Boundaries

Definition

When you define a pressure at a boundary node, FluidFlow calculates the resultant flow rate. When you define a flow rate at a boundary node, FluidFlow calculates the resultant pressure. Every hydraulic model must have at least one inlet boundary (where the model begins or fluid is sourced) and one outlet boundary (where the model ends or fluid is delivered).

FluidFlow provides five boundary types:

• Known Pressure — sets a fixed pressure at the boundary node. Used for pressurised supply headers, pressurised vessels, or any point where the pressure is known.
• Known Flow — sets a fixed flow rate at the boundary. Used for pump-delivered flows, metered supplies, or positive-displacement pump outputs.
• Tank or Vessel Reservoir — sets the boundary pressure as surface pressure plus the static head of the liquid column (ρg·h). Used for open or closed tanks, reservoirs, and vessels. Acts as a source or a destination — not a pass-through.
• Atmospheric Ends (Open Pipe) — sets the boundary to atmospheric pressure. Used for pipe discharges open to atmosphere or for open reservoirs at atmospheric surface pressure.
• Sprinkler — a specialised boundary for fire-protection and sprinkler network analysis.

Engineering context

The choice of boundary type and the accuracy of the values assigned to it determine the trustworthiness of every result in the model. For the Tank or Vessel Reservoir boundary, four inputs must be set correctly:

1. The elevation of the boundary against the model datum.
2. The liquid surface level that sets the static head contribution.
3. The surface pressure — atmospheric for a vented tank, or the gas or blanket pressure for a closed vessel.
4. The net-flow sign convention — whether the boundary is acting as a source or a destination.

An error at any of these propagates to every flow, pressure, velocity, and operating-point result in the connected network, with no warning from the solver that the starting condition was wrong. See Tank and Vessel Boundaries: Why Your Boundary Pressure Is Wrong for the full setup sequence.

For closed-loop systems — where a tank closes the return leg — both the supply and return boundaries must be consistent in elevation and pressure so the solver can resolve the driving head correctly.

Related definitions

Static head · Elevation · NPSH available · Stagnation pressure · System curve

See it in FluidFlow

FluidFlow’s boundary components are selected from the component database and placed at the start and end nodes of the network. The Tank or Vessel Reservoir boundary automatically computes the effective boundary pressure from the surface pressure and liquid-column static head. The Stagnation Pressure model is recommended for these boundaries because it correctly represents conditions at a point where fluid velocity is negligible.