Causes of Reduced Flow and Insufficient Discharge in Screw Pumps

Causes of Reduced Flow and Insufficient Discharge in Screw Pumps

Screw pumps are widely used for transferring viscous, lubricating, and shear-sensitive media such as oil, resin, sludge, syrup, and chemical solutions. As a type of positive displacement pump, they are designed to deliver a nearly constant flow rate. However, in practical operation, users may encounter reduced flow and insufficient discharge. This issue is usually the result of hydraulic leakage, mechanical wear, suction limitations, or system design problems rather than a single failure point.

Increased Internal Wear and Clearance Expansion

One of the most common reasons for flow reduction is internal wear.

As the rotor, stator, or screw elements wear over time, internal clearances increase. This leads to higher internal leakage, allowing fluid to flow backward from the discharge side to the suction side.

Increased internal clearance directly reduces volumetric efficiency, resulting in a noticeable drop in output flow even if pump speed remains unchanged.

This condition develops gradually and is often mistaken for system blockage or suction issues.

Suction Side Restrictions and Poor Inlet Conditions

Screw pumps rely on stable inlet conditions to maintain continuous filling of the pumping chambers.

If the suction pipeline is partially blocked, too long, or too small in diameter, inlet resistance increases and the pump cannot fully fill its cavities. This results in reduced effective displacement.

Air leakage at suction joints can also disrupt continuous flow and cause intermittent discharge.

Fluid Viscosity and Temperature Effects

Fluid properties significantly influence screw pump performance.

When viscosity increases due to low temperature, flow resistance rises sharply. The pump may struggle to draw sufficient fluid into the chamber, especially during startup.

Excessively high viscosity or low temperature can significantly reduce suction efficiency and apparent discharge flow.

Conversely, very low viscosity may increase internal leakage and reduce effective output.

Rotor and Stator Wear or Damage

In progressive cavity screw pumps, the rotor and stator are key components that form sealed cavities.

Wear, deformation, or chemical degradation of the stator elastomer can reduce sealing effectiveness. Similarly, rotor surface damage affects the integrity of the pumping chamber.

This leads to internal slip and reduced volumetric output.

Speed Reduction or Drive System Issues

Pump flow rate is directly proportional to rotational speed.

If the motor speed is reduced due to frequency converter settings, belt slippage, or electrical issues, the output flow will decrease accordingly.

Incorrect speed control settings are a common and often overlooked cause of reduced pump capacity.

Air Entrapment and Gas Content in Fluid

Air or gas mixed in the pumped fluid can significantly affect performance.

Gas compressibility reduces effective displacement and causes irregular discharge. In severe cases, the pump may lose prime or operate inefficiently.

This is especially common in systems with poor suction sealing or high turbulence.

Relief Valve or Bypass Malfunction

Many systems include a relief or bypass valve to protect the pump from overpressure.

If this valve is stuck open or improperly adjusted, a portion of the flow may be diverted back to the suction tank, reducing net output.

Contamination or spring fatigue can also affect valve operation.

Pipeline Leakage or System Backflow

Leakage in discharge pipelines or improper check valve installation can lead to backflow.

This reduces effective discharge volume and creates the impression of pump underperformance.

System pressure balance should always be verified during troubleshooting.

Diagnostic Approach

A systematic diagnosis should include checking rotational speed, suction pressure, fluid temperature, pipeline integrity, and internal wear condition.

Measuring differential pressure and flow rate trends helps distinguish between mechanical wear and system-related issues.

Proper diagnosis requires separating hydraulic problems from mechanical degradation to avoid unnecessary component replacement.

Preventive Measures

Regular inspection of wear components, maintaining proper suction conditions, controlling fluid temperature, and ensuring correct speed settings are key preventive strategies.

Installing filtration systems also helps reduce abrasive wear and prolong pump life.

Conclusion

Reduced flow and insufficient discharge in screw pumps are typically caused by internal wear, poor suction conditions, fluid property changes, drive system issues, or bypass valve malfunctions. A structured diagnostic approach combined with proper maintenance and system optimization can effectively restore performance. Maintaining stable inlet conditions and controlling internal wear are the most important factors for ensuring long-term volumetric efficiency in screw pump systems.

References

1. Pump Handbook, Fourth Edition, McGraw-Hill Education

2. Hydraulic Institute Standards for Positive Displacement Pumps

3. API Recommended Practices for Progressive Cavity Pumps

4. Industrial Screw Pump Operation and Maintenance Manual

5. Machinery Reliability and Fluid Handling Engineering Guide