Accident analysis

Model of electric gate valve: Z960Y -- P W54 10V; Working pressure: 10 MPa; The highest working temperature: 540 ℃; Material iZG20CrMoV: nominal size: 225 mm; Water pressure test: room temperature 29. 4. The MPa overpressure water pressure test is qualified; Date of manufacture: August 2005.

(1) macroscopic examination

Visual inspection of the burst electric gate valve shows that the internal and external surface of the electric gate valve is basically smooth without obvious porosity or impurity of surface casting. The electric gate valve crack is located in the abdomen of the valve body. The crack is slightly curved and the crack is serrated. The crack length is 145 mm and the maximum width is 4 mm. The crack through the valve body wall is thick, the edge of the outer wall surface of the crack has slight depression deformation, and the fracture is brittle fracture. On the casting mark on the outside surface of the valve parallel to the above crack, there is an obvious small crack parallel to the crack, with a length of 25 mm and an opening slightly less than 0. 5 mm, the crack is also brittle cracking. The valve body surface on the cracked side of the gate valve abdomen has obvious drum mutation, and the corresponding inner wall valve core guide bar has obvious deformation. No obvious defects such as loose tissue, porosity or inclusion were found on the inner metal surface.

(2) wall thickness measurement

Wall thickness measurements are made on the outside of the valve body. Through the wall thickness measurement, understand the valve body thickness, check whether there is any abnormal wall thickness. Wall thickness measurement results show that the valve body thickness is 38. 2 mm ~ 43. 1 mm, no abnormal wall thickness was found. For cast valve body, the measured wall thickness is normal.

(3) hardness measurement

Hardness measurement of material of electric gate valve is carried out on the cracked side of valve body. The hardness measurement results of valve body show that the material hardness is 137 HB - 200 HB. According to the hardness test data, the hardness values of the measuring points l and 9 near the welding joint with the main steam pipe are relatively low, respectively 137 HB and 142 HB. The analysis shows that this is related to the post-welding heat treatment process of butt welding joint, and the hardness value is completely normal. In zbj98015-89 "technical conditions for pressurized cast steel parts in boiler pipeline accessories", there is no definite stipulation on the hardness value of ZG 20CrMoV cast steel, but in relevant technical standards, the hardness range of ZG0CrMoV cast steel is required to be 140 HB ~ 201 HB.

(4) metallographic analysis

Prior to the installation of the electric gate valve, according to the relevant regulations, the power plant has conducted the disintegration spectrum analysis of the gate valve and confirmed that the materials used in all parts of the gate valve are correct. There are 3 sampling points of metallographic complex membrane. The tissue is ferroid + carbide and ferroid + sorbite. At present, the results of metallographic analysis on 3 parts of the valve body are iron cable body + carbon and ferrite body + soxhlet body. It can be considered that the metallographic structure of the valve body material is still basically normal.

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