How to Calculate Pump RPM?

The provided context gives a formula related to flow rate changes due to impeller diameter changes, which can indirectly influence RPM calculation in certain scenarios, but it doesn't directly give you how to calculate RPM. Generally, pump RPM is determined by the driver (usually an electric motor) and the pump's design. However, if you're looking to understand how changes in impeller diameter affect flow and, by extension potentially, required RPM, here's a breakdown:

While the initial snippet doesn't provide a direct RPM calculation, it sets up relationships that influence it. For a more complete answer, consider the following:

Affinity Laws for Pumps

Affinity laws describe the relationship between pump speed (RPM), flow rate, head, and power. These laws are essential when dealing with variable speed pumps or when trimming pump impellers. Here's how they apply:

• Flow Rate and RPM: The flow rate is directly proportional to the RPM. This means if you double the RPM, you double the flow rate.
  • Q₁ / Q₂ = N₁ / N₂
    • Where:
      • Q₁ = Initial Flow Rate
      • Q₂ = New Flow Rate
      • N₁ = Initial RPM
      • N₂ = New RPM
• Head and RPM: The head (pressure) is proportional to the square of the RPM.
  • H₁ / H₂ = (N₁ / N₂)²
    • Where:
      • H₁ = Initial Head
      • H₂ = New Head
• Power and RPM: The power is proportional to the cube of the RPM.
  • P₁ / P₂ = (N₁ / N₂)³
    • Where:
      • P₁ = Initial Power
      • P₂ = New Power

Calculating RPM from Flow Rate

If you know the initial flow rate (Q₁), initial RPM (N₁), and desired flow rate (Q₂), you can calculate the new RPM (N₂) using the first affinity law:

N₂ = N₁ * (Q₂ / Q₁)

Example:

Let's say a pump is running at 1750 RPM (N₁) and producing a flow rate of 100 GPM (Q₁). You need to increase the flow rate to 120 GPM (Q₂). What should the new RPM (N₂) be?

N₂ = 1750 RPM * (120 GPM / 100 GPM) = 2100 RPM

Impeller Diameter and RPM

The text you provided mentions the impeller diameter. While not directly calculating RPM, changes to the impeller diameter affect the pump's performance, which in turn can influence the required RPM for a desired flow. The relationship extracted from the video snippet can be written as:

Q_new = Q_old * (D_new / D_old)

Where:

• Q_new = New Flow Rate
• Q_old = Old Flow Rate
• D_new = New Impeller Diameter
• D_old = Old Impeller Diameter

If you change the impeller diameter, this will impact the flow rate at a given RPM. To maintain the original flow rate after changing the impeller diameter, you might need to adjust the RPM. To find the new RPM needed after changing the impeller to maintain the original flow rate, you'd combine these principles.

Example:

You reduce the impeller diameter and want to maintain the same flow rate. First, calculate the new flow rate (Q_new) resulting from the smaller impeller at the original RPM. Then, use the affinity law to calculate the new RPM (N₂) required to get back to the original flow rate (Q_old).

Practical Considerations

• Pump Curves: Always refer to the pump's performance curve provided by the manufacturer. This curve shows the relationship between flow rate, head, power, and RPM for a specific pump. The affinity laws are theoretical and don't account for real-world losses and inefficiencies.
• System Head: The system head (resistance to flow) will also affect the pump's performance. Affinity laws assume the system head remains constant or changes predictably.
• Variable Frequency Drives (VFDs): VFDs allow you to control the speed of the pump motor, providing a convenient way to adjust the RPM and flow rate.

In summary, while the provided text directly discusses flow rate and impeller diameter, calculating pump RPM relies primarily on the affinity laws and an understanding of the pump's performance curve. Consider impeller diameter changes when calculating flow or needing to adjust RPM to compensate for impeller modifications.