Failure Forms and Preventive Measures of Parts and Components of Shell-and-Tube Heat Exchangers
I. Composition and Structure of Shell and Tube Heat Exchanger
Shell-and-tube heat exchanger is a traditional and widely used heat exchange equipment. Because of its strong structure and the ability to choose a variety of materials, it has strong adaptability. Shell-and-tube heat exchangers are widely used in various industries. They are often used as oil coolers in dilute oil stations in cement production enterprises and oil coolers in vehicle engines.
For a long time, steel shell-and-tube heat exchangers have been dominant in the production and use of heat exchangers because of their strong structure, high reliability, strong adaptability and wide selection of materials. With the development of heat transfer enhancement technology, the manufacturing technology and heat transfer performance of shell-and-tube heat exchangers have been improved. However, due to the complexity of the structure and the diversity of operating conditions, there are often local failures or even overall scrap of heat exchangers.
The main parts of shell-and-tube heat exchanger include cylinder, head, bundle, tube sheet, baffle plate, nozzle, flange, etc. In different working conditions and medium environment, various forms of failure may occur. From the structural analysis, the vulnerable parts of failure are the joints between components, such as pipes and tubesheet; from the point of view of force, the surface of the structure is discontinuous, especially at the stress mutation point, which is often caused by the existence of additional stress, such as the weld of the tube and tubesheet; from the operational condition analysis, it is caused by high temperature and high pressure. Thermal stress or additional stress, corrosive working medium, frequent shutdown and fluid-induced vibration of heat exchanger tubes will cause the failure of cylinder, heat exchanger tubes and even the whole machine.
II. Reasons for Shell and Tube Heat Exchangers
1 Tube Bundle Corrosion and Abrasion Failure
The failure of heat exchangers is mostly caused by corrosion. The most common corrosion sites are heat exchanger tubes, followed by tube sheets, heat exchanger heads and small diameter nozzles.
The main causes of tube bundle corrosion and abrasion failure are as follows:
- (1) fouling and corrosion;
- (2) The fluid is corrosive;
- (3) Local corrosion occurs due to accumulation of foreign matter in the inner wall of the pipe;
- (4) Crevice corrosion occurs at the end of the pipe.
Preventive measures include:
- (1) Regular cleaning of tube bundles;
- (2) Reasonable selection of materials;
- (3) Adding corrosion inhibitor to the fluid;
- (4) Setting up filter device and buffer structure at the entrance of fluid;
2. Decreased heat transfer capacity
During the operation of heat exchanger, because of the high hardness of working medium or the presence of particulates and suspended solids in the fluid, serious scaling will occur on the inner and outer walls of the tube bundle. With the increase of fouling layer, the heat transfer resistance increases rapidly. In severe cases, fouling will block the working medium runner, resulting in the rapid reduction of heat transfer capacity.
- (1) Fully grasp the easily polluted parts, polluting substances and the degree of dirt, and carry out regular inspection;
- (2) When the fluid is easy to scale, the equipment or structure which is easy to inspect, disassemble and clean must be adopted.
3 Bundle leakage
In the case of heat transfer medium corrosion, stress corrosion, gap corrosion or collision, wear and tear, microcracks will occur in pipes. If there are high tensile stress or alternating stress, the cracks will rapidly expand and leak. At this time, the common method of blocking pipe on site is used as an emergency repair measure. In fact, because of the increase of temperature difference stress after blocking, the stress corrosion of the pipe itself is accelerated, so that the pipe will be damaged more seriously soon and the whole bundle will be scrapped. The methods of preventing pipe bundle leakage should be considered from material selection, anti-corrosion, anti-damage, reducing tensile stress and preventing vibration. Once the leakage of the tube bundle is found in operation, the tube should be removed and replaced as far as possible, instead of blocking the tube.
4 Failure of connection between tube bundle and tube sheet
According to the different working conditions of heat exchanger, the connection between tube bundle and tube sheet can be divided into three types: welding, expanding and expanding. The failure modes of joints vary with different types of joints. The welding method can be changed to strengthen the weld strength between heat exchanger tube and tube sheet during overhaul.
- (1) Change one-sided welding to two-sided welding;
- (2) Changing the flat welding structure of the front of tube sheet and tube bundle to a certain length of corner welding structure of tube bundle, manufacturing special tooling will change the horizontal position of tube bundle and tube sheet to vertical position, and changing the vertical welding form of heat pipe and tube sheet to horizontal welding form to meet the welding requirements of angle welding.
The two methods mentioned above can improve the welding process, and good results can be obtained. The working environment of cylinder failure shell and tube bundle is basically the same, so the failure form and preventive measures of shell can refer to tube bundle.
Corrosion is the main failure mode of shell and tube heat exchangers. In addition to the corrosiveness of the heat transfer medium itself, the corrosiveness of the working medium, the existence of tensile stress in the shell or tube, and the gap between the tube and the tube sheet will accelerate the corrosion. The main preventive measures for the failure of heat exchangers are as follows: reducing the gap, selecting corrosion-resistant materials, improving the quality of the weld, cleaning regularly, and fluids. With corrosion inhibitor, the temperature fluctuation of the control system can be reduced and the flange connection can be reduced.
III. On-line Failure Detection Method for Shell-and-Tube Heat Exchangers
After a period of operation, if the scale on the wall of the heat exchanger is serious, the heat transfer capacity decreases, and the outlet temperature of the heat exchange medium can not meet the requirements of the design process parameters; the dirt will reduce the inner diameter of the tube; the flow rate will increase correspondingly; and the pressure loss will increase. At this time, scaling can be determined by checking the operating records of flow, pressure and temperature.
2 Corrosion and Wear
The corrosion and abrasion of the inner and outer surfaces of heat exchanger shell and tube can be caused by the action of heat transfer medium and dirt. Thickness gauge is usually used to measure and estimate the corrosion and thinning of shell parts from outside.
Because of corrosion and induced vibration in the middle of the pipe, the end of the pipe is broken and the joint between the pipe and the tube sheet is leaked due to corrosion and fatigue damage. Fluid sampling can be used to check the leakage and damage of the bundle.
From the above analysis, it can be seen that the failure of shell-and-tube heat exchangers is related to many factors, such as material structure, heat transfer medium and working conditions, and sometimes is the result of the combined action of several factors. Therefore, in the process of material selection, design, manufacture, assembly and use of heat exchangers, various influencing factors should be considered comprehensively in order to prevent accidents in the future.
Source: Network Arrangement – China Tube Sheet Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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