Elevation of the Fatigue Strength of Pump Rods as a Result of Treatment with a Special Medium
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ELEVATION OF THE FATIGUE STRENGTH OF PUMP RODS AS A RESULT OF TREATMENT WITH A SPECIAL MEDIUM B. V. Kopei,1,2 O. I. Zvirko,3 T. P. Venhrynyuk,1 Z. V. Slobodyan,3 and I. P. Shtoiko3
UDC 539.431, 662.276
We analyze the causes of the in-service loss of integrity of the steel rods of pumps due to the fatigue fracture of their main part as a result of propagation of the surface cracks. To increase the service life of the rods, it is proposed to use a special liquid working medium inhibiting the process of propagation of fatigue cracks as a result of the artificial creation of the effect of crack closure. As a result of scaleddown experiments on the high-cycle fatigue of rods made of 35KhM and 20N2M steels performed after long periods of operation, it was discovered that, on the one hand, a 3% NaCl solution exerts a noticeable negative influence on the fatigue strength and, on the other hand, the working medium exerts a significant positive effect. This effect manifests itself in the increase in the fatigue strength of the rods not only in corrosive media but also in air. For the practical realization of the method under the field conditions, we propose a design of a special chamber, which is fixed on the rod and covers its part. Keywords: pump rods, fatigue strength, artificial closure of fatigue cracks, arrest of the cracks.
Introduction Pump rods often operate under severe conditions, including the action of cyclic loads, corrosive media, high pressures, high temperatures, etc. This leads to frequent breaks of the rods and, hence, to significant losses of time and costs required for the catching and descending-ascending operations. Therefore, it is necessary to develop new methods that can be used to increase their service life. The analysis of the causes of breaks of the rods for the last five years at the plants of the “Dolynanaftogaz” Oil-and-Gas Extracting Concern shows that the observed failures can be split into the following five groups: breaks in the body of the rod, in the thread, in the stem, and in the coupling, as well as the unscrewing of the rods (Fig. 1). Since accidents most often occur due to the fractures of the body of rods, it is especially important to increase its fatigue strength. This can be done by using various available approaches [1] one of which is based on the inhibition of propagation of fatigue cracks initiated on the surface of the body of a rod as a result of long-term operation. In this connection, we can mention some methods developed earlier and based on the artificial creation of the effect of crack closure, which leads to a decrease in the effective amplitude of the stress intensity factor (SIF) [2–4] regarded as the driving force of the process of fatigue crack growth [5, 6]. In order to artificially induce the effect of crack closure in structural steels, it was recently proposed to use a special liquid working medium (WM) [7]. The liquid penetrates into the crack mouth and leads there to the intense formation of solid products due to the interaction with the metal. 1 2 3
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