Causes of leakage of evaporator tube sheet and heat exchange tube joint and repair

Hu Wanming Guo Shijie

(Hangzhou Electrochemical Group Co., Ltd., Hangzhou 310053, China)

Abstract: This paper introduces the causes of leakage of the evaporator tube sheet and heat exchange tube joint of chlor-alkali enterprises and the repair method. Slotted expansion of the stainless steel heat exchanger is difficult to repair, and the tube end is repaired by the casing method.

Key words: pipe joint; cause of leakage; repair

CLC number: TQ051.5 Document code: B Article ID: 1009-1785 (2010) 01-0026-03

1 Cause of leakage

Shell-and-tube heat exchangers are one of the most widely used equipment in petrochemical enterprises. The key to the quality of equipment manufacturing lies in the quality of the joints. Leakage of equipment such as caustic soda evaporator heating chambers, chlorine liquefiers, vinyl chloride converters, and condensers in chlor-alkali plants can affect individual plant shutdowns or reduce production loads. Although the heat exchanger used by chlor-alkali enterprises is not subject to high temperature and high pressure in the heat exchanger of petrochemical enterprises, the medium is corrosive and the flow is severely abraded. Some fixed tube and plate heat exchangers have temperature difference stress. If the design, manufacture and use are unreasonable, The pipe joint is extremely leaky, causing the heat exchanger to fail.

The caustic soda evaporator heating chamber is a shell-and-tube heat exchanger for enriching caustic soda with a mass fraction of about 17% to about 25%, wherein the shell side is heated by a secondary steam of about 0.15 MPa, and the tube process contains chloride. And impurities such as chlorate. The heating chamber tube plate and the heat exchange tube are both made of 0Cr18Ni10Ti, and the thickness of the tube sheet is 50 mm. Considering the influence of the heat resistance of the heat exchange tube wall, the heat exchange tube is made of thin-walled tube (覫57mm×2mm), arranged in an equilateral triangle, and the center distance 72 mm, 276 roots. Due to the influence of lye concentration and temperature, the heating tube of the heating chamber is subject to scour wear, stress corrosion, alkali embrittlement, pitting and crevice corrosion of sodium chloride. GBl5l-l999 "Shell-and-tube heat exchanger" stipulates that the strength expansion joint is suitable for design pressure ≤ 4 MPa, design temperature ≤ 300 ° C, no severe vibration, no excessive temperature change and no stress corrosion [1]. The ductility of the stainless steel tube is good, the tube expansion performance is very good, and the tube plate and the heat exchange tube are connected by a strong expansion. The equipment has been used for about 4 years, and the alkali content of the shell condensate exceeds the normal value. After inspection, it is found that the heat exchanger tube wall at the joint of the lower tube sheet is very thin, less than 0.6 mm, and the tube sheet and the heat exchange tube joint are shelled. A plurality of heat exchange tubes on the side of the process are damaged, and four heat exchange tubes and tube sheets are loosely leaked, and the thickness of the inner heat exchange tubes in the shell side is very uneven, and the upper tube sheet does not have this phenomenon. One heat exchange tube is taken out and the thickness is from 0.5 to 1.5 mm from bottom to top. The analysis has the following reasons.

(1) The reason why the heat exchange tube wall at the junction of the lower tube sheet is very thin is that the heating chamber is connected to the circulation pump, and the crystal salt contained in the lye is washed away by the heat exchange tube at the tube plate, and is washed under the heating chamber. The largest, the wall is thin.

(2) Proper expansion must be maintained in the expansion joint. Under-expansion cannot guarantee the sealing of the expansion. If the expansion is too large, the tube will be broken and the tube sheet will be deformed. In the case of over-expansion, the heat exchange tube and the tube sheet expand plastically, and plastic deformation occurs under the action of the expansion joint force, and the heat exchange tube becomes thinner, and the deformation of the heat exchange tube which is expanded and expanded is larger. The device uses a thin-walled heat exchange tube that is easily damaged by the wear and tear [2].

(3) When the strength is expanded, the tube plate and the heat exchange tube should have a proper hardness difference, that is, the hardness of the tube plate should be slightly higher than that of the heat exchange tube by about 30 HB, otherwise the tube rebound is larger than the tube plate, causing the expansion joint to be tight. . 0Cr18Ni10Ti stainless steel tube has a large tendency to work harden. When it is expanded and bent, the austenite structure in the material will undergo martensite transformation, which will greatly increase the yield strength of the deformed part.覫57 stainless steel heat exchange tube corresponding tube tube hole diameter is 覫57.55 mm, the maximum deviation is allowed to be 0.25 mm, so the maximum gap between the tube outer diameter and the tube sheet tube hole is 0.8 mm, to achieve the purpose of expansion, the tube There must be a large amount of deformation. However, the hardness of the 0Cr18Ni10Ti pipe increases as the expansion deformation increases, so that the hardness of the heat exchange tube is more than the hardness of the tube sheet, making the expansion joint less tight. Sometimes, to reduce work hardening, a tighter tube outer diameter and fit between the tube holes can be used.

(4) Stress corrosion of pipe joints. The quality of the expansion depends mainly on the radial residual compressive stress at the pipe end. The value is related to the material and size of the pipe and the tube sheet, whether it is slotted, the expansion rate, the radial clearance of the tube and the tube sheet, and the surface roughness. . The expansion process of the heat exchange tube is roughly divided into three steps. The first step is the process of inserting the heat transfer tube into the tube sheet. The heat exchange tube and the tube sheet are left with a certain gap, and the gap is in accordance with the requirements of Class I tube bundle in GB151; The step is to use the expander to make the heat exchange tube and the tube plate hole closely adhere together. In this process, the heat exchange tube gradually transitions from elastic deformation to plastic deformation, and the tube plate hole is in the elastic deformation stage; the third step is to complete the heat exchange tube. The expansion joint, the heat exchange tube and the tube plate hole are permanently deformed, the tube plate hole is permanently deformed, the heat exchange tube inner diameter is increased, and the heat exchange tube wall thickness has a certain amount of thinning, as shown in FIG. For steel pipes and steel plates, the rate of strength expansion is 12% to 18%; the sealing expansion is 7% to 10%; and the expansion is 3% to 7%.



In order to obtain good and stable expansion performance, in addition to strict control of the processing accuracy of the tube sheet, to ensure the proper hardness difference between the tube sheet material and the tube material, it is necessary to correctly select the expander, tube expansion power and control means to ensure proper expansion. Degree and take a reasonable expansion sequence. Since the plastic tube is plastically deformed at the end of the expansion joint, residual stress exists, and as the temperature rises, the residual stress gradually disappears, so that the sealing property and the bonding force at the tube end are lowered. In addition, the additional stress of the strength expansion is large, and the additional stress of the mechanical strength expansion is larger, and the stress corrosion corroding tree crack is easily broken at the step of mechanical expansion of the pipe head portion.

(5) The medium contains chloride ions, which are highly destructive to stress corrosion of austenitic stainless steel. The stress corrosion of austenitic stainless steel is affected by temperature, medium and chemical composition of stainless steel. Under the action of cold deformation, these stresses cause the stable internal structure of the metal to be destroyed, and the grains are displaced under the action of the stress direction to form a slip. Corrosion will form.

(6) The expansion process is unreasonable [3]. The heat exchanger is mechanically expanded, and the stress distribution of the heat exchange tube and the tube sheet is uneven over the entire length. Before the expansion, check whether the joint surface between the pipe hole and the pipe end has oil stains and foreign matter. After the pipe is inflated on the pipe plate, additional stress will be generated at the pipe joint due to the elongation of the pipe. For example, the expansion sequence is unreasonable. This will result in excessive additional stress on the joint and excessive deformation of the tube sheet, thereby reducing the quality of the expansion joint. When expanding the second tube sheet, it is generally considered that the reasonable order is from the outermost tube of the tube sheet. Gradually swell to the center, otherwise the tensile stress of the tube at the center will increase a lot and even be destroyed.

Among the above reasons, two points (1) and (2) are the main reasons. In addition, there are reasons for the use. If the evaporator is severely salted and the circulating pump is not operated properly, the steam-water impact tube in the shell process may induce the tube bundle to vibrate, resulting in leakage or fatigue damage at the sudden change of the pipe joint, as shown in Figure 2. Show.

2 repair program

After determining the leakage of the heating chamber, it is considered to replace the heat exchange tube with the tube plate and the shell, but the processing performance of the stainless steel cold working is poor, mainly because the chip breaking is difficult, the tool is easy to wear, and it is difficult to obtain a high surface quality. One heat exchange tube was tested with a drill press, but the heat exchange tube was drilled with high plasticity, high toughness, and was not easy to be broken. It was easy to block during the cutting process, which affected the smooth surface of the processing surface and long processing time. Since the groove strength was expanded during the initial processing, it was not appropriate to replace the heat exchange tube, and it was not appropriate to rework one unit. Add a length of sleeve to all the joints at the lower tube sheet joint, and tightly fit the sleeve to the heat exchange tube by means of squeezing. The length of the casing exceeds 30 mm of the breakage and 10 turns of the outer diameter of the pipe head, as shown in Figure 2.



After all the heat exchange tubes of the lower tube sheet were finished, the loosening and leakage of the four heat exchange tubes and the tube sheets were compensated by argon arc welding. After the repair, the hydrostatic test was carried out on the shell side at a pressure of 0.2 MPa. It was found that the inside of the original joint was damaged and tightly fitted to the casing and no longer leaked. It has been used for nearly 8 months and it is all normal.

3 Improve manufacturing methods

In the pipe joint connection method of the equipment, since the thickness of the pipe plate is 50 mm, the method of strength expansion and expansion can be considered, and the structure is as shown in Fig. 3.

The service life of the evaporator heating chamber is generally about 10 years. The equipment has only been used for 4 years. Although most of the reasons are due to the production process, there are problems in the design and manufacture of the equipment. If there is a requirement for slotting in design, the strength expansion requirement in GB151-1999 requires that the extension length of the heat exchange tube is not less than 3 mm, so that the tube plate and the heat transfer tube are not swollen and worn, and the length of the expansion joint is not required. Exceeding the back of the tube sheet and leaving 3 mm to avoid plastic deformation and damage during the expansion of the heat exchange tube [4], but it is not guaranteed at the time of manufacture. There is also a requirement for the degree of expansion when the heat transfer tube is subjected to strength expansion. The strength expansion must have a suitable degree of expansion, and the gap between the tube hole and the heat exchange tube should be controlled. The gap is the most important factor affecting the quality of the pipe joint expansion. The larger the gap, the easier it is to over-expansion. In GB151, Class I tube bundles are high-grade, high-grade cold drawn steel tubes, and stainless steel tubes should be selected with high precision and high precision cold drawn steel tubes.



references:

[1] GB151-1999, shell-and-tube heat exchanger.

[2] Lu Yi, Zhang Suolong. Heat exchanger leak cause investigation. Chemical Equipment Technology, 2007, 24(5). 59-61.

[3] Yang Yun. Leakage and preventive measures for the expansion joint of the heat exchanger. Pressure vessel, 2001, (03).67-68.

[4] National Boiler and Pressure Vessel Standardization Technical Committee. Pressure Vessel Design Engineer Training Course. 2006.