Investigation of torsional fatigue failure of a centrifugal pump shaft

doi:10.1016/j.engfailanal.2020.104511

Engineering Failure Analysis

Volume 112, May 2020, 104511

https://doi.org/10.1016/j.engfailanal.2020.104511 Get rights and content

Highlights

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Shaft of a GCP pump failed prematurely within 2.5 years of service leading to the breakdown of blast furnace for 10 h.
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Investigation shows, the keyway root radius and dimensions were non-compliant with standards, thus crack formation.
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The crack which initiated from the root radius caused premature failure due to faulty heat treatment.

Abstract

Shafts of centrifugal pumps are both load bearing and power transmitting component which are usually composed of martensitic stainless steel. In this paper, an attempt is made to manifest how minute deviations from the standard dimensions of the keyway of a shaft leads to the occurrence of premature torsional fatigue along the keyway seat. Microstructural characterization via optical microscope, SEM and EDS further revealed faulty heat treatment of the material that resulted in inferior mechanical properties which assisted in the accelerated crack propagation and ultimate failure of the shaft within 2.5 years of service leading to a shutdown of blast furnace for approximately 10 h. Comparative material analysis was carried out on samples of shaft from other pumps in service (which has not failed) to manifest that proper heat treatment of the shaft material can bring about desired mechanical properties of component and procrastinate the failure.

Introduction

Shafts are cylindrical metallic bars, mostly provided in forged condition. It is generally subjected to torsional and bending loads [1], [2], [3] and are used in wide variety of applications like mounting of wheels in all types of machines, mounting of impellers in pumps and even mounting of bearings in many structures. Shafts may fail due to numerous reasons that include incorrect keyway curvatures, brittle microstructure, machine marks [2], inclusions at stress concentration points [3], loading condition higher than sustainable loading condition [2], [4]. A pump shaft is designed to transmit high rotational motion to the impeller as well as sustain the self-weight of the impeller along with the dynamic load of water. Most common cause of shaft failure in pump is fatigue [2]. Hydraulic forces and weight of the impeller that act on pump shaft determine its level of deflection. It is determined solely by shaft diameter, hardness, metallurgy, fillet radius at the keyway and distance between the bearings. Since hydraulic force action is uneven and load distribution is maximum at the keyway of the pump shaft, fatigue failure is most likely to occur due to marginal design deviations from the standard parameters. Mechanical properties of a shaft material can be tailored to suit the application. It has been found that oil quenching and tempering at suitable temperatures provide optimum microstructure and hardness [5]. Tempered martensitic stainless steel, mostly AISI 410 or 420 grade, is a popular choice as a material for construction of such shafts since it has both dynamic load bearing capacity, high fatigue strength, high impact toughness as well as excellent resistance to corrosion in aqueous application environment [2].

Section snippets

Component history

The shaft belonged to the pump of scrubber of gas cleaning plant (Fig. 1) of blast furnace.

Primary function of a blast furnace gas cleaning plant is:

To remove particulate matter from this gas and leave behind carbon mono oxide gas after cleaning.

Control furnace high top pressure.

The blast furnace gas emanating from the hearth, passes on to the cyclone dust catcher then to the scrubber. A part of this gas then goes to maintain the high-top pressure of the furnace and the rest of it is used to

Site observation

Site observation revealed that the shaft had fractured from exactly the mid location where the impeller was mounted (Fig. 3(a,b)). The shaft was in operation from the commissioning stage of the Blast furnace and failed after 2.5 years of operation. Since shafts are used in almost all sectors of the plant, determining the root cause of such failures can prevent similar failures, breakdown times and financial losses.

Experimental procedure

For analysis of the failure, properties of the failed shaft were compared with another in-service (not failed referred here as good shaft) shaft sample referred from a standby GCP pump, which was found to be in good condition even after running for 3 years. Visual inspection, stereoscopic observation, chemical analysis, microstructural analysis, hardness measurement, EDS analysis, XRD analysis, were carried out both on the failed shaft sample as well as the good shaft samples. Tensile testing

Visual observation

Visual observation of the sample was carried out after the removal of dirt and grease from sample surface. Fatigue cracks were observed to emanate from the key way corners. The fatigue crack propagated along the longitudinal region of the sample along the keyway. The fatigue crack propagating from the keyway side marked ‘A’ had peeled off the sample along the circumferential region (Fig. 4(a)) while only a crack penetrating through the keyway is visible on side ‘B’. Region ‘C’ marks the final

Discussion

The crack originated from the root radius of the key seating location (Fig. 16(a)). The fatigue strength of the keyway region is much less than the actual strength of the material [13]. The key seat is subjected to combined torsional and rotational bending loads due to high number of rotations per minute (high torque from high RPM) and dead weight of the impeller and shaft. It was observed that one side of the keyway of shaft had been peeled off along the keyway while the other side showed

Conclusion

The fatigue strength of a shaft consisting keyway hub connection is much lower compared to the material strength. Due to the presence of sharp notches at the keyway region, the stress concentration is extremely high, causing the material to fail at a stress much lower than the strength of the material. In service, it seems that the stresses increase approximately thrice due to inertia effect during the transient start-and-stop operation. The Goodman Diagram is constructed to show that the

Recommendation

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The dimensions and dimensional tolerances of key-seat as well as the key of the shaft should be manufactured following the guidelines of BS 4235: Part 1:1972 (1986) standard.
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Material should be supplied in quenched and tempered condition. It imparts optimum strength and toughness to the material for sustaining the applied load. For a shaft, manufactured using AISI 410 stainless steel, having a diameter of 160 mm, austenitizing should be done within a temperature range of 925 °C–1010 °C, soaked

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

Authors are grateful to Tata Steel Limited, Jamshedpur, for the financial support to carry out this work. Authors extend their gratitude to Smitali Pal, Kaushal Kishore and Rishav Ghosh for support in investigation. The funding source is TATA STEEL, Jamshedpur and Kalinganagar, RD and Scientific Services Department.

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View full text

Investigation of torsional fatigue failure of a centrifugal pump shaft

Highlights

Abstract

Introduction

Section snippets

Component history

Site observation

Experimental procedure

Visual observation

Discussion

Conclusion

Recommendation

Declaration of Competing Interest

Acknowledgment

Failure analysis of water pump shaft

J. Fail. Anal. Prev.

Failure analysis of a bridge crane shaft

Case Stud. Eng. Fail. Anal.

Fatigue failure analysis of a centrifugal pump shaft

Investigation of stress concentration factor for keyway on shaft under different loading. Conditions: A Case Study

Univ. J. Res.

Failure analysis of counter shafts of a centrifugal pump

Eng. Fail. Anal.

Pump shaft failures - a compendium of case studies

Eng. Fail. Anal.

Diffractografic Analysis of AISI 420 Steel

Solid State Phenom.

Stress concentrations in keyways and optimization of keyway design

J. Strain Anal. Eng. Des.