What is Friction Head Pressure?
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Just like changes in elevation cause resistance that the pump must overcome (static head pressure), the pump must also overcome resistance caused by friction in the pipes and equipment. Friction is the ease with which something slides along a surface. For example, if you slide your cell phone across a metal table, it will slide a lot easier than if you slide it across the carpet. Strange though this might seem, water works the same way. If water travels through a ribbed pipe, it will take more pressure to push it through than if the pipe was smooth. The speed of the water through the pipes has a lot to do with how much friction is generated as well. Just like rubbing your hand quickly against carpet generates heat, fast- moving water generates more friction than slow-running water.
The more friction there is in the pipes and equipment, the more friction head pressure will increase, and the larger a pump will be needed. The larger the pump, the more money it will cost to run the pond. So from that standpoint, it makes sense to try to keep friction head pressure to a minimum.
What causes friction?
1) Fittings
Fittings like Ts, Ys and corners cause the most friction and should be kept to the minimum you need to effectively plumb a pond. 90-degree corners are usually the worst culprits for friction since the water must change directions quite quickly, slowing the water down and creating turbulence. You can lessen this friction by using sweeping corners instead of 90- degree corners. A sweeping corner spreads the sharp corner out into a longer arc, making the turn less abrupt. The more gradual a corner is, the more easily water will flow through the corner, and the less resistance will be created. This is where rigid PVC electrical sweeps, which we mentioned earlier, work really well. They can take a 90-degree corner and spread it out into a 3’ long sweep, which really helps with head pressure. Using flexible PVC pipe instead of rigid PVC pipe can also help eliminate numerous fittings by running one piece of flex pipe instead of a rigid pipe with numerous 90-degree fittings. To learn more about the difference between flexible and rigid PVC, please see our section on plumbing earlier in the guide.
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2) Pipe Size
The size of the pipe determines how fast the water will travel through it. For example, if 2,000 gph (gallons per hour) of water were coming out the end of a 2” diameter pipe, the water would be traveling at a certain speed. If you then swapped in a 1” pipe instead, but still wanted 2,000 gph, the water would have to travel much faster inside the pipe, since the volume inside the pipe is now smaller. This is an important distinction; the speed at which water travels inside a pipe isn’t really determined by the gallons per hour coming out the end, but by the size of the pipe.
At this point you might say, “So what? Why does it matter how fast the water is traveling inside the pipe if I get the same flow rate in the end?” The big reason is that friction and water speed don’t increase at the same rate. Doubling the water speed in a pipe will lead to 4 times as much friction. That means that increasing the speed of water through a pipe can make friction head pressure add up very quickly.
Luckily for us, there’s an easy way to combat rising water speeds inside a pipe; use a larger diameter pipe! Moving from a 2” diameter pipe to a 3” diameter pipe can dramatically reduce the overall friction head pressure if the flow rate is high enough. Here’s an example that will hopefully help clarify this:
Let’s say we have a pond with a 3,000 gph waterfall—that means that 3,000 gallons of water is flowing through the pipes each hour. If we use a 1-1/2” pipe, the water will flow through the pipe at 8.1 feet per second to produce 3000 gph.
• If we use 2” pipe, the water will flow at 4.9 feet per second to produce 3,000 gph.
• If we use 3” pipe, the water will flow at 2.2 feet per second to produce 3,000 gph. As you can see, doubling the pipe diameter from 1-1/2” to 3” causes the water to travel 4 times slower! If we assume for a moment that we have 50’ of pipe, the equivalent head pressure caused by friction for the various sized pipes would be: 1-1/2” pipe - 9.2’ of head pressure
• 2” pipe - 3.4’ of head pressure
• 3’ pipe - 0.6’ of head pressure
The above numbers mean that if you used a 1-1/2” pipe to pump 3,000 gph, you would create an equivalent almost 10’ of head pressure due to friction. This is extremely high, and would be equivalent to pumping to the top of a 10’ high waterfall! As the pipe diameter increases, the friction head pressure numbers drop dramatically until we hit a 3” pipe and there is only a very small 0.6’ of head pressure. That means that the water is traveling quite slowly inside of the pipe and is not creating too much friction. This is what we want.
Most people don’t realize that if one were to go ahead with the above 3,000-gallon pond using 1-1/2” pipe, a very large pump would be needed to overcome the 10’ of friction head pressure. Moving to 2” pipe—or even better, 3” pipe—would allow the use of a much more energy efficient pump to get the 3,000 gph of required output.
At this point you might be wondering where the numbers from in the above example came from. We have a tool available called the Pump Selection Wizard that is able to calculate head pressure and pump requirements. It asks a series of questions related to pond size, flow rate, waterfall height, waterfall width, diameter of pipe and filtration type, and then calculates the head pressure for you. For this example, to calculate the head pressure of the pipe size, I entered 3,000 as the flow rate, a waterfall height of 0 feet, a waterfall width of 0 feet, no UV clarifier and no filter. Here is the Pump Selection
Wizard for your own reference: Pump Selection Wizard: http://www.everything-ponds.com/pump-selection-wizard.html
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