Guidance Summary
All pumping station retrofits and modifications regardless of whether they use similar, smaller, or larger capacity pumps should aim to follow standard design practices to the greatest extent possible.
If physical constraints prevent full compliance, designers should integrate as many standard requirements as feasible while applying supplemental strategies to improve suction-side hydraulics.
Key Design Considerations for Retrofits:
1. Enhancing Layout in Space-Constrained Stations:
Performance can often be improved by smart design modifications, such as:
- Valve Optimization:
- Replace existing valves with types that create less flow disturbance.
- This can reduce the required straight pipe length downstream of valves.
- Pipe Size Adjustments:
- Reduce pipe diameter to increase the effective number of straight pipe diameters available within a fixed pipe length.
- Ensure NPSH margin, velocity limits, and other design criteria are still met.
- Reconfigure piping to include reducing elbows vs. reducers and elbow combinations.
- Pump Type Reconfiguration:
- If changing pump type, rework the piping to reduce the number and severity of flow-disturbing fittings.
- This can help achieve better approach flow conditions with minimal straight-run requirements.
2. Risks of Ignoring Existing Disturbances:
- Do not assume historical performance = acceptable performance.
- Legacy issues (e.g., elbows, reducers, valves causing swirl) may not have shown visible effects previously but could impact new pumps.
- Always evaluate and address flow disturbances, even if previous pumps seemed unaffected.
Recommended Solutions When Compliance Is Not Fully Possible:
If standard L2 (minimum straight pipe length) or layout cannot be achieved:
- a. Use Flow Conditioning Devices
- b. Conduct Physical Hydraulic Modeling
- c. Use Proven Designs:
- When possible, replicate a successful existing configuration known to perform well under similar conditions.
Checklist for Retrofitting Pump Stations:
- Follow standard Hydraulic Institute design guidance where possible
- Replace or reconfigure valves/fittings to reduce disturbance
- Consider pipe resizing carefully to improve effective L/D
- Don’t rely on past pump performance as a design benchmark
Explore use of flow conditioners or physical modeling