During a petrochemical plant shutdown for a scheduled turnaround, a two-stage compressor suffered a catastrophic failure.
The compressor ripped into more than 180 metal shards, some of which were recovered as far as 100 feet away. KnightHawk Engineering (KHE) performed a root cause analysis to determine the cause of the explosion.
Process
The analysis began with an onsite assessment, disassembling the compressor, preserving the evidence and inspecting the components and machinery. KHE then analyzed the processes of the system. The process data was examined in conjunction with KHE’s metallurgical lab analysis. KHE discovered an imbalance between the stages of the compressor, presumably caused by inaccurate reporting of Annubar flows from the detection systems. This finding suggested a leak occurred between the two stages, however, metallurgical analysis revealed no evidence of leaks prior to the incident. Despite the absence of leaks, the inaccurate readings proved problematic. The incorrect molecular weight measurements utilized by the controls systems were significantly higher than reasonable values, considering the system was being switched to a nitrogen purge for shutdown. The reported molecular weight was an estimated weight rather than a live reading for the calculations. Unfortunately, this reported molecular weight became inaccurate during the dynamic shutdown process. In addition to the mass flow differential between stages one and two, KHE discovered a rising discharge temperature in relation to inlet temperature, strongly suggesting a loss of efficiency in the compressor. KHE considered the cause to be internal seal failure allowing for back flow, but further analysis determined this was the result of a surge.
The lab also studied the microscopic structures of the metal to determine the origin and propagation of the fractures. After finding no signs of impact damage, KHE collected 20 of the samples for metallurgical analysis.
The lab review of compressor pieces found no evidence of historical wear, impact damage or casting defects. However, the inlet guide vane revealed signs of corrosion. Chemical testing identified the residue and corrosive element present at the time of the failure. Structural failures were thoroughly examined and determined to be recent, eliminating pre-existing defects as the cause of the catastrophic failure. Hardness tests and tensile strength tests were also performed to identify yield stresses.
KHE used data obtained from the metallurgical and process analyses and the onsite assessments to create an FEA model of the compressor. The model, which incorporated the locations of stresses, was used to determine whether the process or design may have caused the failure of the unit. The model showed, under normal conditions, the unit operated well within design and expected tolerances, and the areas of high stress did not coincide with fracture locations.
KHE reviewed the controls system assessment and the recorded data focusing on the unit’s surge controllers for stage 1 and 2. Although stage 2 did not appear to have entered surge leading up to the incident, it appeared that stage 1 entered surge a number of times in the hours leading up to the failure. Valve data revealed the system was still responding to the surge controller, although the system response time was too slow to avoid surge. It appeared the recycle system controller was insufficiently designed to fully recover the compressor back to the desired conditions.
Solution
The cause of the failure was a combination of factors which contributed to the bypass of an insufficient surge control system, causing compressor surge and compressor failure. The spillback valve was fully open, contributing to rapidly rising temperatures inside the compressor and making surge recovery difficult. To correct the problem, KHE recommended implementing faster response time systems in the compressor controls systems and including polytropic discharge temperature data collection inside the compressor. KHE also recommended the computerization of the compressor performance curve for operator clarity, and the installation of vent lines valves to prevent accidental valve control during the compressor’s operation.
For more information, visit knighthawk.com or call (281) 282-9200.
