A charge gas compressor train at an ethylene plant was exhibiting high vibration levels in the low-pressure casing before crossing first critical speed. These vibration levels prevented the train from reaching its design operating speed of 5,800 rpm. With only a 45-day window to identify the cause of the vibration and fix it, Petroquimica Mexicana de Vinilo (PMV) asked Dresser-Rand engineers to have a look.
PMV, a flagship joint venture between PEMEX and Mexican petrochemical firm, Mexichem, represents the first joint venture between PEMEX and a private company. The merger joined Mexichem’s salt, chlorine and caustic soda operations with PEMEX’s ethylene and vinyl chloride monomer (VCM) operations.
PMV revamped an old PEMEX plant in the Pajaritos petrochemical complex to produce an expected 120 tons of VCM, the key material used to manufacture polyvinyl chloride (PVC), commonly used for electrical insulation, films and pipes.
Francisco Moncayo, Dresser-Rand Services director for Mexico, describes the circumstances. “Initially, PMV representatives asked us to perform a vibration analysis on the compressor train, consisting of a 3 MX compressor, 3M compressor and a 4M compressor, to determine possible causes. The fact that there was no historical data on maintenance or vibration levels compounded the problem because in the past PEMEX had sourced third-party, non-Dresser-Rand parts and service.”
At the outset, the compressor was operating at 5,150 rpm; however, increasing the speed to 5,220 rpm significantly increased vibration levels whereby the protection systems would trip the compressor train and shut it down. Data analysis during the machine trip showed high vibration (6.85 mils peak-to-peak) at the 5,220 rpm level, so the machine could not operate at speeds above 5,150 rpm. Such interruptions affected plant operation and resulted in lost production and revenue.
Internal friction in the low-pressure casing, lack of rigidity in the system, train misalignment, and process piping were found to be the main causes for vibration. In addition, Dresser-Rand engineers found corrosion in the compressor casing, and in the suction and discharge flanges, a half-inch crack on one of the shaft journals, impeller pitting, and coupling gear teeth and spacer flange pitting. In addition, several of the components (installed over the years by third-party parts manufacturers) were found unsuitable and not within Dresser-Rand OEM dimensions.
PMV accepted Dresser-Rand’s proposed solutions to repair the train which included shutting down the train to inspect and repair internal compressor components; measuring bearing clearances and compressor shaft run-out; inspecting laby seals; aligning the compressor and turbine, and aligning the suction and discharge piping to the compressor flanges; and stiffening the compressor supports and discharge lines. Upon inspecting the proximity vibrations system and bearing clearances on the 3M compressor, D-R recommended replacing it and supplied its OEM parts for the overhaul, capital spare parts and 3MX shafts.
When all was done, Dresser-Rand completed the agreed upon scope, aligned the train, commissioned it, and started it up within the 45-day deadline. The Houston, Texas service center was able to accommodate other major repairs to the 3M and 3MX rotors in a short period of time to avoid longer delays to the shutdown.
Moncayo concludes, “Meeting the project’s extraordinary time constraints was a result of many different functions working together within Dresser-Rand, including our Services team in Mexico (Sales, Proposals, Field Services, Reliability and Predictive Maintenance Team), Houston Service Center, Dresser-Rand Turbine Technology Services, Olean, N.Y. Operations upgrades and parts team, and our Technical Support Team in Venezuela.”
According to Moncayo, vibrations on the compressor train are now well below 1 mil.