It is important to calculate this accurately in order to determine the correct sizing and scale of pumping equipment for your needs.
To calculate Total Dynamic Head, also known as TDH, we need to calculate two things:
A) The Vertical Rise.
B) The Friction Losses of all the pipe and components the liquid encounters on the discharge of the pump.
C) After calculating both, add them together to calculate TDH.
Let us show you how to calculate these together and then you will be able to complete this on your own! For the purpose of this walkthrough, we will determine the Total Dynamic Head for 25GPM to go from the Pump to Tank B in the example below.
How to Calculate Vertical Rise
A) Vertical Rise: It must be determined what the vertical rise is from the liquid’s starting point to its ending point. As the liquid level in the tank decreases, the vertical rise will increase, and consequently, the total dynamic head will increase. To simplify matters, assume the tank is empty for the worst case situation.
In the above example, if Tank A is full and going to the top of Tank B, the vertical rise is 10 feet. If Tank A is half empty and there is only 5 feet of liquid in Tank A, then the vertical rise is 15 feet. If tank A is entirely emptied, then the vertical rise will be 21 feet. With the vertical rise being anywhere from 10-21 feet, it is easiest just to use 21 feet to be on the safe side unless you are certain the liquid level will not go below a certain height.
How to Calculate Friction Loss
B) Friction Loss: To calculate the friction loss you first need to know what your desired flow is. Each flow rate will have a different friction loss. The more flow going thru a pipe, the more friction loss there will be, so 5GPM going thru 1 inch pipe will have a higher friction loss than 1GPM going thru 1 inch pipe. After your flow rate, you need to know what type of pipe you are using, the schedule of the pipe, and the length of the pipe, both vertically and horizontally. You also need to know how many elbows, valves, connections, and anything else that comes into contact with the liquid.
Using the above example, let’s calculate the friction loss for 25GPM. There is 1.5 inch PVC Schedule 40 pipe. The horizontal pipe distance from the pump to Tank B is 120 feet, and the vertical pipe distance from the pump to the tank B is 21 feet. There are 2 90 degree long radius elbows and 2 gate valves.
Once this information is calculated, take the following steps:
Step 1) Add the horizontal and vertical discharge pipe together.
120 feet+21 feet= 141 feet
Step 2) Go to this website: http://www.freecalc.com/fricfram.htm
Step 3) Enter pipe size, pipe schedule, piping material, piping length, valves, and fittings.
For this example, the numbers are:
1.5 Inch, Schedule 40, PVC Material, 141 Piping Length in Feet, 2 90 LR Elbows, and 2 Gate Valves.
Step 4) Press “Calculate Pressure Drop.” After pressing “Calculate Pressure Drop,” the calculator states the head loss is 5.6 feet.
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The Result: Total Dynamic Head Calculation
C) Total Dynamic Head: The worst case scenario for the vertical rise is 21 feet. The friction loss for 25GPM is 5.6 feet. Adding these two numbers together, the Total Dynamic Head is 26.6 feet for 25GPM to go from the Pump to Tank B.
What if the liquid level in the tank never goes below 5 feet and the user now requires 20GPM?
If the tank is never emptied more than 5 feet, then the vertical distance between the liquid in Tank A and the top of Tank B is 15 feet.
15 feet of vertical distance + 3.8 feet of friction loss = 18.8 feet of total dynamic head.
Other Considerations When Calculating Total Dynamic Head
Other factors which can affect the friction loss include specific gravity, viscosity and temperature. The more information you have on the system, the more accurate your friction loss number and by extension your Total Dynamic Head will become.
A liquid’s specific gravity can change the friction losses slightly.
If the specific gravity is between 1.0 and 2.0 (water is 1.0), it is not necessary to use that information in your calculations. If it is less than 1.0 or more than 2.0, it is suggested to use an online calculator.
Viscosity on the other hand can greatly increase friction losses. If the liquid is viscous, determine the viscosity by either using a viscous specific gravity chart or an online viscous specific gravity calculator.
As always, March Manufacturing encourages you to contact a March distributor or an engineer at March Manufacturing to review your application before any purchase.
Updated on 5/25/2016