Screw Pumps (Pump Handbook)

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Screw Pumps (Pump Handbook)

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SECTION 3.7 SCREW PUMPS G

J . R . . J . C J . K . L A . J

B R Z A IP P . P

E N N R N I N C R A N

A N E C K I O T T G

Screw pumps 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 (see Figure 1). In all other rotary pumps, the liquid is forced to travel circumferentially, thus giving the screw pump with its unique axial flow pattern and low internal velocities a number of advantages in many applications where liquid agitation or churning is objectionable. The applications 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 processes; 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 a delivery flow characteristic, which is essentially independent of pressure, provided there 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 (see Figure 2) 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 centerline. 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 rotors. Several manufacturers have two basic configurations available: single-end (in Figure 3) and double-end (in Figure 4) construction, of which the latter is the better known.

3.99

3.100

CHAPTER THREE

F I G

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E

1

Diagrams of screw and gear elements, showing (a) axial and (b) circumferential flow.

F I G

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2

The single-screw or progressive cavity pump

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F I G

U

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E

3

Multiple-screw single-end arrangement.

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 a d v a n t a g e s of a screw pump design are as follows: • A wide range of flows and pressures • A wide range of liquids and viscosities • 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

3.7 SCREW PUMPS

F I G

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4

3.101

Mutliple-screw double-end arrangement.

• 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 d i s a d v a n t a g e s are as follows: • A relatively high cost because of close tolerances and running clearances • Performance characteristics sensitive to viscosity changes • High pressure capability requires long pumping elements

THEORY ____________________________________________________________ In screw pumps, it is the intermeshing of the threads on the rotors and the close fit of the surrounding housing that creates one or more sets of moving seals in a series between the pump inlet and outlet. These sets of seals or locks, as they are sometimes referred to, act as a labyrinth and provide the screw pump with its positive pressure capability. The successive sets of seals form fully enclosed cavities (see Figure 5) that move continuously from inlet to outlet. These cavities trap liquid at the inlet and carry it along to the outlet, providing a smooth flow.

Delivery

Because the screw pump is a positive displacement device, it will deliver a definite quantity of liquid with every revolution of the rotors. This delivery can be defined in terms of displacement volume VD, which is the theoretical volume displaced per revolution of the rotors and is dependent only upon the physical dimensions of the rotors. It is generally measured in cubic inches (cubic millimeters) per revolution. This delivery can also be defined in terms of theoretical capacity or flow rate Qt measured in U.S. gallons per minute (cubic meters per hour), which is a function of displacement and speed N: Qt 

In SI units:

Qt 

VDN

In USC units:

231 6 

10

8

VDN

If no internal clearances existed, the pump’s actual delivered or net flow rate Q would equal the theoretical flow rate. Clearances, however, do exist with the result that whenever a pressure differential occurs, there will always be internal leakage from outlet to inlet. This leakage, commonly called slip S, varies depending upon the pump type or


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