Causes and preventive measures of metal corrugated hose cracking
This unit has three gasifiers, and each one has four process nozzles. The cooling water outlet pipelines of its burner all adopt metal corrugated hose connection to improve maintenance efficiency. After the metal corrugated hose runs for a certain period, leakage occurs, which seriously restricts production. The specifications of metal corrugated hoses are DN50mm, PN10MPa, the wall thickness is about 4mm, and the length is 3m. There are 24 metal corrugated hoses are manufactured according to GB/T14525-1993, and the materials used include: 304 loose flange, 304 connecting pipe, 1Cr18Ni9 wire mesh, 304 corrugated hose. The internal medium of the metal corrugated hose is demineralized water, and the outer pipe is atmospheric. The outer metal corrugated hose is covered with stainless steel wire mesh, and the tail end is welded with the pipe, but the metal corrugated hose and the pipe are not effectively protected, the working temperature is normal temperature and the working pressure is 2.1MPa. The connection structure of corrugated metal hose, wire mesh and connecting pipe is shown in the figure below:
Corrugated pipe: It is a cylindrical thin-walled folded shell with multiple transverse corrugations. Bellows are elastic and can produce displacement under the action of pressure, axial force, transverse force or bending moment.Bellows are commonly called corrugated pipe or corrugated box in America and metallbulgen or metal bellows in Europe.
Analysis of causes of damage to metal corrugated hoses
- 1) Environmental factors, austenitic stainless steel is more sensitive to the stress corrosion of chloride ions, while the corrugated metal hose may have the possibility of stress corrosion failure caused by chloride ions when working in a humid atmosphere environment, A large number of point corrosion pits and stress corrosion cracking are caused by chloride ion on the wire mesh, and quite a lot of steel wires are broken. Some of the steel wires produce point corrosion pits, which leads to the reduction of effective sections. The strength of the remaining steel wire is not enough to bear the internal pressure of the metal corrugated hose and overload pull off, so that the wire mesh is broken on a large scale (actually the whole circle of wire is broken), it causes the internal metal corrugated hose to lose the protection of the wire mesh and the constraint of the axial extension, and the metal corrugated hose suffers from the excessive bending, which leads to the overload and fracture at the weak point of the welding between the metal corrugated hose and the short tube. The chloride ion on the steel wire comes from the wet atmosphere with a large amount of chloride ion, which leads to a large number of point corrosion pits and stress corrosion cracking of the steel wire.
- 2) The smelting quality and organization state of the material do not meet the requirements. Through the analysis of the composition and metallography of the wire mesh, it is shown that the steel wire of this batch of wire mesh does not belong to 1Cr18Ni9 in the product specification, and the Cr content is low, the Ni content is significantly low, and the Mn content is high. Although the metallographic analysis of the steel wire shows that it belongs to the single structure of austenite, the stainless steel with this composition neither belongs to the commonly used Cr-Ni stainless steel neither belongs to the standard stainless steel metal corrugated hose with the material of 0 Cr18Ni9, that is, the welding of 304 stainless steel metal corrugated hose body including pipe end does not find any chloride ion stress corrosion phenomenon. Practice has proved that metal corrugated hose has no stress corrosion fracture in the same external environment as the net cover.
- 3) Welding factors there are serious defects in the welding place, which will promote the generation and expansion of cracks. The outer surface of metal corrugated hose, part of the fracture scratched by extrusion has a clear dendritic loose spot in the middle, and the welding quality of metal corrugated hose is not good, with porosity and dendritic loose defects, the effective area of wall thickness is reduced.
- 1) In order to completely solve the problem of stress corrosion failure of metal corrugated hoses, it is recommended to change the wire mesh material from 1Cr18Ni9 to UNS434 (domestic brand is 1 Cr17Mo) the material of ferrite stainless steel metal corrugated hose was changed from 0Cr18Ni9 to UNS316 (domestic brand is 0Cr17Ni12Mo2). The material containing Mo can resist pitting corrosion, and the occurrence and development of stress corrosion cracking are relatively slow; at the same time, strictly control the welding quality of metal corrugated hose, strengthen the effective protection of welding parts of metal corrugated hose and pipe, so as to effectively avoid damage during installation.
- 2) A regular inspection system for metal corrugated hoses should be established, and all hoses should be inspected. Inspection of broken wires is the focus of inspection. If there are too many broken wires, water pressure test or replacement should be carried out as appropriate.
Take the above measures, the current unit is running 100% of the load, all operating parameters are relatively stable, in line with the process indicators, fully meet the production needs, for the company “safe, stable, long, full, excellent” Operation laid a solid foundation.
Source: China Corrugated Hose 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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