First put flow rates on the same basis
FAD or standard flow must be converted to the volume at operating conditions. Only then can velocity be calculated from that volume flow and the cross-sectional area. Reference conditions are therefore part of the specification. Changes in density must also be considered when calculating pressure losses; a simple approximation is valid only within a limited range.
Do not forget the connections
The main pipe may be adequate, yet a narrow branch, hose or quick coupling can cause problems. Local resistance can be described by coefficients or documented flow characteristics. In a ring main the flow split between branches must also be determined; a simple calculation for one pipe does not represent the entire network.
What to check in practice
- Use the design simultaneous demand and documented peaks, not just the daily average.
- Define the allowable loss from the source to the critical point of use.
- Use the calculated internal diameter to select a documented pipe size for the specific system.
Example scenario
The calculation requires an internal diameter of at least 36 mm. A pipe with an outside diameter of 40 mm and a 3 mm wall has an internal diameter of only 34 mm, so it does not meet this requirement. A trade designation without a dimensional table is not sufficient.
What to watch out for
- Using the outside diameter to calculate the cross-sectional area.
- Ignoring fittings and their actual clear bore.
- Recommending a trade size without knowing the material and dimensional series.
Why does the calculator not select a DN size directly?
Different systems can have different internal diameters under the same nominal designation. The tool determines the requirement, but the actual size must be checked against the technical data for the chosen pipe system.