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Speed is ultimately limited by the applications/system capability to deliver flow to the pump inlet at a sufficient pressure to avoid cavitation. This is true of all pumps. Three screw pumps tend to be high speed pumps, not unlike centrifugal pumps. Two pole and four pole motors are most commonly used.

Slower speed may be necessary when dictated by large flows, very high viscosities or low available inlet pressures. High speed operation is desirable when handling low viscosity liquids since the idler rotors generate a hydrodynamic liquid film in their load zones that resists radial hydraulic loads, very similar to hydrodynamic sleeve bearings found in turbomachinery.

To achieve the highest pressure capability from three screw pumps, it is necessary to control the shape of the screws while under hydraulic load. Five-axis NC profile grinding accomplishes this best, through complete dimensional control and a high degree of repeatability. Opposed loading of the idler rotor outside diameters on the power rotor root diameter dictate that these surfaces be heat treated to withstand the cyclic stress.

Again, profile thread grinding produces the final screw contour while leaving the rotors quite hard, in the order of 58RC. This hard surface better resists abrasive wear from contaminants and extends the service life.

Because some three screw pump applications range to pressures of 4500-psi (310-bar), pumping element loading due to hydrostatic pressure can be quite high. With hydraulic balance, the forces are balanced in two planes such that bearing loads are minimal to increase operating life.

Single ended pumps use two similar but different techniques to accomplish axial hydraulic balance. The center screw, or power rotor, incorporates a balancing piston at the discharge end of the screw thread. The area of the piston is made about equal to the area of power rotor thread exposed to discharge pressure.

Consequently, equal opposing forces produce zero net axial force due to discharge pressure and place the power rotor in tension. The balance piston rotates within a close clearance stationary bushing, which may be hardened or hard coated to resist erosive wear. The drive shaft side of the piston is normally internally or externally ported to the pump inlet chamber. Balance leakage flow across this running clearance flushes the pump mechanical seal, which remains at nominal pump inlet pressure.

The two outer screws, idler rotors, also have their discharge ends exposed to discharge pressure. Through various arrangements, discharge pressure is introduced into a hydrostatic pocket area at the inlet end of the idler rotors.

The effective area is just slightly less than the exposed discharge end area, resulting in approximately equal opposing axial forces on the idler rotors. The idler rotors are therefore in compression. Should any force cause the idler rotor to move toward discharge, a resulting loss of pressure acting on the cup shoulder area or hydrostatic land area tends to restore the idler rotor to its design running position.

The upper view in Figure 6 shows a stationary thrust block (cross hatched) and a stationary, radially self locating balance cup. Discharge pressure is brought into the cup via internal passages within the pump or rotor itself. The lower view shows a hydrostatic pocket machined into the end face of the idler rotor. It, too, is fed with discharge pressure. The gap shown is exaggerated and is actually only a few thousandths of an inch.

For some contaminated liquid services, the hydrostatic end faces of the idler rotors are gas nitride hardened or manufactured from solid tungsten carbide and shrink fitted to the inlet end of the idler rotors. When the cup design is used, the cup inside diameter and shoulder area are normally gas nitride hardened. Both techniques are used to resist wear due to the fine contaminants.

In a radial direction, three screw pumps achieve power rotor hydraulic balance due to symmetry. Equal pressure acting in all directions within a stage or wrap results in no radial hydraulic forces since there are no unbalanced areas. The power rotor frequently has a ball bearing to limit end float for proper mechanical seal operation, but it is otherwise under negligible load. Idler rotor radial balance is accomplished through the generation of a hydrodynamic liquid film, in the same fashion as a journal or sleeve bearing.

The eccentricity of the rotating idler rotors sweeps liquid into a converging clearance, resulting in a pressurized liquid film. The film pressure acts on the idler rotor outside diameters in a direction opposing the hydraulically generated radial load (see diagonally opposing arrows indicating direction of loading).

Increasing viscosity causes more fluid to be dragged into the pressurized film, causing the film thickness and thus pressure supporting capability to increase. The idler rotors are supported in their respective housing bores on liquid films and have no other bearing support system. Within limits, if differential pressure increases, the idler rotor moves radially towards the surrounding housing bores.

The resulting increase in eccentricity increases the film pressure and maintains radial balance of the idler rotors. For high suction pressure applications (as in boost stations) special balancing techniques, such as changing the balance piston area or double extending the power rotor, can reduce radial forces to a minimum.

As versatile as three screw pumps are, they are still not suitable for some applications. While many advances in materials engineering are taking place, state-of-the-art three screw pumps have too great a galling tendency on very corrosion resistant materials, such as high nickel steels. A twin screw pump should be considered for corrosive liquid applications in order not to lose the efficiency advantage of screw pumps. 

Twin Screw Pumps: Principle Applications

Two screw or twin screw pumps can handle applications that are well beyond many other types of pumps, including three screw designs. Twin screw pumps are especially suited to very low available inlet pressure applications, and more so if the required flow rates are high.

Services similar to three screw pumps include crude oil pipeline, viscous product processing, synthetic fiber processing, barge unloading, fuel oil burner and fuel oil transfer. Unique applications include:

Table 1. Applications for Twin Screw Pumps