Get Up to Speed on Gear Pumps

These positive-displacement devices can provide a variety of benefits

By Amin Almasi, rotating equipment consultant

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Gear pumps rely on two meshing gears to cause liquid flow. They are precision machines with extremely tight fits and tolerances, and are capable of working against high differential pressures. They come in two types: two gears with external teeth or one gear with external teeth within another with internal teeth.

The most common uses for gear pumps are to move chemical and petrochemical liquids that have relatively high viscosity; to supply fuel oil for burners or other facilities; and to transfer gasoline, kerosene, fuel oil and diesel oil. They also feature in hydraulic devices such as actuators, damper controls and elevators. In addition, they pump coolants, paints, bleaches, solvents, syrups, glues, greases, asphalt, petroleum and lubrication oils, and handle many general industrial applications.

Gear pumps, when properly designed and engineered, can offer many advantages. These include compactness, simplicity, easy serviceability, bidirectional and pulseless flow, self-priming, low net positive suction head requirement, high mean time between maintenance, high-pressure and high-temperature capability, precise and accurate metering, and availability in multiple seal configurations or with sealless magnetic drives.

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The pumps can tolerate suspended small solids in abrasive applications but gradually will wear and lose performance.

Gear pumps are available in a variety of materials of construction, including cast iron, cast steel, stainless steel, high-nickel alloys and lightweight advanced aluminum alloys. They usually come in numerous optional designs, such as close-coupled, abrasion-resistant and API-standard compliant.

External Gear Pumps

In external gear pumps, a casing encloses two meshing gears with external teeth (Figure 1). A rotating shaft drives one gear, which then moves the other gear. The turning of the gears causes liquid to enter the inlet (suction) port, flow into and around the outer periphery of the gears, and then exit through the outlet (discharge) port. The size of the cavity (volume) between the teeth and the speed of the gears regulates this flow. The amount of liquid that slips back to the inlet port also affects the discharge flow. The degree of slip depends upon the side clearance of the gears to the casing, the peripheral clearance of the gear and bore in the casing, gear-to-gear clearance, developed pressure and the viscosity of the liquid — the lower the viscosity, the greater the slippage.

Most external gear pumps use spur, helical or herringbone gears. The helical and herringbone gears deliver more flow and higher pressure. They are quieter than spur gears but may require more net inlet pressure.

As very rough indication for common applications, the rated performance range is 1–500 m3/h with discharge pressures of 3–90 Barg (higher pressures are possible) and power ratings of 0.3–300 kW. Small external gear pumps frequently operate at four-pole motor speeds (1,800 rpm) and have operated at two-pole speeds (3,600 rpm). As the pump capacity per revolution goes up, speed usually is reduced; it could be less than 500 rpm. Operating speeds and flow rates generally are decreased as the liquid viscosity increases.

Internal Gear Pumps

In internal gear pumps, a gear with internal teeth meshes with a gear with external teeth (Figure 2). These pumps may come with or without a crescent-shaped partition. Designs are available to provide the same direction of flow regardless of the direction of shaft rotation.

The gears demeshing on the inlet side draw liquid into the pump. The mechanical contacts between the gears form a part of the moving seal between the inlet and outlet ports. The meshing of the gears forces the liquid out the discharge port.

The pumps’ relatively low speed and inlet pressure requirements often make them more-efficient alternatives to centrifugal pumps, especially as liquid viscosity increases. These designs have proven reliable, simple to operate, and easy to maintain.

As a rough indication, commercially available internal gear pumps can provide flows from 1–400 m3/h and discharge pressures to 40 Barg (sometimes more). The pumps typically contain at least one bushing in the liquid. They can be damaged when pumping large solids.

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