Do I need a pump for my sprinkler zones that are minus 70 feet elevation?

Question

I have a water tank on a hill and some lawns to water at the bottom of the hill.

The pipe that connects those two is a 2.5" pipe (yes, really) and when I attach a pressure gauge at the bottom of this 2.5" pipe I see a pressure of about 35 PSI, but when I connect the water tank, the head will be another 10 feet higher (it's a 10 foot tall tank) so ... figure 40 PSI.

I think my flow rate in GPM is extremely high. I tried some calculators online but am not sure if I am getting it right but it appears that I have hundreds and hundreds of GPM. And again, the pressure is 35-40 PSI.

It would be really nice to not have to purchase, install, and maintain a pump for this application, and it seems like I would not need one ... but may I sanity check:

If I connect a sprinkler zone at the bottom, feeding in 35-40 PSI at (god knows what huge GPM) and that sprinkler zone is maybe 200 ft. of 3/4" PVC and say 4 impact sprinkler heads that are designed to run at 30 PSI ... good to go ?

I lose 1-2 PSI on the 200 ft. of 3/4 PVC run ... do I lose any PSI on each of the impact heads (they are in a series) ?

Am I missing something here ?

Answer 1

The size of your lawn, the throw of the heads, the run time, the number of heads per zone and overlap between their spray patterns, the amount of sun and evaporation during watering, and the water pressure at each head all together determine the answer to your question.

You can twiddle all of these factors except the size of the lawn. A pump is one way to control one of these factors. If you do what you can with all the rest and find that you need an excessive number of zones, maybe a pump will be necessary. There are calculator tools for this.

I haven't tried it but there is a swish-looking phone app planner here.

Answer 2

The 2.5" pipe down the hill is good for gravity feed (minimal pressure loss due to pipe friction.) The GPM if you opened the 2.5" pipe in that location does not matter unless you do that (and would be at 0 PSI.) The GPM will be the GPM of your sprinkler heads (+ any leaks, so don't leak.)

The 200 feet of 3/4" feed to the sprinklers is likely where the problem will be; much more pipe friction (dynamic head) so pressure when flowing will be lower or much lower. How much lower will depend on the flow rate of the chosen sprinklers. Larger pipe will reduce the dynamic head at any given flow. There are plenty of irrigation calculators to help with that on the web. Here's one with links to others. http://irrigation.wsu.edu/Content/Calculators/General/Pressure-Loss-With-Outlets.php

I get 28 PSI loss (so 12 PSI remaining of your 40) for 200 feet of 0.75" pipe (beware of bad results on that calculator if you enter 3/4 instead) with 4 outlets, and 2.5 GPM per head is pretty low for a lot of sprinklers, but you haven't given a number.

To get to "1 PSI loss in the pipe" as you've guessed, your total flow would be 1.65 GPM so 0.41 GPM per sprinkler head, which seems - unlikely.

You can sort what size lateral pipe you need by applying that calculator (if the outlets are equally spaced - if not, you'll have to get more complex with those calculatons) to your actual sprinkler head flow requirements.

Evidently the first sprinklers you mention have a range from 1.5-8.6 GPM.

4 @ 1.5 GPM = 6 GPM in 1.25" pipe gives just under 1 PSI loss with 4 outlets.

4 @ 8.6 GPM = 34.4 GPM requires 2.5 inch pipe to achieve similar dynamic head, which means you'll have to start accounting for additional head loss in your pipe down the hill at that rate. 2 inch pipe would only be 2.3 PSI loss in the lateral. 1.5" would be 9.5 PSI loss in the lateral and might or might not work for you, depending on minimum required pressure and how much additional you lose in the length of 2.5" pipe down the hill.

The minimum rated pressure (which also makes the circle covered smaller) for your brass ones is 30 PSI with 5.5GPM flow per head, so 22 GPM for four which you might just hit with 1.25" pipe at 10.09 PSI loss. 1.5 inch pipe would be safer at 4.15 PSI loss in the lateral. You'd want to examine the length (not vertical drop) of your 2.5" feed for additional losses, though they might be fairly low still (but I don't know how long that pipe is, as you haven't told us.)

Answer 3

You shouldn't need a pump with that much head. Here's the secret to getting enough flow to sprinklers. You don't need to run them all at the same time.

So if you have 4 sprinklers and only have the flow to run 2 of them happily, then simply have 2 zones. Run one until it's done. Then run the other.

Best to do it at night - you'll lose less water to evaporation.

Another trick you can do is reduce (ratchet back) the run-time of the sprinklers. Lots of people vastly, vastly over-water their lawns. Reduce the runtime by 15% a week until the lawn starts to show distress... then increase it back up one notch.

Answer 4

From the position of the sprinklers you have 70 feet of head. You have shown that you have sufficient pressure.

What you need to consider is that if you have too many sprinklers then the pressure drops and so does the performance.

You will only need a pump if the area to be irrigated is too large and you need too many sprinklers....

Answer 5

What is the pressure off your house? 2.5 inch pipe??? Why? If you only have 4 heads on that zone. Also how far away is the main line from valve? Every head on average uses 2 gpm per minute wo 4 heads = 8 gpm a little high but average to a zone. There could be many things wrong with your system the way your describing this issue.

1. Leak in main line or zone.
2. If your on farm, clean heads and flush zones. Well or sistern systems will build up rust animal (duck) crap, calcification etc.
3. Valve not properly firing.
4. Tree roots smothering pipe. Would make sense showing the flow (gpm) but no pressure.
5. Mainline doesn't run long enough to other individual valve boxes.

If your water flow is coming from 500 feet away and your zone is 100 feet. You'll be waiting forever and just think your valve is stuck. Options

1. last 20- 50 feet of zone line reduce pipe size to 1.5 inch from 2.5 inch.
2. Listen for stuck flow by tree roots.
3. Continue the main from shut off to every box.
4. Get rid of maxis go 5000s. 2.5 inch pipe you should have pressure to run 30 heads but your main line will hammer cause you house can't produce that much water fast enough.

If you have 2.5 inch zone lines your main is either same or 3 inch. Which is absolutely absurd. Are you running your system off a fire hydrant? lol. I assume from a well so yes you need a pump.