SEWER FIXTURE UNITS

[jwooten]

Hello all. I am working with an architect that gave me sewer flow in sewer fixture units. Please provide a conversion factor or chart for SFU to gpd or gpm. Thanks.

 

[SlideRuleEra]

Drainage Fixture Units (DFU) are more comprehensive than a flow rate (GPD or GPM). They take into account both the probability of multiple plumbing fixtures being used a one time and the fact that most plumbing loads are really "impulse" not "steady state". Sewer piping has to be sized to work correctly under these "impulse" loads, even if that means they are empty 95% (or more) of the time.

Suggest that you work with DFUs rather than making a conversion to a flow rate - otherwise the sewer system may look oversized in the your calcs, but be undersized in practice.

Here is a chart to help

http://www.engineeringtoolbox.com/drainage-fixture-units-piping-load-d_1078.html

http://www.SlideRuleEra.net

 

[jwooten]

It seems that DFU applies more to interior conduit. I have never designed a sanitary sewer system based on DFU.

The classic sanitary sewer scenario is a community of houses producing X gpm, which is based on population. That scenario is not a good model for several mixed-use buildings tied to one main sanitary sewer system. I would like to take the fixture units for each connection to the buildings and convert to gpm (to convert to cfs). Then I can use Manning's equation to size my pipe.

I understand that sewage flow is not laminar, steady-state flow, and that it is more of an impulse. However, max flow would be produced by all the fixture units releasing continually until a steady-state flow is achieved.

Please correct me if I am off-course with this line of thought.

 

[bimr]

Suggest you find a copy of a piping or plumbing handbook. The Piping Handbook by Nayyar published by McGraw Hill has a Table C13.4 Building Drains and Sewers that show the number of fixture units that may be connected to a certain diameter of drainage pipe. You can probably find the book on bookfinder.com.

For example, a 4" dia pipe at 1/4 inch slope can handle 216 fixture units.

The fixture unit is an arbitrary, comparative value assigned to a specific plumbing fixture, device, or piece of equipment. Fixture Unit values represent the probable flow that fixture will discharge into a drainage system, compared to other fixtures.

 

[jwooten]

Thanks. I will look into the book you suggested.