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Browse Forums Eco Living 1 Feb 23, 2023 1:43 pm Hi I've come to this forum as I've noticed it's a wealth of knowledge of all things rainwater harvesting. I work in water tank sales and often have to help customers with their tank fitout. Usually it's just advise on where to position their various inlet/outlets, as most people order a standard fitout, however sometimes people have unusual requirements that really require someone with plumbing knowledge to advise. That is not me but I'd like to learn more about rainwater harvesting and help out where I can also. Anyway, so was wondering if anyone could help with this query. My client has a 150mm downpipe from his house that he wants to feed into his tank. His plans showed 100mm downpipes and 100mm overflow, which is normally fine. But since he increased his downpipe size we'd normally recommend increasing his overflow to match. However, his tank also has stormwater detention fittings on it, and upgrading everything to 150mm will just make the setup too costly. So, I was going to suggest he consider using 2x 100mm overflows on the tank instead, one going straight to stormwater, the other with his OSD fittings connected to it. So my questions are, 1) is that sound advice? and 2) how many 100mm overflows would actually be needed? At first I thought 2x 100mm overflows would be more than enough, but a colleague informed me that the water flow through 2x 100mm pipes would be less than 1x 150mm pipes. I tried to find this flow rate calculator online, but the calculators coming up were a bit confusing asking for details I wasn't sure about. I just wanted to give a diameter and get an answer. Can anyone please assist with this question? Thanks! Albri Re: 150mm downpipe into RWT, how many 100mm overflows needed 2Feb 23, 2023 5:46 pm Will the 150mm downpipe discharge to a charged system with a 150mm DWV pipe culminating as a vertical riser or will the downpipe drain as a 'dry system' pipe to the tank? I hope for your customer's sake that he/she doesn't have a 150mm subsurface pipe all the way. If so, a large section of the pipework will lack vital sediment flushimg velocity! The important things to know when designing a tank's required overflow capacity are: (1) The regions 1:20 Average Recurrence Interval (ARI). (2) The roof plan area when all downpipes drain to the tank. (3) If only one or some downpipes drains to the tank, the roof harvest area based on a roof slope multiplier found in AS 3500.3 that factors wind driven rain must be used. Knowing the tank size and its roof shape also helps determine the tank's mitigation capacity above the overflow outlet as a detention tank's regular overflow outlet's required discharge capacity should still be calculated as it would be for a normal retention tank. Do you know the property's Permissable Site Discharge (PSD) through the tank's smaller detention overflow? 1 and 2 will let you calculate the amount of water draining to a tank during a minimum qualifying 1:20 ARI but I would add a minimum 20% safety margin to this figure. The smaller detention outlet may provide this margin as an added reserve should the tank fill during a prolonged high intensity rain event. Having a 150mm DWV pipe suggests a high inflow rate. Does this downpipe drain an eaves gutter or does it drain a box gutter? A vertical overflow pipe drains faster than a horizontal overflow pipe with the same height of water above their respective crest or invert.AS/NZS 3500.1 Storage Tanks has two diagrams with tables that show the unmeshed overflow capacity of various size horizontal and vertical pipes. If the overflow outlet is meshed, there will be a flow restriction and taking 15% off any arrived calculations (assuming the mesh is clean which is rare) would be wise. Always err on the side of caution.A (nominal) 100mm SN6 DWV overflow pipe has an internal diameter (ID) of 104mm and an outside diameter (OD) of 110mm. After allowing for the usual flow restricting 'flashing' on the mesh fitting's outer extremity and the mesh itself, this makes the 100mm horizontal flow rate figures in the AS tables roughly 8% optimistic when applied to a 100mm DWV overflow pipe's meshed horizontal outlet.To rid a horizontal outlet of the restrictive mesh to improve the flow capacity, simply fit an Air Gap filter to the exterior overflow pipe. https://rainharvesting.com.au/products/ ... s/air-gap/ A vertical intake that connects via a 90 degree elbow inside the tank to a meshed horizontal outlet is not flow restricted by a meshed fitting unless the mesh is substantially blocked. This is unlikely due to the high velocity turbulent water falling down the vertical intake.If you do fit an internal vertical overflow elbow, you will need to drill the overflow outlet's hole in the tank lower than you would when fitting a standard horizontal outlet so that the top of the vertical pipe (elbow) inside the tank is level to where the bottom of the hole (the invert) would be if fitting a standard horizontal outlet. If the overflow capacity is excessive, the top of the elbow can be higher which will increase a tank's storage capacity if used only as a retention tank. Below are discharge rates shown as L/sec for the differing heights of water above an unmeshed 100mm vertical pipe's crest as per AS/NZS 3500.1 Storage Tanks. Add about 8% for a nominal100mm (104mm true internal diameter) DWV pipe.100mm....125mm....150mm....175mm....200mm 6.60 L/s...7.38 L/s...8.08 L/s...8.73 L/s...9.33 L/s The respective flow rates through an unmeshed 100mm horizontal overflow outlet are:4.67 L/s...5.72 L/s...6.60 L/s...7.38 L/s...8.08 L/s. If a 100mm vertical drain pipe still proves to be insufficient for your customer, there are ways to increase a tank's overflow capacity.The first is automatic if the tank has a roof shape that allows for a large mitigation capacity between the vertical pipe's crest and the tank's top meshed inlet's overtopping level. Secondly, you can increase the weir wall length by bellmouthing the weir. As an example, a 150mm ID vertical bellmouth will flow at 15.84 L/s (950.4 lpm) with a water depth of 100mm above the crest. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ A third way is to create more weirs to supplement the top weir. The easy way to do this is to fit the (nominal) 100mm DWV upturned elbow so that its crest is about the 1/2 way mark up where the horizontal outlet otherwise would be. Before fitting the elbow, you would drill a horizontal row of spaced holes about 15mm below the top of the elbow to supplement the existing weir. 7 x 32mm holes are a good choice. albri His plans showed 100mm downpipes and 100mm overflow, which is normally fine. But since he increased his downpipe size we'd normally recommend increasing his overflow to match.You need to stop giving this blanket advice! Depending on the amount of inflow via, for example, a wet system pipe, the same size overflow which is subjected to a vortex that maintains a large air core will not discharge with the same capacity as a charged inflow pipe that is full of water. When the inflow is from a dry system pipe, the gutter is drained by a vertical pipe whereas the tank's overflow intake is usually horizontal which is less efficient. The required discharge capacity must be hydraulically calculated. albri At first I thought 2x 100mm overflows would be more than enough, but a colleague informed me that the water flow through 2x 100mm pipes would be less than 1x 150mm pipes. I tried to find this flow rate calculator online, but the calculators coming up were a bit confusing asking for details I wasn't sure about. I just wanted to give a diameter and get an answer. Can anyone please assist with this question? Flow through different size pipes which are subject to friction losses and the discharge rates through a tank's horizontal or vertical outlet that connect to an external overflow standing pipe are two different things. Your colleague was erroneously referring to flow rates through pipes which has nothing to do with an overflow outlet's discharge capacity. Two 100mm horizontal outlets will outperform a single 150mm horizontal outlet right up until the water depth above the invert reaches about 160mm when it becomes reasonably equal.Plain vertical intakes are a different matter as once the water depth reaches 45mm, the 150mm pipe will outperform the two 100mm pipes, for example, with depths of 100mm, 125mm and 150mm, the comparative flow/discharge rates of the 100mm and 150mm vertical pipes are 6.60/15.84, 7.38/16.60 and 8.08/18.19 L/s. I commend you for taking steps to improve your customer service and I hope that this post has been of help to you and other readers. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter CheaperSupa Gutter Pumper. A low cost syphonic eaves gutter overflow solution.Had a new driveway installed with a 6 metre long 150 x 150 channel drain and have since found out it has 4cm of water sitting at the bottom below the side pipe… 0 4719 |

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