Orifice Plate Design 1

[joatmon22]

I'm designing an outlet structure for an underground infiltration bed. The bed consists of perforated HDPE pipes in gravel. The pipes will tie into a precast manhole that will outlet larger storm events.

The outlet pipe will be a 15" diameter RCP that connects to the city sewer.

I calculated that I need a 3" orifice for a 100YR event, and that water will only rise to about 5" above the center of the orifice during this time. I therefore plan to cover the opening to the 15" RCP at the manhole, using an orifice plate that covers it's lower half (leaving the top half open for emergency overflow). However, being very green at this, I'm having trouble trying to figure out the most efficient way to do this.


I found a detail on the internet, at the city of columbus's website. they use a .25" diameter aluminum plate, rolled to match the curvature of the manhole. the plate is anchored to the manhole wall with stainless steel, epoxy anchors.

Has anyone used a detail like this? If so, does it work well? If not, would you have any other suggestions?

Thanks.

 

[bpattengale]

What we do at my firm is to place a weir in the manhole structure. You then put an orafice in the wier at the correct elevation and size that will control the flow to your design constraints. Put the top of the weir above the maximum elevation that occurs. Should the orafice get clogged or the flow excede your design the water overtops the weir.

So in your case set the elevation of the orafice high enough that the system will not have any outflow for the volume you need to infiltrate. Any flow above that will pass through the orafice.

 

[bltseattle]

In these parts orifices of that size are installed in a "flow restrictor oil prevention tee" or FROP-T for short. This configuration keeps the orifice submerged so that oil & flotables are trapped and can be vactored out. If the water depth exceeds the top of the riser, weir flow begins. This configuration keeps the full diameter of the outlet pipe clear, which is a concern in what you've proposed; with the opposing sluice gate it is possible to clean the outlet if it were to clog. This configuration allows orifice plates to be swapped in the future if needed.

You can see a Wash Dept of Transportation detail at:

http://www.wsdot.wa.gov/EESC/Design/designstandards/english/pdf/b10.40-00_e.pdf

You can see another example from King County roads, dwg 2-025 at :

http://www.metrokc.gov/ddes/lusd/cad.htm#E7

 

[Jackscheele]

The WSDOT detail of the flow restrictor will solve a problem I have. Question how is the flow through the submerged orifice calculated?

 

[joatmon22]

Thank you all for your input. I'm sorry for not responding sooner--it's been a rough couple of weeks. This is great information.

I think I did a poor job of describing my proposed system earlier - my infiltration bed will outlet into a manhole (the 24" perforated HDPE pipes in the bed will tie directly into the mainhole) the manhole will also connect to a 15" rcp that will carry the water to the city storm sewer (per city regulations, this will tie directly to the city sewer, this is for a single residence). the invert of this 15" rcp will be above the top of the HDPE pipes. a plate will partially cover the opening of the 15" rcp at the manhole. there will be a 3" diameter hole in the plate, that will serve as the orifice. <to answer jack's question the size of this orifice was determined with an iterative process, using bernouli's orifice formula (Cd = 0.6), where head varied as a function of time, runoff rate and the rate of flow out) >. in emergency situations, the flow will be able to overtop the plate and exit into the pipe.

If anyone has any further suggestions or comments, I'm all ears.

Thank you all, again for your help.