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Screw Pump Guide

Screw pumps are a positive displacement type of design where a set of screws rotate in a 360° rotary motion within a casing.

Fluid enters the inlet before being transferred to the outlet via cavities between intermeshing screws. Due to the tight clearances slip is extremely limited, and mechanical action is highly efficient.

Screw Pump Design

There are typically three design types:

Single Screw

This is usually another name for a progressing cavity pump where a single screw rotated by a motor rotates within a stator. This is covered in our progressing cavity pump guide.

Twin screw / quadruple screw with external Timing Gear

A set of twin screws rotate within a pump casing with screws situated side by side. Fluid is drawn by both screws at the inlet before being pushed towards the outlet in the centre or top of the casing. Twin Screws intermeshing

One screw is driven via the motor, with the other rotated by external timing gears at the opposite end of the unit. Screws can be mounted in pairs meaning up to 4 screws can be in one pump. All screws mesh together ensuring fluid travels along the screws from the inlet towards the outlet.

Twin Screw Pump Design
Triple screw

Triple Screw designs consist of a driving screw and two idler screws. The motor shaft rotates the driving screw which in turns rotates the intermeshing twin idler screws.

Pumps can be driven by a motor coupled to a gearbox, or directly coupled to the head enabling it to be driven at speeds of up to 3600rpm.

The lower the viscosity of fluids being pumped, the higher speeds at which components can be rotated at. Higher viscosity fluids such as Heavy Fuel Oil, molasses, bitumen or other slow flowing viscous liquids must be handled at reduced speeds to enable fluid sufficient time to enter the pumps inlet and ensuring cavitation does not occur. Lowering rotational speed also assists with the NPSH required by pumps.

Screw Pumps Advantages

Efficient

Screw pumps are known to be efficient, due to clearances within the pump being fine. A gearbox is not always required, meaning mechanical efficiency is one of the highest when compared to other pumps such as gear or vane requiring such accessories.

Due to the design of internal parts, they can operate at high speeds due to screws suffering from low inertia during fluid transfer.

2 Idler Screws, 1 Driving ScrewSelf Priming

Due to their design they are self-priming up to 7.5M. Providing screws are lubricated prior to startup, units can be ran dry for a limited amount of time.

Non Pulsating & Proportional Flow

Due to their rotary motion, fluid delivery is non pulsating. Furthermore, due to its design flow is proportional to speed enabling a predictable rate of flow to be achieved.

Reversible

Their design means units can be ran in reverse, with the exception that the relief valve will only operate in one direction. 

Triple Screw Pump DesignIntegrated Relief Valve

Units are typically fitted complete with a relief valve protecting the pump from damage should outlet pipework become blocked, limiting the pumps ability to generate excessive pressure in such cases.

Dry Running

Providing screws are lubricated, units are capable of dry running making them suitable for tank stripping as they can also handle small amounts of entrained gas and air.

Low vibration and noise

Due to their efficient mechanical movement, pumps are subject to low amounts of vibration and noise.

NPSH

Screw pumps NPSH requirement can be as little as 1.5M with designs available for immersion in fluids where viscosity is high or NPSH available is low. 

Wide viscosity handling

Due to the ability to alter pump speed through a gear box, pumps can handle a wide range of viscosities up to 35,000cst with changes in fluid viscosity usually having little effect on pumped flow rate.


Screw Pumps Disadvantages

Solid Handling – Models are not suitable for abrasive solid handling which can shorten screw life due to the tight clearances and abrasive affect. Coatings can be applied to reduce wear, but any hard solids >1mm can not be accommodated comfortably. Soft products such as polymerized rubber, mince, molasses, yogurt and Jams can be handled by such units without issue.

Part Replacement – Internal parts can be expensive to replace, with idler and driving screws needed to be replaced as one to ensure parts intermesh without issue and efficiency is maintained.

Applications

Units can be assembled in a variety of materials from Cast Iron, to ductile, Stainless Steel, Bronze and Duplex Steels enabling them to be used with a large number of liquids across industry which include processes such as:

Industrial: Fuel transfer, lubrication transfer, chemical offloading and transfer

Marine: Lube oil systems, fuel transfer, sludge transfer, bilge pumping, oily water separator feed, cargo loading & offloading, burner feed for inert gas generator

Oil & Gas: Bitumen transfer, lubricant transfer, Benzene, phenol & Toulene production, mud transfer.

Chemical: Resin transfer, polymer transfer, Chemical transfer

Food: Handling of dairy products, meat, creams, fudge and liquid sugar mixtures, syrups, juices concentrates, wine, oils honey greases, mousse and mayonnaise

Pharmaceutical – cosmetic, personal care and biopharma product transfer

Screw Pumps vs other designs

Our comparison tables below detail how this type of design compares to other pump technologies:         

Screw Pump VS Gear Pump

Non Pulsating & High efficiency

Pulsating flow and less efficient

Can be operated without gearbox. Smaller footprint. Screws are designed   to operate up to 3600rpm

Require gearbox. Gear teeth efficiency limited by speed.

Internal parts require precise machining

Internal parts are cheaper

 

Limited to around 35,000cst

Higher range of viscosity handling – up to 55,000cst

 



Screw Pump VS Progressing Cavity

Non pulsating flow

Pulsating flow

Designs can be assembled without rubber parts enabling the pump to be   used with solvents and chemicals

Pump contains a large stator which is manufactured from rubber   meaning the unit can not be used with solvents and certain chemicals

Viscosity handling limited to 35,000cst

Handle up to 1Mcst

Fluids must be relatively clean

Handle Solids

Compact Design

Long footprint

Machined Screws can be costly

Part cost can be lower through use of plated rotor, with hollow   rotors used for lighter duties.

Wide Temperature range handling

Temperature must be within specified band due to stator clearances   being tight. Stator can swell at high temperatures.

 

Screw Pump Vs Vane Pump

Higher flows available.

Limited models available and operating speed limited by gearbox. Vanes   can not operate >900rpm

Pressures up to 20 bar

Typically limited to 15 bar

Can handle viscosities up to 35,000cst

Limited viscosity handling to 10,000cst

Part cost higher due to part tolerances and tight clearances required   for intermeshing. Efficiency lost as screws wear

Vane replacement is easy. Vanes self-compensate for wear



Screw Pump vs Diaphragm Pump

Higher Pressure Handling – up to 20 bar

Limited to 8 bar

No Valves which can clog

Valves can clog internally. Valves can open if back pressure is too   higher

Clean fluids only

Solid handling

Spare part replacement costly

Spare parts relatively cheap

Built in relief valve for pressure relief

No built in relief valve

Cleaning in Place (CIP) Friendly. Not easy to disassemble

Easy disassembly and CIP Friendly designs avaiable



Screw Pump Vs Piston Pump

Absence of valves means clogging is not an issue

Valves can clog

Handle up to 35,000 cst

Up to 500cst

Max Flow 1200M³H

Max Flow 64M³H

Max Head 1200M Head

Max Head 3450M

Flow is non pulsating

Flow pulsates



Screw Pump VS Lobe Pump

Max Flow 1200M³H

Max Flow 300M³H

Higher pressures   available – 1200M Head

Max Head 100M

Solids must be less   than 1mm if hard, but soft solids can be larger

Handle solids up to   50mm

Very short dry   running periods accepted

Higher Dry Running   capability especially when fitted with lubricated seal meaning can potentially   run dry indefinitely

Always Self Priming

Non Self Priming   with Stainless Lobes

 

Screw pump P&ID Symbols:


Screw Pump P&ID SymbolScrew Pump P&ID Symbol2

Having Issues with your pump? View our troubleshooting guide.

View our complete range of Screw Pumps


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