If you are replacing a chronically failing pump or selecting a pump for a new application, a crucial tool in helping you choose and size a submersible pump is the submersible pump system curve. Developing a curve for your desired performance helps you determine if a pump will be right for your application. In this post, we’ll show you how to create a pump system curve.
Submersible Pump System Curve Data Points
With the data in the example below, we’ll develop the submersible pump system curve. Note the key data points because we’ll use them in the formulas.
- Desired flow rate: 250 gallons per minute (GPM)
- Total elevation: 42’
- Total length of 3” PVC pipe: 50’ with:
- (5) 90° 3” elbows
- 3” flapper check valve
- 3” gate valve
- Includes a 3” high rate sand filter
- Includes a 3”x 6” PVC pipe increaser
- Total length of 6” PVC pipe: 32’ with:
- 90° 6” elbow
- Includes 6” inlet to water tank
Calculating Total Dynamic Head
Total dynamic head (TDH) is the combination of the static lift (the elevation difference between the water level and the end of the pump discharge piping) and friction loss through the piping system. Calculating TDH is the first step in creating the curve with the following formula:
Total Dynamic Head = Pressure Head + Static Head + Velocity Head + Friction Head
- Pressure Head is the discharge pressure the pump creates, also referred to as “centrifugal pump energy”. In our example, pressure head = 0 as it is atmospheric pressure only.
- Total Static Head is the elevation difference between the liquid level in the tank you are pumping from to the elevation of the water in the discharge tank or pipe, and is 42’ in our example
- Velocity Head is the motion of the water expressed in terms of the head required to produce that velocity. The formula to convert the energy to head in feet is
Velocity Head = Velocity2/2G
In our example, the velocity of the water through the 6” pipe is 2.8’ per second. “G” refers to the gravitational constant and is 32.2 ft/sec2. Expressed in the formula, it’s (2.8ft/sec)2/(2×32.2ft/sec2) = .12’ for the velocity head.
- Friction Head loss is the pressure loss caused by components of the system (i.e. pipe material, distance, etc.). The result is that you need more energy to move the water all the way through the system. The formula to calculate the friction loss is
Friction Loss = Friction Factor * Velocity2/2G
You need to calculate friction loss through every flow transition from fitting to fitting. The table below includes the data needed to calculate the friction loss at GPM, 250 GPM, and 300 GPM.
With all of the values calculated, here is our completed TDH formula:
0 + 42’ + 0.12’ + 27.5’ = 69.62′ TDH (Rounded up to 70’)
Following the same formulas for 200 GPM and 300 GPM, you can determine additional points on the system curve.
- 0 GPM: 42′
- 200 GPM: 60′
- 250 GPM: 70′
- 300 GPM: 82′
Develop the System Curve
Now that you have the calculated points, you can now develop the system curve. With the X axis showing Gallons per Minute (GPM) and the Y axis showing Head (in feet), plot the points above starting with static head at 42’. Add the remaining point to arrive at the curve below:
Using Your Created System Curve
Because the desired flow rate is 250 GPM and the corresponding TDH is 70’, you can check the developed system curve against the performance curves of possible pumps to determine if they will be suitable for your application.
In this case, BJM’s 7.5HP JX55CSS Stainless Steel submersible pumps provide 238 GPM at 64’ TDH. Because this operates near the best efficiency point (BEP), this is the pump we would recommend for this application.
The submersible pump system curves are an important aspect and will get you close to what you need for a pump, but there are additional variables that may influence the specific pump you need. BJM Pumps and our expertly-trained distributors work with you to determine the exact submersible pump you need for your specific application. Since 1983, we’ve helped our customers create pump solutions for the harshest applications with unparalleled customer service. Contact us or call 860-399-5937 now.