Screw Pumps-Water

Water Treatment Primer

Theory

Screw pumps are a unique type of rotary positive displacement pump in which the flow through the pumping elements is truly axial. The liquid is carried between the screw threads on one or more rotors. The liquid is then displaced axially as the screws rotate and mesh. In other types of rotary pumps, the liquid is forced to travel circumferentially, however the screw pump has an axial flow pattern and low internal velocities.

Circumferential Flow

It provides a number of advantages in many applications where liquid agitation or churning is objectionable. Screw pumps are classified as two different types: the single rotor and the multiple rotor. The multiple rotor is further divided into timed and untimed categories. Timed rotors rely on outside means for phasing the mesh of the threads and for supporting the forces acting on the rotors. Untimed rotors rely on precision and accuracy of the screw forms for proper mesh and transmission of rotation (Fraser, et. al., 1986.).

History

The screw pump is the oldest type of pump. The first applications, dating back to the third century B.C., included irrigation and land drainage. The screw pump is thought to have been first used in Egypt (Ewbank, 1972). After several other types of pumps were invented, the screw pump was not used as much because these other pumps could handle higher head capacities. However, later it was found that these pumps could not handle wastewater like the screw pump could. Because of this, the screw pump became widely used for such an application. The Dutch were the first to design a spiral lift screw in 1955. After this, double screw units were put into operation for flood control in the Netherlands and in municipal sewage installations in Europe. Based on excellent results from the pumps used in Europe, the trend extended to Canada and United States and are currently used today (Cheremisinoff, et. al., 1992).

Applications

There are several applications of the screw pump that include a wide range of markets: utilities fuel oil service, industrial oil burners, lubricating oil service, chemical processes, petroleum and crude oil industries, power hydraulics, and many others (Fraser, et. al., 1986). Listed below are some typical situations where a screw pump is used. The benefits of using a screw pump in each of these situations are discussed (Cheremisinoff, et. al., 1992).

* Raw sewage lift stations: Can handle variety of raw sewage influent, are non-clogging, require little attention, are resistant to motor overloads, and are not affected by running dry

* Sewage plant lift stations: Used for sewage lifts up to 40 feet and have self-regulating lift capacity (Normal lifts are 30 feet, while high lifts are 40 feet high.)

* Return activated sludge: Little floc disintegration, nonturbulent discharge into effluent channel, low horsepower requirements, improved activated sludge treatment.

* Stormwater pumping: Are ideal because of large capacity at low heads, no prescreening necessary

* Land Drainage: Used for flood control, can pump large volumes of water over levees.

See a Screw Pump in Action!

Three Basic Types

Single Screw

The single screw pump is more commonly known as the Archimedean screw. It is quite large; typical dimensions include a diameter of 12 inches or greater, and a length up to about 50 feet. It is normally used as a water-raising pump with the screw arranged at an angle of 30 degrees. It can also be used for handling liquids containing solids in suspension with either vertical lift or horizontal transport. The design of single screw pumps allows very little fracturing of particles and little abrasion damage to the pump. One disadvantage is the considerable bulk necessary to achieve high capacities since rotational speeds are of the order of 30-60 rpm (Warring, 1984).

Intermeshing Screw Pump

The intermeshing screw pump is commonly called a rigid-screw pump. This type of pump is suitable for a wide range of sizes, and can be run at high speeds. The larger screw pumps are used for bulk handling of oils and similar fluids. The basic type is suitable for handling most clean fluids with low flow velocities and at low heads (Warring, 1984).

Eccentric screw pump

The eccentric screw pump is versatile. It is capable of handling a variety of liquids and products with high efficiency. It comprises of a rigid screw form rotor rolling in a resilient internal helical stator of hard or soft rubber with a moderately eccentric motion. It can handle viscous liquids, slurries, pastes, solids in suspension, and delicate products. This is because of the low flow velocities through the pump (Warring, 1984).

View Pictures of Other Screw Pumps!

Capacity

The delivered capacity of any screw pump is the theoretical capacity minus the internal leakage. In order to find the capacity of a screw pump the speed of the pump must be known. The delivered capacity of any rotary screw pump can be increased several different ways. The capacity can be increased by simply increasing the speed, increasing the viscosity, or decreasing the differential pressure. The capacity of the pump depends on several factors (Cheremisinoff, et. al., 1992):

* Diameter of the screw

* Speed of the screw

* Number of flights mounted on the screw shaft

* Flights: Single, double, and triple flights are often used. Flights are also known as helixes. With each increase in flights, there is a 20% increase in capacity. Therefore, a single flight pump has a capacity that is 80% of a double flight pump, which in turn has a capacity that is 80% of a triple flight capacity. The three-flight pump can handle the most capacity in the least amount of space.

* Angle of inclination of the screw

* The greater the angle of inclination, the lower the output. The output lowers approximately 3% for every degree increase over a 22° inclination.

* Level of influent in the influent chamber

* Ratio of the diameter of the screw shaft to the outside diameter of the screw flights

* Clearance between screw flights and trough

Advantages

1. Wide range of flows and pressures
2. Wide range of liquids and viscosities
3. Built-in variable capacity
4. High speed capability allowing freedom of driver selection
5. Low internal velocities
6. Self-priming with good suction characteristics
7. High tolerance for entrained air and other gases
8. Minimum churning or foaming
9. Low mechanical vibration, pulsation-free flow, and quiet operation
10. Rugged, compact design -- easy to install and maintain
11. High tolerance to contamination in comparison with other rotary pumps (Fraser, et. al., 1986)

Disadvantages

1. Relatively high cost because of close tolerances and running clearances
2. Performance characteristics sensitive to viscosity change
3. High pressure capability requires long pumping elements (Fraser, et. al., 1986)

REFERENCES

* Cheremisinoff, Nicholas P. and Paul N. Cheremisinoff. Pumps and Pumping Operations. Englewood Cliffs: Prentice Hall, Inc., 1992.
* Ewbank, Thomas. A Description and Historical Account of Hydraulic and Other Machines for Raising Water. New York: A New York Times Company, 1972.
* Fraser, Warren H., Igor J. Karassik, and William C. Krutzch. Pump Handbook. New York: McGraw-Hill Book Company, 1986.
* Warring, R.H. Pumps: Selection, Systems and Applications. Houston: Gulf Publishing Company, 1984.

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