Rotary Pumps

Rotary pumps differ from reciprocating pumps in that they have rotating parts instead of reciprocating (back and forth) moving parts to move fluids. T

Rotary pumps differ from reciprocating pumps in that they have rotating parts instead of reciprocating (back and forth) moving parts to move fluids. There are many types of rotary pumps, such as external and internal gear pumps, lobe pumps and screw pumps. Rotary pumps are best suited for low to medium pressure applications. Because of their construction and operation, rotary pumps tend to have a more even discharge pressure than reciprocating pumps and require less maintenance. Most hydraulic systems use gear-type rotary pumps. Other rotary pump service includes compressor and bearing lubrication, engine oil pumping, and chemical injection. Screw pumps are usually used as transfer pumps because of their large capacities.

Gear pumps

There are two types of gear pumps; the external gear pump and the internal gear pump.

External gear pumps are probably the most common type of rotary pump used in the petroleum industry. Its construction is relatively simple when compared to other positive displacement or centrifugal pumps.

The parts of an external gear pump include the casing, gears, suction port, and discharge port. There are two gears: one is driven by a motor or other prime mover and is called the power or drive gear and the other gear is driven by the power gear and is called the idler or driven gear.

The fluid is picked up by the teeth of both gears. As these gears rotate, fluid is trapped in the spaces between the gear teeth and the wall of the pump casing. The fluid is carried to the discharge side of the pump and is prevented from leaking back to the suction side by the teeth of the two gears meshing together.

The pump casing is usually made of cast iron or steel; however, stainless steel or other materials may be used for corrosive service. The center section of the pump casing is machined flat on both surfaces. The internal portion of the center section is machined out to resemble a figure eight. Both suction and dis-charge ports are located at this center point of the pump. Two gear-type impellers are found in the figure eight area of the pump casing.

Both end sections of the pump casing are machined flat on the surface that contacts the center section. They have drilled holes which are fitted with sleeve bushings or bearings. These end caps are generally supplied with dowels or pins to ensure proper alignment. The outer ends of the pump maybe fitted with small cover plates. Usually a thin gasket is placed between all contacting surfaces to prevent fluid leakage.

There are three types of gear impellers used in the rotary gear pumps; spur, helical and her-ringbone. Although spur gears are generally used because they are cheaper and cost less to maintain, helical and herringbone gears are also used because they provide a smoother transfer of power which results in smoother fluid flow. Also, helical and her-ringbone gears are used in larger pumps which handle large volumes and operate at higher speeds.

Since gear pumps normally handle hydraulic fluids or oil, the bearings are sufficiently lubricated by the fluids being pumped. However, if the pump fluid is non-lubricating, seals are used along with some method of external lubrication. Shaft seals may be of the mechanical or packing gland type, depending on the fluid being pumped, pump design, or type of service.

External gear pumps can be operated in either direction without any special modification to the pump. By observing the rotation of the shafts, the suction side of an operating pump can be determined. To do this, you must remember that the fluid is being pumped around the outer edges of the gear impellers-not between them.

In contrast to the external gear pump, the internal gear pump has one gear rotating within the circumference of the second gear. The outer or external gear is larger than the internal gear. Depending on pump design either the external or internal gear can be the driver.

Fluid is trapped between the teeth of these gears and is moved to the discharge section of the pump. The seal formed by the two sets of the teeth meshing together forces the fluid out the discharge and prevents it from leaking back to the suction side of the pump.

A crescent, commonly called a spacer, keeps the two gears separated. In some models, the crescent is movable allowing the pump to be operated in either direction. The suction and discharge ports are usually elliptical or oval in shape to permit smooth discharge of the fluid.

The materials of construction in internal gear pumps are similar to external gear pumps. The driving gear is usually made of steel while the driven gear may be made of steel or some softer metal. The pump casing is made of cast iron or steel. Because the driving gear rotates around the driven gear, the internal gear pump casing has a more round, compact construction than the external gear pump casing.

Lobe Pumps

Lobe pumps are similar to external gear pumps in that fluid is moved from the suction side to the discharge side in spaces between the rotors and the pump casing. The term lobe comes from the rounded parts of the rotors, which permit the rotors to remain in contact and form seals as they rotate. The shape and smoothness of the rotors prevent one rotor from driving the other, so timing gears are used to synchronize and drive both lobes.

Screw Pumps

Another rotary type pump frequently used in the petroleum industry for pumping oil or other heavy fluids is the screw pump. The screw pump is able to pump large amounts of fluid with ease; however, they are expensive and subject to damage from abrasive materials.

Rotary screw pumps can be divided into two categories; single screw pumps and multiple screw pumps. Single screw pumps only have one rotor (screw) and are not commonly used for industrial service. Therefore, this discussion will concentrate on multiple-screw pumps.

In operation, these pumps have a power rotor (screw) which is driven by a motor or other prime mover. The power rotor in turn drives one or more other rotors, referred to as idler rotors, which are meshed with the power rotor. As the power rotor turns, fluid is drawn in one end of the pump. Since the threads of the power rotor and idler rotors mate, the fluid is forced along the threads to the discharge side of the pump.

All of the rotors are usually made of the same type of metal, such as high carbon steel, steel alloys, or non-corrosive metals like stainless steel. The casing of a rotary screw pump is usually made of cast iron or steel and machined internally to provide a close fit around the rotors. The power rotor shaft extends outside the casing with some type of seal to prevent leakage.

Idler rotor shafts do not extend outside the casing. The casing ends are drilled out to receive the rotor shaft bearings.

The bearings, which support the shaft, are usually sleeve or bushing-type bearings. They are either pressed onto the shaft or inserted in the casing ends. However, the bearings of some screw pumps are located in the center casing instead of the ends. Lubrication of the bearings is provided by the fluid being pumped.

• [accordion]
• 1. Rotary pump Types : Screw Pump, Gear Pump, Lobe Pump, Vane Pump
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• 2. Centrifugal and Rotary Pumps - Nelik
• 3. API 682 Pumps Shaft Sealing Systems for Centrifugal and Rotary Pumps
• 4. API 682 4th edition piping plans. Eagleburgmann