@SaveH20 - Is there an upper limit of tank size before the water pressure exceeds the burst pressure (of joints more likely) of 100mm DWV? I am looking at a single 200KL Rhino tank
The two issues I have with a low point connected wet system is
a) failure of the joints in the 100mm DWV sections. If my 200KL tank is full and the pipe burst or joint failed, I will have 150KL+ worth of flooding! plus the loss of all that precious water.
b) the lack of ability to place a first flush device into a fully charged wet system. In my opinion a wet system must have a first flush device fitted just prior to the tank inlet. In times of low/nil flow, the water in underground 100mm components could be very stagnant. However I am unable to envisage how you could do that and still have a fully charged system under tank pressure.
RiH
Hi RiH,
A standard wet system that fills through the tank's top meshed inlet is taller and will always exert more pressure than a bottom fed system because the vertical riser and downpipe(s) remain charged at a height above the tank's inlet unless flushed whereas a low feed system only retains water to the level of the water in the tank.
Re tank size, the weight of the water in the tank and the head pressure are both separate. One metre of head is 9.81 kPa or 1.422 psi. If your 200k Rhino tank was half full, the water would weigh 100 tons but if that water was only 1.5 metres deep, the tank's pressure exerted would only be about 14.7 kPa plus the additional depth of the buried wet system pipe. If you had a standard wet system with a vertical riser and the riser was 3.5 metres high, it would be exerting about 34.3 kPa plus the additional depth of the buried wet system pipe.
Plumbers habitually (but unwisely) bury 90 mm PVC-u stormwater pipe for use with wet system and generally get away with it despite the flimsy 2 mm thick walls. Manufacturers also use it for first flush pipe. Hmmm!!!
The 100 mm (104.2 mm internal diameter) DWV pipe generally used is the SN6 that has a wall thickness of 3 mm and is very strong but it is not pressure rated. Given the low pressures involved, there is no need for concern. As an aside, I manufacture a first flush kit that requires 100 mm pipe and there has never been a hint of trouble. The 100 mm PVC-u pipe has about 46% more volume than the 90 mm PVC-u stormwater pipe.
I have seen quite a few 90 mm pvc stormwater pipes cracked, distorted and leaking at the fittings and this is usually caused by ground movement. This exerts more pressure on pipes than the internal water pressure ever would.
The question about water loss in the event of leakage is a good one and I don't have a definitive answer. I guess that a leak would soon be seen and maybe something like the valve in the link below might offer some protection but as to whether the low water pressure in the tank would (ironically) negate its effectiveness is something that maybe someone else can comment on.
http://www.spec-net.com.au/press/1208/swt_101208.htm
Having a first flush fitted just prior to the tank inlet is actually the worst place to have one. Lets say you have one a couple of metres long, a 90 mm pvc pipe would hold 11.6 litres and a 100 mm pvc pipe will hold 17 litres. The worst anaerobic water (in a standard wet system) is held in the horizontal pipe with decaying debris and no air/water interface. When it rains, the water that enters the flush pipe draining off the vertical riser will be the water retained in the riser and not the worst of the water stagnating away in the pipe or the first flush that will still be in the downpipe at the other end.
The best place to have a flush kit is at the downpipe but sometimes it is difficult when there is limited available head. One unit to look at is the Safe Rain http://www.saferain.com.au/ as it is able to divert larger amounts of flush but the best solution is to drain off the wet pipe during periods of little or no rain and put that water to good use.
You can incorporate a DIY sediment trap come first flush into the horizontal pipe and I discussed it in the thread below.
viewtopic.php?f=35&t=59393
The catch with diverting a small branch pipe into a low plumbed inlet is that there are a lot of different pipes and fittings and you can't 'directly' connect from one type of pipe to another. Things can be adapted and for most installations that use 100 mm DWV pipe, using DWV pipe reducers and smaller DWV female threaded fittings with poly threaded fittings (poly directors etc) will adapt the DWV pipe to many other fittings.
EDITED 13/08/2014: Made head pressure references more precise.