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

Centrifugal Pump with Journal Bearing

A journal bearing is essentially for a viscose pump, and it devices load capacity by pumping the lubricant through a small clearance region. To generate pressure, the resistance to pumping must increase in the direction of the flow. The journal moves to form a converging tapered clearance in the direction of the rotation or flow.

The eccentricity e is the total displacement of the journal from the concentric position. The attitude angle γ, is the angle between the load direction and the line of centres. Note, that because of the necessity to form a converging wedge, the displacement of the journal is not along a line that is coincident with the load vector. A positive pressure is produced in the converging region of the clearance. Downstream from the minimum film thickness which occur along the line of centres, the film become divergent. The resistance decrease in the direction of pumping, and either negative pressure occur or the air in the lubricant gasifies or cavitates and a region of atmosphere pressure occurs in the bearing area. This phenomenon is known as fluid film bearing cavitation. It should be clearly distinguished from other form of cavitation that take place in pumps, such as in the impeller, for example. The fluid is travelling at a high velocity and the inertia forces on each fluid element dominate. Implosion occur in the impeller and can cause damage.

In a bearing the viscose forces dominate and each fluid particle moves at a constant velocity in proportion to the net shearing forces on it. Thus cavitation on the bearing is more of a change of a phase of the lubricant that occurs in a region of lower pressure that permits the release of entrained gases. Generally bearing cavitation does not causes damage.

Bearing Types

The most common type of journal bearing in the plain cylindrical bushing. It can be split and have lubricating freed groove at the parting line. A ramification is to incorporate axial grooves to enable better cooling and to improve whirl stability. The principle advantages of cylindrical bearing are (1) simple construction ad (2) a high load capacity relative to other bearing configuration.

 

This type of bearing also has several disadvantages:

• Whirl instability: this is prone to sub synchronous whirling at high speed and also at low loads. Whirling is an orbiting of the journal (shaft) centre in the bearings, a motion that is superimposed upon the normal journal rotation. The orbital frequency is approximately half the rotating speed of the shaft. The expression half-frequency whirl is commonly used. The reason for the occurrence of this whirl and more details concerning bearing dynamics are presented in the section on bearing dynamics.
•  Viscose Heat Generation: because of the generally larger and uninterrupted surface area of this bearing, it generates more viscose power loss than some other types.
• Contamination: The cylindrical bearing is more susceptible to contamination problems than other because contaminants that are dragged in at the leading edge of the bearing cannot easily dislodge because of the absence of grooves or other escape paths.

 The advantage of simplicity and load capacity make the plain journal a leading candidate for most applications, but performance should be carefully investigated for whirl instability and potential thermal problems. Cylindrical bearings are generally used for medium speed (500 in/s (200 mm/sec) surface speed) and medium to heavy load application (250 to 400 lb/in2 (17 to 28 bar) on a projected area.

 

Cylindrical Bearing with Axial Grooves

 

A typical configuration of this type of bearing is a plain cylindrical bearing with four equally spaced longitudinal groove extending most of the way through the bearing. Usually, a slight land area exist at either end of the groove to force the inlet flow to each groove into the bearing clearance region, rather than out the groove ends. This configuration is a little less simple than the plain cylinder bearing, and because the grooves consume some land area, this configuration has less load capacity than the plain bushing. Since oil is fed into each of the axial grooves, this bearing requires more inlet flow but also will run cooler than the plain bushing. The grooves act as convenient outlets for any contaminant in the lubricant, and thus the grooved bearing can tolerate more contamination than the plain cylindrical bearing.

 

Empirical and Lobe Bearings

 

Elliptical and lobe bearing have noncircular geometries. As elliptical bearing is simply a two lobe bearing with the major axis along the horizontal axis. Thus at the leading edge region, a converging clearance produces positive pressure, but downstream from the minimum film thickness, a divergent film thickness distribution can be found with resulting negative, or cavitations, pressure.

 

The canted lobe, generally develop positive pressure throughout the lobe because the bearing is constructed with a completely converging film thickness in each lobed region. This design has excellent whirl resistance (superior to that of the symmetric lobe bearing) and a reasonable good load capability. A 2:1 ratio between leading and trailing edge concentric clearance is generally a reasonable compromise with respect to performance.

 

Elliptical and lobe bearing are often used because they provide better resistance to whirl than cylindrical configurations. They do so because they have multiple load producing pads that assist in preventing large attitude angles and cross coupling. Elliptical and lobe bearing are generally used for high speed, low load applications where whirls might be a problem.