How to Calculate the Pump Size Needed for Lifting and Discharging Water
When it comes to transferring water from one location to another, such as lifting it 20 feet and discharging it 40 feet away, selecting the right pump is critical. This involves understanding the total dynamic head (TDH), flow rate, and pump efficiency. Here, we'll break down the process step by step to help you choose the correct pump size.
1. Calculating Total Dynamic Head (TDH)
Total Dynamic Head (TDH) is the total amount of energy needed to lift the water from its source to the discharge point. TDH consists of the static head and friction losses in the piping system.
1.1. Calculating Static Head
The static head is the vertical distance the water needs to be lifted, which in this example is 20 feet.
1.2. Calculating Friction Losses
Friction losses depend on the flow rate, pipe diameter, length, and material. For rough estimation, you can use the following formula:
Friction Loss Length of pipe in feet times; (Friction loss per foot per 100 feet / 100)
For instance, if you have a pipe length of 40 feet and a friction loss of 2 feet per 100 feet of pipe, the friction loss would be:
Friction Loss 40 ft times; (2 ft / 100 ft) 0.8 ft
1.3. Calculating Total Dynamic Head
The TDH is the sum of the static head and friction losses:
TDH Static Head Friction Loss
2. Determining Flow Rate
The desired flow rate in gallons per minute (GPM) is critical. This will influence the pump size and type you need. The appropriate flow rate depends on your specific application requirements.
3. Pump Selection
Once you have the TDH and flow rate, you can select a pump following these steps:
3.1. Refer to Pump Manufacturer Curves
Use pump manufacturer curves to find a pump that can provide the required flow rate at the TDH calculated.
3.2. Consider Pump Efficiency
Pump efficiency usually ranges from 50 to 90%. Higher efficiency pumps require less power to achieve the same output. For instance, a pump with 70% efficiency means it needs less power to lift the water.
4. Power Requirement Calculation
Power requirement can be calculated using the following formula:
Power HP (Flow Rate GPM times; TDH ft times; 0.433) / (Pump Efficiency times; 3960)
Where 0.433 is the conversion factor, accounting for the weight of water (1 GPM lifts 1 ft of water requires approximately 0.433 HP), and 3960 is a constant for converting to horsepower.
Example Calculation
Assuming a flow rate of 10 GPM, a friction loss of 0.8 ft, and an efficiency of 70%:
TDH 20 ft 0.8 ft 20.8 ft
Power HP (10 GPM times; 20.8 ft times; 0.433) / (0.7 times; 3960) ≈ 0.14 HP
Conclusion
To lift water 20 feet and discharge it 40 feet away, you need a pump that can handle the calculated TDH and flow rate. Always check pump specifications and curves from manufacturers to ensure you find the right match for your requirements.