Failure Analysis of Gas Compressor Head Bolts

Following is a summary of an instance in which this associate provided Failure Analysis assistance, demonstrating specific industry expertise in a matter involving failure analysis of gas compressor head bolts.

A metallurgical examination was performed on several cylinder bolts from a gas compressor used in pumping natural gas from the oilfields; the partially disassembled unit is shown in Figure 1. The bolts had a variety of stamped markings, some of which were per the applicable standard, ASTM A 193. The investigation included visual inspection at 1X, macroscopic inspection, Rockwell hardness, chemical analysis, and metallography.

All the tested bolts met the appropriate mechanical and chemical requirements as stated in the ASTM standard. Some bolts were threaded by machining and other bolts by rolling; the stronger bolts were threaded by machining. All of the various bolt configurations experienced fatigue failures (hard and soft bolts, machined and rolled threads) as shown in Figure 2; ratchet marks were present at the thread root. The factors that probably contributed most heavily to the fatigue failures are the pump characteristics, which could include: vibration, torque at installation, and misalignment that would cause bending. Based on calculations, the source of stress that has the most detrimental influence on bolt performance is vibration and bending (alignment).

One obvious improvement to bolt performance is the use of rolled threads to minimize stress raisers as compared to corners of cut threads, minimize presence of flaws due to thread machining or rolling as exemplified in Figure 3. The results of this investigation cannot identify a bolt issue as the cause of the fatigue failures. Other factors, as mentioned above, appear to have a strong influence on bolt performance.

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