Pumped transfer — where the pump curve meets the system

Water pumped from an atmospheric tank through a control valve and a 150 m line into a tank held at 3 bar — the everyday hydraulic question every plant asks of a centrifugal pump: at what flow does it actually run? The pump is given the way a vendor gives it, as three points off the datasheet curve (45 m at shut-off, 39 m at 2 L/s, 21 m at 4 L/s), not as a flow or an outlet pressure.

Design (Steady State). The flow is specified (150 mol/s, about 9.7 m³/h) along with each block's outlet pressure; the solver reports the pump's 1.25 kW shaft duty and the pressure profile down the line.

Rating (Run Mode > Pressure Driven). Now nothing specifies the flow. The pump follows its curve, the valve its Cv law, the pipe Darcy-Weisbach friction, and the flow is wherever the pump's head equals what the valve, the line and the 2 bar lift into the receiving tank take away — the operating point on a pump-and-system-curve chart. The valve is sized so that point is the design duty: 150 mol/s, 34 m of head, 4.32 bar at the discharge.

Throttle it. Set FCV101's opening to 0.5 on its Input sheet and re-run: the flow falls to 93 mol/s and the discharge pressure rises to 4.98 bar, because a pump delivers more head at less flow and rides back up its curve. That is the behaviour the curve encodes and a fixed pressure rise cannot show.

Honesty notes. The curve is a quadratic fitted through the three points (H = H0 - a·Q - b·Q²), which is how vendor curves are usually reduced; it carries no NPSH check and no efficiency curve, so the shaft duty uses the one efficiency given.