Help Correcting Current Irrigation Setup

[Pmac16]

Hi all,
Here's my situation. I have a current sprinkler system from the existing homeowner that was set up to pull water from a lake.
The homeowner took the pump so I have purchased a simer 3415p as a replacement.
The setup (picture attached) is 1/2" schedule 40 pvc, single zone, with 6 Hunter PGP Ultra heads. I'm not sure how he made this work and expect to have issues due to pressure loss resulting from the pipe size. What's the simplest fix? A)different pump, B) different heads, C)run 1.5" across the middle creating smaller runs of .5"? Is there any scenario in which I can make the current piping work?

 

[wwhitney]

So what are the pressure and flow requirements for each sprinkler head?

Cheers, Wayne

 

[Sponsor]

 

[Pmac16]

So what are the pressure and flow requirements for each sprinkler head?

Cheers, Wayne

Pressure is listed at 25-70 for those heads (radius 17-46) so I'm guessing closer to 25 to hit my needed 20'. They currently have 3gpm nozzles but I can change those to whatever I need if that's a way to make it work.

 

[wwhitney]

OK, so if your heads are constant flow rate at 3 gpm, and increased pressure just makes them throw farther, then here's a quick analysis of the current layout:

1/2" Schedule 40 has an ID of 0.622". This pressure loss calculator will tell use the pressure drop on each segment, given the length, ID, and flow:
http://irrigation.wsu.edu/Content/Calculators/General/Pipeline-Pressure-Loss.php

1) The 40' segment carrying 15 gpm drops 25.8 psi
(assumes the line from the pump connects next to the lower right head; provide additional distance info if this is not approximately true).
2) The 20' segment carrying 12 gpm drops 8.5 psi
3) The 20' segment carrying 6 gpm drops 2.4 psi
4) The 40' segment carrying 3 gpm drops 1.3 psi.
5) We can skip the 20' segment to the middle head, as it's not the controlling case.

So if you need 25 psi at the bottom left head, then your pump, plus the piping from the pump to that first tee, has to be able to deliver 18 gpm @ 63 psi. Again, this assumes the heads are self-limiting to 3 gpm; if the closer heads will flow more water at the elevated psi, then that increased flow will likely starve the farthest head of sufficient pressure. And your heads will be getting different pressures, so you may find that the heads with the higher pressures throw farther.

If 2 gpm heads are an option, then repeating the computation above gives:

1) 12.2 psi
2) 4.0 psi
3) 1.1 psi
4) 0.6 psi

Now the pump and the piping to the first tee only need to provide 17.9 psi over the minimum head psi (so 43 psi if you want 25 psi at the farthest head) at a flow rate of 12 gpm.

Also, running an additional 1/2" (or 3/4") schedule PVC line from that first tee to the middle sprinkler head would do a lot to cut down on the pressure drops 1 & 2, by providing parallel paths to the top middle sprinkler. Parallel paths make the calculations a lot harder, but the improvement would be significant, more than halving the pressure drop to the top middle head.

Cheers, Wayne

P.S. I didn't include an allowance for fittings, so the above lengths should be increased by the equivalent lengths of the actual fittings used.

 

[Pmac16]

OK, so if your heads are constant flow rate at 3 gpm, and increased pressure just makes them throw farther, then here's a quick analysis of the current layout:

1/2" Schedule 40 has an ID of 0.622". This pressure loss calculator will tell use the pressure drop on each segment, given the length, ID, and flow:
http://irrigation.wsu.edu/Content/Calculators/General/Pipeline-Pressure-Loss.php

1) The 40' segment carrying 15 gpm drops 25.8 psi
(assumes the line from the pump connects next to the lower right head; provide additional distance info if this is not approximately true).
2) The 20' segment carrying 12 gpm drops 8.5 psi
3) The 20' segment carrying 6 gpm drops 2.4 psi
4) The 40' segment carrying 3 gpm drops 1.3 psi.
5) We can skip the 20' segment to the middle head, as it's not the controlling case.

So if you need 25 psi at the bottom left head, then your pump, plus the piping from the pump to that first tee, has to be able to deliver 18 gpm @ 63 psi. Again, this assumes the heads are self-limiting to 3 gpm; if the closer heads will flow more water at the elevated psi, then that increased flow will likely starve the farthest head of sufficient pressure. And your heads will be getting different pressures, so you may find that the heads with the higher pressures throw farther.

If 2 gpm heads are an option, then repeating the computation above gives:

1) 12.2 psi
2) 4.0 psi
3) 1.1 psi
4) 0.6 psi

Now the pump and the piping to the first tee only need to provide 17.9 psi over the minimum head psi (so 43 psi if you want 25 psi at the farthest head) at a flow rate of 12 gpm.

Also, running an additional 1/2" (or 3/4") schedule PVC line from that first tee to the middle sprinkler head would do a lot to cut down on the pressure drops 1 & 2, by providing parallel paths to the top middle sprinkler. Parallel paths make the calculations a lot harder, but the improvement would be significant, more than halving the pressure drop to the top middle head.

Cheers, Wayne

P.S. I didn't include an allowance for fittings, so the above lengths should be increased by the equivalent lengths of the actual fittings used.

Wow, great info, really appreciate it.

So based on the chart below for my pump (3415P at 5' elevation), it sounds like I'll be short of the needed pressure at 3gpm but would be good if I switch to 2gpm nozzles then,? If I did that and also run a 3/4" to the center head I'll be good at the lower left(last) head?

 

[wwhitney]

What's the elevation relative to the lake for the highest spray head, and for the lower left spray head?
What is the length and size of the pipe from the pump to the first tee?
Where is that tee located along the 40' run?

Cheers, Wayne

 

[Pmac16]

What's the elevation relative to the lake for the highest spray head, and for the lower left spray head?
What is the length and size of the pipe from the pump to the first tee?
Where is that tee located along the 40' run?

Cheers, Wayne

5' elevation max for the highest spray head, probably less. 1.5" schedule 40 from the pump to the 1st tee, 10' run. And the tee is 10' from the lower right head, so 30' from head #2.

 

[wwhitney]

OK, I looked up the PGP Ultra data at https://www.hunterindustries.com/irrigation-product/rotors/pgp-ultra That data shows that flow is not independent of input pressure, so the calculations are a bit harder. [Perhaps there is a standard way to handle this, but I'm going to first try to figure that out myself.]

Looking at your pump data again, I'm not sure what to make of the fact that at 5' suction pressure, the lowest gpm given is 18 gpm at 45 psi, while for your application you might be better off with, say 15 gpm at a higher psi. I'm also not sure what to infer from the 8 gpm at 45psi with 15' of suction pressure; if suction and discharge pressure tradeoff one to one, that would also mean 8 gpm at 49 psi with 5' of suction pressure (since 2.3' = 1 psi). [My knowledge of pumps is limited.]

There's only one nozzle marketed as 3.0 gpm, so I assume you have the Blue 3.0 nozzle. That lowest rating on that nozzle is 2.2 gpm @ 25 psi with a 35' throw. So the design envelope with that nozzle at the farthest location is whether the system will get 2.2 gpm to that nozzle with 25 psi of pressure.

Also, going back to the OP, all of the ideas you presented are possibilities for a working design; and it appears from my first post that at least one of them will be necessary. What is your order of preference among them?

Option (C), running a 1.5" header 40' to the left on the page, from the current "first tee" to the current farthest 40' segment, with a short (10'?) 1/2" spur to the current middle sprinkler head, will certainly be a huge improvement. I'm pretty sure that would work with your current pump and heads, and will do the calculations if that's your first choice. That will also give you the most balanced system, as without that header, the heads nearer to the pump will definitely be getting a lot more pressure than the farther heads. [Are you sure there isn't already a 1/2" header there already? As that would be a far more rational piping layout.]

Option (A), a bigger pump, would certainly work with the current piping, but will give you the biggest disparity in output between the nearest head and the farthest head; the computations in my first post would let you pick the pump (and I could update them based on the 30' info and adding an allowance for elbows). And option (B), smaller heads, will work, other than my uncertainty about how your pump behaves if connected to a system that would only flow, say, 12 gpm @ 45 psi pump discharge, or 13 gpm @ 50 psi pump discharge.

So as I don't have time/interest at the moment to calculate out all 3 scenarios, tell me your first choice and I'll try to calculate that out.

Cheers, Wayne

 

[Pmac16]

Thanks again, Wayne. You're a huge help.

I think my first choice, if it'll work, would be to just replace the blue 3.0 nozzles with 2.0 nozzles. It would be a quick, easy, and basically free fix, if that solves the problem and obviously depending on how the first few heads handle the excess. If not then I think I'll run 1.5 all the way across and tie into all 3 crossing lines. Little bit of digging but still not terrible and wayyyy cheaper than a new pump.

I, too, am confused by how the pump chart drops off after 18gpm. I would think it would be an arch all the way down to 0 with increasing pressures along the way. Unless they're just thinking that most heads operate under 50psi so no need to list it beyond that.

 

[wwhitney]

OK, my latest thought is this:

You need to get some clarity from the manufacturer on what your 1.5 HP pump will do with 5' of suction head and a system that will only flow 5-15 gpm at 45-50 psi. Because the lowest gpm flow rate on the data sheet for the 5' of suction head row is 18 gpm @ 45 psi, with no entry at 50 psi. While the 3.0 gpm heads need 45 psi to actually flow 3.0 gpm. Since you can't possibly get full pump pressure at every head, your system with the current heads will never flow 18 gpm with a pump that can only put out 45 psi.

Now if the pump with 5' of suction head is okay with, say, 12 gpm @ 48 psi (or whatever) or even better 8 gpm @ 50 psi, then there's definitely no problem using that pump, and you can go ahead and install it and check the system performance. That will be more accurate than reams of calculations. Based on how badly the current system does, you can choose between adding the header and/or changing the heads.

But if that pump will be unhappy or damaged unless the piping will flow at least 18 gpm at 45 psi, then it may be the wrong pump. To actually get your system to flow 18 gpm at 45 psi, I think you'd need to do something like add the header we've talked about, and increase the head size to blue 4.0 heads. Those heads flow 3.0 gpm at 25 psi, and I think the piping with header will do 18 gpm with less than 20 psi pressure drop (will require a calculation to check).

Cheers, Wayne

 

[Pmac16]

Sounds great. Really appreciate your time and insight

 

[wwhitney]

OK, I'll analyze the system crudely drawn below, where blue is 1.5" PVC Schedule 40 and red is 0.5" PVC Schedule 40. Note the top two 20' segments are missing, so the actual behavior will differ from this model, but it won't be worse (no upside to actually removing them). Also, to start I'll assume a constant 3.0 gpm flow to each sprinkler head.

For fitting equivalent length, I'll use this table: https://www.engineeringtoolbox.com/pvc-pipes-equivalent-length-fittings-d_801.html along with some guesses as to the fitting configuration. I get:

A = 10' plus one elbow plus the straight path of a tee = 20'
B = 20' plus the straight path of a tee = 23'
C = 20' plus the branch of a tee = 28'
D = 10' plus the branch and straight path of tees = 15'
E = 20' plus an elbow = 24'
F = 30' plus an elbow = 34'

1/2" Schedule 40 ID = 0.622"; 1-1/2" Schedule 40 ID = 1.61" Pressure loss:

A = 18 gpm / 0.2 psi
B = 12 gpm / 0.1 psi
C = 6 gpm / 0.0 psi

So those are all negligible.

D = 6 gpm / 1.8 psi
E = 3 gpm / 0.8 psi
F = 3 gpm / 1.1 psi

The upshot is that with the 1.5" header, all the heads get the same pressure to within 3 psi. Everything should be pretty well balanced. And as the pump can do 45 psi at 18 gpm, with the current heads spec'ed at 3 gpm at 45 psi each, with 42 psi minimum at the heads, they should be very close to 3 gpm. The calculation say everything should work fine.

That was a more dramatic effect that I expected, probably because I've only done these analyses a couple times. Since C and D are both carrying 6 gpm, it might make more sense to size them both at, say 1", rather than the unequal sizing I drew and analyzed.

Cheers,
Wayne