Before we talk and plan for the plumbing, we need to understand how water moves. Water’s pressure, volume, flow velocity and head are a small part of hydrodynamics; which is very complicated with much more information available online. The succinct data below is to assist when adding features or pumps.
Water Pressure: In a pond, there are multiple factors contributing to water pressure. Without pressure, water tends not to move except due to gravity’s effect on it. At the bottom of the main pool, there is a natural, increasing pressure due to the weight of water above pushing down. Water is heavy and weighs ~8-1/3 pounds per gallon which fits in a ~6-1/8” cube. The pump creates two kinds of artificial pressure: negative pressure (or suction) and positive pressure (discharge). The optimum pressure, velocity and flow of any pump would be the unit sitting in the water and discharging straight out without plumbing. Of course, this would not be very useful. Plumbing is needed – but it has many factors that will reduce the flow rate.
Backpressure: Adding plumbing constricts water. First, water flowing through a pipe moves faster in the center and slower along the walls of the pipe (friction). Therefore, the larger the pipe, the more volume can move through the center unimpeded. Next, the type of pipe material can increase the amount of friction as different materials have different surfaces (smoother is better). Lastly, any turbulence or restrictions within the plumbing will further reduce flow and velocity. A straight piece of pipe with no fittings is optimum (but not necessarily practical). Every turn causes turbulence and slows the water down. Therefore, any turns you make should be as graceful as possible (e.g.: Use two 45’s instead of a 90). Likewise, water passing through fittings such as check valves, shutoffs, connection fittings, etc., will impede the water’s movement. Therefore, plan the plumbing’s size, material and fittings very carefully.
Static Head Pressure: As discussed in Article 9, static head pressure will further reduce flow and velocity. It is a combination of the water density, the length of the plumbing, and the elevation changes within the pathway. Water density affects are minor – such as temperature and clarity. The length of the plumbing from pump to discharge is another factor. The longer the pipe means more water within the pipe. This column of water has weight. Thus, the longer the pipe, the more weight of water the pump must push. Lastly, the elevation changes of the pipes above or below the pump decelerates or accelerates the water due to the affects of gravity. Pumps are advertised with maximum flow (no head) so check their chart.
Calculating Head Pressure: The following contributes 1’ of static head pressure; always round up: Every vertical foot of height above the water feature surface the pump must push; every ten feet of pipe through which water will travel; every check valve; every 90° fitting; every four 45° fittings; every sixteen 22.5° fittings.
Calculating Flow: With the three factors listed above, you can estimate the flow rate, but it will be difficult to determine the exact flow rate. Always plan for you pump to be beyond, not just meet your estimate. Once you have the pump, you can use a stopwatch and measure how long it takes to fill a 5-gallon bucket in your final plumbing configuration. This is important to know exactly what your flow is. In general, flow rates can be broken down into three types: low pressure: velocity ~6’/sec (gravity up to 19 PSI, most pond pumps); average pressure: velocity ~12’/sec (20 - 100 PSI, high head pumps and house plumbing); and high pressure: velocity ~18’/sec (over 100 PSI, assume peak pipe velocity). To calculate velocity: 0.408 x (GPM divided by inside pipe diameter squared). Example: A 59GPM waterfall is fed by a 2” line: (59GPM divided by (22)* 0.408) = 6.018 feet per second (low pressure); PSI = FPS answer (6.018) x 0.433 = 2.6 PSI).
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