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