crack detection.pdf

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FAILURE ANALYSIS TO BLADES OF STEAM TURBINES AT

NORMAL CONDITIONS OF OPERATIONS AND RESONANCE

A.L. Tejeda 1,a , J.A. Rodrguez 1,b * , J.M. Rodrguez 2,c , J.C. Garca 1,d , M.A. Basurto-Pensado 1,e

1Centro de Investigacin en Ingeniera y Ciencias Aplicadas Universidad Autnoma del Estado deMorelos; Av. Universidad 1001, Col. Chamilpa, C. P. 62209, Cuernavaca, Morelos, Mxico.

2Centro Nacional de Investigacin y Desarrollo Tecnolgico; Interior Internado Palmira s/n, Col. Palmira,C. P. 62490, Cuernavaca, Morelos, Mxico

11th International Fatigue Congress, March 2014.


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ABSTRACT

The need to provide quality services, accurate diagnosis, timely rehab and improvement inmaterials and operation of the turbines, have given way to scientific research crack initiation and

your propagation, the impending fracture and estimating life of the turbine components at normalconditions and resonance.

The L-1 stage blades suffer high alternating stresses, for its size, exposure to high temperatures,and mechanical loads under repetitive strain of cyclic load. The last stages of blades are also

subjected to severe centrifugal loads that, when combined with the alternating loads, the stressesgenerated are responsible for fatigue failures.

In this work, the last stage L-1 blades of a steam turbine under cyclic load or fatigue wereanalyzed, in firstly were observed and measured the crack initiation and crack propagation under

normal conditions operation and then in the conditions on resonance.

The results show that in resonance conditions the blade material fatigue strength is decreased. Inthe case of the presence of cracks in the blades, its showed that the natural frequency is decreased.


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L-0 L-1 L-2 L-3 L-4 L-5

Stage of steam turbine Comparative of frequencies


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INTRODUCTION

Thermoelectric plants produce electricity from the heat energy released, the risks of failure are dueto their working conditions, steam turbines present problems and break downs by design or

operation, among the most notable find high levels of vibration and excessive movement of the rotor.

The defects that cause failure can be generated for the extreme conditions such as alternatingloads. The blades of the low pressure are the most important components of the turbine, thebreakage, corrosions or deposits on the blades lead to imbalance in the rotor, which generates high

vibrations and high resonance stress concentration caused by natural frequencies and the harmonicspeed.

Adequate diagnostics and timely rehabilitation have allowed studies of initiation and propagation ofcrack, impending fracture and the estimation of the components of the steam turbine useful life, but

there is the question of how avoid fracture by resonance. The L-1 stage blade, which is mainlyexposed to centrifugal and axial forces, subjected to fatigue was studied.


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Maxximum stresszone

Dynamic Stress Distribution

0.15 m

Inner forces in the end section of the blade


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MATERIALS AND DEVICES

pecifications of materials, devices and equipment

MaterialsType Materials Type Materials

pecimen test Commercial steel Stage L ! blades A"S" #!$ stainless%evices and Equipment

niversal testing machine Shaker electronic deviceLoad capacity &'$ () *requency range + !&$ -

Max. displacement !$$ mm /aveform Sinusoidal*requency

operation$ '$ -

tressing type T T, T C, C CT T0Tension Tension, T C0Tension Compression, C C0Compression Compression


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MATERIALS AND DEVICES

Sha(er Electronic %evice

1niversal Testing Machine


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METHODOLOGY

1. Specimens for fatigue test: pieces of 380 mm and delimited the edges to 40 mm and sectionedto each 50 mm. A parts of specimens were cracked in transversal direction with 5 mm length to

150 mm from the blades root.2. Fatigue equipment: Stressing type T-T, maximum displacement 100 mm, frequency 20 Hz.3. Calculation of centrifugal force (maximum load fatigue) and axial force.

4. Centrifugal forces simulated in an universal testing machine.5. Obteining natural frequencies considering the case as a cantilever blade.6. Blade test: First blade without any crack, the second blade with a crack of 15 mm length to 115

mm from the blades root and thrid blade with a crack to 193 mm from the blades root.7. Normal operation and resonance conditions.8. Experimental tests of cyclic load, using a external excitation device by contact in specific point.

*t 0 m 2 r 2 3 &

*a 0 $.& *t


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RESULTS

Centrifugal and axial forces

Fatigue test

Axial and centrifugal forces%ATA4 3 0 5+6.77 rad8s *t 0 &!$.'5 ()

m 0 !+'& g r 0 $.9#'' m *a 0 #&.!! ()

*atigue strength f:normal operation; M


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RESULTS

Natural frequencies Blades models

)atural frequency of blade =ithout and =ith crac( >lades =ith crac(


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THANK YOU

crack detection.pdf

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