Understanding Hydraulics: Calculating Pressure Loss and Pipe Friction
How much head your filter pump really needs is not determined by the pool depth, but by the sum of all flow losses in the circuit.
Head: the Sum of All Losses
A common misconception is that the pump has to overcome the pool depth or the height of the water level. In a closed circulation circuit, where the suction and return lie at almost the same level, these geodetic heights largely cancel each other out. What the pump actually has to do is overcome the flow resistances along the entire path.
The required head H is made up of four components:
- Geodetic height (hgeo) – the height difference between inlet and outlet; in a closed circuit often close to zero.
- Pipe friction (hfric) – the linear loss in the straight pipe runs.
- Local resistances (hloc) – bends, elbows, tees, valves, unions.
- Component losses (hcomp) – filter, heat exchanger or heat pump, salt cell, measuring cell as per the data sheet.
So H = hgeo + hfric + hloc + hcomp. All components are usually given in meters of water column (m WC), because pump curves are also plotted in meters against the flow rate. For conversion: 1 bar corresponds to about 10.2 m WC.
Calculating Pipe Friction per Meter
The friction loss in a straight pipe follows the Darcy–Weisbach relationship. Written as head loss per meter: hfric/L = λ · (1/d) · v² / (2·g). Here λ is the pipe friction factor, d the clear inner diameter, v the flow velocity and g the acceleration due to gravity (9.81 m/s²).
It is crucial that you calculate with the inner diameter, not the nominal size. A d63 (DN50) pipe in PN10 has an inside diameter of around 57 mm, a d50 (DN40) pipe around 45 mm. The flow velocity results from the volume flow and the cross-section: v = Q / A, with A = π/4 · d².
For smooth PVC-U in the turbulent range (Reynolds number roughly 10,000 to 100,000), a practical approximate value for λ is about 0.02. Manufacturer catalogs provide ready-made pressure loss diagrams for their pipes, from which you can read off the value per meter directly – this is the most reliable source.
Because v enters as a square and d is in the denominator, the loss reacts very sensitively to the nominal size: if you route the same volume flow through d50 instead of d63, the velocity rises considerably and the loss per meter roughly triples. On long runs, the larger dimension therefore almost always pays off.
Local Resistances: Bends, Valves, Fittings
Every fitting and every valve creates an additional, local loss. It is captured via the resistance coefficient ζ (zeta): h = ζ · v² / (2·g). The term v²/(2·g) is the so-called velocity head and, at a given flow velocity, is the same for all local resistances – so you only need to add up the ζ values and multiply once by the velocity head.
The following values are guide values; when in doubt, the manufacturer’s specification for the specific valve is decisive:
| Fitting / valve | ζ (guide value) |
|---|---|
| 90° bend (wide radius, solvent socket) | 0.3 – 0.5 |
| 90° elbow (tight) | 1.0 – 1.3 |
| 45° bend | 0.2 – 0.3 |
| Tee, straight run | 0.3 |
| Tee, branch | 1.0 – 1.3 |
| Ball valve, fully open | 0.1 – 0.3 |
| Check valve | 2 – 4 |
| Union | 0.2 – 0.5 |
Two points are important: a throttled ball valve has a coefficient many times higher than an open one – it is better to control the flow through proper sizing than through throttling. And check valves vary widely; here you should use the data sheet value wherever possible.
Operating Point on the Curve
Pipe friction and local resistances both depend on the square of the velocity and therefore on the square of the volume flow. If you plot the required head against the flow rate, the result is an upward-opening parabola through the origin – the system curve.
The pump curve runs the opposite way: the more flow, the less head the pump can deliver. The operating point is the intersection of the two curves. It determines which volume flow Q and which head H actually establish themselves in this particular system – not the pump’s maximum data sheet value.
In practice, this means you calculate H for your desired design flow rate and look for a pump whose curve reaches this point with some reserve. As the sand filter becomes dirty, the system curve moves upward and the intersection shifts toward a lower flow rate. If you size for the freshly backwashed filter without any reserve, you will quickly fall below the minimum throughput in operation, for example for the dosing technology.
Worked Example Step by Step
Example system: 48 m³ pool, desired circulation flow Q = 12 m³/h, d63 pipe throughout (DN50, inner diameter around 57 mm), closed circuit.
- Flow velocity: Q = 12 m³/h = 0.00333 m³/s; A = π/4 · 0.057² = 0.00255 m². This gives v = 0.00333 / 0.00255 ≈ 1.3 m/s – a good value for the pressure side. Velocity head v²/(2·g) = 1.3² / 19.62 ≈ 0.087 m.
- Geodetic height: suction and return at about the same level, hgeo ≈ 0 m.
- Pipe friction: total straight pipe length, suction plus pressure side = 25 m. Loss per meter at v ≈ 1.3 m/s and d63 around 0.03 m/m (λ ≈ 0.02). This gives 25 · 0.03 = 0.75 m.
- Local resistances: assuming 4 × 90° bends (0.5 each), 2 ball valves (0.3 each), 2 straight-run tees (0.3 each) and unions (0.4 in total) give Σζ ≈ 3.6. This gives hloc = 3.6 · 0.087 ≈ 0.31 m.
- Components: sand filter (clean) and heat pump together, according to the data sheets, e.g. 2.5 m.
- Total: H = 0 + 0.75 + 0.31 + 2.5 ≈ 3.6 m WC (about 0.35 bar) at 12 m³/h.
You size the pump for this operating point (12 m³/h / 3.6 m) – plus a reserve for filter fouling, so that the pump curve provides more like 5 to 6 m at 12 m³/h. For comparison: if the same 12 m³/h were routed through d50 instead of d63, the velocity would be around 2.1 m/s and the friction loss per meter around 0.10 m/m – the 25 m of pipe alone would then give around 2.5 m instead of 0.75 m. This shows how much leverage the right pipe dimension has on energy demand.
Bill of materials
Example – please verify with a specialist. Parts and quantities depend on the installation; sizes shown are examples.
| Component | Size | Art. No. | KLN No. | Qty |
|---|---|---|---|---|
| Filter vessel | – | – | – | Include the unit's pressure loss from the data sheet in the total head; allow a reserve for fouling |
| Heat pump / heat exchanger | – | – | – | Account for the unit's pressure loss per the data sheet as component share h_komp |
FAQ
In which unit is the head given – meters or bar?
In pool technology, usually in meters of water column (m WC), because pump curves are also shown in meters against the flow rate. For conversion: 1 bar corresponds to about 10.2 m WC.
How strongly does the pipe diameter affect the pressure loss?
Very strongly. At the same flow rate, the flow velocity rises as the inner diameter gets smaller, and the friction loss grows disproportionately. Switching from d63 to d50 can roughly triple the loss per meter.
Do I have to include the pressure loss of the filter and heat pump?
Yes. Equipment pressure losses from the data sheets go directly into the head and often even dominate. For the sand filter, you should also plan a reserve for the increasing fouling.
Why does my pump deliver less flow than stated in the data sheet?
The data sheet values apply to defined heads. Your real system has a certain resistance; the actual flow rate only results at the intersection of the system and pump curves and is almost always below the maximum value.
Which flow velocity should I not exceed?
As a guide value, about 1.5 m/s applies on the suction side and around 2 m/s on the pressure side. Higher values drive up the pressure loss and noise generation significantly.
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.
