Positive-Displacement Pumps: Modeling Fixed Flow and Discharge Pressure

Engineering context

The centrifugal pump workflow covers machines where flow and head are linked by a performance curve — the operating point falls where that curve crosses system resistance. Positive-displacement (PD) pumps behave fundamentally differently. A PD pump delivers an essentially fixed volumetric flow per revolution, largely independent of discharge pressure. The system resistance does not set the flow: it sets the pressure the pump must develop to push that fixed flow through the route, and therefore the power it draws.

Booster pumps add head in series with an existing driver to reach a higher operating pressure or recover head lost along a long route. The combined behaviour is the series sum of what each device contributes at the shared flow.

The modeling mistake to avoid: Treat a PD pump like a centrifugal one and the predicted flow will wrongly chase the system curve. Treat a centrifugal booster like a fixed-flow device and you will miss how it rides its own performance curve. Getting the device type right is the first step.

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Engineering workflow

Decide which device you are modeling: a fixed-displacement source (PD pump) or a curve-following machine added in series (centrifugal booster).
For a PD pump, use FluidFlow’s dedicated Rotating Positive Displacement Pump component — distinct from the centrifugal pump element — to represent the fixed-flow behaviour.
Lay out the route and assign pipe, fitting, and fluid data so the system resistance is correctly represented. For a PD pump, this resistance determines the discharge pressure and power directly.
For a booster, add a second pump component in series and define its performance curve, so the solver resolves the combined operating point at the common flow.
Set the boundary conditions at the supply and delivery ends: pressures, elevations, and free-surface levels where relevant.
Solve the steady-state network and read the developed pressure, velocities, and power for the PD case; or the combined operating point for the booster arrangement.
Check the developed discharge pressure against the equipment’s pressure rating and the route’s design pressure.

⚠ Overpressure protection is out of scope for this page: PD systems must have independent overpressure protection because a blocked or closed discharge has no flow-relief mechanism in the pump itself. Route any relief or PSV scope to the FluidFlow engineering team for review.

Why fixed-flow behaviour changes the model

A centrifugal pump trades head for flow along its curve; a PD pump holds flow and lets pressure rise to whatever the system demands. The system resistance calculation matters more for a PD pump because it determines pressure and power directly — not just shifts the operating point. Modeling the fixed-flow behaviour correctly lets you check the developed pressure against equipment and pipe ratings before commissioning.

How FluidFlow helps

FluidFlow models PD and booster arrangements within the same steady-state network as the rest of your system. The Rotating Positive Displacement Pump component represents the fixed-flow machine; a centrifugal pump component added in series for a booster rides its own performance curve. Route resistance and fluid properties solve together, so you see developed pressure and power directly rather than iterating a spreadsheet.

Go deeper

Knowledge Base Modeling Pumps in Parallel & Series The series arrangement behind a booster pump — how to set up the combined performance and read the shared operating point. Knowledge Base Composite Pump Curves: Series & Parallel Performance Visualising combined operating points for series and parallel pump arrangements — applicable to centrifugal booster configurations. Knowledge Base The Flow of Liquid via a Pump Worked pump example — boundary setup, pump component inputs, and reading flow and developed head from the results.