How to design Drains, Stacks and Building Sewers with DESAGÜES (DRAINS)?

The Fixture Unit (F.U.) concept (HUNTER´S method) used in the design engine of our software DESAGÜES (DRAINS) allows, instead of determining the diameters directly from a sewage flow value, to take as a reference the ones – stated usually on tables on standard codes regulations- that sets the maximum allowable capacity, in Fixture Units, for each available pipe diameter.

Certainly, behind those F.U. values on the aforementioned standard tables, exists the definition of a flow or, rather, the maximum allowable flow for a certain condition (pipe at half capacity, for example) that each diameter is capable to convey. But, in our case, in order to facilitate to the user the data entry and also the posterior analysis and design, we have chosen to use only Fixture Units in the program.

Thus, the HUNTER’S method for designing drains in buildings, will only require that you have the following information:

• Fixture Units values recommended for assignment to each plumbing fixture that discharges into a sanitary drainage system.
• Maximum Permissible Loads (in terms of Fixture Units) for Sanitary Drainage Piping (Drains, Stacks and building Sewers).

This information, as already outlined in the first paragraph, is generally reflected in the country´s standard codes regulations for the design of drains in buildings and, in its defect, in technical textbooks.

How to Personalize DESAGÜES (Drains) to the specific values of the sanitary code regulations of each country?

Like PLUMBER, our software for the design of water supply systems in buildings, we have included in DESAGÜES the Fixtures’ Manager, as well the Pipes’ Diameter Manager, dialogs where a user can change the default Fixture Units values ​​and create her normative catalog for all of the actual and future Sanitary Drainage projects created with the software.

Let´s start with the Pipes’ Diameter Manager:

DESAGÜES Pipes’ Diameter Manager

Thus, the task of personalization will start with clicking on the button in the Configuration tab → Diameter and F.U. panel, to access this dialog:

Here you´ll find three tabs:

Sanitary Tab: Diameters and Maximum Fixture Units for Sanitary Drainage Piping

In this tab you will notice, in addition to the Nominal and Interior diameters columns, there are 6 columns in which you should specify, for each diameter, the maximum number of fixture units that each diameter can convey.

Specifically, there are three categories of Fixture Units depending on the drainage piping component:

Max. F.U.s in Horizontal Branches: A horizontal branch is a drain to which the sanitary fixture traps (1 on the next image) are connected in the same building story; this component is denoted with number 2 on the next image. So, in this column you must introduce de F.U.s that are allowed for each pipe size.

In DESAGÜES a sanitary Network comprises all horizontal branches that receives de fixtures’ discharges in the same building’s story.

Max F.U.s in Stacks: A stack is a vertical section of pipe extending two or more stories with branches therefrom to serve the stories through which it passes (shown with number 3 on previous figure).

The Fixture Units Method discriminates the diameter definition in Stacks according to the number of intervals that it has, i.e. according to the number of levels that it will serve. Thus, if it receives only the contribution of three networks in the same number of building levels, is considered to be a two intervals stack and its diameter shall be determined according to the values in Column 4.

Of receiving major number of networks in more than three levels, values in column 5 will be used, that is to say it will be a more than a two story Stack.

Max F.U.s in pipes: Values in the last three columns will be used for sizing sewers pipes or building drains which are the part of the sanitary system which receives the sewage from waste stacks and conducts it to the building’s sewer. This component is depicted on previous figure with the number 4.

For the sections of pipe in this network, the sizing is done not only according to the fixture units conveyed, but also according to the longitudinal slope of the branch in question.

That is why in last three columns you must specify for each diameter in the project library, the maximum fixture units for slopes of 1%, 2% and 4%.

Pluvial Tab: Diameters and tributary areas for building’s Storm Drainage Piping

When it comes to vertical (Leaders) and horizontal drainage pipes used for the conveyance of rainwater that fall in the exposed areas of the building, the design is based on the maximum permissible of projected drainage area that can be conveyed by these elements and is also based upon a rainfall rate (mm / hr).

DESAGÜES will require, as you can see in the Pluvial tab, the introduction of these projected drainage areas values based on two categories, as we see in the following figure:

Rain Water Leader Drainage Area (column 2): corresponds to the maximum drainage area that can be conveyed by the vertical pipes (Leaders) in charge to transport the rain water from the roof of the building to the ground level.

Drainage Area (columns 2 to 5): Sizing of horizontal storm network has the same characteristics as the sanitary building drains. Therefore, it will be necessary to specify the maximum drainage area that can be handled by each pipe diameter and for four longitudinal slopes´ values: 1%, 2%, 4% and 6%.

General Tab: Automatic Design options and general data

Finally, the General tab contains, besides the fields to specify the name and a descriptive text for the Library of diameters to use in your projects, two options to set the way the program will select the diameters for sewers pipes or building drains (both Sanitary and Pluvial):

As shown in the above figure, you can choose to give priority to obtain the smaller possible diameter, in such a case it will tend to get higher slopes for the selected diameter. Another option is to look for the lowest possible slope at the expense, of course, of larger diameters. So you must study the results with each option well.

The Sanitary Fixtures Manager

When clicking on the Sanitary Fixtures button in Configuration tab, the Sanitary Fixtures’ Manager will be shown:

Here it is necessary to specify only the number of Fixture Units for each sanitary fixture, the minimum diameter of fixture’s trap (both parameters must be obtained from sanitary codes in your country) and the geometric height of the fixture’s discharge point.

With regard to the trap’s diameter you must keep in mind that the value you enter here will be used by the program to check and set, if it’s the case, the diameters in the pipes to which the fixture drains in sewage network, in such a way that these diameters are equal to or greater than the entered here. This is of particular importance in fixtures such as the Water Closet (WC), where the majority of the standards set a minimum diameter of 100 mm (4 “) while the required diameter by fixture unit’s value is generally lower.

The fixture’s discharge point height value is used to define the quantities (length) of pipe to generate bills of materials in your projects. Most of the pieces discharges onto the floor, which is why its value will usually be zero (0).

Regarding bills of materials, you can specify for each sanitary fixture, the fittings that will be installed on it, by placing the mouse cursor in the respective row and clicking onto the button at the top right of the dialog:

For example, as displayed in the previous figure, a bathtub, in which the discharge is directly to the floor, includes a 45° elbow and a floor trap. When the total of bathtubs in the project is accounted for, these two accessories will be included for each fixture in the grand total.