Sep 25, 2016 1:48 pm
I have been reading many posts and getting more confused by the minute! I am looking at connecting 2 x 90mm round downpipes up to a new tank. Currently one (orange, patio roof) is not connected to anything as it is new. The other connects to an old rubble pit. The water is mainly used for garden use and for the toilet.
Total roof area of the house is around 140sqm with 3 downpipes. Only one of these however will be connected to the new tank (around 60sqm). I will also connect the new patio roof (around 40sqm) to the tank.
It will be a wet (charged) system and I was originally planning on running 2 x 100mm pipes to the tank from the bottom of the downpipes (as they will be stronger) however am now thinking I may be able to join them like the attached diagram. i.e. the 2 orange downpipes will be 90mm round and will then go into 100mm stormwater under ground, joining near the second downpipe then into the tank. The house is in Clontarf, Northside Brisbane.
Is running these 2 x 90mm downpipes into the 1 x 100mm going to be sufficient? Would it be necessary to have a silt trap if it is just being used on the gardens and toilet and there are no trees nearby? Is there anything else I need to be aware of?
i have also read about low restriction inlet and not sure what this is?
Appreciate any advice..
Re: Rainwater Tank Design - North Brisbane2
Oct 01, 2016 2:31 am
The first thing you need to do is Google your council's requirements. Council mandates however are often poorly considered but there are some basic design principles that should apply. These are:
The area's 1:20 Average Recurrence Interval (ARI). The Brisbane 1:20 ARI is 240 mm/hr, this is based on an average rainfall intensity over a 5 minute duration and 240mm/hr = an average of 4mm per minute over the 5 minute duration. Eaves gutter and downpipe size compliance is calculated so that gutters do not overflow during a 1:20 ARI and the rainwater harvesting design capacity must be able to handle this inflow AND overflow.
The roof area harvested. You have a total of 100 sq m that you intend harvesting and during a 1:20 ARI, this will collect 400 litres per minute if the rain falls vertically. You should however add a safety margin for heavier rainfall or for wind driven rain dumping more rain on a sloping roof area facing the weather. I always suggest a minimum 20% safety margin, making your design divert requirement 480 litres per minute during a 1:20 ARI.
Wet system. Having a remote located tank or when more than one downpipe is diverted to the tank invariably involves a wet system. A wet system describes a downpipe that is plumbed underground to the tank and then up the side of the tank with a vertical riser to the tank's top meshed inlet. Unless drained, the wet system pipes and downpipe(s) will retain water to the height of the vertical riser. Wet systems are notorious for accumulating sediment and holding anaerobic water. Wet systems need to be drained and this requires a flush valve at the lowest point but most wet systems are buried and the common Inspection Outlet (IO) that is fitted is often 250-300mm above the buried pipe. Not good!
Leaf Diverters. Organic matter must be kept out of the wet system to prevent a build up of gunk and prevent stagnation. Most areas north of Sydney mandate leaf diverters on wet systems to prevent mosquito access and you will almost certainly be required to have them fitted. A disadvantage of leaf diverters is that they reduce the available head.
Overflow pipe. Queensland is notorious for the overflowing rainwater tanks during heavy rain. You need to have an overflow capacity that is equal to or greater than the inflow capacity during heavy rain but don't expect a plumber or even the tank seller to be able to tell you what the tank's overflow capacity is.
Water tanks have their overflow pipe exiting the side of the tank and the outlet also has mosquito proof mesh that presents a flow restriction. Most mosquito proof wire mesh has an open surface area of little more than 50%.
The flow through a side outlet without any restriction is calculated by Torricelli's law that accounts for the height of the water above the overflow's invert (bottom of the pipe). A 100mm orifice with a 30mm, 40mm and 50mm height above the invert will flow at 0.53, 0.90 and 1.34 litres per second (LPS) respectively and even 100mm above the invert only gives 4.67 lps. When you factor the mesh and the fact that tank manufacturers now fit the overflow very high on the tank which reduces mitigation, you can see that your 100 sq m roof harvest area would cause a problem during heavy rain if a standard sized meshed overflow outlet was used.
The weir flow down a vertical pipe is greater than a side outlet, for example, the comparative flow rates with a 30mm, 40mm, 50mm and 100mm height of water above a 100mm vertical pipe's crest are 3.25, 4.17, 4.67 and 6.60 lps. A vertical pipe opening is easily made by having a 90 degree elbow/neck inside the tank but you would still have an inadequate discharge rate and more so with the overflow mesh fitted. The mesh however can be removed if the exterior discharge pipe has a flap valve fitted at an air break and I believe that overflow air breaks are a mandated requirement in QLD.
To ensure that the overflow has an adequate capacity, the best solution is to have an internal bell mouth overflow fitted. Doubling the opening's diameter gives four times more cross sectional area and fitting a common 200mm bell mouth to a 100mm DWV pipe will give plenty of flow even with the mesh fitted.
Hydraulic head. This is the force that drives the water through the pipes when you have a wet system. The head pressure must be able to transfer water to the tank, if the pipe doesn't have the flow capacity during heavy rain to drain the roof, then the water will back up and the incoming water will overflow at the leaf diverter.
Fitting a low restriction inlet would eliminate the above problem and increase yield.
You will need to seal the bottom of the leaf diverter to the downpipe and calculate the head from the bottom of the leaf diverter to about 50mm above the invert at the top of the vertical riser. Don't do what so many do and calculate it to the height of the tank's top meshed inlet.
It is important to have as smooth a passage as possible to reduce friction losses through the pipes, every elbow or tee adds an equivalent length of pipe as friction loss to the hydraulic calculations. I would have a 45 degree junction rather than an 90 degree tee where the diagram shows the two pipes merging, the reason being that the short pipe from the larger 60 sq m roof area is shown as communicating with the other pipe at 90 degrees which would generate the equivalent of 6.75 metres of equivalent pipe length in friction loss.
I would have two 90mm pvc stormwater pipes coming from both roof areas all the way to the 100mm DWV 45 degree junction as the smaller pipes will reduce the volume of water held in the wet system. 90mm pvc stormwater pipe is also available with 2.5mm walls whereas the standard pipe has flimsy 1.9mm walls. The 90mm stormwater pipe would connect to the junction via a 100mm x 90mm reducer.
I can only do a hydraulic calculation guesstimate because I don't know the pipe lengths but I would guess about 34 metres including the vertical riser at the tank. With the fitting's friction losses, the equivalent pipe length would be at least 55 metres. If a 45 degree junction was plumbed further to the right of the diagram's tee, the dominant 60 sq m roof area would boost the flow and a 350mm head would then suffice.
A low restriction inlet has been described in many posts and it can only be fitted if leaf diverters are fitted at the downpipes because water entering a holding tank must first pass through mosquito proof mesh. Instead of having an elbow fitted at the base of the vertical riser, a tee is fitted. The extra opening is then reduced to usually 40-50 mm and the smaller pipe then connected to a same size tank inlet via a flexible coupling. The tank's inlet valve must be fitted a minimum of 100mm above the bottom of the tank at the valve's lowest point and within 75 degrees either side of the pump's draw valve.
The low restriction inlet will be the priority flow path to the tank because it operates with greater head pressure than the vertical riser's flow path to the top of the tank, allowing the wet system to flush every time it rains. It also delivers oxygen rich water to the anaerobic zone and the retained water level in the wet system will be the same height as the water level in the tank and not the higher water level at the top of the vertical riser as per archaic standard wet systems. The additional inlet also supplements the flow capacity to the tank, allowing a lesser head to be used if necessary, for example, to fit leaf diverters.
A low restriction inlet is usually accompanied by a DIY sediment trap that uses off the shelf parts. This must be fitted to a non turbulent section of pipe (preferably at least 5 metres away from elbows and tees) so that the suspended sediment settles as bed load, in your case it would be fitted close to the tank. The sediment trap collects the bed load which is later discharged at high velocity through a reduced size pipe. The trap makes a huge difference to the amount of sediment that would normally find its way to the tank and a quick flush every so often that wastes minimal water is all that is required. The water that remains in the wet system is the best (last) water to enter the pipes.
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