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Drainage Calculations

Drainage Fixtures Unit (DFU) Values
DFU is a measure of the probable discharge into the drainage system by various type of plumbing fixtures. The drainage fixture-unit value for a particular fixture depends on its volume rate of drainage discharge, on the time duration of a single drainage operation and on the average time between successive operations.   Drain Fixture Unit, or DFU, is a plumbing design factor, or a relative measure of the drain wastewater flow or load for various plumbing fixtures.

1 DFU = 1 cubic foot of water drained through a 1 1/4" diameter pipe in one minute.
              (1 ft³ /min water drained through a  1 1/4" diameter pipe).

1 DFU ≈ (approximately) 7.48 US GPM or ≈ 0.47 liters/second.

Refer to following tables for Drainage Fixture Unit (DFU) Value:

Table 11.4.1 (Drainage Fixture Unit (DFU) Values - NSPC 2006

Table 7-3 (Drainage Fixture Unit (DFU) Value - UPC 2006

Notice in the table above that the DFU factor for a plumbing fixture will vary depending on the drain and trap size or diameter.

Some standard DFU values from NSPC Table 11.4.1 for other than dwelling units:
a) Kitchen Sink, Domestic, with one 1-1/2" trap = 2 DFU
b) Lavatory , 1-1/4" Waste                                 = 1 DFU
c) Shower, group, per Head (Continuous flow) = 5 DFU
d) Urinal, 1.0 GPF (Gallons per Flush)              = 4 DFU
e) Water Closet, 1.6 GPF Gravity                      = 4 DFU
f) Floor Drain (FD)                                             = 0 DFU

From Table 11.4.1 (NSPC) and Table 7-3 (UPC),

we can calculate total DFU for horizontal and vertical drainage pipes in each toilets.

UPC 7-3.png


Fixture Unit Equivalents

The unit equivalent of plumbing fixtures shown in Table 7-3 (UPC) shall be based on the size of trap required, and the unit equivalent of fixtures and devices not shown in Table 7-3 shall be based on the rated discharge capacity in gpm in accordance with Table 7-4.

Fixture Trap Loading.png
Table 7-4.png

Size of Drainage Piping

The minimum sizes of vertical and horizontal drainage piping shall be determined from the total of all fixtures units connected and additionally, in the case of vertical drainage pipes, in accordance with their length. Table 7-5 (UPC) and Table 11.5.1B (NSPC) shows the maximum number of fixture unit allowed on any vertical or horizontal drainage pipe, building drain, or building sewer of given size; the maximum number of fixture units allowed on any branch interval of a given size.

Table 7-5 UPC Pipe size.png
Table 11.5.1B.png

For example, from Table 7-5 (UPC), it states 2 in (50 mm) pipe can carry total fixture units unto 16 DFU in vertical and 8 DFU in horizontal position. Maximum length can be 26 m in vertical and unlimited in horizontal position. Similar 4 in (100 mm) pipe can carry total fixture units unto 256 DFU in vertical and 216 DFU in horizontal position. Maximum length can be 91 m in vertical and unlimited in horizontal position.

Similarly, Table 11.5.1B (NSPC) states 2 in pipe can carry total fixture units up to 6 DFU in horizontal and 10 DFU in vertical position (stack). 4 in pipe can carry total fixture units upto 160 DFU in horizontal position and 240 DFU in vertical (stack) pipe.

Minimum Trap(Pipe)  Size

Using table 5.2 (NSPC), we get the minimum pipe size to be used for each plumbing fixtures. For example, one Water Closet (WC) = 4 DFU. From table 11.5.1B up to 6 DFU pipe size shall be 2”. But for WC minimum pipe size shall be 3 inch. So even one WC DFU is 4, pipe will be 3 in and not 2 in.

Table 5.2 min trap size_Page_1.png

Examples for Sizing Drainage Piping

Example -1 : Drainage Pipe (Soil) Pipe Sizing

Example - 2: Drainage Pipe (Waste) Pipe Sizing

Example - 3: Sizing of building drainage sewer (3D Isometric)

Example - 4: Sizing Vertical Stacks

Example - 5: Vertical Offset

Example - 6: Horizontal Branch Drain must be at least 8 feet center to center

Example - 7: Horizontal offset

Example - 5.png
Example - 6.png
Example - 7.png


Use Table 12.16 (NSPC) & Table 7-5 (UPC) to size vent pipe.

Table 12.16 (National standard plumbing code illustrated) we find that 1-1/2” vent pipe can connect up to 8 drainage fixture units. And 2” vent pipe can connect up to 20 DFU. Each WC has a 1-1/2” vent pipe connected to it as each WC has a value of 4 DFU. As the vent pipes are connected together as shown in figure 1.1 the DFU value exceeds 8 DFU which means that a 2” pipe has to be used.

Table 12.16 Vent Pipe Size.jpg


Rainfall Intensity : The amount and rate of rainfall expected in the area is the most important factor. You can usually get the precipitation data from your local code book or online weather bureau.

Once you have found the maximum anticipated rainfall and square feet of roof area, you can calculate the diameter of rainwater pipe from Table 11-1 (Sizing roof drain and vertical rain water piping).

Spacing of Roof Drains : Spcing of roof drain is required to eliminate the puddling on roof surface after a rainfall i.e. in no case shall the surface in the vicinity of the drain be recessed to create a reservoir.

Below figure suggest one way to arrange roof drain and size the storm water pipes for typical roof.

Table 11-1 Rain Water Sizing.jpg
Figure 8-25 strom water drinage system.p
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