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Wet Behind the Ears



Wednesday, April 23rd 2008, 11:44am

Design Pressure Question

Hey guys,

This is the first system I've designed and I've spent alot of time (probably too much) at I am left with a question though, and it's about pressure in the system.

Static Pressure: 65psi - 3/4" CPVC (Mainline)

GPM Flows (11GPM theory: 8GPM measured at hose bib, unsure of size);

I measured the pressure with a gauge atleast 40' farther downstream from the point I intend to tap in, so I would assume that pressure would be marginally higher upstream at the tap in point.

Assuming the above numbers and about a 5psi loss at the anti-siphon valves, and a max of about 6.2 psi loss through the laterals (longest run is just shy of 100'). This should put atleast 53 psi in my laterals at any given point (elevation change is minimal). I will be using Rainbird 1800 spray heads, at about 30psi max.

Question: How does the rest of the pressure get regulated? 1800 spray heads come with or without pressure regulators. If I purchased the non regulated ones, would they just mist if they saw an inlet pressure of ~53psi? Is it more "standard procedure" to purchase 1 pressure regulator and install just before the valve manifold? Is the adjustment on the top of the sprinkler head enough to reduce pressure down to 30psi without a regulator in the system?

I guess the part that confuses me is that the 1800 literature says it takes an inlet pressure of 15-70psi. Am I wrong to assume that the adjustment screw will reduce the pressure to the desired levels?


Supreme Member

Posts: 482

Location: Houston, Texas


Wednesday, April 23rd 2008, 8:44pm

If you want to regulate the pressure on the zone that has spray heads on it you could use valves with flow control on them or a pressure regulator for only the valves that are using spray. If you have rotors on the same system but different valves you can let them fly normal they use normally around 50 PSI.

The flow control valves will allow you to adjust the flow through the valve to compensate for higher pressure.

I hope this helped.

Dan Todd

P.S. Don't use the set screw on the nozzle to crank it down to compensate for high pressure. Just Don't Do it.
:thumbup: :thumbsup:
LI0006121, BPAT0011021, CI0009500


Supreme Member


Thursday, April 24th 2008, 8:34am

RE: Design Pressure Question

...Am I wrong to assume that the adjustment screw will reduce the pressure to the desired levels?

I'm going to say yes... sort of... at least technically... (on the wording)...

The purpose of the adjustment screw isn't to "reduce the pressure at the head". Its purpose is to adjust how the head reacts to various pressures. So when "1800 literature says it takes an inlet pressure of 15-70psi", that means that the device will function within these pressure ranges. Less than 15psi and the head likely will not popup. More than 70psi and you're likely to start damaging the device. What you really need to look at is the specs for the nozzle. It will tell you approximately how far the nozzle with throw water at various pressures AT THE NOZZLE assuming the adjustment screw is fully open. So an example spec for a nozzle might be 15' @ 60psi, 12' @ 30psi, and the adjustment screw can reduce that by 30%. For this situation, it would mean that if you have 60psi at the nozzle, the water will throw from 10'-15' depending upon the adjustment screw, but at 30psi, it will throw from 8'-12'.

Todd is correct that a way to regulate the pressure on a zone is to use valves with flow control. Basically that is just a valve with a screw that is used to restrict the water flow (by restricting the water flow, you reduce the pressure... a pressure regulator uses the exact sample principle, but it opens and closes the restriction so that the exit pressure remains constant). Pressure regulators for the entire system usually are not used unless you have a starting pressure of something greater than 80psi. Then to protect the system, you install a pressure regulator at the point of mainline connection to reduce damage to the system from excessive pressures.

If you try to go the path of using flow control to adjust the pressure, changes to the input pressure (such as changes in local water demand if you are on city water) will effect the pressure at the nozzle and hence the throw of the spray heads. So rather than trying to tweek pressure with flow control alone, I personally decided to purchase the 1800s with the pressure regulator built into the pop up and valves with flow control. That way, I only had to design my system to insure I was getting about 35psi at the pop-ups, I can still use the flow control on my valves to make gross pressure adjustment, and I don't suffer major effects from pressure losses in lateral lines since the pressure reaching each nozzle is about 30psi.

The other thing to using the pressure regulators in the 1800s is that you will have less flow at each nozzle. Water pressure not only affects the distance the water is thrown by the nozzle but also the flow rate at the nozzle. So if all the heads are spraying at 30psi, your flow will be lower than if they were spraying at 50psi. After all, it sounds like you're designing this system sort of blind because you don't really know what the maximum flow rate is. (You've measured using the bucket test at a hose bibb not at the point of connection, and you're ASSUMING you'll gain 3gpm between your test spot and your actual connection). You'll do much better as designing a little more conservatively. As a suggestion, you might want to consider using the MPRotators in the 1800s. They will use much less GPM than spray heads. They are more expensive than nozzles, but you can also design head placements at 15-20 foot spacing rather than 12-15 foot spacing for typical nozzles.

Wet Behind the Ears



Thursday, April 24th 2008, 11:24pm

Guys, thank you so much. I really appreciate that you both took the time to answer my questions.

Todd, I'm using only spray heads. The lawn will be a reasonable size of about 1800 sq ft, but it's really very curvy and has some trees scattered throughout. I may be able to get away with a couple of rotors in the system, but for simplicity and keeping a spare head around, I opted to go with only spray heads.

HooKooDooKu, your explanation really hit home as it relates to my exact scenario. I went to the hardware store tonight and came home with some Rainbird anti-siphon valves (with flow control). I also purchased enough materials to build my valve manifold and tap into the mainline. Once the mainline is tapped, I intend to "measure" the flow using a bucket again. I based my design off of the 8GPM limit and currently my highest zone requires about 6.5GPM. I intend to verify my calculations once the mainline is tapped. Either way, I thought shooting low was the better side to err on.

I also decided to do as you suggest and get the 1800 series with pressure regulators. I was on the fence about them, trying to decide if I needed the redundancy of two pressure regulators, and you pretty much threw me off on the side of "Get them!"

I'm spending a little more than I originally anticipated, but really, I only want to do this once. I guess my mentality to to go a little overboard rather than look "new" and be digging up my yard twice.

I'm originally from Seattle, WA and if you've ever been there, you know that it's green. Where I am now is a desert and pretty much everything is brown brown brown... Man, I really REALLY want a green lawn this year. Once the sprinkler system is in place, I'll be tilling the ground and spreading seed. Can you tell I'm excited? 8o

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