Son-in-law question

[Skogsgurra]

My son in law is building a little "shed" for truck and lorry repairs. There is a quite large area for parking these big trucks and it accumulates water when it rains. The authorities say that he is allowed to let 3 litres/second into the sewer when it rains heavily. He is going to build a reservoir where rainwater can accumulate and then make a hole in it some 500 mm down. He now asks ME(!) what diameter that hole shall have to guarantee that water flow at maximum head shall not be more than 3 l/s.

Help! I can't tell him I don't know. So I think I should cheat and get help from the Pub.

Anyone?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[MikeHalloran]

An array of five (5) holes, 20.0 mm diameter, in a metal sheet or barrel wall, should be about right. The calculation is valid only if the holes are sharp-edged, i.e., no burrs and especially NO CHAMFER.

Deburring the holes or 'breaking the edges' will cause them to flow more, by an amount that I can't calculate because it depends very strongly on the achieved edge radius at a microscopic scale.



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

An alternative would be a single sharp-edged hole of 44.71mm diameter.



Mike Halloran
Pembroke Pines, FL, USA

 

[Skogsgurra]

Thanks a lot! Just what he asked about. I will be the hero tomorrow!

But, if pressed, I will reveal the true source of my wisdom.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[zdas04]

I always attribute to eng-tips.com. If you pretend to know everything, you appear to be an idiot. If you know who to ask any technical question in the world, then you appear to be one smart individual. I'd rather look smart than look like a know-it-all moron.

David

 

[dhengr]

Skogs:

If you don’t know that MikeH is pullin your leg just a little bit, you might have to reveal the “true source of your wisdom” and more than that, your personal lack thereof and mine too, on this particular matter. Whatever will happen when/if the sharp edges on those holes rust a little and become chamfers? But, he’s pointing you in the right direction. No offense Mike, I didn’t check your calcs. for size and number of holes. And, Skogs you oughta know all this stuff, isn’t their some analogy btwn. flow rate, pressure, and pipe size; and amps, volts, and wire size? Get out your electric meter out and figure this out, does that darn thing just read in units of amps and volts, switch it to l/s and pressure head.

There won’t be more water falling on the site from a given design rain storm, but its runoff rate from the shed roof will be quicker than exists now. So, your retention pond is the right idea and must be large enough to hold this difference in runoff/time to the sewer. Will the local soils absorb the water over a few days so that you can argue that you are actually reducing the flow to the sewer? Maybe install a few crushed rock filled drainage trenches across the flow lines on the site, to distribute and retain the water to a larger soil area, as in a septic drainage field. Is it a sanitary sewer where they dislike adding storm water to total treatment volume, or is it a storm sewer where they might be a bit more lenient and negotiate a bit? Can you run the water over a longer route to the sewer and through higher grass, or some such, which slows the flow. Can you do some regrading of the site to change the flow rate to sewer or change the direction of flow away from the sewer? Otherwise, a properly sized pipe set a foot above the bottom of the retention pond, or some such, will drain 3 l/s, with some increase in flow as the water level goes above that one foot elev. Is the little shed and the parking area all new impervious area? Or, is the little shed just replacing some impervious parking area, in which case the runoff rate wouldn’t change much. Talk with one of your local Civil Engineering friends, they should be able to help you resolve this with the city.

 

[Ron]

Gunnar....I would suggest a vertical overflow pipe (like a weir) instead of a hole in the bottom...which will clog with time...An overflow is much more controllable. Let the reservoir fill to a certain point then overflow to control the off-site flow. Slots in the pipe can be adjusted to create the restriction for the flow.

 

[francesca]

Why is he trying to send rain water into the sewer instead of letting it run off to the same place it's going now. It's roof water, right? So clean. Only dirty water needs to go into a sewer.

 

[Skogsgurra]

OK, he already knows I'm not so smart. So don't worry and don't preach. My reputation is since long gone in that family. Remember he married my daughter. And daughters know you aren't very clever. Enuff said.

Why it is necessary to do this? Probably because the site is wilderness now and the Örebro city wants to take care of the precipitaion in an orderly way. They just told him to do what he is doing. Build a large enough reservoir and something to control the flow rate.

OK, I know about the sqrt(2gh) thing and also about hole size and flow. But to take it from there is a little like knowing ohms law and then try to build something practical. I like to have the expert's view rather than fumble myself.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[rowingengineer]

I do like Ron's idea of the weir.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[zdas04]

Stormwater runoff is a big deal with the greenies right now. I somehow got on the subscription list for a monthly magazine on the subject. I don't read it, but thumbing through I was amazed at the lengths that municipalities are making people go to control it. One idea is to put huge cubes (about 3 m on a side) in the ground (the top is solid with a permeable material and the sides and bottom are a honeycomb), then park cars over the new retention pond. The stuff they're talking about in this magazine is horribly expensive. It sounds like they're letting Gunnar's son-in-law off fairly lightly.

David

 

[francesca]

I'm certain that the city does not want rain water entering the sanitary sewer. It dilutes the effluent, inundates the waste water treatment plant and makes it harder to treat the effluent. What you need most likely is a detention basin that slows the rate of run-off to meet the existing rate. However, concentrating runoff in a narrow jet significantly increases the velocity and makes the runoff erosive -- exactly what you are trying to prevent! Your outlet should send water out in a sheet, rather than a channel. A weir overflow is preferable, or a level spreader at the outlet.

 

[VEBill]

...the city does not want rain water entering the sanitary sewer...

Correct everywhere. But many locations unfortunately are stuck with one combined rain and sanitary sewer system; so they often have no choice.

 

[BigInch]

Won't you have a lot of oil drips? An oil-water separator sounds like exactly what you should have. No oil should be running off to anywhere to land or sewer.



Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[MikeHalloran]

I'm guessing that the requirement was described to SIL in a particular way that makes any device's performance easily verifiable with a bucket and a watch and no serious math or physics.

Weirs are too subtle for greenazis to accept.




Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

skoggs, here's your oil-water separator attached.

You can check with the authorities to see of they will allow more flow from the water side for what should be a clean(er) water stream than what they have allowed so far for a combined oil-water flow.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[btrueblood]

Back to Mike's advice - start with 4 holes, and add the 5th if the flow is not high enough...

Regarding clogged holes - just add more when the old ones plug up (note that .177 rounds might be an effective hole-making device in a thin walled barrel...and a good subject for further experimentation).

 

[msquared48]

Big Inch:

I like the oil overflow bin.

With enough oil in the container, you could sell tickets to mud (oil) wrestling contests.

Or another idea, put a pig in the oil and have it be the prize for anyone who can get it out of the pit in 30 seconds.

Or not...

Mike McCann
MMC Engineering

 

[Skogsgurra]

Mike

Those are all very fine ideas that I shall forward to my son-in-law. He likes different sports and was the one who actually did the infamous beer can shooting, which I have been unrightfully accused of. I did the filming.

We have triathlons (or any higher number -athlons) during the summer. Not so much mud wrestling. One popular sport is eel heaving. Bring as many eels as you can from one tank over to another. Many variations of this popular sport - two teams that heave eels back to the opponent's tank and also with pushing opponents into any tank are popular. Axe-throwing (without killing yourself or anyone else) is big. But the Finns are better at it than we are. Wife-lugging is popular. Finns better, again. Very-heavy-car-battery-throwing is for the pros. I have a problem even getting a very-heavy-car-battery off ground.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[KENAT]

So you just stood by and watched the crime, doesn't that make it worse?

I just have to know more about Wife-lugging.

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