Analysis on the failure of shell-and-tube heat exchangers and related discussion

Abstract: This paper describes the common failure modes of shell-and-tube heat exchangers, analyzes the causes of various failures, proposes preventive measures, and introduces the on-line detection method for failure of shell-and-tube heat exchangers, which is a shell-and-tube heat exchanger. The use of the company's production in the future provides a useful reference.

Key words: shell-and-tube heat exchanger; failure analysis; preventive measures; on-line detection CLC number: TQ172.6 Document code: BArticle ID:1003-1324(2008)01-0024-03

The shell-and-tube heat exchanger is a traditional, widely used heat exchange device. Due to its sturdy construction and the ability to be manufactured from a variety of materials, it is highly adaptable. Shell-and-tube heat exchangers are widely used in various industries. They are often used as oil coolers for equipment oil stations in cement production enterprises, and are used as vehicle engine oil coolers. For a long time, steel shell-and-tube heat exchangers have always dominated the production and use of heat exchangers due to their strong structure, high reliability, adaptability and wide selection of materials. With the development of technologies such as heat transfer enhancement, shell-and-tube heat exchangers are also constantly improving in manufacturing technology and heat transfer performance. However, due to the complexity of the structure and the variety of operating conditions, local failure or even total scrap of the heat exchanger often occurs.

2 failure modes and preventive measures for shell and tube heat exchanger parts The main components of shell and tube heat exchangers include: cylinder, head, tube bundle, tube sheet, baffle, nozzle, flange, etc., in different Various forms of failure may occur in working conditions and media environments. From the structural analysis, the part that is prone to failure is the joint between the components, such as the joint between the pipe and the tube sheet; from the perspective of the force analysis, the surface of the structure is discontinuous, especially at the sudden change of stress, due to the existence of additional stress. And cause failure, such as the weld of the cylinder and the tube sheet; from the use condition analysis, the thermal stress or the additional stress caused by the high temperature and high pressure, the working medium is corrosive, and the fluid is frequently started and stopped to cause the fluid induction of the heat exchange tube. Vibration, etc., will cause the cylinder, heat exchange tube and even the whole machine to fail.

2.1 Tube bundle failure (1) Tube bundle corrosion and abrasion failure Most of the heat exchanger failure is caused by corrosion. The common corrosion site of zui is the heat exchange tube, which in turn is the tube sheet, the heat exchanger head and the small diameter tube.

The main reasons for tube bundle corrosion and abrasion failure are: 1 fouling corrosion; 2 fluid is corrosive; 3 foreign matter accumulates on the inner wall of the tube and local corrosion occurs; 4 crevice corrosion occurs at the tube end.

Precautionary measures include: 1 regular cleaning of the tube bundle; 2 reasonable selection of materials; 3 addition of corrosion inhibitor to the fluid; 4 filtration device and buffer structure at the fluid inlet.

(2) Decreased heat transfer capacity During the operation of the heat exchanger, due to the high hardness of the working medium or the inclusion of particulate matter and suspended matter in the fluid, the inner and outer walls of the tube bundle may be seriously fouled. As the fouling layer is thickened, the heat transfer resistance increases rapidly. In severe cases, the dirt will block the working medium flow passage, resulting in a rapid decrease in heat exchange capacity.

The prevention method is:

1 Fully grasp the degree of dirt, dirt and dirt, and conduct regular inspections; 2 When the fluid is easily fouled, equipment or structure that is easy to inspect, disassemble, and clean must be used.

(3) Tube bundle leakage In the case of heat exchange medium corrosion, stress corrosion, crevice corrosion or collision, wear, etc., micro-cracks will be generated on the tube. If there is high tensile stress or alternating stress, the crack will expand rapidly and leak. At this time, the method of plugging the pipe is often used as an emergency repair measure. In fact, after the pipe is blocked, the temperature difference stress is increased, thereby accelerating the stress corrosion of the pipe, and the pipe will soon be more seriously damaged, so that the tube bundle is completely scrapped. The method of preventing tube bundle leakage should be considered in terms of material selection, corrosion prevention, damage prevention, reduction of tensile stress and prevention of vibration. Once the tube bundle leak is found during operation, the tube should be replaced as much as possible without blocking the tube.

2.2 Failure of the connection between the tube bundle and the tube sheet According to the different working conditions of the heat exchanger, the connection form of the tube bundle and the tube sheet can be divided into three types: welding, expansion joint and expansion welding. The form of the joint is different and the form of failure is also different. For welding products that are welded by conventional methods during welding, the welding method can be changed during overhaul to strengthen the weld strength between the heat exchange tubes and the tube sheets.

1 Change the single-sided welding to double-sided welding; 2 The flat-welded structure of the front end of the tube-changing tube and the tube bundle is an angle-shaped welded structure with a certain length extending from the tube bundle, and the special tooling is used to change the tube bundle and the tube sheet from the flat position to the flat position. In the vertical state, the vertical welding form of the heat pipe and the front side of the tube sheet is changed to a horizontal welding form to meet the welding requirements of the angular welding. Good results can be obtained by improving the welding process by the above two methods.

2.3 The cylinder failure shell and the tube bundle are in the same working environment. Therefore, the failure mode and preventive measures of the shell can be referred to the tube bundle.

Typical failure modes and preventive measures for 3 shell-and-tube heat exchangers Corrosion is the main failure mode of shell-and-tube heat exchangers. In addition to the corrosive nature of the heat and acid medium itself, the corrosive nature of the working medium, and the tensile stress of the shell or tube, the gap between the tube and the tube sheet will accelerate the corrosion, which will cause the heat exchanger to fail. The main preventive measures are: reducing the gap, selecting the corrosion-resistant material, improving the quality of the weld, cleaning regularly, adding corrosion inhibitor to the fluid, controlling the temperature fluctuation of the system, and reducing the flange connection.

4 On-line detection method for shell-and-tube heat exchanger failure 4.1 After a period of operation of the scale heat exchanger, if the fouling of the tube wall is serious, the heat transfer capacity will decrease, and the outlet temperature of the heat exchange medium will not meet the design process parameters; the dirt will The inner diameter of the tube becomes smaller; the flow rate increases correspondingly; the pressure loss increases. At this time, the fouling can be determined by checking the operation records such as flow rate, pressure, and temperature.

4.2 Corrosion and wear of heat exchange medium, dirt and other effects will cause corrosion and wear on the heat exchanger shell and the inner and outer surfaces of the tube. A thickness gauge is usually used for the casing to measure and estimate the location of the casing which is corroded and thinned from the outside.

4.3 The middle of the leaking pipe is broken due to corrosion, induced vibration, etc., and the pipe end leaks due to corrosion, fatigue damage and the like. The leak and damage of the tube bundle can be checked by fluid sampling.

5 Conclusions From the above analysis, it can be seen that the failure of the shell-and-tube heat exchanger is related to various factors such as material, structure, heat exchange medium and working conditions, and sometimes it is the result of several factors working together. Therefore, in the process of material selection, design, manufacture, assembly and use of heat exchangers, various influencing factors should be considered in order to prevent problems before they occur. For our users, only by actively understanding the performance of the heat exchanger, strictly conducting online testing, analyzing the cause of failure, and rationally applying preventive measures, can we make good use of heat exchange equipment to better serve the production.

Fried Series

Rushan Jinguo Food Co., Ltd , https://www.jinguofood.cn