Troubleshooting Sudden Pressure Drop Under Load in Screw Pumps

Troubleshooting Sudden Pressure Drop Under Load in Screw Pumps (Normal No-Load Operation)

Screw pumps are positive displacement pumps widely used in lubrication systems, oil transfer, chemical processing, and high-viscosity media transport. A common failure phenomenon is that the pump operates normally under no-load conditions, but when connected to the system and loaded, the discharge pressure drops sharply or fails to build up. This indicates a mismatch between internal pump capability and external system resistance, or the presence of leakage, suction instability, or mechanical degradation.

Internal Leakage Due to Wear or Clearance Expansion

One of the primary causes is increased internal clearance between rotors, stators, or screw elements.

When the pump is unloaded, resistance is low, so flow appears normal. However, under load, internal leakage increases significantly, preventing pressure buildup.

Under-load pressure collapse is a typical symptom of advanced internal wear and volumetric efficiency loss.

This condition is often gradual and may be misdiagnosed as pipeline issues.

Relief Valve or Bypass System Failure

If the relief valve is stuck open or incorrectly adjusted, the pumped medium is continuously returned to the suction side or tank.

Under no-load conditions, pressure may still appear normal, but once system resistance increases, the bypass flow dominates, causing pressure collapse.

Spring fatigue, contamination, or improper calibration are common causes.

Suction System Limitation and Air Ingress

Insufficient suction conditions become more critical under load.

Air leakage in suction piping, clogged filters, or excessive suction lift can prevent full chamber filling. Under load, this leads to incomplete displacement and rapid pressure drop.

Even minor suction-side air ingress can cause severe pressure instability when system backpressure increases.

Excessive Fluid Viscosity or Temperature Drop

High-viscosity fluids are highly sensitive to temperature.

When temperature decreases under operating load, viscosity increases sharply, causing excessive suction resistance and incomplete filling. Conversely, uneven heating may create unstable flow conditions.

This leads to pressure drop when system demand increases.

Gas Entrapment and Cavitation

Gas or vapor in the fluid compresses under pressure, preventing stable pressure transmission.

Under load conditions, gas expansion becomes more pronounced, resulting in sudden pressure collapse and irregular discharge.

Cavitation caused by insufficient inlet pressure can further worsen performance and damage internal components.

Rotor or Stator Degradation

In progressive cavity screw pumps, stator wear, swelling, or chemical degradation significantly reduces sealing efficiency.

At low load, the pump may still perform acceptably, but under higher backpressure, internal slip increases rapidly, causing pressure failure.

Rotor surface wear has a similar effect by increasing leakage paths.

Drive System or Speed Limitation

If the motor or frequency converter cannot maintain rated speed under load, the pump output will decrease.

Overload protection settings, voltage drop, or belt slippage may reduce effective torque and speed.

Load-dependent speed reduction directly results in insufficient discharge pressure.

Pipeline Backflow or System Leakage

System-side leakage or incorrect check valve installation may allow backflow when pressure increases.

This reduces effective discharge pressure even if pump operation appears normal.

Undetected system leaks are often mistaken for pump failure.

Diagnostic Procedure

A structured troubleshooting process is essential.

First, verify motor speed and electrical load. Second, inspect suction conditions and check for air leakage. Third, test relief valve operation. Fourth, evaluate fluid temperature and viscosity. Finally, assess internal wear through differential pressure and flow comparison.

Accurate diagnosis requires separating hydraulic system faults from mechanical degradation inside the pump.

Corrective Measures

Solutions include repairing or replacing worn internal components, recalibrating relief valves, improving suction conditions, stabilizing fluid temperature, eliminating air leaks, and ensuring stable drive system operation.

Severely worn pumps may require overhaul to restore volumetric efficiency.

Preventive Strategies

Regular monitoring of pressure-flow curves, maintaining stable suction conditions, controlling fluid temperature, and periodic inspection of wear components can effectively prevent recurrence.

Proper system design and correct pump sizing are also critical for avoiding overload-related pressure failure.

Conclusion

Sudden pressure drop under load in screw pumps is typically caused by internal wear, relief valve malfunction, suction instability, gas entrainment, temperature-induced viscosity changes, drive system limitations, or pipeline leakage. A systematic diagnostic approach combined with mechanical inspection and hydraulic optimization is essential for restoring stable performance. Maintaining volumetric integrity and stable suction conditions is the key to ensuring reliable load operation 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 Screw Pump Systems

4. Industrial Pump Reliability and Troubleshooting Guide

5. Machinery Condition Monitoring and Fluid System Engineering Manual