Hey
What size down pipe would I need, and how many if my roof area equals 1224.78 metres squared and I have average rainfall of 173kl per year?
Also how much plumbing would I need from my hypothetical tanks?
Down pipes are typically about 10 cm diameter. That way, they fit the rain gutter bottom width, and can handle heavy rainfall if the gutters are clear of leaves.
Your rainfall should be in meters per year, not kiloliters per year, unless you have already taken the roof area into account.
The size downpipe does NOT depend on average rainfall per year, but the rate of rainfall (inches/hour). That is a statisctical exercise, because you probably wouldn't want to design for the max rainfall rate, but rather something like design to catch the flow of something like 90 percent of maximum rainfall. There is a cost tradeoff here.
If you lived in the desert, you might want to design for max rainfall.
Thanks. Talked to my Dad. Told me to assume that the down pipes had a width of 200mm as this is for a school auditoriam.
@ drwls. Yes, the roof area has already been taken into account
@ bobpursley. how would i design for max rainfall? The school is in the desert.
Max rainfall? That is what weather records are for. Look them up.
Then, you have the flow /min, and the length of pipe, and the pressure head (due to gravity). Bernoulli's equation rules, or you can use any number of online pipe flow calculators to figure this.
To determine the size of the downpipe and the number of downpipes you would need, you need to consider the roof area and the average rainfall. Here's how you can calculate it:
1. Roof Area:
If your roof area is given in square meters, which is 1224.78 m², there is no need for any additional calculations.
2. Rainfall:
If the rainfall is given in kiloliters (kl) per year, which is 173 kl, we need to convert it to meters. To do this, you'll need to know the average annual rainfall in your area in millimeters (mm). Let's assume the average annual rainfall in your area is 1000 mm.
To convert kiloliters (kl) to cubic meters (m³), we multiply by 1000:
173 kl * 1000 = 173,000 m³
To convert cubic meters (m³) to millimeters (mm), we divide by the roof area in square meters:
173,000 m³ / 1224.78 m² = 141 mm
So, your average rainfall is approximately 141 mm per year.
3. Downpipe Size:
The size of the downpipe is determined by the maximum rainfall intensity in your area. This measurement reflects the amount of rainfall that can occur within a specific time frame, usually expressed in liters per second per square meter (l/s/m²) or millimeters per hour (mm/h).
You'll need to research the maximum rainfall intensity for your location or consult local building codes, as this value can vary significantly. Let's assume the maximum rainfall intensity for your area is 10 mm/h.
Based on your average annual rainfall (141 mm) and the maximum rainfall intensity (10 mm/h), you can calculate the required downpipe size. Here is the calculation:
Required Downpipe Size (in liters per second):
(Roof area in square meters * Maximum rainfall intensity in mm/h) / (3,600 seconds per hour)
Required Downpipe Size (in millimeters per hour):
Required Downpipe Size (in liters per second) * 3,600 seconds per hour / Roof area in square meters
Once you calculate the required downpipe size, you can choose a standard downpipe diameter that matches or exceeds this value. Common downpipe sizes range from 75 mm to 150 mm in diameter.
4. Number of Downpipes:
To determine the number of downpipes needed, you'll need to consider the total area that each downpipe can handle effectively. This value also varies depending on the material and design of the downpipe.
Again, you'll need to refer to local building codes or consult an engineer to determine the appropriate downpipe capacity. As a rough estimate, a downpipe with a diameter of 100 mm can handle approximately 1 liter per second of rainfall.
Divide the required downpipe size (in liters per second) by the capacity of a single downpipe (in liters per second) to find the number of downpipes needed.
Now, regarding plumbing from the tanks, you'll need to provide more specific details about the hypothetical tanks: their capacity, location in relation to the downpipes, desired flow rate, and distance from the point of use. With this information, it will be easier to determine the necessary plumbing requirements.
Remember, these calculations provide a general guideline, but it's always best to consult a professional plumber, engineer, or local authorities for accurate sizing and design based on your specific requirements and local regulations.