3.4.2 Screw pump

Screw pump

The most common type of screw pump has three rotating screws, figure 3.42a. The central screw is the driver whilst the basic function of the two idler screws is to provide sealing and are driven largely by fluid pressure. The screw helixes form a number of sealed elements with the pump casing which transports the fluid axially when the screw rotates. The fluid flow is smooth without disturbing pulsations. The screw pump is quiet running and can be run with a direct drive at 1400 r/min or 2800 r/min. By constructing pumps with longer screws more sealed elements can be obtained resulting in greater pressure increases.

Figure 3.42a Screw pump with three screws. Note the pressure relief valve on the pressure side of the pump.

Screw pumps are used primarily for oil, which should be free from contaminants. They are self-priming but must not be run dry as this can cause damage to the bearings. Axial force is taken up partly by hydraulic balancing and partly by thrust bearings. With regard to the construction of thrust bearings, screw pumps are normally designed for one direction of rotation and flow. Some manufacturers supply screw pumps with two screws and an external synchronising gear. In this case fluids containing small quantities of solid particles can be handled.

Screw pumps are made from grey cast iron and carbon steel. Special surface treatment of components can improve wear resistance. Shaft sealing is by means of stuffing boxes or mechanical seals.

Screw pumps are available for volume flows ranging from 0,3 m³/h to about 1000 m³/h with normal pressure increases of up to about 3 MPa and of up to 30 MPa for high pressure models. Viscosity should not be too low and should be in excess of 5 to 10 mm²/s.

The maximum permissible viscosity is about 1000 mm²/s. Screw pumps can be used for fluid temperatures ranging from minus 40°C to plus 90°C and up to 250°C with increased internal clearances.