More plumbing math

In my preceding post "Rain barrel design using two bunghole system":

The cross-section of a two-inch pipe is 3.14 square inches (if you're doing the math, remember the radius of a 2-inch diameter pipe is 1 inch).  The cross-section of a 3/4 inch pipe is 0.44 square inches.  If you have a 2-inch bottom pipe then it can feed 7 or 8 pipes with a 3/4 inch diameter.

 I'm actually going to be feeding my closed barrel system through the 1.5 inch overflow from an upright rain barrel.  The cross section of a 1.5 inch pipe is 1.76 square inches.  I'll have four barrels, being fed through 3/4 inch diameter pipe.  Each of the 4 pipes has a cross-section of 0.44 inches, for a total of 1.76 square inches.  Perfect in theory, but this will not be theory.  There will be losses such as friction, but this isn't rocket science, even if it might be rocket fuel tank science.

I am aware that I'm playing with engineering when things won't be perfect.  Usually a rainstorm causes a downspout to have a lot of falling water mixed with air, and it is not completely full of water.  Feeding a 2x3 inch downspout into a 3-inch diameter pipe adds much more capacity than is usually needed.  The overflow needs to be able to handle what the 2x3 inch is delivering, and how quickly the barrels can take the water will determine how much of a sudden cloudburst it can catch.

We're dealing with a situation where hundreds of gallons will arrive as quickly as an inch of rain can fall.    If the water tends to arrive as an all-day trickle, then the plumbing only needs to deal with a trickle.  You have to design the plumbing to deal with the amount of water that you're willing to catch or lose.  A little planning can reduce the amount of water that will be lost through the overflow.

Rain barrel design using two bunghole system

I have some barrels that I'm going to put on their sides, so I can use them without making new holes.  The barrels can be filled and emptied using the two bungholes.

Sorry for the crude drawing, I hope it shows the design well enough.

First of all, why not only use the bungholes on the bottom?  Because air has to be able to escape.  You can see that in this design, the top are connected with a pipe which connects to an air vent pipe so air can escape and enter as the water level changes.  Remember to put mosquito screen on the air vent.

The pipe coming in should be as big as possible, because a lot of rain may arrive at once.  The overflow pipe should be as big as the pipe coming in, so water won't back up into the downspout.  This is a sealed barrel system, so you can use an automatic diverter which can deal with the tap pipe being full of water, but the air vent pipe opening must be higher than the diverter is.

The air vent is connected to both the top and bottom pipes so air can easily escape as the pipes fill with water.  The separate vent should reduce gurgling and speed the flow of the water.  The same principles are used in household drain systems.

The large vertical pipe on the left carries the water down from the intake, so it should be at least as large as the intake pipe.  The horizontal pipe at the bottom should also be that large, so all the water coming in can easily reach the bungholes and flow in to the bottom of the barrels.

This is where the first bottleneck can be expected.  Your downspout is probably 2x3 inches or 3x4 inches, so a 3 or 4 inch diameter pipe can handle it all; if your diverter has a smaller pipe then that's the largest size that you need (you might consider a larger pipe across the bottom in case you might be cross-feeding from a second downspout).  But your bunghole has a two inch diameter -- or if you're using 3/4 inch pipe threaded into the bungs then you only have the 0.75 inch diameter to fill the barrel through.  Obviously you want the horizontal pipe to be larger than the openings into the bungs, otherwise all the water will try to go in the first barrel and the other taps will be starved.

The cross-section of a two-inch pipe is 3.14 square inches (if you're doing the math, remember the radius of a 2-inch diameter pipe is 1 inch).  The cross-section of a 3/4 inch pipe is 0.44 square inches.  If you have a 2-inch bottom pipe then it can feed 7 or 8 pipes with a 3/4 inch diameter.

If you're using two-inch feeds into the bungs, but only have a 1.5 inch diverter pipe coming in, then you won't have enough water coming in to fill a single 2-inch opening, so there's not much of a problem in that case.

If you're paying attention, you might have noticed that I haven't explained why I'm showing the water-filled vertical pipe on the left having a connection to the top horizontal pipe.  That's because I'm using that to cheat.  If the bottom bungs can't let water into the barrels as fast as it is coming in, then the water will flow across and also try to fill the barrels through the top bungholes.  There will be a steady flow of air coming out, but some water will probably be able to flow in through the top.  There is a risk of bubbling or gurgling noise from this, but I don't know how it compares to the sound of the rain.

Also, even if you're feeding two-inch diameter taps into the bottom bungholes, a 3/4 inch pipe is sufficient for air to flow out of the top.  This is useful because barrels tend to have bungs with different threads -- the bunghole with the NPT thread ("fine thread") can fit two-inch diameter threaded pipe.  But the other bunghole has buttress thread ("coarse thread") and it's easier and cheaper to get buttress bungs with a 3/4 inch NPT knockout, then use that for 3/4 inch air venting.  If you tap the barrels while they're upright, it will be easier to keep track after that which side should be up.

EDIT: Notice that this design is based on using rigid plumbing.  Using hoses requires extra care to not trap too much air or water in curved hose sections.

When it rains, it sometimes pours

I've decided to catch the rain from the drainpipe by using barrels with a large open top, so I know that I'll be able to get the first 50 gallons of a sudden cloudburst.  Well, 50 gallons per barrel if they're empty.  I have some 20-gallon barrels that I'll also be connecting in the system, and figuring out how to connect them has been fun.

While I'd like to have two-inch pipe running everywhere, it looks like it will be more practical to feed the extra barrels through a garden hose and 1¼" sump pump tubing (from my overflow pipe).  Obviously this will work better if the rainfall is a trickle rather than a downpour.

So I was wondering how quickly the rain falls... there was a light rainfall yesterday, so let's see what the rate was.  The local National Weather Service office is in Chanhassen, Minnesota.  No rainfall info.  And it says "The weather observations depicted here are not official, and are not quality controlled. For the nearest official observation, check out the data from Flying Cloud Airport in Eden Prairie."  Well, I can understand why the weather office's weather readings might not be official, but it just doesn't seem right.  OK, off to the airport.

During that light rainfall yesterday, there was one hour with 0.14 inch of rain, one hour with 0.05, and several hours at a lesser rate.  Good, that was a nice trickling rate, so collection should work fine with days like that.

When there are brief burst of sudden rain, I should be able to catch much of it also.  The barrels will all be connected together at the bottom, so if the system is half full, each collection barrel can handle about 25 gallons of water at once before overflowing into the 1¼" tube.  The water can then trickle into the other barrels through the bottom hose during the time between cloudbursts.

I'm hoping that will work well enough.  In case I need more capacity, I will have the plumbing designed so I can add more connections.