Calculating the appropriate capacity for a submersible pump involves determining the specific requirements of your water system to ensure the pump can deliver the necessary amount of water at the required pressure. It's not a single formula but rather a process of evaluating several key factors: the required flow rate, the total dynamic head (TDH) the pump must overcome, and the capacity of your well.
Understanding Required Pump Capacity
The "capacity" of a pump is typically expressed as a flow rate (e.g., gallons per minute or GPM, liters per minute or LPM) at a specific pressure or head (vertical lift). When sizing a submersible pump, you are determining the capacity it needs to have to meet your demands.
Key Factors for Determining Required Capacity
To calculate the required submersible pump capacity, you need to assess the following:
1. Determine Your Required Flow Rate
This is the amount of water you need at peak demand. It depends on the number of fixtures (faucets, showers, toilets, appliances) that might be used simultaneously in your home or application. Fixture flow rates are often measured in GPM. Summing up the potential peak usage gives you a target flow rate for your pump.
2. Calculate Total Dynamic Head (TDH)
Total Dynamic Head is the total resistance the pump must overcome to move water from the well to the discharge point. It is the sum of several components:
Static Head
This is the vertical distance from the water level in the well (when the pump is running) up to the highest point the water needs to reach at the discharge.
Pressure Head
This is the pressure required at the discharge point, often needed for pressure tanks or specific applications. Every PSI (pounds per square inch) of required pressure adds approximately 2.31 feet of head.
Friction Loss
As water flows through pipes, fittings (elbows, tees, valves), and other components, it loses pressure due to friction. This pressure loss is converted into an equivalent "head" value. You should refer to friction loss tables for the pipe that you have used; this information is also available via the Internet. These tables provide friction loss values (in feet of head per 100 feet of pipe) based on the pipe material, diameter, and the flow rate. Friction loss increases significantly with higher flow rates and smaller pipe diameters. You must calculate the total friction loss for the entire piping system.
TDH is calculated as:
TDH = Static Head + Pressure Head + Friction Loss
3. Assess Your Well Capacity
An extremely critical factor, often overlooked, is the well's capacity. Well capacity is how fast water flows into the well from the aquifer. Your pump's flow rate should not exceed the well's sustainable yield. If you pump water out faster than it flows in, you risk pumping the well dry, which can damage the pump. A well yield test is typically performed to determine this rate.
Selecting the Right Pump
Once you have determined your required flow rate and calculated the TDH, you can look at pump performance curves provided by manufacturers. These curves show the flow rate a specific pump model can deliver at various heads. You need to select a pump whose curve shows it can deliver your required flow rate at your calculated TDH, and whose maximum flow rate does not significantly exceed your well's sustainable capacity.
Example:
Let's say you need 10 GPM, your static head is 150 ft, you need 40 PSI (approx. 92 ft head) at the house, and your piping calculations using friction loss tables show 30 ft of friction loss at 10 GPM.
- Required Flow Rate = 10 GPM
- Calculated TDH = 150 ft (Static) + 92 ft (Pressure) + 30 ft (Friction) = 272 ft
You would look for a pump that can deliver at least 10 GPM at 272 ft of head. Additionally, if your well yield test shows the well can only sustain 8 GPM, you would need to adjust your water usage expectations or consider storage solutions, as installing a 10 GPM pump would likely pump the well dry.
In summary, calculating the appropriate submersible pump capacity involves evaluating your water needs (flow rate), the physical demands of the system (TDH including friction loss), and the well's ability to recharge (well capacity).
