Materials Science, Vol. 37, No. 5, 2001 EVALUATION OF THE ADMISSIBLE DEPTH OF CRACKLIKE DEFECTS IN PIPELINES OF POWER-GENERATING UNITS I. M. Dmytrakh, A. M. Syrotyuk, and R. S. Hrabovs’kyi UDC 620.191.33: 620.193 We estimate the admissible depth of cracklike defects a * in pipelines made of 08Kh18N12T and 12Kh1MF steels by using the characteristics of cyclic corrosion crack-growth resistance and the actual parameters of working media, in particular, the presence of irregular detrimental impuri- ties. It is shown that the values of the parameter a * depend on the material, shape of the defects, and the composition of the medium. For the analyzed type of pipelines, the admissible range of this parameter can be approximately specified as 6–9 mm. Moreover, the lowest values of the admissible crack depth a * are observed for pipelines of 12Kh1MF steel in aqueous solutions of ammonia with admixtures of acetic acid (pH 5.9) used as working media. The experience of operation of power-generating equipment shows that a large part of its failures is caused by the formation of corrosion pits, pitting, cracks, and other defects whose development leads to the fracture ei- ther of the entire structure or of some of its elements [1, 2]. This observation, clearly, remains true for the im- portant and critical objects, such as pipelines of various power-generating units. Thus, numerous events of cor- rosion-mechanical damage to the inner surfaces of the pipes of feed pipelines of the supercritical-pressure pow- er-generating units of thermal power plants seem to be especially dangerous [3]. These defects have the form of corrosion pits and grooves some of which turn into cracks 5–7 mm in depth (Fig. 1). The service life of the in- vestigated equipment can be prolonged and its reliable operation guaranteed if we perform the diagnostics of the actual state of the metal and determine the characteristics of its resistance to subsequent propagation of cracklike defects [4, 5]. It is very important to take into account the great variety of in-service factors affecting the pre- dicted development of the discovered defects. The admissible depth of cracklike defects in pipelines of 08Kh18N12T and 12Kh1MF steels is estimated by using the characteristics of their cyclic corrosion crack resistance taking into account the characteristics of the actual working media, including the possibility of appearance of irregular detrimental impurities. Method for the Evaluation of the Admissible Depth of Cracklike Defects To solve the posed problem, it is necessary to formulate a proper criterion for numerical calculations be- cause different stages of the fracture processes in pipelines can be regarded as basic [4, 5]. This depends on the general purposes of calculations specified by the operating conditions of a structure and the rules of its in-service maintenance. We compute the admissible crack depth guaranteeing the reliability of operation of the pipeline between two consecutive regular preventive inspections. To this end, we use the following criterion imposed on the in- crement Δ a of the depth a of a revealed cracklike defect: Δ a ≤ Δ a * for Δ N = Δ N * , (1) Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv; Franko Pedagogical University, Drohobych. Trans- lated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 37, No. 5, pp. 69–74, September–October, 2001. Original article submitted January 11, 2001. 1068–820X/01/3705–0769 $25.00 © 2001 Plenum Publishing Corporation 769