What are the functions of Single Screw Pumps?

The Three Core Functions of Single Screw Pumps

A single screw pump (also known as a progressive cavity pump) performs three primary functions: metered flow of viscous fluids, low-pulsation transfer of shear-sensitive materials, and self-priming lifting of multi-phase mixtures. Unlike centrifugal pumps, a single screw pump delivers a constant, linear flow rate directly proportional to its rotational speed—typically achieving volumetric efficiencies above 95% across a wide pressure range.

This unique performance makes it indispensable in industries ranging from oil & gas to food processing and wastewater treatment. Below, we explore each function with measurable data and practical examples.

Function 1: Accurate Metering and Dosing of High-Viscosity Fluids

Single screw pumps function as positive displacement metering devices. Each rotation displaces a fixed volume (from 0.005 to over 2,500 liters) regardless of discharge pressure. Flow rate accuracy typically stays within ±0.5% to ±1% when properly sized.

Real-World Example: Polymer Injection in Oil Fields

In enhanced oil recovery (EOR), operators inject polymer solutions with viscosities up to 10,000 cP. A single screw pump doses at precisely 2.5 L/min against pressures of 40 bar (580 psi). Centrifugal pumps would lose efficiency above 1,000 cP and cannot maintain linear dosing. The result: consistent polymer injection reduces chemical waste by up to 15% annually per well pad.

• Viscosity range: 1 cP to 1,000,000+ cP (e.g., heavy crude, molasses, sludge)
• Flow rate linearity: R² > 0.999 from 10% to 100% motor speed
• Pressure capability: Up to 48 bar (700 psi) in single-stage, multistage versions exceed 240 bar (3,480 psi)

Function 2: Low-Pulsation, Shear-Sensitive Material Handling

The gentle pumping action of a single screw pump is one of its most valued functions. Unlike piston or lobe pumps, the fluid moves axially without turbulence or rapid pressure spikes. Shear rates are typically below 200 s⁻¹—comparable to low-shear mixing. This preserves product integrity for delicate materials.

Data on Shear Reduction

In a controlled test pumping yogurt cultures (a shear-sensitive biological fluid), a single screw pump maintained viable cell counts above 95% after 10 hours of recirculation. A lobe pump under identical conditions reduced viability to 72%. For food processors, this means longer shelf life and better product consistency.

Function 3: Self-Priming and Multi-Phase Mixture Lifting

Single screw pumps can lift fluids from suction lifts as high as 8.5 meters (28 feet) of water column under dry conditions, and 9.5 meters (31 feet) when primed. More importantly, they handle gas, liquid, and solid mixtures simultaneously—a function called multi-phase pumping. Gas volume fractions (GVF) up to 80% are possible without vapor lock.

Practical Example: Wastewater Scum Pumping

In a municipal treatment plant, scum contains 40% air, 55% water, and 5% grease/solids. A single screw pump transfers this mixture from a DAF (dissolved air flotation) tank to a digester at 12 m³/h. Centrifugal pumps lost prime within 10 minutes, while diaphragm pumps clogged every 2 hours. The single screw pump ran for 2,000 hours without failure, reducing maintenance costs by €8,000/year.

• Maximum solid particle size: up to 15 mm (depending on stator elastomer hardness)
• Self-priming dry run time: typically 30-60 seconds (with proper material selection)
• Multi-phase GVF limit: 0% to 80% without auxiliary equipment

FAQ: Common Questions About Single Screw Pump Functions

1. Can a single screw pump run dry?

Yes, but only for short durations (30-60 seconds). The stator (typically elastomer) requires fluid lubrication and cooling. Extended dry running damages the stator and rotor. For applications with intermittent dry periods, specify stators made of high-temperature elastomers (e.g., FKM or HNBR) or add a dry-run protection sensor.

2. What is the typical efficiency range?

Single screw pumps offer volumetric efficiency between 85% and 98% under normal operating conditions. Overall mechanical efficiency (including motor and gearbox) ranges from 55% to 75%, which is 10-20% higher than centrifugal pumps when handling viscosities above 1,000 cP.

3. How does flow rate change with viscosity?

Unlike centrifugal pumps, flow rate of a single screw pump remains nearly constant with viscosity changes. Doubling viscosity from 1,000 cP to 2,000 cP reduces flow by less than 2% at constant speed. This makes them ideal for dosing additives where feed viscosity varies with temperature.

4. What are the common failure modes affecting these functions?

1. Stator wear – reduces volumetric efficiency; typical lifespan 4,000-8,000 hours in abrasive slurries.
2. Rotor corrosion – leads to slip and pressure loss; use 316SS or duplex materials for pH < 4 or > 10.
3. Universal joint failure – causes pulsation; replace every 10,000-15,000 hours in continuous service.

5. What is the maximum pressure per stage?

A single-stage single screw pump typically delivers 6 to 12 bar (87 to 174 psi) differential pressure. Each additional stage adds similar pressure. For high-pressure applications like reverse osmosis feed (up to 40 bar), use a 4-stage or 6-stage configuration. Each stage contains one set of rotor-stator cavities.

Selection Guide: Matching Functions to Application Needs

To select the correct single screw pump for your function, evaluate three parameters against the table below:

For most industrial applications, the single screw pump’s combination of metering precision, low shear, and multi-phase capability makes it the most versatile positive displacement pump. When your fluid is too thick for centrifugal pumps or too delicate for piston pumps, a single screw pump is the functional answer.