Three screw pumps are commonly used as lube oil pumps for compressors, gear boxes, turbines and other rotating equipment. Their popularity stems from the efficient, reliable and long lasting service they provide. While most lube oil pumps are only exposed to ambient pressure at the pump inlet port there are some applications where the pump sees high inlet pressures. This poses a huge challenge for the pump seal solution and pump construction. In many pump designs it can also lead to high bearing and seal loads that negatively affect the pump and seal life. High inlet pressure applications for lube oil pumps can be found in flooded screw compressors and turbo expanders.
Three screw pumps are often driven by an electric motor which is connected to the shaft of the main screw. The shaft exits through the pump casing. Therefore the main screw must be sealed to prevent leakage. Most common sealing solution is a mechanical seal. Due to the high inlet pressure, only special sealing arrangements, often with balanced mechanical seals, can achieve this task. The pump cost is driven up even further.
The pressure of the lube oil entering the pump is at the same level as the compressed gas which can reach 100 bar or more. Such high inlet pressures can create high axial forces on the main pump drive screw (power rotor) that lead to high bearing loads. This needs to be considered in the pump design. Reinforced bearings, liquid channels for hydraulic balance, mechanical bracing and other measures tend to significantly increase the pump cost.
There is an innovative new approach to high inlet pressure applications that is able to meet the technical challenges and even provides lower cost solutions: Magnetically coupled lube oil pumps.