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Author: David Bazel, Sales Engineer
Within the power generation sector, screw pumps are commonly found in applications such as bearing and gear lubrication, high pressure fuel injection, fuel forwarding, jacking oil, and hydraulic power for hydro plant control. The 3-screw pump type from Leistritz is the most common for power generation applications and features a single hardened steel power rotor along with two idler rotors acting as rotating seals. The rotors operate on a thin product fluid film within the pump liner that functions as a large journal bearing, providing support and preventing metal-to-metal contact. Furthermore, an internal balance line acts to hydrodynamically balance the rotor set, nullifying any axial forces and eliminating the need for a thrust bearing. In fact, the 3-screw pump requires only a single bearing for positioning of the rotor set, and a single mechanical seal that is subject only to suction pressure.
One of the most important benefits of the screw pump in power applications is the fact that it is a positive displacement machine. It has a capacity rating based on pump geometry and speed of rotation and will efficiently deliver a nearly constant volume of critical lube oil or fuel oil flow regardless of fluctuations in back pressure. This means that machinery bearings, gears, or combustion systems will not be starved in the event of a disturbance such as a clogged inlet strainer.
The screw pump’s versatility in handling a wide range of viscosities is also noteworthy. It can efficiently handle very thin, low lubricity fuels such as kerosene, ultra low sulfur diesel, and bio-diesel, as well as viscous #6 heavy fuel oil. Additional benefits provided by the screw pump design for power applications include low noise, low vibration, low shear, tolerance for entrained air, and a non-pulsating flow.
Flexible configuration options optimize a screw pump’s location within a power plant. In addition to the standard horizontally baseplate mounted pump & motor, screw pumps can be horizontally flange mounted off a motor, vertically tank mounted, or vertically pedestal mounted. These direct motor drive configurations are possible because screw pumps operate at motor speeds and do not need the added complexity and cost of a speed reducer.
Certain screw pump models feature a pump cartridge that slides into a fabricated steel casing. This design simplifies maintenance as the cartridge can be removed without disturbing the pump mounting or piping connections, but also enables this pump type to be used as a drop-in replacement for older or obsolete pumps that may be very costly to maintain. For example, a 3-screw pump cartridge with custom fabricated casing can replace an older twin screw pump with timing gears, four bearings, and four seals, replicating the original pump’s flange, shaft, and mounting dimensions. Installation cost is low, pump cost is generally below that of an existing pump overhaul, and the plant benefits from the latest 3-screw pump technology.