The positive displacement technology handles backpressure fluctuations, cavitation and air entrainment.
by David Bazel
December 21, 2015

Rotary positive displacement screw pumps are used by a wide range of industries, including oil and gas production, pipelines, refineries, terminals, chemical processing, marine and power generation. Specifically in 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.

Image 1. Re-engineered drop-in replacement pumpImage 1. Re-engineered drop-in replacement pump (Images and graphics courtesy of Leistritz Advanced Technologies Corp.)

Screw pumps can excel in varied industries and services because of their design. They are relatively simple, low-maintenance machines with few moving parts. They can efficiently pump a fixed volume of product within a wide range of capacity, pressure and viscosity.

The three-screw pump type is among the most common for power generation applications. It features a single hardened-steel power rotor that performs the bulk of the pumping action, along with two idler rotors—typically hardened steel or cast iron—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.

Additionally, an internal balance line acts to hydrodynamically balance the rotor set, nullifying any axial forces and eliminating the need for a thrust bearing. The three-screw pump requires a single bearing for positioning the rotor set and a single mechanical seal subject only to suction pressure.

Screw pumps tend to have a record of high reliability and low maintenance. Available casing materials include aluminum alloy, cast iron, ductile iron, steel and stainless steel. Integral relief valves are offered on certain models, directing overpressure flow either back to suction or to an external bypass line.

Operating as a positive displacement machine is one of the screw pump's top benefits in power applications. It has a capacity rating based on pump geometry and rotation speed, minus a small amount of product slip. The actual slip is a function of rotor/liner clearance, differential pressure (increasing as pressure increases) and product viscosity (decreasing as viscosity increases).

Image 2. Vertical pedestal-mounted pump/motorImage 2. Vertical pedestal-mounted pump/motor
Image 3. Vertical tank-mounted pump/motorImage 3. Vertical tank-mounted pump/motor

Unlike a centrifugal pump in which pressure increases result in reduced flow—as well as movement away from the best efficiency point (BEP)—a screw pump will efficiently deliver a nearly constant volume of critical lube oil or fuel oil flow regardless of fluctuations in backpressure. As a result, 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 biodiesel, as well as viscous No. 6 heavy fuel oil. By comparison, a centrifugal pump's efficiency drops as viscosity increases. A screw pump can be ideal for operators working with heavy fuels or high-viscosity gear oil and for applications using a common pump for varying grades of fuel oil.

Air entrainment and cavitation are other key areas where screw pumps have advantages over other technologies. Air entrainment involves air or vapor bubbles in the entering fluid, while cavitation refers to vapor bubbles that form in the pump's low-pressure section because of insufficient suction pressure and implode in the high-pressure section. Centrifugal pumps can have difficulty handling these issues. Capacity drops significantly—2 percent entrained air can reduce flow by about 10 percent—and the resulting noise, vibration and possible damage to the seal, shaft and impeller present serious challenges to the operator. In severe cases, excessive entrained air can cause a centrifugal pump to vapor lock or lose its prime and ability to pump.

Three-screw pumps are self-priming machines with a significant ability to tolerate entrained air and cavitation, and they are not vulnerable to potential vapor lock. Also available are deaeration features that act to smooth the pressure rise within the pump, preventing the sudden implosion of bubbles that would occur during an otherwise rapid pressure spike.

Additional benefits provided by the screw pump's design for power applications include low noise, low vibration, low shear and a non-pulsating flow. The virtually pulsation-free flow eliminates the need for pulsation dampeners, simplifies piping system design and ensures a smooth and steady fuel supply to sensitive combustion systems. A screw pump's low-shear effect comes from its gentle axial movement of product from suction to discharge. For lube systems with water contamination, low shear prevents the formation of emulsions and their negative impact on downstream filters, strainers and machinery.

Flexible configuration options can optimize a screw pump's location within a power plant. Screw pumps can be horizontal baseplate-mounted with the motor, flange-mounted off a motor horizontally or tank- or pedestal-mounted vertically.

The vertical tank-mount design saves space as the pump, with axial inlet and strainer, sits submerged inside the oil tank. Discharge flow is piped above the tank through a mounting plate, which also supports the vertical motor and relief valve.