Worked example: a PE100 diesel rising main

A rising main is a pump lifting a fluid up an elevation, and its character is different from a long flat pipe: most of the pump head goes into raising the fluid, not into friction. This worked example transfers diesel up a PE100 main, and it also shows the suction-side check that matters whenever a pump lifts a fuel.

The setup

A pump draws diesel from a low tank at 2 m of head and lifts it up a 250 m PE100 rising main to an elevated day tank at 37 m, so the static lift is 35 m. The fluid is the diesel preset (density 840 kg/m3, kinematic viscosity 3.5 x 10^-6 m2/s), which carries a vapour pressure, so the solver can report the available net positive suction head. The pipe is PE100 OD110 SDR11, a 90 mm bore, with a very low roughness of 0.003 mm, so it is hydraulically smooth. The pump curve runs from 40 m at shut-off to 22 m at about 37.5 L/s, at 75 per cent efficiency, with a required net positive suction head of 3 m.

The physics and the method

Fluid Network Studio solves the network for the flow where the pump curve meets the system curve. The system curve here is dominated by the 35 m of static lift: of the roughly 32 m of head the pump develops at its operating point, the great majority is spent raising the diesel, and only a small part on friction, because the smooth PE bore loses little. That is the signature of a rising main.

Because the diesel preset carries a vapour pressure, the solver computes the available net positive suction head at the pump inlet from the suction static head and the friction to that point, and compares it against the required net positive suction head of 3 m. If the available margin falls below the required value, the pump would cavitate, and the model flags it. This is the check that a flat-pipe friction calculation cannot give you.

What you learn

Solving the example shows how a rising main splits its head between elevation and friction, the low friction of a smooth PE bore, and the suction margin between available and required net positive suction head. Lower the tank level or lengthen the suction side and re-solve to watch the available margin fall towards the required value. The wider pump-sizing workflow, including the operating point against a system curve, is on the pump system design page.

Open this example in FNS and lower the supply tank to watch the suction margin shrink.