About Pipe Network Calculation for Waste Water

Explains the concept of the Pipe Network Calculation Waste Water.

Detection of Network Geometry and Components

Before calculation, the pipe network is detected from the drawing in all detail. This is done automatically by clicking Calculate. Only components of the layers that have been selected for component detection in the Settings dialog are detected.

To analyze the geometry of the pipe network, the pipe sections are detected first. Pipe sections in the pipe network are delimited by T-pieces or by components of dimension changes (e.g. transitions).

The program then searches for the starting component of the pipe network. It can consist of a sewer junction or a partial network end.

On this basis, the flow paths of the pipe network are determined. Flow paths always lead from an initial component to a drainage element, a rainwater feed, a partial network start or termination (exhauster). The number of flow paths in a regular waste water pipe network is thus always equal to the number of installed end components.

Note: When considering initial- and ending components, flow direction of the waste water is not taken into account. This leads to sewer junction or partial network end being considered initial components and drainage elements or partial network start ending components.

Calculation Open Drainage

Prior to calculation you can determine a number of parameters in the Settings dialog, minimum slope for various pipe types or rainfall rate data for example.

Initially, the program determines the waste- and rainwater drain in the network based on the settings for drainage elements and rainwater feed. These depend on the selected building type or on the discharge index, which was either set globally for all pipe sections in the Settings, or which was assigned individually to the individual pipe sections.

The thereby determined drains are the basis for the dimensioning of down-, collecting- and ground pipe. Single feed- and collector pipes are always dimensioned according to tables 5 and 9 of DIN EN 12056-2.

If some pipe sections are without slope information, minimum slope requirements are used for dimensioning. The same applies if a slope is smaller than the minimum slope and the checkbox When drawn slope is too small, insert a minimum slope is activated in the Settings dialog. With this checkbox deactivated, the dimension of the pipe section is increased until the required hydraulic performance is achieved.

Determining the dimension of a collecting- or ground pipe always leads to a recalculation of the pipe slope. Down-pipes are calculated like a horizontal pipe with minimum slope.

When dimensioning, the filling degree of pipes normally is 0.5 for waste water ground- and collecting pipes and 0.7 for rain- and mixed water collecting pipes. The filling degree can be increased to 0.7 or 1.0 for ground pipes on the outside of buildings. To do this, activate one of the checkboxes Ground pipes behind lifting stations with increased filling level or Ground pipes behind open pit with increased filling level in the Settings dialog. All pipe sections that have been set to be ground pipes outside of the building will be dimensioned according to DIN EN 12056-2.

Beyond that extend, the program does not create hydraulic certificates or checking for full filling.

Calculation Siphonic Drainage

Prior to the calculation of the pipe network, you can set some parameters in the Settings dialog.

If the checkbox hydraulic balance is deactivated, the pipe network will be dimensioned using available pressure, as far as pipe sections do not have their dimensions fixed. The algorithm dimensions the network according to an available R-value and varies pipe sections until a dpResid. smaller than +/- 100 mb is reached. As soon as this goal has been achieved, the hydraulic load according to DIN 1986-100 is proven. It may very well happen, that a dpResid. smaller +/- 100 mb can not be held.

The reasons for that can be manifold:

  • selection of pipe dimensions is too small or restricted
  • too small the volume flow rate per flow path / too many roof drains
  • too stark the length asymmetry between the longest and the shortest flow path
  • too little length of single feed pipes, etc.

It is generally to be taken into account to keep flow paths similar in length when constructing; and making single feed pipes appropriately long. Furthermore, the drains have to be sufficiently loaded. The dimensioning proposal from the roof drain assistant usually presents good starting values.

If the network cannot be changed anymore topographically and no solution lower than 100 mb can be found, the pipe network can be fixed and completely balanced, which provides two different methods of reaching the proposed design target.

Note:

Due to the error pressure difference dpResid., asymmetries in the static pressure calculation along shared points of the stream line of the flow paths will occur automatically. Only with a complete hydraulic balancing can it be considered balanced and thus correct; also refer to the commentary for EN 12056-3. When dimensioning, the static pressure profile along the flow paths is thus to be considered only as a rough approximation.

Simulation Siphonic Drainage

If the checkbox hydraulic balancing is activated, the calculation will be carried out immediately. The first step consists of dimensioning, the second of correcting volume flow rates until the resulting dpResid. disappears. Thereby one has a completely balanced system. If all dimensions of the system have been determined, the step for dimensioning is skipped and the volume flow rate is calculated.

The pipe network is considered approved for existing networks, if the actual volume flow rate reaches a minimum of 95 % of the desired volume flow rate. New systems must reach a minimum of 100 % of the desired volume flow rate.

DIN 1986-100 demands the keeping of startup conditions: A certain pipe inside diameter can not be exceeded. Otherwise, the " slamming " of the down pipe - and thus the intended operation - cannot be guaranteed.

Note:

If all dimensions of the system have been fixed, the checkbox hydraulic balancing is automatically activated. This allows for hydraulic certificates even for existing systems. Another use case would be the optimization of the pipe network according to user-defined conditions.

The complete balancing is explicitly recommended (see commentary EN 12056-3), due to it providing a better insight into volume flow rate distribution and prohibiting deviations of the static pressure profile within flow paths. The dimensioning process as such has been designed for quick handwritten calculation of siphonic drainage systems and is thus suitable for predimensioning of siphonic drainage systems.