Jump to content
psyclones

Screw downpipe

Recommended Posts

2 hours ago, J.C.MacSwell said:
On 10/10/2020 at 7:08 PM, studiot said:

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

That doesn't make sense.

 

Don't ask me, you wrote it and I merely quoted it.

2 hours ago, J.C.MacSwell said:

You still haven't replied to this:

Well you haven't specified the issue properly and I'm not a mind reader so here is your answer.

Just identify where on the chart your barrels fall.

drain1.jpg.495c90e545c8e5a379e226b12227eff6.jpg

18 minutes ago, Endy0816 said:

@psyclones:

I've been reading where vortex breakers are used to improve roof drainage, but don't see any examples where a screw inside the pipe is used or know if a screw would work.

https://en.wikipedia.org/wiki/Vortex_breaker

http://www.roofingmagazine.com/retrofit-roof-drains-feature-integrated-vortex-breaker/

 

Interesting find. Thank you +1

Edited by studiot

Share this post


Link to post
Share on other sites

From Endy's last link was this one to the manufacturers site;

http://www.omgroofing.com/

PRODUCT DESCRIPTION

"The OMG Hercules-Plus RetroDrain features the OMG vortex breaker technology integrated into the cast aluminum strainer dome. With vortex breaker technology, the Hercules-Plus can provide up to 2.5 times greater water flow, compared to the existing Hercules drain, which means that water and excessive weight can leave the roof quickly. In addition to faster water flow off the roof, the vortex breaker can greatly reduce "chugging" which can damage plumbing systems."

Roof-Drains-OMG-Roofing-Products.png.47ff814a58f524154299e022984cdf07.png

This is pretty much the opposite of the screw design of the original post. Probably a different solution to the same problem though. That chugging mentioned above is that oscillation that slows the flow that I think both ideas are trying to eliminate.

On 10/8/2020 at 9:37 PM, arc said:

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. 

 

Share this post


Link to post
Share on other sites
3 hours ago, studiot said:

 

Don't ask me, you wrote it and I merely quoted it.

 

An unbroken column of water can exist at partial vacuum pressures, generally down to the vapour pressure of the water. I have no idea why you would suggest otherwise.

 

3 hours ago, studiot said:

 

Well you haven't specified the issue properly and I'm not a mind reader so here is your answer.

Just identify where on the chart your barrels fall.

As I suspected from your posts....you don't know the answer.

Share this post


Link to post
Share on other sites

My apologies I haven't chimed in yet. (I need to switch on notifications from this forum if that possible). 

To answer your question studiot, I don't have any more information - this was just a story told to me.

What made me question it was the credentials of the guy who designed it, who I'm guessing know very little if anything about fluid mechanics.  

I just thought it was a very interesting problem.

But as the discussion has unfolded - modelling the drain pipe as an open channel flow can get quite tricky. So details an assumptions about the problem are critical to solving it, which is what I thought. 

But thank-you all. 

 

Share this post


Link to post
Share on other sites
1 hour ago, psyclones said:

My apologies I haven't chimed in yet. (I need to switch on notifications from this forum if that possible). 

To answer your question studiot, I don't have any more information - this was just a story told to me.

What made me question it was the credentials of the guy who designed it, who I'm guessing know very little if anything about fluid mechanics.  

I just thought it was a very interesting problem.

But as the discussion has unfolded - modelling the drain pipe as an open channel flow can get quite tricky. So details an assumptions about the problem are critical to solving it, which is what I thought. 

But thank-you all. 

 

knew not know.

Share this post


Link to post
Share on other sites
On 10/20/2020 at 8:35 AM, psyclones said:

But as the discussion has unfolded - modelling the drain pipe as an open channel flow can get quite tricky. So details an assumptions about the problem are critical to solving it, which is what I thought. 

Yes the devil is in the detail.  Sorry there was not more of it. +1

 

On 10/12/2020 at 4:56 AM, J.C.MacSwell said:

As I suspected from your posts....you don't know the answer.

I stopped posting in this thread because of the long absence of psyclones and the personal turn of your conversation in respect of matters of which you have no knowledge.

FYI I am currently developing (elsewhere) an mathematical model of flow in a  similar situation, that of the drip rate of medical giving sets.
This is about a pipe stuck into a bag (not a barrel) of fluid and how to achieve a predetermined rate of flow.

So yes, I do understand the fluid mechanics of the situation.

Share this post


Link to post
Share on other sites
2 hours ago, studiot said:

 

So yes, I do understand the fluid mechanics of the situation.

You seemed fixated on the air blockage, or potential air blockage.

So I asked this question:

 

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?

It's not complicated, there is no air blockage to distract from the effect I was describing, and you can't answer it correctly without considering the effect I was describing.

 

Edited by J.C.MacSwell

Share this post


Link to post
Share on other sites
30 minutes ago, J.C.MacSwell said:

This question suggests otherwise:

 

If you weren't so know-it-all, holier-than-thou rude about this then we could perhaps have a more dignified conversation about a statement you made but falsely attributed to me.

 

When you introduced the unbroken column of water in the pipe I take this to mean that the pipe is full to the brim with water.

Therefore there is simply no space left for air, vacuum or anything else.

I even talked about a plug of water (as being a continuous segment of the pipe full bore with water) which drops down the pipe.

We do not know the pressure conditions below such a plug.
If the pipe connects directly to further pipework below then the pressure will be increased, not reduced.
If there is a vent then air can escape and this will have a slightly lower standing pressure as that air now has velocity.
If there is a long drop then the usual situation is that the air and water become partially mixed and a gurgling of water/air mixture comes out at the bottom.

But I repeat that we do not know the conditions of discharge.

Equally we do not know the conditions of entry.

Water leaving (the bottom or actually any point) of a reservoir of liquid forms various pressure patterns/ regimes depending upon the approach shape of the entry region to the exit.

A pipe, for instance, stuck up into the fluid in the  reservoir will induce an entirely different exit flow regime from one flush with or even tapered to the walls or floor.
There are even tables of coefficients for sharp edged transitions v gradual transitions between pipe and wall/floor.

So when I said that you did not provide full details of your barrels I only told you what any fluid mech engineer would have told you.

In fact I provided a wide ranging chart for a whole range of different entry conditions at your pipe.

Your question was rather like my brother asking me for directions to Newport, without even telling me which continent he was sailing for.

Share this post


Link to post
Share on other sites

image.png.c038b5b28655d1469451491f176da802.png

I think this is the effect they may be trying to achieve.

1 hour ago, studiot said:

Your question was rather like my brother asking me for directions to Newport, without even telling me which continent he was sailing for.

His brother might point that out, while they were vacationing together in Rhode Island.

 

Share this post


Link to post
Share on other sites
41 minutes ago, J.C.MacSwell said:

image.png.c038b5b28655d1469451491f176da802.png

I think this is the effect they may be trying to achieve.

Hopefully I will not be called upon to criticise/analyse these sketches.

Where did they come from ?

 

You have repeatedly mentioned siphonic action.  I see no siphonic action in those diagrams.

Here is Wikipedia on the subject. Please note what they say.

 

Quote

https://en.wikipedia.org/wiki/Siphon#Vacuum_siphons

 

Siphonic roof drainage

Despite the name, siphonic roof drainage does not work as a siphon; the technology makes use of gravity induced vacuum pumping[66] to carry water horizontally from multiple roof drains to a single downpipe and to increase flow velocity.[67] Metal baffles at the roof drain inlets reduce the injection of air which increases the efficiency of the system.[68] One benefit to this drainage technique is reduced capital costs in construction compared to traditional roof drainage.[66] Another benefit is the elimination of pipe pitch or gradient required for conventional roof drainage piping. However this system of gravity pumping is mainly suitable for large buildings and is not usually suitable for residential properties.[68]

 

I did wonder why the screw drawing carried the screw all the way down the pipe.
Surely it would be enough to have a short section of helix to induce the vortex, perhaps combined with an entry baffle.
Although entry baffles would surely have been fitted anyway; debris protection is not restricted to 'siphonic'  systems of any pedigree.
I also wondered if it would be cheaper to simply insert new smaller pipes if the existing ones were hydraulically oversize. After all formed helices don't come cheap.

Share this post


Link to post
Share on other sites
7 minutes ago, studiot said:

Where did they come from ?

This site. Scroll down about 1/3 of the page to find the diagram.

Share this post


Link to post
Share on other sites
9 minutes ago, Area54 said:

This site. Scroll down about 1/3 of the page to find the diagram.

Thank you. +1

 

Share this post


Link to post
Share on other sites
On 10/10/2020 at 2:40 PM, J.C.MacSwell said:

 

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.

 

Share this post


Link to post
Share on other sites

Here is a quote from the UK Technical Manual of siphonc drainage.

Perhaps not all non UK companies do not bear this in mind.

Quote
They must not use misleading advertising,make false claims about benefits of a particular product, or denigrates another.

http://www.siphonic-roof-drainage.co.uk/images/SRDA_An_introduction_to_siphonic_drainage_Nov2014_lowres.pdf

 

In general such systems must be designed from the outset of the building, especially as 'siphonic' drainage can only work on a flooded roof.

Share this post


Link to post
Share on other sites

 

2 hours ago, studiot said:

Hopefully I will not be called upon to criticise/analyse these sketches.

God forbid.

 

2 hours ago, studiot said:

 

You have repeatedly mentioned siphonic action.  

 

I have repeatedly tried to describe it to you. I hadn't used the term. I did refer to breaking siphon and maintaining the siphon early on and when you pointed out there was no siphon I suggested I may be using the wrong term as the water is never "drawn" uphill.

The principle is the same. 

Share this post


Link to post
Share on other sites
On 10/20/2020 at 8:28 PM, psyclones said:

What made me question it was the credentials of the guy who designed it, who I'm guessing know very little if anything about fluid mechanics.  

This statement was NOT aimed at you studiot.

It was talking about at the guy who told me the story - which I realize now maybe I shouldn't have mentioned.

Knowing the details of the problem I think is sort of beside the point. What I was trying to find is simple conceptual flaw in the screw design – if there was one.  

You don't have to insult me. 

Share this post


Link to post
Share on other sites

 

On 10/24/2020 at 10:18 PM, studiot said:

I stopped posting in this thread because of the long absence of psyclones and the personal turn of your conversation in respect of matters of which you have no knowledge.

I realise I have not been as involved in the discussion as every one else. I may have created the thread, but in my opinion I'm not the "Arbitor" of the thread, people can say what they want. All insights and comments are welcome!

I realised I read you post it out of context - So again, sorry.

 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.