Advantages and Disadvantages of Screw Pumps Paper

HISTORY  The screw pump was described by Archimedes by Archimedes on the occasion of his visit to Egypt. to  Egypt. This  This

suggests that the apparatus was unknown to the Greeks before Hellenistic before Hellenistic times.  A cuneiform inscription of  Assyrian  Assyrian king Sennacherib king Sennacherib (704 –681 BC) has been interpreted

by Stephanie by Stephanie Dalley to describe casting water screws in bronze some 350 years earlier. This is consistent with classical author Strabo, author Strabo, who  who describes the Hanging the Hanging Gardens as watered by screws.  In 1588 Sliding vane water pump technology is described by Italian engineer Agostino Ramelli in his book “The Diverse and Artifactitious Machines of Captain Agostino Ramelli,” which also included other pump and engine designs.

INTRODUCTION

Screw Pump  A screw pump is a positive-displacement pump that use one or several screws to

move fluids or solids along the screw axis. In its simplest form, a sin gle screw rotates in a cylindrical cavity, thereby moving the material along the screw's spindle.  Since there needs to be some clearance between the liner and the screws, it is possible for any fluid that is pumped to slip backwards into the pump to lower pressure zones. How Does it Work?

Diagram of the Screw Pump

 The fluid enters from the suction side of the pump and moves linearly along these intermeshing screws to the discharge side of the pump. The clearances between the screws and the liner are very small hence the fluid gains pressure while moving through the pump.

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Vane Pumps

 A positive-displacement pump that consists of vanes mounted to a rotor that rotates inside a cavity. In some cases, these vanes can have variable length and/or be tensioned to maintain contact with the walls as the pump rotates.  While vane pumps can handle moderate viscosity liquids, they excel at handling low viscosity liquids such as LP gas (propane), ammonia, solvents, alcohol, fuel oils, gasoline, and refrigerants. Vane pumps have no internal metal-to-metal contact and selfcompensate for wear, enabling them to maintain peak performance on these nonlubricating liquids. How does it Works?

 A slotted rotor is eccentrically supported in a cycloidal cam. The rotor is located close to

the wall of the cam so a crescent-shaped cavity is formed. The rotor is sealed into the cam by two side plates. Vanes or blades fit within the slots of the impeller. As the rotor rotates (yellow arrow) and fluid enters the pump, centrifugal force, hydraulic pressure, and/or pushrods push the vanes to the walls of the housing. The tight seal among the vanes, rotor, cam, and side plate is the key to the good suction characteristics common to the vane pumping principle.  The housing and cam force fluid into the pumping chamber through holes in the cam (small red arrow on the bottom of the pump). Fluid enters the pockets created by the vanes, rotor, cam, and side plate.  As the rotor continues around, the vanes sweep the fluid to the opposite side of the crescent where it is squeezed through discharge holes of the cam as the vane approaches the point of the crescent (small red arrow on the side of the pump). Fluid then exits the discharge port.

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TERMINOLOGY  Screw Pump-a pump in which the working pressure is created by means of screw-

shaped impellers in the vertical water column  Vane Pump

a pump in which a liquid is displaced through a shaped stator by a shaped rotor.  Archimedes Screw- a machine used for transferring water from a low-lying body of

water into irrigation ditches.  Two/Double screw pump-These pumps operates by using two intermeshing screws as

described above.  Three screw pump-They utilize one driving screw intermeshed with the other two

screws to create pressure and move fluid.  Shaft Seal - Component mechanical seals, industry-standard cartridge mechanical seals,

and magnetically-driven pumps.  Externals (head, casing) - Cast iron, ductile iron, steel, and stainless steel.  Vane, Pushrods - Carbon graphite.  End Plates - Carbon graphite  Packing - Available from some vendors, but not usually recommended for thin liquid

service

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ADVANTAGES AND DISADVANTAGES OF SCREW PUMPS ADVANTAGES  Wide range of flows and pressures  Wide range of liquids and viscosities 

Built-in variable capacity

 High speed capability for freedom of driver selection  Low internal velocities  Self-priming with good suction characteristics  High tolerance for entrained air and other gases 

Minimum churning or foaming

 Low mechanical vibration, pulsation-free flow, and quiet operation  Rugged, compact design - easy to install and maintain  High tolerance to contamination in comparison with other rotary pumps

DISADVANTAGES  Relatively high cost because of close tolerances and running clearances  Performance characteristics sensitive to viscosity change  High pressure capability requires long pumping elements

ADVANTAGES AND DISADVANTAGES OF VANE PUMPS ADVANTAGES  Handles thin liquids at relatively higher pressures  Compensates for wear through vane exte nsion  Sometimes preferred for solvents, LPG  Can run dry for short periods  Can have one seal or stuffing box  Develops good vacuum

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DISADVANTAGES

 Can have two stuffing boxes  Complex housing and many parts  Not suitable for high pressures  Not suitable for high viscosity  Not good with abrasives

ADITIONAL ADVANTAGES IN SCREW PUMPS + Slow Speed, Simple and Rugged design

Probably the main and overall advantage of a screw pump is its superb reliability. The simple design, open structure and slow rotation speed makes it a heavy-duty pump with minimal wear that operates for years without trouble. + Pumps raw water with heavy solids and floating debris

Because of the open structure and large passage between the flights a screw pump can pump raw sewage without the need for a coarse screen before the pump. Both floating debris and heavy solids are simply lifted up. This saves considerably on equipment costs for a coarse screen or maintenance! + Can run without water

A screw pump can operate even when there is no water in the inlet. Therefore it is not necessary to install expensive measures (level control etc.) to prevent ‘dry-running’’. The lower bearing does not need cooling. + No collection sump required = minimum head A screw pump ‘scoops’ the water directly from the surface and does not need a collection

sump. This keeps the pump head to a minimum. + Constant high efficiency with variable capacity

The efficiency-curve of a screw pump is flat on the top. Due to that efficiency characteristic, the screw pump offers even high efficiency when it works at 50% of its capacity.

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+ Pump capacity is self-regulating with incoming level

When incoming water-level goes down, at dry weather flow, the screw pump ‘automatically’ pumps less water. Ergo: no control system required to adapt pump performance.

+ 'Gentle handling' of biological flock

The activated return sludge on STP’s is a delicate biological substance. Because of the low rotational speed and large opening between the flights, screw pumps do not damage this biological flock (whereas the high-speed rotating centrifugal pumps will completely shred the biological flock). + Easy maintenance (no 'high skilled' staff required)

A screw pump requires very little maintenance. Compared to (submersed) centrifugal pumps it is next to nothing. Besides that, no ‘highly skilled’ maintenance staff are required which makes this type of pump very suitable for remote locations. +Long lifetime (> 20-40 years)

Screw pumps with typical lifetimes of between 20-40 years are not unusual.

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