To keep your pool water clean and healthy, it’s important to have the right size pump to ensure proper circulation and filtration. But with so many pump sizes available, it can be difficult to know which one is right for your 20000 gallon pool. In this article, we’ll show you how to determine the size of pump you need for your pool.
To size the correct size of pool pump need, you need two things
Flow rate in Gallons per minute (GPM)
Total dynamic head (TDH)
Flow rate and dynamic head are explained very well with example for every person to understand and size a correct pool pump.
Table of Contents
Determine the flow rate in gallons per minute (GPM)
Flow rate in gallons per minute (GPM) in a swimming pool refers to the speed at which water is circulated through the pool’s filtration and circulation system.
It measures how many gallons of water pass through the system in one minute. This parameter is essential for maintaining water cleanliness, evenly distributing pool chemicals, and preventing issues like stagnant water or algae growth.
A proper flow rate ensures that all the pool water is filtered and treated within a specific time frame, typically 8 to 10 hours, to keep the pool water in good condition for swimming.
Step 1: Determine the Number of Gallons to be Pumped per Hour
To calculate the number of gallons that should be pumped per hour to clean the entire pool water, you should decide the turnover time. Basing on the standards, the turn over should be between 8-10 hours. In our example, we are going to use 8 hours.
To do this, divide the total number of gallons of water in your pool by 8.
For a 20000 gallon pool, the calculation is as follows:
20,000 gallons ÷ 8 hours = 2,500 gallons per hour
So, you would need to pump 2,500 gallons per hour to clean all the water in your 20,000-gallon pool in eight hours
Step 2: Convert Gallons per Hour to Gallons per Minute (GPM)
Since pumps are usually rated in gallons per minute (GPM), divide the gallons per hour obtained above by 60. The 60 represents the number of minutes in an hour.
We obtained 2,500 gallons per hour, so:
2,500 ÷ 60 = 41.67 GPM (gallons per minute), which is the flow rate you need for your pump.
So we round off this to 42 Gallons Per Minute (GPM)
So for a pool of 20000 gallons, you will need a pump of approximately 42 GPM for a turnover of 8 hours.
Note: If a turnover of 10 hours is considered, the GPM required will differ. Here's how to calculate it:
Step 1: Divide the gallons in the pool by 10
20,000 ÷ 10 = 2,000 Gallons per hour
Step 2: Convert the Gallons per Hour to Gallons per Minute (GPM)
2,000 ÷ 60 = 33.33 Gallons Per Minute (GPM)
So we round off this to 33 Gallons Per Minute (GPM)
So, a pool of 20000 gallons requires a pump of 33 – 42 GPM (gallons per minute)
For a pool of 20000 gallons, you need a pump of 33 – 42 GPM (gallons per minute) depending on the turnover time you choose (turn over of 8 – 10 hours respectively )
Now lets determine the Total dynamic head (TDH)
Total dynamic head
Total Dynamic Head (TDH) is the sum of various factors that determine the energy required by a pump to move fluid through a system. It represents the total resistance the pump needs to overcome to maintain the desired flow rate. Here are the factors considered when calculating Total Dynamic Head:
Static head (HS) refers to the vertical distance between a reference point and the water level in a fluid system, typically a liquid like water. It represents the potential energy that the fluid possesses due to its height above a specific point.
Losses due to friction in pipe length (Hf), often referred to as major losses, occur when a fluid flows through a pipe. These losses result from the friction between the moving fluid and the interior surface of the pipe. This can be done using the Darcy-Weisbach equation or empirical methods.
Losses in fittings (H fittings), also known as minor losses refer to the reduction in pressure or energy that occurs when a fluid flows through various components in a piping system, such as bends, valves, elbows, and other fittings.
The formula for calculating Total Dynamic Head (TDH) is:
TDH = HS + Hf + H fittings
Where:Top of Form
Hs is the static head
Hf is the head loss due to friction
H fittings is the head loss due to fittings.
Example used in the calculations
For easy understanding we are going to be using an example in all the formulars , we going to use our example in all the calculations.
The pipe length from the swimming pool to the pump and from the pump to the pool is 26 meters, the pipe diameter is 2 inches, the piping system has 6 in number 90° Standard Elbows, 3 in number 45° Standard Elbows, 4 fully open Ball Valves. Vertical distance from the lowest pool bottom to the centre of the pump is 2 meters and vertical distance from the pump’s centre to the highest discharge point into the pool is 0.5 meters.
The swimming pool is of 20000 gallons of water, we obtained the flow rate 33 -42 GPM for a turnover time of 8-10 hours, respectively. Here we are using GPM of 42 gallons per minute hence a turnover of 8 hours. It depends upon your choice; you can choose 33 GPM if you want a turnover of 10 hours.
Size the correct pool pump needed?
To size the pump correctly we need the Total dynamic head in feet and flow rate in Gallons per minute
We have already obtained the GPM as 42 gallons per minute for a turnover of 8hours, no we need TDH In feet.
TDH = HS + Hf + H fittings
Lets start with static head.
Static head
Refers the cumulative vertical distance that water needs to overcome as it navigates through a fluid system. It is divided into two, suction static head and discharge static head.
Suction Static Head: This refers to the vertical distance between the water level in the pool and the pump’s inlet. It represents the height that the pump needs to lift water from the pool. A greater suction static head requires the pump to work harder to overcome the gravitational force and draw water into the system. Factors such as pool elevation and the height of the water column contribute to the suction static head.
Discharge Static Head: This is the vertical distance between the pump’s outlet and the highest point where the water needs to be delivered. It represents the height that the pump needs to push water against gravity to reach its intended destination, such as the pool’s return jets or other equipment. Similar to the suction static head, the discharge static head contributes to the overall resistance that the pump must overcome to move water effectively.
Pool pump static head illustration
Static head = suction static head + discharge static head
From the example
Static suction head is 2 meters
Static discharge head is 0.5 meters
Static head = suction static head + discharge static head
Static head = 2 + 0.5 = 2.5 meters
Static head is 2.5 meters
Head loss due to friction/ major losses
Hf = (f*L*V^2)/(D*2*g)
Where
Hf is the head loss in the pipe due to friction
f is the Darcy friction factor.
L is the length of the pipe.
D is the diameter of the pipe: 2 inches = 0.0508 meters
V is the velocity of the water (V): Q / (π * D^2 / 4)
g is acceleration due to gravity (g): 9.81 m/s²
Obtain the Darcy friction factor (f)
From the example above of a pool
Flow rate (Q): 42 gallons per minute
Pipe diameter (D): 2 inches = 0.0508 meters
Pipe length (L): 26 meters
Calculate the fluid or water velocity
V = (Q*4)/(ℼ*D^2)
Given 42 gallons per minute.
83GPM = (42/60) Gallons per second = 0.7 Gallon Per second
1 gallon = 0.0037854m³
0.7 Gallons = 0.7 x 0.0037854 = 0.00264978 m³/s
Q = 0.00264978m³/s
Water flow rate is 0.00264978m³/s
Velocity of water
V = (Q*4)/(ℼ*D^2)
Substitute in the formular V = (0.00264978*4)/(𝝅 x 0.05082)
V = 1.307m/s
Having got the velocity of water, now let’s obtain Reynolds number
Calculate Reynolds Number (Re)
Re is used to determine the type of flow pattern as laminar or turbulent while flowing through a pipe.
Re = ρVD)/μ
ρ is the density of the water
V is the fluid velocity
D is the diameter of the pipe
μ is the dynamic viscosity of the water
A table that provides Darcy friction factors (f) for different Reynolds number (Re) ranges in various flow regimes:
Reynolds Number (Re)
Flow Regime
Darcy Friction Factor (f)
Re < 2000
Laminar
16 / Re
2000 < Re < 4000
Transitional
Transition Zone
Re > 4000
Turbulent
Empirical correlations
Density of water (ρ) is 1000 kilograms (kg/m³) The dynamic viscosity of water at room temperature (around 20°C or 68°F) is approximately 0.001 kg/(m·s)
μ = 0.001 kg/(m·s)
lets substitute in the formular for Re
Re = (1000*1.307*0.0508)/ 0.001 = 66395.6
Re = 66396
So from the table above Re > 4000, the flow regime is turbulent, therefore use Empirical correlation to obtain the Darcy Friction factor (f)
Express the Reynolds number Re obtained in scientific form
Re = 66,395
Re = 6.6×104
use the moody diagram to obtain the friction factor (f)
To use the moody diagram, you have to be with the Reynolds number (Re), and the type of material of the pipes used for example plastic, steel etc. Relative pipe rougness varies depending on the material used as shown in the key of the moody diagram
In the example of this article we used plastic pipe (PVC)
Look for where the Reynolds number (Re) meets with the relative Roughness of your pipe on the moody diagram
In the example Re is 6.6×104 and the relative roughness for plastic (PVC) is 0.0025
Draw a straight line from their meeting point to the side of friction factor and read off the friction factor (f)
In the example friction factor (f) is 0.027
moody diagram
Now calculate the head loss due to friction
Hf = (f*L*V^2)/(D*2*g)
All the parameter in the above equations have been obtained and some given, i.e.
F = 0.027
L= 26m
V = 1.307m/s
D = 0.0508 meters
Hf = (0.027*26*1.3072)/(0.0508*2*9.81)
Hf = 1.203 meters of water.
The head loss due to friction is 1.203 meters of water.
Head loss due to fittings (Hfitting) or Minor losses
Refers to the reduction in pressure or energy that occurs when a fluid flows through various components in a piping system, such as bends, valves, elbows, and other fittings.
Hfitting = (N*K*V2)/2g
Where
Hfitting is the minor head loss due to fittings ie valve, bends, tees etc
N is the number of fittings
V is the velocity of water
g is acceleration due to gravity
K is the friction factor. The fiction factor is different for various types of fittings
Resistance Coefficient K of commonly used fittings
Friction Losses in Pipe Fittings
Resistance Coefficient K (use in formula hf = Kv2/2g)
Head loss due to fittings is 0.2557 meters of water.
Total Dynamic head (TDH)
Total dynamic Head = static head + head loss due to friction + head loss in fittings
From the above calculations
Static head is 2.5 meters
Head loss due to friction is 1.203 meters of water.
Head loss due to fittings is 0.2557 meters of water.
Total dynamic head = 2.5 + 1.203+ 0.2557 = 3.9587 meters
Convert the total dynamic head (TDH) into feet because the performance curves for various pumps are in feet
1 meter = 3.281 feet
3.9587meter = 3.9587 *3.281 = 12.99 feet
TDH = 13feet
Total dynamic head is 13 feet, using data from the example given above
Choosing pool pump for your pool
Having got the total dynamic head has 13 feet and flow rate of 42 gallons per minute, we can now easily choose a pump that suites this pool using performance curves of different pumps.
These values of TDH and GPM vary according to the design of your pool and your specifications, so make sure that you have calculated the TDH and GPM of your pool, don’t just use the values we have calculated in our example.
Using pool performance curves to choose a pump involves matching the calculated Total Dynamic Head (TDH) and the desired flow rate (in GPM) to a point on the pump’s performance curve. Here’s a step-by-step guide:
Calculate Total Dynamic Head (TDH) and Flow Rate: You’ve already calculated the TDH as 13 feet and the flow rate as 42 gallons per minute (GPM). These two values represent the key factors in determining the appropriate pump for your pool. Let me hope you have also calculated the TDH and GPM of your pump. Note that TDH varies for various pools and design.
Collect Pump Performance Curves: Obtain the performance curves for the pool pumps you’re considering. These curves are usually provided by pump manufacturers and show the relationship between flow rate and head loss (TDH) for specific pump models. Below are these performance curves for various pumps and models to choose from. Each performance curve is linking to that specific pump.
Locate the Intersection Point: On the performance curve graph, find the value on the x-axis (flow rate in GPM) that corresponds to your calculated flow rate in the example it is 42 GPM. Then, find the value on the y-axis (head loss or TDH in feet) that corresponds to your calculated TDH in the example it is 13 feet.
Choose a Suitable Pump: Identify the point where the flow rate and TDH intersect on the performance curve. This point represents the specific operating conditions of your pool. Now, look at the nearest pump curve available.
Select a Pump Model: Choose a pump model whose performance curve passes closest to or slightly above the intersection point you found. This indicates that the pump can provide the necessary flow rate and head pressure for your pool’s specific conditions.
Check Efficiency and Power Consumption: Alongside the performance curve, manufacturers often provide efficiency and power consumption information. Ensure that the pump’s efficiency aligns with your energy efficiency goals.
How do I know what size pool pump I need?