Shallow Well Pump Sizing: A Comprehensive Guide

Shallow well pump sizing is a critical aspect of designing an efficient and reliable water system for a home or building. It involves determining the appropriate pump capacity to meet the water demand while considering the well’s yield and the total dynamic head (T.D.H.) of the system. The T.D.H. consists of the static water level, friction head, and elevation head, all of which must be carefully calculated to ensure the pump’s optimal performance.

Understanding the Total Dynamic Head (T.D.H.)

The T.D.H. is the total amount of pressure the pump must overcome to deliver water from the well to the point of use. It is calculated by adding the following components:

1. Static Water Level: The height of the water in the well from the ground level.
2. Friction Head: The pressure loss due to the flow of water through the pipes. This can be calculated using the Hazen-Williams formula, which takes into account the flow rate, pipe size, and pipe length.
3. Elevation Head: The height difference between the water level in the well and the point of water use.

To calculate the friction head, you’ll need to know the following:

• Flow Rate: The maximum amount of water the system is expected to deliver, typically measured in gallons per minute (GPM).
• Pipe Size: The diameter of the pipes used in the system, which can range from 1/2 inch to 2 inches or more, depending on the application.
• Pipe Length: The total length of the pipes from the well to the point of use.

Using these factors, you can calculate the friction head using the Hazen-Williams formula:

Friction Head (in feet) = (10.44 × Q^1.85 × L) / (C^1.85 × D^4.87)
Where:
Q = Flow rate (GPM)
L = Pipe length (feet)
C = Hazen-Williams coefficient (typically 140 for new PVC pipes)
D = Pipe diameter (inches)

For example, if you have a system with a flow rate of 14 GPM, a pipe length of 100 feet, and a 1-inch PVC pipe (C=140), the friction head would be:

Friction Head = (10.44 × 14^1.85 × 100) / (140^1.85 × 1^4.87) = 8.5 feet

Determining the Pump Capacity

The pump capacity, or flow rate, needed to meet the water demand of a household or system can be estimated based on the number of bathrooms and the type of fixtures used. As a general rule, a two-bathroom house with modern fixtures will have a peak water demand of around 98 GPM.

To meet this demand, you would need a pump with a capacity of at least 14 GPM, which is the minimum-sized pump required to meet peak demand without supplemental supply. However, it’s important to consider the well’s yield and the T.D.H. of the system to ensure the pump can deliver the necessary flow rate.

Selecting the Appropriate Pump

Shallow well pumps are designed for wells with a pumping depth of less than 25 feet. They can be either jet pumps or submersible pumps, with jet pumps being more common for shallow well applications.

Jet Pumps

Jet pumps can be further classified into two types:

1. Shallow Well Jet Pumps: These pumps are designed specifically for shallow well applications and can typically handle a maximum depth of 25 feet.
2. Convertible Jet Pumps: These pumps can be used for both shallow and deep well applications, with a maximum depth of around 90 feet.

Jet pumps work by using a venturi effect to create suction and draw water from the well. They are generally less expensive and easier to install than submersible pumps, but they can be noisier and less efficient.

Submersible Pumps

Submersible pumps are also an option for shallow wells, and they offer the advantage of being quieter than jet pumps. They come in two main configurations:

1. 2-Wire Submersible Pumps: These pumps are simpler to install but require replacement if the electronics fail.
2. 3-Wire Submersible Pumps: These pumps offer the advantage of easier repairs if the electronics need to be replaced.

When selecting a shallow well pump, it’s important to consider the pump’s total head rating, which should be close to the T.D.H. of the system. As a general guideline, allowing a variation of plus or minus 15% on the total head rating is recommended.

Factors to Consider in Shallow Well Pump Sizing

1. Well Yield: The amount of water the well can produce, typically measured in GPM. This will determine the maximum flow rate the pump can deliver.
2. Water Demand: The peak water usage of the household or system, which can be estimated based on the number of bathrooms and the type of fixtures used.
3. Total Dynamic Head (T.D.H.): The total amount of pressure the pump must overcome, including the static water level, friction head, and elevation head.
4. Pump Type: The choice between a jet pump or a submersible pump, based on factors such as noise, efficiency, and ease of installation.
5. Pump Configuration: For submersible pumps, the choice between 2-wire and 3-wire configurations, with the latter offering easier repairs.
6. Pump Total Head Rating: The pump’s maximum pressure rating, which should be close to the T.D.H. of the system.

By considering these factors and following the guidelines outlined in this comprehensive guide, you can ensure that your shallow well pump is properly sized and capable of meeting the water demands of your household or system.

References

1. Pump Fundamentals – Shallow Well Pump Sizing
2. WellCare Information Sheet – Sizing a Well Pump
3. Red Lion Products – Shallow Well Pump Sizing