Gene Vogel
Pump & Vibration Specialist
Electrical Apparatus Service Association
St. Louis, MO
In his paper presented at the EASA Convention 2017, Gene Vogel discusses the advantages of using variable-speed drives (VSDs) to enhance the efficiency of pump systems. Vogel begins by contrasting the sizing of electric motors and centrifugal pumps, noting that while a larger motor on a smaller load runs efficiently, a larger pump on a smaller load can lead to inefficiency and increased energy consumption. Motors convert electrical power into mechanical power, while pumps convert mechanical power into hydraulic power, with each conversion involving some energy loss primarily to heat.
Pump efficiency varies significantly across its operating range, typically between 65% and 85%, but can be as low as 20% to 30% when the pump does not match the application. This variability presents a substantial opportunity for improving efficiency. Vogel explains the relationship between head and flow in centrifugal pumps, using the analogy of raising different volumes of water to different heights. The efficiency of a pump is influenced by the curvature of its impeller vanes, which are designed to match specific flow rates. When the flow rate changes, turbulence results, reducing efficiency.
Vogel introduces the concept of pump curves, which illustrate the relationship between head, flow rate, efficiency, and power. The best efficiency point (BEP) is the flow rate at which the pump operates most efficiently. Operating outside the preferred range can lead to increased vibration, reduced efficiency, and overheating. The power required for a pump can be calculated from the head, flow rate, and efficiency values, and this calculation is crucial for troubleshooting issues related to motor current and overheating.
The paper discusses the impact of system head on pump performance, noting that factors such as pipe length, diameter, and fittings can affect system head. Changes in suction head due to varying sump levels also impact system head. Vogel compares two common methods for controlling flow: throttling a discharge valve and varying the pump rotating speed. Throttling increases friction head and reduces flow, while varying the pump speed reduces both head and flow, leading to significant energy savings.
Vogel highlights the benefits of using VSDs to control pump speed, including reduced stress and wear on the pump and system, and mitigation of cavitation issues. However, he also notes potential disadvantages, such as structural resonance at specific operating speeds and voltage spikes from non-sinusoidal power applied by VSDs. Motors designed for VSD operation have protections to mitigate these negative effects.
In conclusion, Vogel emphasizes that VSDs can provide substantial energy savings and improve pump system efficiency, especially in applications with varying demand. Understanding pump curves and the impact of system head is essential for optimizing pump performance and ensuring reliable operation.
Key Points Covered:
- Differences between motor and pump sizing
- Variability of pump efficiency across operating range
- Relationship between head and flow in centrifugal pumps
- Importance of pump curves and best efficiency point (BEP)
- Impact of system head on pump performance
- Comparison of throttling and varying pump speed for flow control
- Benefits and potential disadvantages of using variable-speed drives (VSDs)
Key Takeaways:
- Larger pumps on smaller loads can lead to inefficiency and increased energy consumption
- Pump efficiency varies significantly and presents opportunities for improvement
- Understanding pump curves is crucial for optimizing performance
- VSDs can provide substantial energy savings and reduce stress on pump systems
- System head is affected by various factors, including pipe length and fittings
- Motors designed for VSD operation have protections to mitigate negative effects
- VSDs are particularly beneficial in applications with varying demand
Print
