Engineering context

A network is only as trustworthy as the boundary conditions that bound it. Tanks, reservoirs, and vessels fix the pressure at the point of connection: the surface pressure (atmospheric for a vented tank, or a defined gas pressure for a closed vessel) plus the static head of the liquid column from the free surface down to the outlet, ρg·h.

Set the level or the elevation wrong and every downstream flow and pressure result inherits the error. This is one of the most common sources of disagreement between a hand calculation and a network model — not a pipe friction mistake, but an error in how the boundary pressure was defined.

Engineering workflow

1. Place the Tank or Vessel Reservoir boundary where it bounds the flow path. Set its elevation against the same datum used for all other components in the network. Mixing elevation references is one of the most common sources of false static head in a model.
2. Define the surface pressure: atmospheric for a vented tank, or the specified gas or blanket pressure for a closed vessel. This is the pressure at the liquid surface — not at the nozzle.
3. Set the liquid surface level. FluidFlow uses this, together with the nozzle elevation, to calculate the static head contributed by the boundary. The greater the height between the surface and the nozzle, the more head is available to drive flow.
4. Connect the pipe at the nozzle’s true elevation. The difference between the free surface and the nozzle elevation is what determines the static head term in the energy balance. Entering the wrong nozzle elevation directly corrupts the boundary pressure.
5. Confirm the boundary acts as the correct source or destination. For Tank/Vessel Reservoir and Known Pressure boundaries, FluidFlow resolves this automatically — the node with higher total energy (pressure plus elevation head) becomes the inlet. No sign convention input is required. If you are using a Known Flow boundary, set the flow direction to “Into Network” to designate it as the inlet explicitly.
6. Select the correct fluid from the FluidFlow database. Fluid density drives the ρg·h calculation. The fluid must be defined at the inlet boundary; properties entered at outlet boundaries are ignored by the solver.
7. Solve the network and review the results. Check flows, pressures at each boundary, velocities, and any design alerts. Confirm the operating point is physically plausible — unexpected results at this stage usually trace back to an elevation or surface pressure input error, not a pipe friction issue.

Why the boundary — not just the pipe — sets the result

Two identical pipe routes connected to tanks at different levels or surface pressures will carry different flows, because the boundary supplies the head that drives or resists the flow. An error at the boundary propagates to every result that depends on it — flow rates, velocities, pressure drops, pump operating points — with no warning from the solver that the starting condition was wrong.

How FluidFlow helps

FluidFlow represents tank and vessel boundaries with the Tank/Vessel Reservoir boundary, used as a source or destination in the network. For fixed-pressure connections such as a header tie-in or a controlled vessel, use a Known Pressure boundary. For open or atmospheric discharge scenarios, use an Open Pipe Ends boundary. In each case, you set the elevation, liquid level (for reservoir types), and surface pressure; the solver uses the resulting boundary pressure — surface pressure plus static head — to resolve flows and pressures across the connected system.

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