outlet can not meet the design requirements; The other is that the hydrophobic level of the heat exchanger is kept too low or no water level or the hydrophobic temperature is much higher than the design value, or the hydrophobic flow resistance is larger or the suction pressure suddenly decreases, etc. , when the drainage into the next stage of the heat exchanger with steam, washing heat exchanger tube damage; High-pressure water supply from the leakage at great speed will be rushed out of the adjacent pipe or diaphragm erosion damage. Another reason is the direct impact of steam or hydrophobic water. Because the anti-impact plate material and fixed way is not reasonable. In operation, it breaks or falls off and loses the function of anti-erosion protection; the area of the anti-erosion plate is not big enough, and the water droplets move with the high-speed airflow, impacting the tube bundle outside the anti-erosion plate; the distance between the shell and the tube bundle is too small, make the steam flow at the entrance very high.

Stress corrosion cracking (SCC) is the cracking of metal or alloy caused by the combined action of tensile stress and specific corrosion medium. It is characterized by the fact that most of the surface is undamaged and only a portion of the fine cracks penetrate the interior of the metal or alloy. Stress corrosion cracking can occur within the range of commonly used design stress, so its consequences are serious. The important factors causing stress corrosion cracking are temperature, solution composition, metal or alloy composition, stress and metal structure.

2.2 Pipe vibration

When the water temperature is too low or the unit is overloaded, when the steam flow rate and velocity between the tubes of the heat exchanger exceed more than the designed value, the tubes with certain elasticity will vibrate under the action of the fluid disturbance force on the shell side, when the frequency of the exciting force coincides with the natural frequency of the tube bundle or its multiple, it will cause the tube bundle to resonate and increase the amplitude greatly, the mechanism of vibration damage of tube bundle is as follows: 

(1) because of vibration, the stress of tube or the joint between tube and tubesheet exceeds the fatigue endurance limit of material, which causes the fatigue fracture of tube; 

(2) the vibrating pipe in the hole of the pipe supporting the baffle will rub with the baffle metal, so that the pipe wall will become thin, and finally lead to the rupture; 

(3) when the vibration amplitude is large, the adjacent pipes in the middle of the span will rub with each other, wear or fatigue the pipe.

2.3 Erosion of water inlet of pipe

The corrosion damage of the inlet pipe end only occurs in the carbon steel heat exchanger, which is a combined process of corrosion and erosion: the mechanism is that the oxidation film formed on the surface of the pipe wall metal is destroyed and taken away by the high turbulence water supply, the metal material is losing. Eventually the pipe broke. Sometimes the damage surface can be extended to the pipe end weld and even to the tubesheet: when the pH value of feed water is low (less than 9.6) , the oxygen content is high (more than 7μg/L) , the temperature is low (less than 260 ° C) , and the turbulence degree is high, the erosion is easy to occur.

2.4 Corrosion

When the tube of low pressure heat exchanger is copper, the low adding copper tube is often forced to be replaced because of serious leakage. The corrosion rate of copper is the lowest at pH 8.5 ~ 8.8. Carbon steel requires a pH of at least 9.5. The high pH value of boiler feed water leads to the corrosion of copper pipe. The main factors affecting the corrosion of carbon steel tube bundles are: oxygen content and feed water pH value: when the dissolved oxygen in feed water is too high or the pH value is too low, the inner wall of the high-pressure tube will be corroded, therefore, the concentration of dissolved oxygen in feed water should not exceed 7 pg/L, and the pH value should be maintained between 9.3 and 9.6. If there is oxygen on the shell side, it will cause oxygen corrosion on the outer wall of the tube bundle. Copper deposition: can cause pitting corrosion, pitting pits. Temperature affects the formation of FE3O4 oxide film on the surface of carbon steel. It is generally considered that the FE3O4 oxide film is relatively stable when the temperature is above 260 ° C% . Below this temperature, the degree of protection of FE3O4 oxide film depends on the pH of feed water and other environmental factors. When pH is above 9.6, safe.

2.5 Poor material and workmanship

The material of the tube is not good, the thickness of the tube is not uniform, the tube has defects before assembling, the expansion mouth is over-inflated, the outside of the tube has tensile damage, etc. .

Third, deal with the countermeasures

1.After the occurrence of leakage treatment measures

When the leakage occurs, the feed water pressure decreases and the feed water to the boiler decreases. Therefore, when the leakage of heat exchanger pipe system is found, the heat exchanger should be stopped immediately to reduce the number of tube damage and reduce the extent of damage. Unit shutdown, should check whether GAWGA leakage, and find ways to eliminate.

For the leakage of the end, the original weld metal should be scraped off before repair welding, and proper heat treatment should be carried out to eliminate the thermal stress: for the leakage of the pipe itself, the form and location of the leakage of the pipe bundle should be checked first, and select the appropriate pipe plugging process, plugging the two ends of the pipe. No matter what plugging technology is used, in order to ensure the quality of plugging, the end of the blocked pipe must be well treated to make the tube sheet and hole round and clean, and have a good contact surface with the plug. In the case of crack or erosion at the joint of tube and tubesheet, the original tube material and weld metal must be removed at the end to make the plug in close contact with tubesheet.

2. Preventive measures

2.1 Port leakage precautions

The heat exchanger should have enough thickness tubesheet, have good pipe hole processing, surfacing welding, pipe expansion, welding process, the operation of the heat exchanger at the start and stop temperature rise rate, temperature drop rate not exceed the provisions, the water side should have safety valve to prevent overpressure, maintenance to have the correct pipe plugging process.

2.2 Preventive measures for leakage of the pipe itself

(1) Measures to prevent erosion, to limit the flow rate of steam or drainage on the shell side and to prevent flash in the cooling section; to ensure that there is sufficient residual overheating of steam at the outlet of the steam cooling section; to ensure that the plate is securely fixed and of sufficient area; Material to be good; keep the shell side water level normal, prohibit low water level or no water level operation.

 (2) Preventive measures against pipe vibration, installation of a steam side safety door on the high-pressure side, restriction of the flow rate of steam or drainage on the shell side, and sufficient pipe spacing to reduce the flow rate on the shell side, on the other hand, it reduces the possibility of tube collision and friction damage: it limits the length of free section of tube bundle.

(3) The corrosion prevention measures at the inlet of water supply pipe, the flow velocity of fluid in the pipe side or in the pipe side, not only affect the value of convective heat transfer coefficient, but also affect the heat resistance of dirt, so as to affect the total heat transfer coefficient. Especially for the fluid containing sediment and other easily deposited particles, low flow rate may even lead to pipe blockage, which seriously affects the use of equipment. However, the pressure loss increases significantly with increasing flow velocity. Therefore, it is very important to choose the proper flow rate. When the flow rate of feed water is limited, the flow rate in the tube will increase obviously when a row of heat exchangers are stopped up or the number of blocked tubes is large The oxygen content of feed water is controlled to be 7 μg/l, and the pH value of feed water is controlled to be 9.2-9.6.

(4) Corrosion prevention measures

To eliminate stress, stress can come from a variety of sources, such as external stress, residual stress, welding stress and corrosion products generated stress. When the material is selected, the unit will be changed into a copper-free system, which is beneficial to the anticorrosion of the whole unit and the control of steam and crystal quality, in order to prevent non-condensing gas in the lower pressure heat exchanger accumulation, to ensure the normal work of the vent air system, in the start-up, water side, steam side should be discharged clean air, water quality to qualified; Good anti-corrosion measures should be taken before leaving the factory to prevent corrosion during storage and transportation. Nitrogen-filled anti-corrosion methods are usually adopted for carbon steel tube heat exchangers, both steam side and water side, the anticorrosion measures of filling water, filling gas or filling nitrogen are adopted respectively, and the pH value of deaerating water is adjusted appropriately at the water side to play a protective role.

(5) Preventive measures for pipe leakage caused by poor material and technology

The tube wall shall be at least 2.0 mm to improve the erosion resistance. Before assembling, each tube should be inspected by means of flaw detection and water pressure test, the tube bundle should be heat-treated without visual defects, and the tube holes on the tube sheet should be maintained with certain roughness, tolerance and concentric degree, pipe Hole chamfer or rounded should be smooth without burrs.

(6) Preventive plugging

Perform preventive plugging. It is suggested that a certain size of bypass hole should be made on the tube sheet while blocking a part of the tube in order to reduce the feed water flow rate and reduce the corrosion. This method has been used in many power plants at home and abroad, and it has been proved that it can extend the life of heat exchangers appropriately, reduce the number of leaks.

(7) Process selection

In a heat exchanger, which fluid flows through the tube side and which flows through the shell side, the general principle of choice may be to consider the following:

a)materials that are unclean or easily decomposed and scale should flow through the side that is easy to clean. For the straight tube bundle, the above-mentioned materials should generally go inside the tube, but when the tube bundle can be removed for cleaning, it can also go outside the tube.

b) fluids that require increased flow rates to increase their convective heat transfer coefficient should travel through the tube, as the cross-sectional area in the tube is usually smaller than that between the tubes, and it is easy to use multiple tube lengths to increase the flow rate.

c) the corrosive material shall be carried inside the pipe so that the shell can be made of ordinary materials, only the pipe, tubesheet and head shall be made of corrosion-resistant materials.

d) high-pressure material goes inside the pipe so that the housing can not withstand high pressure.

e) materials at very high or very low temperatures should be run through the tube to reduce heat loss. Of course, if in order to better cooling, you can also allow high-temperature material shell journey.

f) the steam generally passes through the shell side, because it is convenient to discharge the condensate, and the steam is cleaner, and its convective heat transfer coefficient has little relationship with the flow rate.

g) the viscous fluid generally flows in the shell side, because the cross section and the flow direction of the channel are constantly changing when the fluid flows in the shell side with baffles, and the surge flow can be achieved at the low Re number (Re＞100) , which is beneficial to improving the convective heat transfer coefficient of the fluid outside the tube. The above points can not be met at the same time, and sometimes contradictory, it should be based on specific circumstances, grasp the main aspects, make appropriate decisions.