Travelling Screen Head Loss

[ceprab]

I am working on sizing a seawater intake sump. Design of teh sump itself according to HI standards has been systematic, but I have drawn a blank on the anticipated head loss across the travelling screen.

I have proposed two dual flow screens for the duty (28000 m3/h ) and I am hoping for a relationship that will allow me to firstly size the screen adequately (which could be done without knowing head loss if I allow a large margin) and preferably to be able to relate flow, head loss and screen area to the hole size in the screen plates.

Extensive googling has turned up a rough guide on the back of the brackett green brochure for 10 or 5mm holes but nothing else.

Anyone got any ideas? Information for 3mm hole size would be especially welcome as that will eliminate further downstream strainers.

Thanks
ceprab

 

[Latexman]

In Perry's, check out the section on "Perforated Plates and Screens" in the fluid flow section. My 6th Edition has it.

Good luck,
Latexman

 

[bimr]

You would be best served if you go that information directly from a manufacturer. Try these firms:

http://www.water.siemens.com/SiteCo...nes/Headworks/Brochures/ZP-Intake-BR-0406.pdf

http://www.weatherford.com/weatherf...ts/johnsonscreens/js_johnsonscreens.hcsp?js=1

http://www.hendrickscreenco.com/hme/hme.asp

 

[ceprab]

Thanks Latexman, bimr.

I found the Perry's section but wasn't sure how well it would apply to flow in open channels.

I have managed to get through to a manufacturer. Learning is to avoid the corporate sales line if it doesn't respond fast enough and ring up until you get a response.

I have good enough data now.

ceprab

 

[rmw]

So what does your data tell you? This one has me curious.

I've looked at many plant CW flow balances that had traveling screens and never saw the head loss taken into account. By the time you account for fluctuations due to tides, river levels, lake levels, etc, the head loss of the screen seems negligable.

I assume your worse case would be when your tide was at the lowest and the total flow has to travel through less screen open area.

rmw

 

[ceprab]

rmw,

What getting the vendor data told me was that the head loss I had allowed was very generous. I had allowed for 1 m (being conservative) for the trash rack and screen based on comments I had seen in a couple of (old) texts and a dose of caution due to wanting 3mm mesh size.

The data received indicate that overall head loss of about 10 cm on the dual flow screen is acheivable even with the 3 mm mesh. At least at the flow rate I require, and the way to increase flowrate is generally to add more channels so the drop is unlikely to be greater than this. So, in conclusion, this is a negligible concern as you suggest, but not because tide is more of a factor - head loss has to be allowed for under the worst (low) tide condition to ensure adequate NPSH.

ceprab