Pipe Sizing: d50 or d63 on the Suction and Pressure Side
It is not the nominal size that matters, but the flow velocity in the pipe. By sizing the suction and pressure side separately, you choose confidently between d50 and d63 – and avoid cavitation, flow noise and unnecessary pressure loss.
Flow Velocity as the True Sizing Parameter
The question “d50 or d63?” cannot be answered by the delivered flow rate alone. What matters is the flow velocity in the pipe, that is, the volume flow divided by the free cross-sectional area. The faster the water flows, the higher the pipe friction, pressure loss and noise generation become – and on the suction side there is the additional risk that the pump draws air or cavitates.
Important for sizing: PVC-U pipes are designated by their outside diameter d. d50 (DN40) has an outside diameter of 50 mm, while d63 (DN50) measures 63 mm on the outside. The inner diameter that governs the velocity depends on the pressure rating: with common pressure pipes rated PN10, the clear bore is around 45 mm for d50 and around 57 mm for d63. Stepping up from d50 to d63 increases the cross-sectional area by more than half – so at the same flow rate the velocity drops accordingly and noticeably.
Sizing therefore always follows this path: first define the planned circulation volume flow, then choose the pipe dimension so that the velocity stays within the recommended range.
Limits for the Suction and Pressure Side
The suction and pressure side are deliberately sized differently because the physical conditions differ. On the suction side there is negative pressure; high velocities there promote cavitation and the drawing in of air at leaky points. In planning practice, a velocity of about 1.5 m/s is taken as a guide value that should not be exceeded.
On the pressure side the pump works against the resistance; here a higher velocity of around 2.0 m/s (within limits up to about 2.5 m/s) is acceptable. Anything above that shows up as noticeable flow noise, growing pressure loss and higher power consumption by the pump.
These different ranges lead to a practical consequence: The suction line is often run one size larger than the pressure line. A system with d50 on the pressure side thus often gets d63 on the suction side. Treat the values given as guide values for the preliminary selection – binding are the specifications of the pump manufacturer, in particular regarding the permissible suction-side inflow.
Sizing Table: m³/h for d50 and d63
The following table gives guide values for the maximum flow rate per pipe dimension, separated by suction and pressure side. These are based on the velocity ranges mentioned and the inner diameters of common PVC-U pressure pipes rated PN10.
| Pipe (outside d / DN) | Inner Ø (PN10) | Suction side (~1.5 m/s) | Pressure side (~2.0 m/s) |
|---|---|---|---|
| d50 (DN40) | approx. 45 mm | up to ~8 m³/h | up to ~11 m³/h |
| d63 (DN50) | approx. 57 mm | up to ~13 m³/h | up to ~17 m³/h |
As a rough orientation: a typical filter pump of around 8 to 10 m³/h is already in the borderline range of d50 on the suction side and is clearly on the safe side with d63 on the suction side; on the pressure side, d50 usually suffices in this case. The values apply to short, straight runs. For longer or angled lines, the length allowance in the next section must be included.
Length, Number of Bends and the Length Allowance
The total pressure loss of a line consists of two parts: the pipe friction per metre over the length and the individual resistances of bends, elbows and valves. Both increase with velocity. That is why it is not enough to consider the plain pipe length alone.
Bends and elbows can be estimated as an equivalent pipe length. As a rule of thumb for planning, a 90° elbow acts roughly like one to two additional metres of pipe; a wide 90° bend considerably less, a sharp elbow rather more. Add the individual resistances to the pipe length in this approximate way.
For practical purposes, a length allowance as a decision rule is sufficient:
- Up to about 10 m of simple, barely angled run: the table value applies directly.
- Beyond that, or with many bends: choose one pipe size larger or calculate with the lower end of the velocity range.
- When in doubt with long lines: d63 instead of d50 – the larger dimension costs little more but permanently reduces velocity, noise and pump load.
The Reducing Bush Before the Pump
Many filter pumps have connections in d50 or 1½” or 2″. If the suction line is – as recommended – generously run in d63, it must be reduced to the connection before the pump. The rule is: Keep the suction line in d63 for as long as possible and only reduce it directly at the pump port. A reducing bush placed early needlessly lengthens the fast, lossy run.
Make sure that no air pockets form at the reducing bush, where air can collect and disturb the suction. A pipe section that is as straight and short as possible directly before the pump inlet calms the flow. Avoid several consecutive reducing bushes and elbows close to the pump – every disturbance has a particularly unfavourable effect on the suction side.
On the pressure side, the reducing bush is less critical: going from d63 to d50 at the filter or valve connection costs only a little additional pressure loss.
Separate Lines for Each Suction Point
The skimmer and main drain should be run to the pump with separate suction lines and brought together and controlled there via one ball valve each. This allows the suction to be balanced between surface skimming and the main drain, and individual points to be shut off for maintenance or winter.
What is decisive for sizing: because at times the full pump flow runs through a single suction point (for example when only the main drain is open), each individual line must be sized for the entire volume flow – not just for a partial flow. A shared collector line that is dimensioned too tightly is the most common sizing error on the suction side.
With several skimmers the same principle applies: each supply line is sized so that the velocity stays within the suction-side range, and the distribution is handled via valves at the pump. Only after the pump, on the pressure side, may you consistently move down to the smaller dimension.
FAQ
Is d50 sufficient for my filter pump?
Up to about 8 m³/h, d50 is usually sufficient on the suction side, and up to around 11 m³/h on the pressure side. If the circulation is higher or the line is long, you should run at least the suction side in d63.
Why is the suction side dimensioned larger than the pressure side?
On the suction side there is negative pressure. A lower velocity of about 1.5 m/s prevents cavitation and the drawing in of air. On the pressure side, around 2.0 m/s is permissible, which is why the smaller dimension is often sufficient there.
May I reduce the pressure line from d63 to d50?
Yes. At the filter or valve connection, reducing from d63 to d50 on the pressure side is uncritical and causes only a small amount of additional pressure loss.
How do I account for bends and elbows when sizing?
Add each 90° elbow to the length as roughly one to two additional metres of pipe. With many bends or long runs, it is better to choose one size larger.
Can I connect the skimmer and main drain via a shared line?
Better not. Separate lines with one ball valve each allow suction control and the shut-off of individual points. Each individual line must be sized for the full pump volume flow.
This guide provides general technical orientation and does not replace professional planning. Safety-, electrical- and permit-relevant work must be carried out by qualified specialists.
