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Rain tank overflow plumbing

(16 posts) (5 voices)

Started 6 years ago by monchochos
Latest reply from Eco

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monchochos
Member

I have a rain tank at the rear of my block, behind the house. The only storm water connection is at the front of the block. I don't think it's feasible to plumb 90mm pipe all the way from overflow to the front because the only access is down one side of the block which is a driveway. Plus I'd have to cross under the driveway and back up to the storm water point.

Anyone got a good solution?

What I'm thinking of doing is connecting some type of poly pipe to a bottom outlet in the tank, running that around to the storm water point then going up with a riser that is hight adjusted to lower than the tank overflow, then down to the storm water point. The idea being that when water level reaches the hight of the vertical riser out the front of the house then it will drain down to storm water, rather than going out the overflow outlet.

Will this work? What size pipe can I get away with? I'm hoping that something like 25mm poly might do.

Posted Tuesday 22 Mar 2016 @ 1:00:13 pm from IP #
Diver
Member

Check out the Supadiverta, you simply height adjust it on the downpipe to be a couple of cm below the bottom of the overflow outlet and the tank will never overflow...you could harvest 100 downpipes to the tank and still not need an overflow.

http://supadiverta.com.au/overflow-pipe.html

Note that water diverted directly into a tank must pass through mosquito proof mesh (less than 1,000 microns). The Supadiverta has a 750 micron meshed filter that water passes through to enter a mosquito proof reservoir.

Disclosure: The Supadiverta is one of my company's products.

Posted Tuesday 22 Mar 2016 @ 4:11:22 pm from IP #
monchochos
Member

I've seen the supadiverta and I plan to use it for a new installation soon. However it's not going to work for this case. I guess I'd like to use the height adjusting overflow principle of the supadiverta with just standard fittings. Would 25mm pipe be enough for a run of about 35m?

Posted Tuesday 22 Mar 2016 @ 10:00:26 pm from IP #
ghostgum
Member

I would use the same diameter for the tank overflow pipe as you use for the pipe from the downpipes to the tank. They need to carry the same flow rate.

Posted Tuesday 22 Mar 2016 @ 11:57:16 pm from IP #
monchochos
Member

It's not easily done with 90mm. I was hoping I could use a sort of wet system with smaller diameter pipe that can support the similar flow rates.

Posted Wednesday 23 Mar 2016 @ 1:42:40 am from IP #
Benny
Member

Diver is the real expert here monchocos but I don't think you'd get a good flow through the 25mm pipe. When you install it with "going up with a riser that is height adjusted to lower than the tank overflow" this means you will only have a very small head to push the water through the 25mm pipe - just the difference in height from top of riser to bottom of tank overflow. Also the 25mm pipe will be 35m long!

Of course the flow rate you need through this pipe depends on the rain flow rate in to the tank - how many downpipes are servicing the tank ? what roof area ? what potential rainfall rate ? In my experience even just a short length of 25mm pipe used directly from the overflow outlet (with full tank depth as the head pressure) can back up the water in the tank.

Posted Wednesday 23 Mar 2016 @ 2:06:12 am from IP #
Diver
Member

monchochos said:
I've seen the supadiverta and I plan to use it for a new installation soon. However it's not going to work for this case.

Is the block flat and do the downpipes currently divert water to the stormwater? If those answers are yes, then why wouldn't it work when your simplest solution is to eliminate the need for an overflow pipe on the tank?

monchochos said:
Would 25mm pipe be enough for a run of about 35m?

I take it that you are asking about your 25mm internal standing drain pipe idea.

There are three simple rules:

1. Doubling a pipes internal diameter increases the internal volume by X4.

2. If you double the pipe's internal diameter and the head remains the same, you will increase the flow rate by approximately X6.

3. To double the flow rate, you need to increase the head X4.

Have a play with the simple flow rate calculator that I have linked below. You can change the settings, for example the diameter from cm to mm and the discharge rate to litres per minute. You also have to make an allowance for the pipe not being perfectly straight and so your 35 metres may well be an effective 38-40 metres when expressed as friction loss.

http://www.calctool.org/CALC/eng/civil/hazen-williams_g

Posted Wednesday 23 Mar 2016 @ 4:10:30 pm from IP #
Diver
Member

Benny said:
I don't think you'd get a good flow through the 25mm pipe. When you install it with "going up with a riser that is height adjusted to lower than the tank overflow" this means you will only have a very small head to push the water through the 25mm pipe - just the difference in height from top of riser to bottom of tank overflow.

The head will be generated in the pipe's vertical drop and measured from the top of the water above the riser to the stormwater connection. This confuses many, you are not alone.

A submerged pipe in the scenario described would also need to be submerged approximately 3.5 times the pipe's internal diameter to stop drawing a free surface vortex, this in turn would lower the internal standing overflow pipe's height as well as the tank's highest water level. If it was set to a height that was just below the overflow, the vortex would prevent the pipe from operating at anywhere near peak efficiency. The pipe will also operate with a much reduced efficiency until the height of water above the standing pipe's orifice stops the vortex.

Of interest, smaller (not micro) bubbles have an ascendency of about 0.25 metres per second. A 25mm diameter pipe has an internal volume of little more than 0.5 litres per metre, this means that if the pipe was flowing at one metre per second, the flow rate would be approximately 30 litres per minute...or would it? The answer is no if there was a vortex delivering air to the pipe.

There are four stages of weir flow pipe purging...weir, plug, bubble and full flow. Bubble flow is reasonably high velocity with no air pocket in the standing pipe but even 10% air entrainment will result in a 16-17% lower flow rate than there would be if there was no vortex and a resultant higher flow velocity.

Posted Wednesday 23 Mar 2016 @ 4:53:19 pm from IP #
Diver
Member

Hi monchochos,

I have understood your 25mm water tank overflow pipe idea as it having a riser inside the tank due to the reference to connecting the hose to an outlet "in the tank', the same as a WaterBoy hose would be connected to an outlet "in the tank".

http://www.crystalclearwater.com.au/waterboy/

After reading Benny's post a few more times and then yours again, it seems that you could actually be intending to connect the hose to an outlet on the outside of the tank (with no internal riser) and then run the hose near to the stormwater connection at the front of the property where you would plumb a riser that would be height adjusted to be just below the height of the water tank's top overflow outlet.

If the above is the actual intention, it will not work are you would require.

The minimum air flushing velocity required for 25mm pipe is 0.48 m/s, a flow rate of 0.24 L/s or 14.4 LPM. If the air at the top of the riser is not flushed, it will act as a physical barrier to impede the flow.

25mm LDPE has an internal diameter of 25.4mm.

If you lowered the riser to achieve a dynamic head that equated to the minimum air flushing flow rate required, you would also have a much lower retention level in the tank.

You would also be adding additional friction losses in the longer pipe length and the 90 degree elbows due to fitting the external riser.

If you did provide the required dynamic head to purge the air at the top of the riser, then the vertical drop from the top of the riser to the stormwater connection would syphon the tank unless you fitted a check valve or provided manual intervention.

Regardless of the above, a 25mm LDPE overflow pipe is too small for your discharge requirement.

Posted Friday 25 Mar 2016 @ 4:41:23 am from IP #
Benny
Member

As I said - you're the expert Diver and thanks for correcting my "head" calculation. My understanding of what he was trying to do is as per your last post - a 25mm pipe connected to outside of tank - at overflow outlet. I did this once and even with the 25mm pipe dropping straight to the ground - no 30mm extension - the tank would back up and overflow out the in heavy rain. Monochros hasn't given us any idea here about what the likely inflow rate might be.

Posted Friday 25 Mar 2016 @ 7:42:17 am from IP #
Eco
Member

I am having trouble understanding your problem. Don't you have downpipes at rear of your house at present? If you do those downpipes would have to connect to the stormwater discharge point at the front of the house now. So there would have to be storm water pipes from the back to the front of the house. Or is the guttering and roof designed to take all the roofing rain water to the front of the house - which would be an unusual design.

Posted Friday 25 Mar 2016 @ 8:46:18 am from IP #
monchochos
Member

Diver, you've basically understand my plan in your last post. Probably needs a diagram to be completely clear though. The riser at the storm water point would have some sort of small reservoir that's open to the air with an outlet to the storm water. Hard to explain. Think of a supadiverta working in reverse, can that work. The block slopes down to the back so air locks would be an issue i guess.

For everyone's info, the rear down pipes harvesting the back half of the house don't have storm water point. They were originally flowing into a small tank with overflow going onto the lawn. The front down pipe drains to the street storm water outlet. So there is no storm water plumbing from back to to front, if there was I'd be using it.

Posted Saturday 26 Mar 2016 @ 10:43:30 am from IP #
Eco
Member

Thanks for the explanation - background is always useful. So if you don't connect you are no worse off than before.

Diver indicated that 25 mm poly with water flowing by gravity is not sufficient. So you may wish to consider novel solutions.

Do you still have the original small tank? (It sounds like you are replacing this tank with a bigger one.) Would you consider using it as a storm surge tank. It would fill with overflow during the storm and slowly discharge over a longer period through the 25 mm pipe. Whether is was sufficient would depend on the size of the tank and the size of the storm and the area of roof that is being collected.

Could consider pumping the excess. Do you have a pump connected to this tank? If so connect a solenoid valve on a dedicated line to the storm water waste. The solenoid valve would be triggered by a float switch. Still might not have sufficient capacity. Active systems are not preferred.

These are not solutions typically used in domestic situations - to my knowledge. (A bigger pipe run by gravity is clearly the preferred but not available option.)

Posted Saturday 26 Mar 2016 @ 11:58:46 am from IP #
Diver
Member

Benny said:
As I said - you're the expert Diver and thanks for correcting my "head" calculation.

Benny...you were correct. I was wrong in understanding that there would be a riser inside the tank.

Posted Saturday 26 Mar 2016 @ 4:34:07 pm from IP #
Diver
Member

Eco said:
Do you still have the original small tank? (It sounds like you are replacing this tank with a bigger one.) Would you consider using it as a storm surge tank. It would fill with overflow during the storm and slowly discharge over a longer period through the 25 mm pipe. Whether is was sufficient would depend on the size of the tank and the size of the storm and the area of roof that is being collected.

After reading the further information, what Eco has so well explained above is the simplest solution provided the original small tank has a sufficient height and detention capacity.

The first step would be to determine the height on the small tank that provides neutral head to the stormwater connection to determine your remaining available head.

Could you still retain the small tank's overflow as an emergency overflow provision during extended heavy rainfall?

Posted Saturday 26 Mar 2016 @ 4:36:45 pm from IP #
Eco
Member

I just re-read Monchochos post - "the block slopes down to the back". Obviously that is why the downpipes at the rear were not connected to the stormwater discharge point. The slope (and height of tank) would also impact on the effectiveness of the surge tank idea - as Diver indicated. If it is too much the surge tank idea is not going to work because it still relies on gravity flows and gravity flows need height difference. Can you tell us the difference in height between the storm water discharge point and the rear of the house - where the tank will be. And the size and height of the small tank, if one exists. (There has really been too much guessing in this - we still haven't been told the area of the roofing being collected from. This is vital to know to get some idea of max flow rates that need to be catered for.)

Posted Saturday 26 Mar 2016 @ 11:50:36 pm from IP #

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