The polypropylene unit in a large Gulf Coast petrochemical plant operates an extruder gear box powered by a 12,700-hp electric motor. The journal bearing was splash-cavity lubricated from a 20-gal. sump.
When extruding standard-density product, the bearing temperature would typically be 81°C. But the high torque required by heavier loading of the gear box when switching to high-density product made the motor work harder, raising the motor bearing temperature higher than 85°C, which resulted in an automatic shut-off of the motor. Extruder flow rates would have to be reduced by eight to 10 metric tons per hour to keep bearing temperatures below the critical level. The solution was to install an oil-circulating system on the bearing to improve cooling.
View more content on PlantServics.com
Before installing the circulating system, the plant tried to mitigate the problem by upgrading from the major-brand ISO 68 synthetic bearing oil being used to Synfilm GT 68 from Royal Purple (www.royalpurple.com) in hopes that its higher film strength would reduce temperatures. Synfilm GT is a polyalfaolefin (PAO) synthetic oil combining properties of premium turbine oils (water separation and long oil life) with non-corrosive high film strength technology. Severe-duty operations frequently produce conditions that cause the desired full-fluid-film lubrication to diminish to the point where mixed-film or even boundary lubrication occurs (the lubricated surfaces are only partially supported by a film of oil).
Under these conditions, the synthetic oil’s high film strength is able to reduce operating temperatures and reduce machine vibrations by restoring the desired lubrication mode.
The bearing lubricant change was made when running-standard density product and resulted in the bearing temperature falling from the historical 81°C to 75°C (an 11°F reduction). More importantly, during the high-torque runs that previously caused problems, full production rates could be maintained with the bearing temperature never exceeding 78°C. The additional 18,000 lbs. per hour of production achieved is valued at $0.20/lb., or $3,600 per hour. The total investment in the oil was $509.
Based on the efficiency improvement documented for the drive motor, the operating unit sought additional efficiency gains by changing the EP gear oil in the extruder gear box to Royal Purple Synergy gear oil. To document the improvement, motor power requirements were monitored with the flow rate to see if any gain could be determined from changing the oil in the gear box.
As shown in the figure, the oil change netted 3.1% energy savings, which represents $1.74 savings per hp per 1,000 hrs. x 12,700 hp = $22,105 energy savings for each 1,000 hours of operation. The plant’s cogeneration facility produces the majority of the electric power the plant uses at a cost of just $0.07/kWh. It would be reasonable to expect slightly higher savings on the higher-density processes.
This case study shows that lubricant selection can have significant economic impact on both the cost of operation and production output. Selecting lubricants that meet the equipment manufacturer’s recommendations in new equipment will maintain new equipment warranties. In older equipment, they provide a guideline for proper lubricant selection. However, most OEM recommendations are strongly influenced by the need to select lubricants that are widely available. And while following the manufacturer’s lubricant recommendation generally results in adequate lubrication, other options might provide significantly improved performance and greater economic value.