1. Definition and Key Impacts of Impeller Gap

The impeller gap refers to the radial clearance between the impeller and the pump casing (or guide vane ring), typically ranging from 0.2 mm to 0.5 mm. This gap significantly affects the performance of  multistage vertical turbine pumps in two main aspects:

● Hydraulic Losses: Excessive gaps increase leakage flow, reducing volumetric efficiency; excessively small gaps may cause friction wear or cavitation.

● Flow Characteristics: Gap size directly influences the flow uniformity at the impeller outlet, thereby affecting head and efficiency curves.

2. Theoretical Basis for Impeller Gap Optimization

2.1 Volumetric Efficiency Improvement

Volumetric efficiency (ηₛ) is defined as the ratio of actual output flow to theoretical flow:

ηₛ = 1 − QQleak

where Qleak is the leakage flow caused by the impeller gap. Optimizing the gap significantly reduces leakage. For example:

● Reducing the gap from 0.3 mm to 0.2 mm decreases leakage by 15–20%.

● In multistage pumps, cumulative optimization across stages can improve total efficiency by 5–10%.

2.2 Reduction in Hydraulic Losses

Optimizing the gap improves flow uniformity at the impeller outlet, reducing turbulence and thus minimizing head loss. For instance:

● CFD simulations show that reducing the gap from 0.4 mm to 0.25 mm lowers turbulent kinetic energy by 30%, corresponding to a 4–6% reduction in shaft power consumption.

2.3 Cavitation Performance Enhancement

Large gaps exacerbate pressure pulsations at the inlet, increasing cavitation risk. Optimizing the gap stabilizes flow and raises the NPSHr (net positive suction head) margin, particularly effective under low-flow conditions.

3. Experimental Verification and Engineering Cases

3.1 Laboratory Test Data

A research institute conducted comparative tests on a multistage vertical turbine pump (parameters: 2950 rpm, 100 m³/h, 200 m head).

3.2 Industrial Application Examples

● Petrochemical Circulation Pump Retrofit: A refinery reduced the impeller gap from 0.4 mm to 0.28 mm, achieving annual energy savings of 120 kW·h and an 8% reduction in operating costs.

● Offshore Platform Injection Pump Optimization: Using laser interferometry to control the gap (±0.02 mm), a pump’s volumetric efficiency improved from 81% to 89%, resolving vibration issues caused by excessive gaps.

4. Optimization Methods and Implementation Steps

4.1 Mathematical Model for Gap Optimization

Based on centrifugal pump similarity laws and correction coefficients, the relationship between gap and efficiency is:

η = η₀(1 − k·δD)

where δ is the gap value, D is the impeller diameter, and k is an empirical coefficient (typically 0.1–0.3).

4.2 Key Implementation Technologies

●Precision Manufacturing: CNC machines and grinding tools achieve micro-meter-level precision (IT7–IT8) for impellers and casings.

●In-Situ Measurement: Laser alignment tools and ultrasonic thickness gauges monitor gaps during assembly to avoid deviations.

● Dynamic Adjustment: For high-temperature or corrosive media, replaceable sealing rings with bolt-based fine-tuning are used.

4.3 Considerations

● Friction-Wear Balance: Undersized gaps increase mechanical wear; material hardness (e.g., Cr12MoV for impellers, HT250 for casings) and operational conditions must be balanced.

● Thermal Expansion Compensation: Reserved gaps (0.03–0.05 mm) are necessary for high-temperature applications (e.g., hot oil pumps).

5. Future Trends

●Digital Design: AI-based optimization algorithms (e.g., genetic algorithms) will rapidly determine optimal gaps.

●Additive Manufacturing: Metal 3D printing enables integrated impeller-casing designs, reducing assembly errors.

●Smart Monitoring: Fiber-optic sensors paired with digital twins will enable real-time gap monitoring and performance degradation prediction.

Conclusion

Impeller gap optimization is one of the most direct methods to enhance multistage vertical turbine pump efficiency. Combining precision manufacturing, dynamic adjustment, and intelligent monitoring can achieve efficiency gains of 5–15%, reduce energy consumption, and lower maintenance costs. With advancements in fabrication and analytics, gap optimization will evolve toward higher precision and intelligence, becoming a core technology for pump energy retrofitting.

Note: Practical engineering solutions must integrate medium properties, operational conditions, and cost constraints, validated through life cycle cost (LCC) analysis.