# Water velocity advice



## merciless (May 13, 2020)

I have an existing system where I need to replace many sprinkler heads that have been neglected for a very long time. As I've researched this, I've become more interested in the nuances of how sprinkler systems work. I started by measuring PSI and GPM and have come to the conclusion that my water velocity is ridiculously high. However, I'm not sure. I'm trying to answer: 1) Is the water velocity as high as I think? 2) Does it matter?

I'll begin with some background information.

The sprinkler system is run from a well and water is driven by a 1.5HP pump. The pipe coming into the pump is 1.5" Schedule 40 PVC (all pipes involved are Schedule 40 PVC). The water leaves the pump through a 1.5" --> 1.25" adapter and enters 1 of 3 zones (controlled by an indexing valve...all 1.25"). Originally, there was a spigot attached to the 1.25" pipe coming out of the pump. I measured 60 psi of static pressure here. When I measured GPM, I realized the spigot is much more restrictive than the 1.25" pipe it is tapped into. I could only get 10-15GPM. So, I cut the PVC and arranged it so I could measure flow directly from the 1.25" pipe. A five-gallon bucket filled up in 5 to 6 seconds, so 50-60GPM. I've dug enough up to see one of the zones (let's say zone 1) continues the 1.25" pipe to the first sprinkler head (about 20 feet away). Most of the system appears to be 3/4" PVC or less from that point (an additional 8 sprinklers and approx 100ft of PVC for the longest run).

Even if all of my laterals were 1.25" PVC, that's 10.73 feet/s @ 50GPM and 12.87 feet/s @ 60GPM (I'm reading < 5 feet/s is ideal but up to 7 feet/s is common). The chart doesn't even have anything for 3/4" line, lol. If the velocity is as high as I believe it is, does it matter since I'm not tapped into my home's water supply line from the city? The system has run for 10-15 years with no perceivable problems.

Below is the chart I referenced.
https://www.rainbird.com/sites/default/files/media/documents/2018-01/ref_PVC_Schedule40_IPS_PlasticPipe.pdf


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## TC2 (Sep 15, 2017)

You're measuring total flow, which is limited under normal conditions by your sprinkler output. Only if the sprinklers in your zone are outputting 50 GPM will the water be moving this fast.


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## g-man (Jun 15, 2017)

^ +1

In other words, count how many heads in that zone. Then get the gpm of each nozzle. Add it all together to get the total actual gpm of that zone.


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## merciless (May 13, 2020)

This makes sense...which is probably why it never occurred to me 

Thanks...


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