Water and Debris Loading on Steel Screen?

[Parosh]

I am designing a screen that will be permanently immersed in water (in spray water pump bays) and that will be able to catch debris... think steel wire mesh with 1/4" x 1/4" openings. I am wondering if anyone has references for what kind of loading from the water flow I need to worry about, and what sort of debris load I should account for.

I am unfamiliar with this topic as I generally do more building design. I have info on flow rates, liquid levels, and design velocity. I'm also located in Canada if that's important.

Thanks!

 

[phamENG]

Seems to me the worst case would be when your screen gets clogged. The weight of a reasonable thickness of whatever debris you're dealing with + weight of water for maximum level.

Others may have a different idea.

 

[jayrod12]

Let's say the grating got fully clogged. What is the height of water it would need to support before the water finds somewhere else to drain? That would be the value I design for.

 

[Parosh]

Thanks for the replies phamENG and jayrod12.

Seems pretty straightforward when you put it that way, maybe I was overthinking it.

 

[jayrod12]

Seems pretty straightforward when you put it that way, maybe I was overthinking it.

Forest for the trees. We've all been there.

 

[JoelTXCive]

I have designed several of these.

The hydrostatic water pressure is the highest loading you will see.

The problem I always run into is that the designs often gets too heavy and member sizes get large once you apply a fully hydrostatic head to the screen. You effectively have a retaining wall or gate holding back water.

I usually meet with the water engineers and try and get them to specify a head (pressure) that they want the debris screen to hold back before failure.

Also, be sure to ask the water engineers how much net free area they need for their screen. They get very touchy if you impede flow or reduce their effective pipe/channel cross section.

Below is an article discussing what happens when a pipe entrance gets clogged. It just says, "the full hydrostatic pressure will be developed"

 

[JoelTXCive]

Here is a design guide that has some background info on debris screens.

It's from our friends across the pond in Britain...

 

[dhengr]

Parosh:
What kind/size of debris do they expect? Quarter inch mesh will get clogged by almost anything a little dirtier than drinking water. Can that fine a mesh be a second or third level screen? Otherwise, someone has to design a water level warning system and an overflow channel to protect the screening system and the pumps. Search here on E-Tips, it seems to me we have addressed this problem before.

 

[JoelTXCive]

The projects I have deal with stormwater, so whenever possible my first choice for a screen is to use off the shelf steel grating versus custom fabricating something.

Are you working in English units or SI units? If English units, I can give you a rundown on the grating that i try and use first. I can also give you a quick summary of how grating is sized if you are unfamiliar with it. I try and use "Type 38-4" grating of varying section depths.

 

[Parosh]

Thanks for all the help JoelTXCive! I typically use SI but can convert.

dhengr - any form of organic matter from trees and such. There are a few screens of different sizes, I just used the one with the smallest openings for the question.

 

[JoelTXCive]

You mentioned 1/4" by 1/4" openings earlier, which is really small. I haven't worked with screens that fine.

I try and use "Type 38-4" steel grating, which means the grating's bearing bars are spaced at 38/16ths (2 3/8") of an inch and the cross bars for lateral torsional buckling are at 4" inch spacing. Nucor and other grating manufacturers have grating capacity tables on their websites where you can look up allowable loads for a certain size.

Below is a snip from Nucor's catalog on how Imperial grating is labelled. I think the SI labels are the exact same, but in millimeters instead of inches.

 

[Parosh]

Sweet, I really appreciate all your help. Nucor seems grate.