Jess Stryker's
Landscape Sprinkler Design Tutorial
Step #5
Pressure Loss
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Jess Stryker's |
This is the part that many people are scared of. I'm going to show you a simplified method I developed in 1979 that is a good cross between quality and ease of use. The method is now commonly used, but I don't lay claim to inventing it. On the other hand, I've never heard of anyone who used it prior to me. But enough of my self indulgent ego stroking! (Little brains require big egos to keep them balanced, right?) First a little primer on hydraulics (ieeee.... come back, please!). This is provided for those who like to know WHY we are doing what we are doing. I'll keep it as non-technical as possible! If you wish to preserve your brain uncluttered by knowledge you can skip the rest of this page. (Yipeee!)
Friction Loss:
As water moves through a pipe it loses pressure due to a phenomenon commonly called "friction loss". Much of this loss is caused by turbulence, but we call it friction loss for simplicity. The amount of friction loss is determined by the type of pipe, the diameter of the pipe, the amount of water flowing through the pipe, and the length of the pipe. These factors are then plugged into a formula (the Williams/Hazen Formula) which gives us the friction loss.The water also loses pressure each time it passes through a valve, a backflow preventer, or anything else it encounters on it's way to the sprinkler head. Even a bend in the pipe causes a pressure loss!
You need lots of pressure at those sprinkler heads!
The sprinkler head needs a minimum amount of water pressure to work properly. You saw that in the manufacturer's nozzle chart. As the pressure increases so does the flow (GPM) and the radius of the throw. So in order to assure that there is enough pressure to run the sprinkler heads, we need to calculate the pressure losses between the water source and the sprinkler head. If the pressure loss is found to be too great, then we must reduce it by going back and making the components, such as the pipe, larger. Thus sizing the pipe is largely a trial and error process. It takes hours and hours of changes and calculations until it's finally right. What a pain!A Minor Shortcut:
This is where my little method comes in to play. It's nothing earth shattering, in fact it's just a matter of looking at the problem from a different perspective. We're going to turn this problem on it's head and work backwards. We'll start by determining the maximum allowable friction loss, and then, using a "cheat chart", we'll work backward to get the smallest possible pipe sizes in one step. None of this trial and error garbage! Enough theory for now, let's get on with the design!
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