I've seen several videos for designing a drip irrigation system, but I'm confused because some use closed loops where the main line circles around and reattaches to itself, while others use main lines that simply terminate. Is there an advantage to using a closed loop?
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In most cases the small advantages in a loop system will be outweighed by the disadvantages of increased cost and increased labor. The advantages are mostly seen in larger systems, residential systems and gardens will rarely see any benefit to a loop system.
The primary advantages (particularly for larger systems) is that the pressure is evened out a bit which can help reduce water hammer (the water hitting a valve when it closes) and can sometimes allow for a smaller diameter of pipe to be used. That second advantage would be impossible in most residential systems since 1/2" tubing is the most common size we see used in residential systems.
The disadvantages are increased cost and labor -- increased cost from needing to purchase more tubing to accomplish the entire loop. The increased labor comes from needing to run additional tubing and get it connected and maintained. If the tubing is to be buried, the additional digging or trenching can get pretty labor intense.
For someone planning a larger system it would likely be a good idea to calculate the benefits of the loop, but for the vast majority of residential systems the loop will offer little to no benefit but will increase cost and labor time.
I hope this helps!
If I exceed my maximum run length of 200' with my 1/2 tubing, but it is in a closed loop, does it ''cancel'' the potentiel pressure problems?
It can help prevent some pressure issues, but they can definitely still crop up -- in theory, pressure lost would be calculated as half the flow going one direction and half the flow going the other, but unfortunately it doesn't really work like that in practice.
Here is a link (not affiliated with us, but his knowledge is vast) that goes deeper into the theory behind looped systems: https://www.irrigationtutorials.com/using-a-looped-mainline-for-irrigation/