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psyclones

Screw downpipe

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Hi,

Someone told me after building a multi storey building. In one section of the building existing downpipe(s) weren't effective at transferring water from roof to ground level - water I believe was pooling on roof. 

To solve this, a screw design (see attached) downpipe was created. The screw design supposedly helped suck the water down the pipe.

There are two problems that appear to me straight away:

1. The velocity ignoring frictional losses down the screw design would be less because gravity forces are not pulling the liquid straight down, the smaller the incline (from horizontal) the slower the inlet velocity of liquid. 

2. The internal screw sheeting and core structure subtracts volume the flow could occupy in the straight downpipe, effecting flow rate. 

Or course this is highly simplified and I'm unsure of how centrifugal effects on water flow swirling around would affect flow rate and head losses.

There would be higher head losses per length of pipe, but I'm unsure if those losses would lower inlet pressure to below that of free falling water flow. 

Trying to fabricate and even clean would be just short of a nightmare. 

Although I think its novel an interesting idea, but I can’t see the design would be more effective than a standard downpipe? 

Like I said this was just a story, I didn’t actually get to see this.  I’ve attached cad model of how I think it would work.

What are your thoughts?

Image (3).jpg

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Hello psyclones,

I've seen a few different sizes of roof drains in action, they start to swirl on their own when the water overwhelms the drain diameter and I believe this is simply the cohesion of the water responding to the effect of gravity, the water acts in almost an elastic manner. Sometimes if the depth of the water is just right in relation to the pipe diameter you will see a oscillating cycle where the velocity increases and pulls the surface down until some air is pulled into the vortex which then suddenly slows the velocity and closes the vortex, then the cycle repeats again and again. The suction is quite dramatic on the larger drains. 

It sounds to me like the screw design is just enough to slow the flow to eliminate that oscillation cycle that has a substantial slowdown until the water accelerates down the pipe again leading to another slowdown. Keeping a steady flow at a speed just below that breakpoint would be the key and the screw angle would be set to that ideal flow rate. 

Like you said it could be a maintenance problem. But consider probably this is their "after the fact" solution for miscalculating the correct drain size.  With regard to the screw, I'm thinking it is just a drop-in insert at the roof and just long enough to dampen the unwanted effects. BTW in the last decade they seem to have doubled the diameter on these drains in newer construction.

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9 hours ago, psyclones said:

What are your thoughts?

 

An interesting topic.

My guess would be that the problem had less to do with downpipe sizes and more to do with inappropriate roof/gutter profiles.

It is necessary to develop a pressure head to force water through a pipe as you observe.
This is known as the backwater curve and often forgotten in  the hands of inexperienced designers.
 

Some information of the roof/gutter profiles would be useful.

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I can see one advantage.
At low flow rates drainpipes are sometimes noisy because water drops fall down them and hit the bottom with a thud.

That wouldn't happen with this.

But I simply can't imagine any way in which putting that spiral baffle in the way will make the water flow faster.

 

 

 

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Posted (edited)

Somewhat counterintuitive but It depends on how much drag it creates vs how much suction it can "hold" onto. A siphon can pull water down much faster than water free falling in some cases. You can get the added benefit of up to or as much as atmospheric pressure. You lose that as air gets introduced and breaks the "train" in the drain (wannabe poet).

Edited by J.C.MacSwell

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Part of me wonders if rifling the inside of the pipe would be better than adding a screw or leaving it alone 

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9 hours ago, J.C.MacSwell said:

Somewhat counterintuitive but It depends on how much drag it creates vs how much suction it can "hold" onto. A siphon can pull water down much faster than water free falling in some cases. You can get the added benefit of up to or as much as atmospheric pressure. You lose that as air gets introduced and breaks the "train" in the drain (wannabe poet).

More to the point, where does the air go ?

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3 hours ago, studiot said:

More to the point, where does the air go ?

If air gets introduced due to the partial vacuum and breaks the siphon it can get dragged down the drain or escape upward. 

Baffles and anti-vortex plates are commonly used to maintain siphon. I've never seen a screw down pipe as described but I think they are trying to achieve a similar result. More "pull" at the drain rather than simply spilling at the drain (somewhat of an oversimplified description)

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5 hours ago, J.C.MacSwell said:

If air gets introduced due to the partial vacuum and breaks the siphon it can get dragged down the drain or escape upward. 

Baffles and anti-vortex plates are commonly used to maintain siphon. I've never seen a screw down pipe as described but I think they are trying to achieve a similar result. More "pull" at the drain rather than simply spilling at the drain (somewhat of an oversimplified description)

There is no siphon effect in this case; quite the reverse in fact.
I was hoping my nudge would make you think about that.

:)

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1 minute ago, studiot said:

There is no siphon effect in this case; quite the reverse in fact.
I was hoping my nudge would make you think about that.

:)

what makes you say that?

Maybe I'm using the wrong term? The effect I'm talking about is the enabling of the water at the drain to accelerate faster than free fall.

I know there is no drawing of the water upwards first but the principle is the same.

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Imagine a down pipe full of air.

Now suddenly fill the top with a plug of water.

Unless the air beneath the water goes somewhere, it must suffer increasing compression as the water falls.

This in turn offers increasing upward force to the water, which is effectively saying a reduction in gravity, thereby slowing it down.

 

So I asked the question; where does the air go ?

Various methods are employed to vent or dispel that air.

 

The siphon/suction effect occurs when a charge of water (and perhaps other material) drops down past a side junction.
This can draw the air out of the side junction after the charge has passed. It can even be enough to empty the water traps in the side branch.

Again various methods can be employed to obviate this.

 

My guess is that the screw is meant to induce a swirl of vortex motion into the water to vent that air.
This is similar to letting the water out of a bath where there is section at the centre of the vortex which has no water and the air beneath escapes upwards.
When the bath is too full there is no upward vortex and the water empties more slowly.

But I also guess that part of the fault lies in the roof/gutter profile creating this plug of water at the entry hopper during heavy rain, whihc was why I asked about it.

@OldChemE  might be a sufficient pipework specialist to comment here, he has probably met this phenomenon many times.
 

Edited by studiot

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The air can be pushed down the pipe, assuming it isn't blocked. Assuming the resistance of that is not excessive, once in motion the unbroken column of water will be "pulling" water down at the mouth of the drain with a partial vacuum.

So it will drain faster than simply spilling in free fall if the conditions are right (the partial vacuum being greater than the resistance due to drag and any back pressure)

If the conditions are not right you don't get the benefit.

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8 minutes ago, J.C.MacSwell said:

The air can be pushed down the pipe, assuming it isn't blocked. Assuming the resistance of that is not excessive, once in motion the unbroken column of water will be "pulling" water down at the mouth of the drain with a partial vacuum.

That doesn't make sense.

How can there be both an unbroken column of water and a (partial) vacuum in the same pipe?

Edited by studiot

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5 minutes ago, studiot said:

That doesn't make sense.

How can there be both an unbroken column of water and a (partial) vacuum in the same pipe?

You can picture a static one readily, can you not? A vertical pipe closed on each end, with air below atmospheric pressure at the top?

Or picture same thing with a cap on the bottom and a pressure gauge at the top showing ambient pressure. Remove the cap.... 

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14 minutes ago, J.C.MacSwell said:

You can picture a static one readily, can you not? A vertical pipe closed on each end, with air below atmospheric pressure at the top?

Or picture same thing with a cap on the bottom and a pressure gauge at the top showing ambient pressure. Remove the cap.... 

If there is anything else in the pipe at all (other than pipe of course), the water column cann be 'unbroken'.

If you mean something else than please be properly specific.

Please also give some consideration to the mechanisms I expanded on for you.

They form part of the UK Building Regulations and I'm sure  there will have been many BRE papers on the subject preceeding the embodiment in regualtion.

Don't forget at the outset there is nothing but air in then pipe, not a column of water.

Before the pipe can be filled with water, that air has to be dealt with.

Edited by studiot

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1 hour ago, studiot said:


When the bath is too full there is no upward vortex and the water empties more slowly.


 

If the drain is attached to a pipe it can drain more quickly...after it starts going

21 minutes ago, studiot said:

 

Don't forget at the outset there is nothing but air in then pipe, not a column of water.

Right. But after it gets going there can be, and that can draw more water thru the drain at a faster rate.

When emptying the bath what do you hear when the vortex first draws air down the drain? A sucking sound.

Drainage generally becomes less efficient at that point as the (partial) vacuum is broken.

If you had two identical barrels of water, one with a 1 inch hole in the bottom and one with a 1 inch ID vertical pipe at the bottom (sticking out the bottom), says 12 inches long, which do you think would drain faster?

35 minutes ago, studiot said:

 

Before the pipe can be filled with water, that air has to be dealt with.

Obviously not much air to deal with in this example, but other than near drain traps there is plenty of volume to allow a head of water to start pushing a column of water into a pipe with little compression or back pressure. Eventually any excess air gets pushed down, entrained in the flow, or finally escapes upward.

Edited by J.C.MacSwell

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2 hours ago, J.C.MacSwell said:

 

Obviously not much air to deal with in this example, but other than near drain traps there is plenty of volume to allow a head of water to start pushing a column of water into a pipe with little compression or back pressure. Eventually any excess air gets pushed down, entrained in the flow, or finally escapes upward.

Obviously you are not listening to what I am saying so I will leave you with with this thought

Say a 10 foot vertical pipe, initially filled with air, experiences a sudden influx of water that blocks upward venting and compresses the air to one foot.

Then unless that air has somewhere to go it will now be at a pressure of 10 atmouspheres.

Thus it will exert an upward force 10 times that of the atmousphere above the water.

But by all means continue to fly in the face of established regulation and practice.

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1 minute ago, studiot said:

Obviously you are not listening to what I am saying so I will leave you with with this thought

Say a 10 foot vertical pipe, initially filled with air, experiences a sudden influx of water that blocks upward venting and compresses the air to one foot.

Then unless that air has somewhere to go it will now be at a pressure of 10 atmouspheres.

Thus it will exert an upward force 10 times that of the atmousphere above the water.

But by all means continue to fly in the face of established regulation and practice.

LOL.

Dynamics aside, It takes a 320 foot water column to get a head of 10 atmospheres. If the air can't push through (including at whatever pressure a real system has in practice)....how does the water?

So if your established regulated system is blocked (it happens) you have a problem. You have nowhere for the water to go either. The system is blocked. It has failed.

Regulations don't allow designing it work that way. I guarantee you.

 

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13 minutes ago, J.C.MacSwell said:

LOL.

Dynamics aside, It takes a 320 foot water column to get a head of 10 atmospheres. If the air can't push through (including at whatever pressure a real system has in practice)....how does the water?

So if your established regulated system is blocked (it happens) you have a problem. You have nowhere for the water to go either. The system is blocked. It has failed.

Regulations don't allow designing it work that way. I guarantee you.

 

 

Has someone repealed Boyle's Law whilst I was out walking the Mendips today?

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11 minutes ago, studiot said:

 

Has someone repealed Boyle's Law whilst I was out walking the Mendips today?

No, and you're still allowed to misconstrue results with it. If air can't get through, neither can the water. The fact that air often gets displaced upwards as well doesn't change that. Nor is it necessary.

I hope you enjoyed your walk though.

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From listening to both J.C. and studiot's opinions;

    In the states, and I assume where you folks are, both ends of these systems are open to the same atmospheres, and the pressures on both the ends are of little difference to each other. As I said earlier the waters cohesion forms and holds the vortex that stretches out until it collapses. But the air can and is coming out the top until the water depth/volume increases to the point the vortex completely forms at which time the atmosphere is drawn into the vortex and the air flow is only downward into the pipe. But the water's cohesion can only sustain itself so much and I'm wondering if that suction sound you hear that begins as the vortex is stretched out to its breaking point isn't air escaping back out the top again and not still going continually downward as I had initially thought. 

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23 hours ago, studiot said:

My guess is that the screw is meant to induce a swirl of vortex motion into the water to vent that air.
This is similar to letting the water out of a bath where there is section at the centre of the vortex which has no water and the air beneath escapes upwards.
When the bath is too full there is no upward vortex and the water empties more slowly.

But I also guess that part of the fault lies in the roof/gutter profile creating this plug of water at the entry hopper during heavy rain, whihc was why I asked about it.

@OldChemE  might be a sufficient pipework specialist to comment here, he has probably met this phenomenon many times.
 

Alas, there are many different types of work in Chemical Engineering and I never spent much time on pipe systems, but its nice to get a mention.  I, too, was thinking that perhaps the goal was to create a vortex action, but have not been able to come up with a proof that it would work.

It wold be interesting to know if anyone has ever actually done an experiment on this.

Edited by OldChemE
added last sentence

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22 hours ago, studiot said:

Say a 10 foot vertical pipe, initially filled with air, experiences a sudden influx of water that blocks upward venting and compresses the air to one foot.

I'm glad to hear the Boyle is not turning in his grave.

In my book this was clear enough.

Since volume = Area cross section x length reducing the length  by a factor of 10 reduces the volume by a factor of 10.

Boyle, rest his soul, requires that this increase the pressure by a factor of 10.

Please read more carefully.

I have, at no point stated the air would escape or be driven upwards, downwards or anywhere else.

I simply asked you where it would go.

Because the alternative is compression, whether for all or just some of it.

The phenomenon that if you try to pour too much liquid down a throat or pipe then the air will push back is well known. It can be observed in bottles, laboratory glassware, peipes, and a myriad of other installations.

The OP states quite clearly that there was too much water.

On 10/9/2020 at 3:13 AM, psyclones said:

I believe was pooling on roof. 

This can't have occurred 24/7 every day of every week of every year.
By psyclones seems to have abandoned this thread so I have no further information.

I restate my conviction that at present we do not have enough information to properly determine the cause.

 

 

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2 hours ago, studiot said:

I'm glad to hear the Boyle is not turning in his grave.

In my book this was clear enough.

Since volume = Area cross section x length reducing the length  by a factor of 10 reduces the volume by a factor of 10.

Boyle, rest his soul, requires that this increase the pressure by a factor of 10.

Please read more carefully.

I have, at no point stated the air would escape or be driven upwards, downwards or anywhere else.

I simply asked you where it would go.

Because the alternative is compression, whether for all or just some of it.

The phenomenon that if you try to pour too much liquid down a throat or pipe then the air will push back is well known. It can be observed in bottles, laboratory glassware, peipes, and a myriad of other installations.

The OP states quite clearly that there was too much water.

This can't have occurred 24/7 every day of every week of every year.
By psyclones seems to have abandoned this thread so I have no further information.

I restate my conviction that at present we do not have enough information to properly determine the cause.

 

 

Maybe you should? Where did I say your decrease in volume would not increase pressure as you suggested? 

I pointed out that it would take a head of 320 feet of water to get your 10 atmospheres of pressure. You or Boyle have a problem with that?

Ambient air at constant temperature needs to be compressed to gain pressure. This creates volume for the water to put additional pressure on the system.

This makes it unlikely for what is described in the OP (with regard to roof drainage, not filling bottles that have only one opening or similar problems) to have anything to do with your line of thought.

You also asked this:

On 10/10/2020 at 3:08 PM, studiot said:

That doesn't make sense.

How can there be both an unbroken column of water and a (partial) vacuum in the same pipe?

That doesn't make sense.

You still haven't replied to this:

On 10/10/2020 at 3:36 PM, J.C.MacSwell said:

 

If you had two identical barrels of water, one with a 1 inch hole in the bottom and one with a 1 inch ID vertical pipe at the bottom (sticking out the bottom), says 12 inches long, which do you think would drain faster?

 

 

Edited by J.C.MacSwell

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