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Wednesday, June 30, 2010

Screw Pump Theory

Screw pump are a special type of rotary positive displacement pump in which the flow through the pumping elements is truly axial. The liquid is carried between screw threads on one or more rotors and is displaced axially as the screws rotate and mesh. In all other rotary pumps, the liquid is forced to travel circumferentially, thus giving the advantages in many applications where liquid agitation or churning is objectionable.

The application of screw pumps cover a diversified range of markets including navy, marine and utilities fuel oil services; marine cargo; industrial oil burners; lubricating oil services; chemical process; petroleum and crude oil industries; power hydraulics for navy and machine tools; and many others. The screw pump can handle liquids in a range of viscosities, from molasses to gasoline, as well as synthetic liquids in a pressure range from 50 to 5000 lb/in2 (3.5 to 350 bar) and flows up to 8000 gal/min (1820 m3/h).

Because of the relatively low inertia of their rotating parts, screw pumps are capable of operating at higher speeds than other rotary or reciprocating pumps of comparable displacement. Some turbine attached lubricating oil pumps operate at 10,000 rpm and even higher. Screw pumps, like other rotary positive displacement pumps, are self priming and have is sufficient viscosity in the liquid being pumped.

Screw pumps are generally classified into single or multiple rotor types. The latter is further divided into timed and untimed categories.

The single screw or progressive cavity pump has a rotor thread that is eccentric to the axis of rotation and meshes with internal threads of the stator (rotor housing or body). Alternatively, the stator is made to wobble along the pump centreline.

Multiple screw pumps are available in a variety of configurations and designs. All employ one driven rotor in a mesh and one or more sealing rotor. Several manufactures have two basic configurations available: single end and double end construction, of which the latter is the better known.

As with every pump type, certain advantages and disadvantages can be found in a screw pump design. These should be recognized when selecting the best pump for a particular application. The advantage of a screw pump design are as follows:
  • A wide range of flows and pressures
  • A wide range of liquids and viscosity
  • High speed capability, allowing the freedom of driver selection
  • Low internal velocities
  • Self priming with good suction characteristics
  • A high tolerance for entrained air and other gases
  • Low velocities for minimum churning or foaming
  • Low mechanical vibration, pulsation-free flow, and quiet operation
  • A rugged, compact design that is easy to install and maintain
  • High tolerance to contamination in comparison with other rotary pumps
The disadvantage are as follows:
  • Relatively high cost because of close tolerance and running clearances
  • Performance characteristics sensitive to viscosities changes
  • High pressure capabilities requires long pumping elements

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