International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 03 Issue: 08 | Aug-2016 www.irjet.net p-ISSN: 2395-0072 © 2016, IRJET | Impact Factor value: 4.45 | ISO 9001:2008 Certified Journal | Page 306 Failure analysis of the multistage de-scaling pump impeller of the hot strip mill plant Arbind Kumar Akela, Devaraja K, Rameshwar Sah and Suraja Yekkar R&D and Scientific Services, JSW Steel Ltd, Bellary-583275, Karnataka-India Abstract - Premature failure of the multistage descaling pump impeller of hot strip mill (HSM) has been investigated. The multistage centrifugal pumps are used in hot strip mill for a large variety of applications, such as roll cooling, strip cooling at run- out table and de-scaling. The impeller is a single piece casting of martensitic stainless steel. The impeller rotates at 4800 rpm with a volumetric flow of 400 m 3 /hr of flue water with a discharge pressure of 185-210 bar. The premature failed impeller has been thoroughly investigated to find out the root cause of the failure. The investigated includes visual examination, chemical analysis, hardness measurement and optical microscopy and electron probe micro analyzer (EPMA).Visual examination of the failed impeller indicated that the crack originated at the vane curvature and the presence of fatigue. The purpose of the investigation was to find out the root cause of impeller blade failure and suggest possible measures, to avoid the failure in future. The investigation results revealed that erosion, fatigue stress and material degradation were found to be the prominent reason for the impeller failure. Key words: Impeller1, Martensitic stainless steel2, Cast3, EPMA4, Erosion4 and Fatigue stress6 1. INTRODUCTION Centrifugal pumps are a sub-class of dynamic axisymmetric work-absorbing turbomachinery used to induce flow or raise pressure of a liquid. Centrifugal pump consists of a rotating five stage impeller inside a stationary volute (casing). The impeller is fastened to the shaft. The shaft passes through the pump casing and out through the stuffing box (portion of the casing through which the shaft extends and where seal or packing is placed). The shaft is supported by two bearing housings and is connected by a coupling installed between the pump shaft and the motor shaft [01]. Impeller has a series of curved vanes fitted inside the shroud plates. These vanes are backward curved. Backward curved vanes have the blade angle less than 90 degrees [02]. A pump configuration may vary depending on the fluid-flow direction that can be radial, semi-axial or axial. Axial flow pumps achieve larger flow rates than radial pumps and are used in drainage control, power plants and process cooling [03]. Figure 1 (a) shows a front view of multistage centrifugal pump. Any pump operation is determined by the flow rate Q (m3/h), the water discharging pressure P2 (bar) and impeller revolutions n (min–1). The design and operation of a pump also depend on the operational efficiency, the stability of the head-capacity characteristic, vibration and noise. An important issue is a possible pump failure due to fatigue, cavitations, hydro abrasive wear or erosion. Cavitation is the removal of material from the surface by formation and rapid collapse of bubbles of gas or vapor in an adjacent liquid. This type of wear is somewhat unique as only one metal surface is involved. The basic mechanism is true pitting fatigue, resulting in pits on the surfaces of the impeller. High-speed flow of liquid in these devices causes local hydrodynamic pressures to vary widely and rapidly. This resulting pit can be quite small or very large and deep depending on the particular circumstances [04]. Cavitation pitting is caused by rapid and repetitive movement between the metal and the liquid [04]. Most centrifugal pumps are out of sight during the operation and, for this reason, it is important to monitor the shaft vibrations and other operational parameters [03]. The average life of the impeller is 5 years while the stainless-steel impeller failed in the 2.5 years of the service. The impeller was a single cast made up of the grade AISI 414 martensitic stainless steel. This stainless-steel grade was a material of choice because of its good corrosion and erosion-resistant properties. In present studies, premature failure investigation of the impeller was carried out. The pump is working in the area of hot rolling mill for de-scaling of the strip. The centrifugal-pump operating parameter were maintained as, water flow rate Q = 400 m3/h, inlet pressure of water P1 = 5 bars, discharging pressure P2 = 200 bars and the number of revolutions n = 4800 min–1. The pump assembly was dismantled for investigation. The damaged impeller is shown in figure 1 (b).