Charged/Wet Rainwater Head

lychees · Jan 28, 2023 2:55 pm

Hi all,

This has been somewhat discussed in other threads but my specific case hasn’t been covered.

I have a 10,000L water tank filled by a charged system that has 6, 100mm downpipes around our house. I have a laser level and have measured roughly 20cm of head from the top of the tank riser to the highest fill point on my leaf diverters. Now I understand this is less than ideal in terms of head as during heavy downpour the downpipes are currently overflowing, but that is how the system has been setup and the tank is on a concrete slab so my only option would be to move it which is less than ideal.

I have read on other threads that head can be increased by adding a low restriction inlet lower in the tank.

1. Can I tee into the existing pump suction outlet to flow into the tank to increase head or should I not be lazy and drill another hole?
2. What options are there for install if I drill another hole, my tank provider only has 50mm inlets as the biggest inlet size? I don’t think this would be large enough compared to my flow rate during heavy downpour.

SaveH2O · Jan 28, 2023 7:33 pm

Having leaf diverters allows you to divert directly into the tank and a 50mm low restriction inlet will provide a good supplementary flow rate because the inlet's available head is the varying height of the water in the tank Vs the constant height of a vertical riser above a tank. There are some additional minor friction losses, for example when the water enters the tank but we can safely ignore them in this instance.

A velocity of 1 mps through a 50mm inlet = a flow rate of approximately 120 lpm. Using the friction loss calculator found at https://www.nationalpump.com.au/calcula ... alculator/ we find that 2.182 m head is required to deliver 120 lpm through a 100m length of 50mm pipe which is 0,02182 m (21.8mm) per metre of pipe.

Note that using a calculator that uses the Hazen Williams flow rate equation is preferred but the one used here is simple, uses metric measures and arrives at a similar result regardless.
Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅
Water at the short branch pipe will be already subjected to the (friction loss) reduced head pressure in the 100mm pipe at the 100mm x 50mm junction's branch offtake and a flow rate calculation which also has to allow for friction losses through fittings calculated as additional pipe lengths will still provide much more head than the 22mm required if using let's say just one m of 50mm pipe.

The linked video demonstrates friction losses at various points along a pipe. Pressure equates to head as can be seen in the piezometers.

https://www.youtube.com/watch?v=_hSL9_eo4n8

What I do is measure the tank's water level height when full and the distance between it and the top of the vertical riser as the guaranteed additional head. There is no need to finess too much with calculations when establishing the effectiveness of using a low restriction inlet to supplement the inflow rate but obviously a 1/4 full tank will have more flow through a low inlet than a 3/4 full tank will.

The low restriction inlet valve needs to be a minimum 100mm above the bottom of the tank at the inlet's lowest point but be aware that many distributors measure the inlet's height from the centre of the hole. Also fit it within 75 degrees of the outlet supplying the pump.