The natural gas transmission industry operates an aging fleet of large reciprocating integral gas compressors. Many of these compressors are more than 40 years old with more than 250,000 operating hours. Crankshaft failure rates are low, however when crankshafts fail the costs in downtime and repairs can be significant.

Reducing the Risk of Crankshaft Failure

One way to reduce the risk of crankshaft failure is by monitoring bearing alignment through the use of static web deflections. Southwest Research Institute (SwRI) developed the strain data capture module (SDCM) to acquire dynamic strain data directly related to crankshaft operating stress. The SDCM is mounted in the root of the web of the crankshaft to monitor variations in compressor loads, compressor speeds, and measure and record the dynamic strain in the compressor throw through a complete thermal cycle (which may be 24 hours). The strain data capture module powers the strain gage, provides signal conditioning, samples at programmable rate, and stores the strain waveform to nonvolatile memory.

Analyzing the Strain Data

When the test program is complete, the data are downloaded from the SDCM to a PC for spectral and waveform analysis.  Combining the captured strain data with finite element (FE) modeling, SwRI engineers are able to correlate the peak stress conditions in the crankshaft web with the unit operating conditions.

Typically, an FE analysis of the crankshaft is completed to determine the peak stress levels based on the measured values at the strain gage location. A spectral analysis of the strain gage data can identify high stress conditions, and cycle counting can aid in damage and life analysis. 

While many machines are operating at stress levels thought to be low enough to produce “infinite life,” several crankshaft failures, combined with data from the SDCM and finite element analysis, have led to a new definition of infinite life for machines that have operated in the 10⁹ cycle range.

Strain Data Capture Module Successes

• Developed by SwRI to allow measurement of dynamic strain on webs in reciprocating compressors

• Programmable sample interval (usually 1 to 5 minutes)

• Acquire data from cold start—vary speed and loading condition and record strain levels

• Data downloaded to PC after test

• Measurements combined with cylinder pressures (compressor or engine) and torsional vibration

• Couple with finite element model to predict peak stresses at likely failure locations

• Identify "problem" loading and speed operating conditions

Right: TLAD Test Set.  Above: GMWTest Set.   Click on graphics for larger views of TLAD test set and GMW test set.