BINSFELD ENGINEERING INC.

Drive Shaft Failure Analysis on a Multistage Vertical Turbine Pump in River Water Supply Service in a Nickel & Cobalt Mine

Drive Shaft Failure Analysis on a Multistage Vertical Turbine Pump in River Water Supply Service in a Nickel & Cobalt Mine
 

Problem Statement

• Pumps installed in 2011. One drive shaft suffered a catastrophic failure on 12/10/11 and was shortly followed by another on 1/26/12.

• Prior to any analysis by the authors, cladding and extra bracing were added to the engine support structure to reduce vibration on all three pumps. Additionally, plates were welded along the I-beams supporting the pump to the discharge on all three pumps. Lastly, grout was added to the internal cavity created by the cladding on one pump.

• The gearbox on one of the pumps suffered a crack on one of its centering feet after one drive shaft incident.

• The goal became to determine the root cause of the drive shaft failure, including identification of any resonance or other stress creators over the pump operating speed range. Based on this, a practical fix would be identified.
 

Photos Drive Shaft Failures


 

 

 

 

 

 

 

 

Failures located at the male end weld (same location each incident) at the engine-side of the assembly.

Outline Drawing
COS Pump Speed: 1760 rpm
COS Engine Speed: 1800 rpm
Engine Cylinders: 12
Gearbox Ratio: 1:1.0217
Condition of Service (COS)


 

 

 

 

 

 

 

 

 

 

Analysis Method and Steps Taken

• Radio Frequency (RF) telemetry strain gages measuring axial bending and torque were installed on the driveshaft of one pump.

• Time-transient vibration testing results on the pump, gearbox, drive shaft, and engine were collected using accelerometers and shaft sticks.

• An Operating Deflection Shape (ODS) test was performed to reveal dynamic behavior of the entire pump system.

• Experimental Modal Analysis (EMA) data was collected to find natural frequencies of the different system components.

• A test-calibrated Finite Element Analysis (FEA) based fracture mechanics analysis approach was used to predict the ability of detected stresses in the drive shaft to encourage initiation and propagation of the crack.
 

Time Domain Plots/ Drive Shaft 1,650 - 1,820 rpm


 

 

 

 

 

 

 

 

 

 

Finite Element Analysis


 

 

 

 

 

 

 

 

 

 

 

Stress Linearization Through Weld Bead


 

 

 

 

 

If the weld on the driveshaft did not penetrate to the inner diameter of the welded components in the region near the weld, this would create a geometry that is basically a crack around the circumference of the shaft. This creates a high stress concentration at the inner edge of the weld. Such a circumstance can be quantitatively evaluated with Linear Elastic Fracture Mechanics (LEFM).

Fracture Mechanics Calculation

The peak membrane plus bending stress located at the region near the “crack” was calculated from the FEA model to be 3118 psi due to the observed alternating torque and axial loading.

Fatigue Analysis Due to Alternating Torque and Axial Load


 

 

 

 

For carbon steels, exceeding a critical stress intensity factor of about 10,000 psi(in)0.5 indicates that the crack or material flaw of radial length “a” has the ability to propagate under alternating load. This analysis calculated a possible kI value of 11,040 which indicates significant but “borderline” probability of failure from fatigue loading of the weld bead.

Computer Model Using ME’Scope Program Plotting ODS Test Results


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The ODS animation at 41 Hz on Pump A (1.5x RPM) indicated a strong motion of the gearbox head towards the engine. According to the strain gage data, it was evident that the drive shaft axial splines were binding in the axial direction when under torque load, allowing this mode to enable driveshaft failure.

Strain Gage Test


 

 

 

 

 

 

 

 

 

 

 

Drive Shaft Torque Spectrum and Torsional Natural Frequencies Natural frequencies at 7.25 Hz and 42 Hz.

FFT Spectra of Vibration at Gearbox and Engine Top


 

 

 

 

 

 

 

 

 

Notice that the ½ x rpm harmonics appear in both the engine and gearbox.

Mangoro Station Pump C Vibration vs. Speed


 

 

 

 

 

 

Signal 2 – Top of the gearbox parallel to the discharge
Signal 3 – Top of the gearbox perpendicular to the discharge
Signal 4 – Top of the pump discharge head parallel to the discharge
*1,750 rpm column piping vibration was interpolated

Conclusions & Observations

1. The failure mechanism of the drive shaft was caused by the elevated axial and torsional oscillation loads in combination with the jammed driveshaft spline.

2. The situation became severe because an axial (horizontal parallel to the crankshaft) pump natural frequency and torsional shaft assembly natural frequency were simultaneously in resonance with an unexpectedly high 1.5x running speed harmonic, which appeared due to a poorly tuned engine (resulting in a 1/2x rpm fundamental and its harmonics).

3. The 2nd torsional natural frequency of the drive shaft was determined to be at 42.0 Hz (Pump C). The separation margin from 1.5x running speed is within 5% from both the pump and engine speed.

4. The torsional oscillation was observed to be as high as 94% zero-to-peak of the mean torque value.

5. The measured axial force oscillation imposed on the drive shaft peaked at 11,000 lbf pk-pk.

6. Since the weld on the driveshaft did not penetrate to the inner diameter of the material the region near the weld, this created effectively a crack around the circumference of the shaft. This resulted in a high stress concentration at the edge of the weld. The peak oscillating membrane plus bending stress amplitude located at the region near the “crack” was calculated from the FEA model to be 3118 psi due to the observed alternating torque and axial loading.

7. For carbon steels, exceeding a critical stress intensity factor of 8,000-10,000 indicates the effective initiated crack length “a” has the ability to propagate under alternating load. This analysis calculated a stress intensity factor value of 11,040 which explained the failure from fatigue loading on the weld bead.

Recommendations & Results

1. The engine was re-tuned due to the observed mis-firing, since it was providing unusually strong torque impulses or "shocks" at the rate of 1/2x RPM, which caused strong frequency harmonics. This left no place to “park” the system natural frequencies to avoid resonance.

2. A torque shock absorbing coupling between the engine and the drive shaft was implemented. The entire drive shaft and coupling assembly was replaced including the u-joints at each end.

3. The highest vibration level dropped from 24 mm/s pk at the gearbox horizontal measurement location near the input shaft to 7.8 mm/s pk after change to a flexible coupling. Failures ceased.