Luo Chongyang a, Zhang Junming a, Yang Dongb, Yang Honga

(Lanzhou Petrochemical Company, a. Research Institute; b. Oil Refinery; Lanzhou 730060, China)

Abstract: The corrosion cracking problem frequently occurring in Hastelloy U-tube heat exchangers has been analyzed from the aspects of material and operating conditions. The anti-corrosion technology measures such as material replacement have been adopted, and good results have been obtained.

Key words: U-tube heat exchanger; Hastelloy; corrosion; fracture

CLC number: TQ 051.501; TQ 050.91 Document code: B

Hastelloy is the commercial name of Haynes International N10665 steel in the United States. It has good resistance to corrosion of hydrochloric acid, acetic acid, sulfuric acid and other reducing or oxidizing media [1]. This nickel-molybdenum-based alloy has High strength, good ductility, high hardness and easy to work harden. Therefore, it is widely used in the chemical industry. However, a U-tube heat exchanger of a company's petroleum resin plant experienced frequent corrosion cracking when using this material.

The material used in the U-tube heat exchanger (hereinafter referred to as H-601 U-tube heat exchanger) is imported from the United States Hastelloy (brand No. N10665), which was put into use in March 2003 and was used only for 6 months. Corrosion cracking occurred at the bend of the heat pipe, causing the unit to stop working. Temporarily replaced with 316L stainless steel pipe, cracking occurred in the curved section only for 3 days.

In order to prevent the occurrence of similar accidents, the author started from the aspects of material chemical composition, macroscopic fracture characteristics and operating conditions, and analyzed the causes of fracture of H-601U tube heat exchanger, and proposed corresponding solutions.

1 Introduction to the equipment

The heat transfer tube size of the H-601U tubular heat exchanger with frequent corrosion cracking is Φ19 mm × 2 mm. The heat exchanger has a process medium of resin liquid, an inlet temperature of 100 ° C, an outlet temperature of 280 ° C, and a tube working pressure of 1.7 MPa. The shell-side medium is a thermal oil with an inlet temperature of 330 ° C, an outlet temperature of 310 ° C, and a shell-side working pressure of 0.8 MPa.

H-601U-tube heat exchanger Hastelloy U-shaped heat exchange tube bending section of the bend is shown in Figure 1. The on-site inspection showed that there was no obvious thinning in the straight pipe section of the tube bundle. The individual tubes were cracked in the U-bend section, the crack direction was radial, and there was no thinning near the crack. No local expansion and bulging were found in the tube bundle tube.



2 failure analysis

2.1 Analysis of heat transfer tube material

The material used for the original heat exchange tube is N10665 (International Steel grade, standard UNS) nickel-based NibMo alloy, the chemical composition of which is shown in Table 1.

Table 1 Chemical composition of Hastelloy of grade N10665 %





The material used for the heat exchange tube is Hastelloy, which is a nickel-based corrosion-resistant alloy (w(Ni) ≥ 50%). Its metallographic structure, physical properties and corrosion resistance are between stainless steel and pure nickel. Body structure metal [2]. Therefore, it is known from the corrosion characteristics of austenitic stainless steel that Hastelloy may have intergranular corrosion in an oxidizing medium and stress corrosion in a medium having a chlorine ion.

2.2 Media Analysis

The H-601U-tube heat exchanger tube-passing medium is a resin solution which is subjected to alkali washing and water washing after polymerization, and the components of the corrosive medium are sampled and analyzed. The test results are shown in Table 2. It can be seen from Table 2 that the mass concentration of chloride ions in the resin solution is 80-130 mg/L, and the mass concentration of sulfur element is 17-33 mg/L.



According to the literature [3], the temperature sensitive zone of stress corrosion cracking caused by chloride ions in austenitic stainless steel is generally 150~280 °C. In this temperature region, the mass concentration of chloride ion is small, which can cause austenite structure. Intergranular corrosion leads to corrosion cracking [4]. The operating temperature of the H-601 U-tube heat exchanger bundle component is generally between 100 and 190 °C, and some areas are in the temperature range where stress corrosion cracking is likely to occur.

2.3 The effect of stress

The stress sources of H-601 U-tube heat exchanger tube bundle are mainly as follows: 1 From the operating conditions, the inlet temperature of the tube-pass medium (resin liquid) is 100 ° C, the outlet temperature is 280 ° C, and the temperature difference between the inlet and outlet is large, resulting in feeding. The expansion and free elongation of the tubes of the segments and the discharge sections are different. However, the two are rigidly joined together by the tube sheets, so that the tubes of the feed section and the discharge section may exert an interaction force due to the difference in elongation, i.e., temperature difference stress is generated. Structurally, the tube sheet is the fixed end and the tube bundle is the free end. When a temperature difference stress is generated, the stress is concentrated on the heat exchange tube at the free end. 2 The bending radius of the heat exchange tube is too small, the minimum is only 4 cm, there is a sudden change in the structural form, the generated stress can not be evenly distributed, and it can not be alleviated or eliminated, which causes the local stress value at the bend to rise sharply. The various stresses generated during the operation of the equipment will concentrate on the stress concentration caused by the curved section of the U-shaped heat exchange tube. Concentrated stress is an important cause of stress corrosion cracking of metal components.

2.4 Causes of corrosion cracking

Based on the above analysis, the medium and temperature environment of the heat transfer tube material Hastelloy and 316 L have the conditions of stress corrosion, and the stress concentration of the U-shaped heat exchange tube is the individual U-shaped heat exchange tube bend. An important cause of cracking in the segment.

3 Improvement measures and effects

According to the above analysis, it is preferred to replace the material of the heat exchanger tube bundle. Considering that the heat transfer area of ​​the original H-601 U-tube heat exchanger is only 32 m2, the erosion of the U-bend of the bundle is severe due to the small heat exchange area. Therefore, according to the corrosion status of the H-601 U-tube heat exchanger and the test results of the laboratory, the scouring of the bend of the bundle is reduced by appropriately increasing the size of the heat exchanger to increase the heat exchange area.

The newly replaced H-601U tube heat exchanger tube bundle is made of 20# steel, and the heat exchange area is increased to 50 m2. The anti-corrosion measures are also implemented during the maintenance of the unit. At present, the newly installed 20-steel H-601 heat exchanger tube bundle has been running for nearly 1 year, no leakage is found, and the operation is good, which can meet the long-term operation requirements of the equipment.

references:

[1] Chen Gongyu. Hastelloy and its application [J]. Shanghai Chemical Industry, 2004, 017 (10): 55- 56.

[2] Le Jing. Hastelloy for corrosion resistant valves [J]. Valves, 2003, 34 (01): 34-36.

[3] Wang Fei, Lin Ying. Steel for chemical equipment [M]. Beijing: Chemical Press, 2003.

[4] Zhang Tiefeng. Discussion on Stress Corrosion Cracking and Protective Measures of Austenitic Steel Tubes[J]. 石油化工设备技术,2002,23 (6):21-28. (杜编)