Possibility of temporary use of pipe bends of steam power plants damaged by creep pores

Abstract

In the operation of the power equipment of steam power plants the steam pipe bends can be damaged at a temperature exceeding 500~ The presence of creep pores (especially pore chains) in the metal of a bend operating for a long time indicates that the service life of the bend has been exhausted. However, it remains unclear whether it is possible to use it again and when it should be replaced. The present analysis of the results of a study of the metal of damaged bends makes it possible to determine the possibility and the time of temporary operation of bends with pores and pore chains.The morphological features of the structure of damaged bends have been studied quite well [1 -5]. The most substantial data on the topic obtained by us 2 are presented in Table 1. We studied bends with cracks having a size varying from microcracks to through cracks and classified them as damaged. The microcracks were determined with the help of plastic replicas 3 by the method of [6]. Bends with visually detectable cracks of any depth were classified as fractured.Analyzing the obtained data we established the following: -The proportion of bends with through cracks depended on the structural state of the metal; in bends with a ferritebainite structure it amounted to 64%, in bends with ferritecarbide structure it amounted to 29%.-The number of fractured bends produced from metals with ferrite-carbide and ferrite-bainite structures was about the same.-No effect of the structural state on the development of the through cracks and the relative depth of nonthrough cracks could be observed. The width of the crack mouths fluctuated from 0.3 to 200 ram, and the relative depth of nonthrough cracks fluctuated from 1.5 to 90% of the thickness of the pipe walls.-In fractured bends the depth of propagation of microdamage over the thickness of the wall did not depend on the A comparatixe study of the metal of eleven cut bends has sho~n that the morphological features ol'lhe microscopic damage (pores, pore chains and microcracks) observed on replicas and on cut nricrosections of the san~e bend are ', irtuall,, the same. 326structural state of the metal. For a ferrite-bainite structure the depth of propagation of pore chains and single pores amounted to about 55 -100 and 80-100% of the width of the wall, and for a ferrite-carbide structure it amounted to 80 -100 and 100%, respectively.-For each studied bend the depth of propagation of pores was usually greater than that of pore chains.-Microcracks were observed predominantly on bends produced from a metal with a ferrite-carbide structure.-The minimum time before failure of bends in different structural states was about the same and amounted to about 45 thousand hours.In order to determine the kinetics of the development of microdamage over the thickness of the wall of the stretched zone of a bend we used additional results of a study of bends cut after long-term operation, the external surfaces of which did not contain micro and macrocracks but had pores or pore chains (determined by the method of re...

Estimating the remaining service life of pipeline bend metal with microdamage based on the results from long-term strength tests of solid and notched samples

“…In [9], data of studies are presented in which the period of time was observed (for seven cases) for which single pores developed to chains of pores in the tensile zone of bends that are in operation. It was found in the course of long term observations that under the oper ating conditions of metal (at a temperature of 540-555°С, stresses ranging from 60 to 80 MPa, and oper ating time equal to 80000-140000 h), this period of time makes from 2500 to 30000 h. In this connection, the period of time from the incipience of individual pores to the occurrence of pore chains was selected as a check one for making a comparison between the results of field observations and results of calculations.…”

“…Notched samples with ω 0 = 0.65 were selected as the basic ones proceeding from the data of [9], in which it was revealed that in the bends on which cracks were found pore chains propagated to exactly this rel ative depth. The smooth part of the samples had a diameter of 16 mm, and their annularly notched part had a diameter of 9.4 mm.…”