## Pipe diameter

It should be noted that the inner pipe diameter can differ considerably from the nominal diameter depending on the material and the standard. A minimum diameter is often required for solids-laden pumping media to prevent clogging or blockage of the pipeline. For example, a minimum diameter of DN 80 is required for sewage containing faeces in accordance with EN 12056 or DWA guidelines, unless a pump with a shredding device is used. Another important parameter is the mean flow velocity . In doing so, contradicting requirements have to be reconciled:
• In the case of media laden with solids, a minimum flow rate is required in order to avoid deposits. In wastewater technology, 0.7 m / s is required for this.
• A small pipe diameter leads to high flow speeds and thus to high pressure losses. This increases the operating costs of the system. In addition, the risk of flap blows and pressure surges is greater at high flow speeds.
• High flow velocities can also lead to noise pollution. This is particularly important for installations inside buildings.
• In contrast, a large pipeline diameter results in higher investment costs due to the higher material requirements.
For many areas of application there are regulations or recommendations for the dimensioning of pipelines in national or international standards. The following values can provide an initial reference point for dimensioning the pipeline for some applications:
• Wastewater / waste water – 0.7 … 2.3 m / s
• Installation in the building – 0.7… 1.2 m / s
• Irrigation – 0.5 … 1.5 m / s
• Heating – 0.3 … 1.5 m / s
• Recommended – 0.5 … 0.8 m / s
• Main water pipes – 1… 2 m / s
• Water pipelines – up to 3 m / s
• Drinking water / service water
• Pressure lines – 1… 2 m / s
• Suction lines – 0.5… 1 m / s

## Flow velocity

v = Q / A v – Average flow velocity Q – Flow rate A – Area flowed through For a circular cross section, the following results are obtained with A = p / 4 – D2 v = 4 – Q / (p – D2) D – Diameter of the circular area flowed through (internal pipe diameter) c or v are usually used as symbols for the average flow velocity. Often v is used for the local velocity, while c is used as a symbol for the mean velocity. The DIN 24260 provides the symbol v for the average speed. The average flow velocity is an important parameter when selecting the optimum pipe diameter for a new pipeline.