HELP: Will my wet feed system backup during a downpour?

We are about to install a wet feed rainwater storage system with a significantly long horizontal pipe run. I'm not sure how to calculate if the downspouts will back up during a downpour. Simple friction loss calculators say I will only lose about 6 inches of head pressure, though I really am not sure. I don't want our downspout feed pipe to backup (overflow) during a downpour and therefore lose a bunch of precious water.

Can you help determine if this system will back up during a downpour when the storage tank is close to its 5 foot max fill height? Since I am naturally curious, how many gallons/hr would this system be able to handle.

Here are the critical values:
Assume a worst case 1,800 gallon/hr downpour
3" PVC pipe (entire system)
4 x 45 degree elbows
2 x 90 degree elbows
1 x shutoff ball valve
Height of downspout wet feed pipe: 10 feet
Horizontal pipe run: 100 feet
Wet feeds into 1500 gallon storage tank with max fill height of 5 feet
Assume bottom of storage tank is at the same level as the bottom of the downspout wet feed pipe

Thank you for helping us save water in Arizona!

I have to ask a few questions, my signature has a link to rainfall catchment and use topics.
- why is the tank 100 feet away?
- 90mm to 100mm pipe is best to use.
- Are you fitting leaf separators at the building?
- are you installing total system drain points anywhere along the system to clear blockages away?
- will the horizontal pipe have any slope?
- what is the ball valve for?
- what size is it?
- a 5000 gal tank is really the starting size needed for a tank if you are using a lot of water. Min. 3 months usage.
- with a 1800 gal downpour your 1500 gal tank will overflow and you will loose the benefits of the whole system.
in Australia with 18 inches of rainfall, I have 2 / 5000gal tanks.

Hi, I get lost with feet and inches, but you write the following:

"Assume bottom of storage tank is at the same level as the bottom of the downspout wet feed pipe"

Meaning that as soon as the tank fills, the water gradient is not as high as before.

Furthermore, you write about many different elbows: why and where? Is the water expected to flow up and down again?

John C Daley wrote:I have to ask a few questions, my signature has a link to rainfall catchment and use topics.
- why is the tank 100 feet away?
- 90mm to 100mm pipe is best to use.
- Are you fitting leaf separators at the building?
- are you installing total system drain points anywhere along the system to clear blockages away?
- will the horizontal pipe have any slope?
- what is the ball valve for?
- what size is it?
- a 5000 gal tank is really the starting size needed for a tank if you are using a lot of water. Min. 3 months usage.
- with a 1800 gal downpour your 1500 gal tank will overflow and you will loose the benefits of the whole system.
in Australia with 18 inches of rainfall, I have 2 / 5000gal tanks.

John,
I omitted a lot of details for simplicity since I am mostly concerned about the pipe system being sized for a worst-case downpour (with a factor of safety) and causing an input-side overflow. I am happy you are interested and your expertise is valuable in case I incorrectly designed or omitted anything. I also looked over the link on your signature block. You certainly are an expert at rainwater collection!
- The tank is 100 feet away since the area close to the downspouts already has plants, trees, and other things that my wife will not be happy if I removed. It is definitely not an optimal system, but 100 feet away is the closest she will allow.
- I was considering the larger diameter pipe 4" (about 100mm) PVC pipe. So that is an option and still wondering how much flow could a 4" system handle?
- The leaf/debris separators are immediately before the vertical enclosed PVC pipe system.
-The horizontal pipes will not have a designed-in gravity slope since it is a wet-feed system and will always have water in them. In practice, there will be about a 2 foot drop from the feed side to the storage tank. The drain/maintenance ports on either side of the horizontal run will allow for infrequent cleaning of the horizontal pipes via pressurized city water or other methods.
- As for the ball valve, your question made me realize I was mixing up details. There will not be a ball valve on the supply-side. The ball valve is for the demand-side and connects the other storage tanks in the overall system. Joining the overall storage system enables one spigot to access the entire system. The ball valve allows us to isolate a tank that needs maintenance. We would pump the water into the other tanks during maintenance.
- I was referencing the 1,800 gallon/hr downpour solely for flow rate. We live in southern Arizona (Tucson). In the past 15 years, I believe the maximum we have ever experienced was 1.5 inches during a single rainstorm which would give us about 700 gallons in the storage tank for that particular part of the roof.

Thanks for your questions.

hans muster wrote:Hi, I get lost with feet and inches, but you write the following:

"Assume bottom of storage tank is at the same level as the bottom of the downspout wet feed pipe"

Meaning that as soon as the tank fills, the water gradient is not as high as before.

Furthermore, you write about many different elbows: why and where? Is the water expected to flow up and down again?

Hans,
I must have been tired since it is hot (107F, 42C) this time of the year in Tucson, Arizona and we just finished installing the much simpler dry feed rainwater system on the other part of the house.

I provided too much detail of the wet feed system since I forgot that the wet feed system still has water entering the storage tank from the top. So the storage tank water level is irrelevant. For some reason, I mistakenly thought the storage tank would be fed from below. I'll blame it on the heat, my old age, or just my stupidity.

I guess some of the details are still relevant, except I can use a simple pipe flow calculator. That makes things a lot easier to calculate.

Hans,
Additionally, yes once in an enclosed PVC pipe system the water will flow vertically down from the downspout section and go horizontally underground through our backyard. The 100 foot (30m) horizontal section underground will go under a block wall fence to the location of the storage tank where it will turn vertically to the storage tank input at the top of the tank. The change in height of the topmost pipe input to the storage tank output is about 5 feet (1.5m).

With a 3" (76mm) inner diameter PVC pipe system, some of my earlier calculations indicated about 1.2 feet (366mm) of lost pressure head at a 1800 gal/hr (6800 l/hr) flow rate. I'll need to double check these, but it seems like I should not experience any pipe flow backup during a downpour.