Too many people–even some industry professionals–believe that using progressively smaller pipe sizes in a lawn sprinkler system will help keep the water pressure high. The argument goes something like this: if water moves through the pipes and past the sprinklers, the pipe should get smaller in order to squeeze the water so that the pressure stays high enough to operate the sprinklers. However, this is not the case.
This misconception largely persists because the reasoning behind the argument seems logical and tallies with experience. For example, when you put your thumb over the end of a hose, you make the opening smaller; and as you do so, you can feel the water pressure behind your thumb increase. This would seem to be proof positive that decreasing the size of an opening increases water pressure and that, by extension, a smaller pipe would also do the same thing.
But the reality is that when you put your thumb over the end of a length of hose, you’re only changing the flow dynamics in the hose. When water moves through a conduit of any kind, it encounters resistance caused by conduit surfaces. But it still moves at the maximum speed possible while still overcoming this friction. When the water leaves its conduit, it has almost no pressure left.
What all this means is that when you put your thumb over the end of the hose, the water flows more slowly, resulting in a loss of pressure due to friction. The more tightly you squeeze your thumb, the more you’ll see reduced flow and feel greater pressure. But the bottom line is, you’ve not created any new pressure. You’ve just made a trade-off (reduced flow for increased pressure).
The same ideas apply to using smaller pipes for sprinkler systems. A smaller pipe would lessen the flow of water as well as reduce the pressure loss in the pipes. This in turn would cause more pressure but render a sprinkler system inoperative. This is because sprinklers need a certain level of flow working in tandem with pressure to function properly.
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