Evaluation and Successful Modification of Impeller Using Forced Response Analysis

Transcription

1 Turbomachinery Symposium 2008 Case study Evaluation and Successful Modification of Impeller Using Forced Response Analysis Bhabesh Thakur Hui Kuang Robert Huffman Murari Singh GE Oil & Gas, Bethlehem, PA 1

2 Background History A compressor impeller failed in the field with multiple cracks and missing chunks of material from the blades were found at the inlet after 1,000~1,200 hours of service. It appeared that a foreign object caused the damage (FOD). The impeller was taken off the line for repair and a spare unit installed. 2

3 Background History (Continued) The spare unit tripped off line following high compressor vibration after 1,700 hours of service. Unit was disassembled and it was discovered that the impeller blades were damaged similar to the first event. Inlet screen and upstream components were investigated to locate signs of FOD. No signs of FOD were found. Pictures of failed blades with missing material are shown on Figure 1. 3

4 Pictures of Failed Blades Figure 1: Pictures Showing Failed Blades with Missing Material 4

5 Evaluation of Original Design (1) The unit has 12 Inlet Guide Vanes (IGV). Modal Analysis indicated possibility of resonance condition with 2x(IGV) forcing frequency and impeller natural frequencies at, 1) 1937 Hz, 5-ND (1C) (5 Nodal Diameter, 1st Order Mode) 2) 2006 Hz, 5-ND (2C) (5 Nodal Diameter, 2nd Order Mode) The FEA Model and Modal Analysis results are shown in Figure 2 through Figure 9. 5

6 Evaluation of Original Design (2) Figure 2: FEA Model of the Original Design 6

7 Evaluation of Original Design (3) Figure 3: Radial Component, Mode Shape 1937 Hz, 5-ND (1C), Original Design 7

8 I Evaluation of Original Design (4) I 1 ANSYS 10.0A1 MAR :23:03 NODAL SOLUTION STEP=9999 UY (AVG) RS':S=l PowerGraphics EFACET=l AVRES=Mat DMX = SMN = SMX = r ~ ~ ~ -.347E-03 ~ ~ L j CJ Figure 4: Tangential Component, Mode Shape 1937 Hz, 5-ND (1C), Original Design 8

9 Evaluation of Original Design (5) Mode Animation Figure 5: Axial Component, Mode Shape 1937 Hz, 5-ND (1C), Original Design 9

10 I Evaluation of Original Design (6) 1 ANSYS 10. 0A1 MAR :58:45 NODAL SOLUTION STEP=9999 ux (AVG) RSYS=1 PowerGraphics EFACET=1 AVRES=Mat DMX = SMN = SMX = o D D D. 312E-03 D D D E-03 I Figure 6: Radial Component, Mode Shape 2006 Hz, 5-ND (2C), Original Design 10

11 I 1 Evaluation of Original Design (7) ANSYS 10.0A1 ~AR :59:10 ~ODAL SOLUTION STEP=9999 UY (AVG) RSYS=1 PowerGraphics E:FACET=1.li.VRE S =Mat DMX = SMN = SMX = J ~ A -.806E-03 ~.172E-03 L._J ~ ~ L._J I Figure 7: Tangential Component, Mode Shape 2006 Hz, 5-ND (2C), Original Design 11

12 I Evaluation of Original Design (8) I NODAL SOLUTION STEP=1 SUB =37 FREQ=2006 UZ ( AVG) RSYS=1 DMX =9.52 SMN = SMX = J\N J AN :41 : Figure 8: Axial Component, Mode Shape 2006 Hz, 5-ND (2C), Original Design 12

13 2,500 Evaluation of Original Design (9) Original Design Impeller Interference Diagram 2,000 24E (2xIGV) 1,500 Frequency, Hz 12E (1xIGV) 1,000 Nmax Speed 5,335 rpm 500 Min Speed 4,000 rpm Nodal Diameter Family Figure 9: Interference Diagram, Original Design 13

14 Forced Response Analysis of Failed Impeller (1) The Results of the Forced Response Analysis conducted at the possible resonance frequencies are as follows, 1) 1937 Hz, 5-ND (1C) Goodman Factor of Safety 0.9 (Figure 10 ). 2) 2006 Hz, 5-ND (2C) - Goodman Factor of Safety 1.3 (Figure 11 ). 3) Goodman Factor of Safety 0.9 and 1.3 for the above cases are less than the acceptable Margin of Safety

15 Forced Response Analysis of Failed Impeller (2) Goodman Diagram Harmonic Response Stresses, 1937 Hz, 5-ND (1C), Original Design Fatigue Strength Material = AISI 4320 Steel Steady Stress Based on: Rotor Speed = 5,100 rpm & Steady Gas Load 40 Fatigue Strength Adjusted for Surface Finish Alternating Stress Based on: Harmonic Response of 5% (peak to peak) of the Gas load Alternating Stress, ksi 30 F.S. = Steady Stress, ksi UTS Figure 10: Goodman Diagram, Response Stress, 1937 Hz, 5-ND (1C) 15

16 Forced Response Analysis of Failed Impeller (3) Goodman Diagram Harmonic Response Stresses, 2006 Hz, 5-ND (2C), Original Design 60 Fatigue Strength Material = AISI 4320 Steel Fatigue Strength Adjusted for Surface Finish Steady Stress Based on: Rotor Speed = 5,100 rpm & Steady Gas Load Alternating Stress Based on: Harmonic Response of 5% (peak to peak) of the Gas load Alternating Stress, ksi F.S. = UTS Steady Stress, ksi Figure 11: Goodman Diagram, Response Stress, 2006 Hz, 5-ND (2C) 16

17 Reason For Failure Possible Resonance condition for 2x(IGV) Forcing frequency with Impeller natural frequencies at 1937 Hz, 5-ND (1C) and/or 2006 Hz, 5-ND (2C) may have caused failure. Considering Forced Response Stress, Goodman Factor of Safety 0.9 and 1.3 for the above cases are less than the acceptable Margin of Safety 1.5, which eventually may have resulted in the failure. 17

18 Design Modifications Design Modification: 1) Weld repair current damaged impeller. 2) Machine back blade Leading Edge 0.75 in. (Re-radius Leading Edge to maintain 0.17 in. normal thickness). 3) Perform Modal Analysis and Forced Response Analysis of new assembly. Affect on Impeller Performance: 1) Decreases head approximately 0.65% 2) Decreases flow approximately 0.12% 18

19 Modified Design (1) The FEA Model with modified design, blade trimmed at leading edge by 0.75 inch, was created. The Frequency variation of the Tested Blades with the FEA model was % (lower). The FEA Model and Modal Analysis results are shown in Figure 12 to Figure 16. The Goodman Diagram Based on the Results of Forced Response Analysis is shown in Figure

20 I Modified Design (2) I 1 Figure 12: FEA Model Modified Design, Blade Trimmed at Inlet 0.75 in. 20

21 I Modified Design (3) I 1 ANSYS 10.0A1 MAR :10:27 NODAL SOLUTION STEP=9999 ux (AVG) RSYS=l PowerGraphics EFACET=l AVRES=Mat DMX =.542E-03 SMN =-.117E-03 SMX =.227E E E-04 Cl -.404E-04 D -.227E-05 D.359E-04 D.740E-04 D. 112E-03 D. lsoe-03 D.lBBE E-03 GE-Conmec Hz (SND) Figure 13: Radial Component, Mode Shape 1974 Hz, 5-ND (1C), Modified Design 21

22 I Modified Design (4) I 1 ANSYS 10.0A1 MAR :10:27 NODAL SOLUTION STEP=9999 UY (AVG) RSYS=l PowerGraphics EFACET=l AVRES=Mat DMX =.542E-03 SMN =-.113E-03 SMX =.107E E E-04 Cl -.641E-04 D -.396E-04 D -.lsle-04 D.935E-05 D.338E-04 D.583E-04 D.828E E-03 GE-Conmec Hz (SND) Figure 14: Tangential Component, Mode Shape 1974 Hz, 5-ND (1C), Modified Design 22

23 I Modified Design (5) I 1 NODAL STEP=1 SUB =18 FREQ=1974 UZ (AVG) RSYS=O DMX =6.344 SMN = SMX =5.768 J\N JAN :42: Model with 0.75 shorter Blade, Trimmed at Inlet Figure 15: Axial Component, Mode Shape 1974 Hz, 5-ND (1C), Modified Design 23

24 Modified Design (6) 2,500 Modified Design Impeller, Blade leading Edge Cut (0.75 inch) 2, (2 X IGV) Frequency, Hz 1,500 1, (1 X IGV) Max Speed = 5,335 rpm 500 Min Speed = 4,000 rpm Nodal Diameter Family Figure 16: Interference Diagram, Modified Design 24

25 Forced Response Analysis Modified Design (1) The Results of the Forced Response Analysis conducted at the possible resonance frequency are as follows, 1) 1974 Hz, 5-ND (1C) Goodman Factor of Safety 2.4 (Figure 17 ). 2) Goodman Factor of Safety 2.4 for the above case is greater than the acceptable Margin of Safety

26 Forced Response Analysis Modified Design (2) Goodman Diagram Harmonic Response Stresses, 1974 Hz, 5-ND (1C), Modified Design 60 Fatigue Strength Material = AISI 4320 Steel 50 Steady Stress Based on: Rotor Speed = 5,100 rpm & Steady Gas Load Fatigue Strength Adjusted for Surface Finish Alternating Stress Based on: Harmonic Response of 5% (peak to peak) of the Gas load 40 Alternating Stress, ksi 30 ` F.S. = 2.4 UTS Steady Stress, ksi Figure 17: Goodman Diagram, Response Stress, 1974 Hz, 5-ND (1C) 26

27 27 Conclusion Original design was modified: 1) Blade trimmed at leading edge by 0.75 inch. 2) Damaged blades repaired. The above modifications resulted in increased Impeller frequencies and acceptable Goodman Factor of Safety: 1) 1937 Hz, 5-ND (1C) increased to 1974 Hz, 5-ND (1C) and Goodman Factor of Safety increased from 0.9 to 2.4 2) 2006 Hz, 5-ND (2C) increased to 2357 Hz, 5-ND (2C) Out side range of the Forcing Frequency. The unit with the modified design has been in service during the past last 3 years without a failure.