Engineering context

Not every pipe system has a pump. Gravity drains, overflow lines, tank-to-tank transfers, and elevated-reservoir feeds are driven entirely by the difference in elevation between where the fluid starts and where it ends. The flow rate settles at the point where the available driving head is exactly consumed by friction and fitting losses across the route.

Three quantities recur on these jobs and are routinely confused.

• Static head is the pressure resulting from a column of liquid acting under gravity — it is the elevation-driven component of the available driving energy, expressed as a head equivalent.
• Net static head is that difference measured between the two free surfaces (or boundary points) that actually bound the flow path — not between arbitrary pipe nodes mid-route.
• Total dynamic head (TDH) is the total head a pump would need to deliver for a given flow — static head plus friction and fitting losses plus any velocity-head and pressure-head differences between source and sink. TDH is the figure to compare against a vendor pump curve when a pump is later considered.

Not every pipe system has a pump. Gravity drains, overflow lines, tank-to-tank transfers, and elevated-reservoir feeds are driven entirely by the difference in elevation between where the fluid starts and where it ends. The flow rate settles at the point where the available driving head is exactly consumed by friction and fitting losses across the route.

The calculation that trips people up most often is static versus stagnation pressure at an open pipe end. At a pipe discharging freely to atmosphere, the static pressure at the open end is atmospheric (zero gauge). The stagnation (total) pressure at that same point is higher by the dynamic pressure term ½ρv², which represents the kinetic energy carried by the moving fluid. Reading the wrong one into a hand calculation is a frequent source of disagreement between a spreadsheet and a network model.

Engineering workflow

1. Identify the bounding points. Locate the two points that bound the flow path — typically two free liquid surfaces, an open discharge, or a fixed-pressure boundary — and fix their elevations against a common datum
2. Establish net static head. Compute the elevation difference between those two bounding points, not between arbitrary pipe nodes mid-route. This is the net static head available to drive flow.
3. Lay out the route. Assign pipe data along the full path: internal diameters, lengths, roughness, and fittings.
4. Select the fluid. Choose the fluid from the FluidFlow database so its density — which sets the static head term ρg·h — carries through the connected model.
5. Set boundary conditions. Enter surface pressure at each tank or open end (atmospheric for vented, gas blanket pressure for closed vessels) and the elevations from Step 1.
6. Solve the steady-state network. With no pump present, the solver finds the flow rate at which available static head equals total friction and fitting losses.
7. Read the results. Record the resulting flow, velocities, and head consumed at each point along the route.
8. Size for a pump if required. If a pump is being considered, read the TDH at the target duty from the same model and apply a design margin before checking a vendor curve.

Why net head — not a single pipe run — decides the flow

In a gravity-driven system the flow rate is not an input: it is the result of the balance between available static head and route losses. Estimating one pipe leg in isolation cannot capture that balance, because the whole route between the two bounding surfaces shares one driving head. A common error is measuring static head from the inlet pipe connection rather than from the free surface above it — missing the liquid column contribution inside the tank. A connected steady-state solver resolves the balance directly and stays consistent when tank level, surface pressure, or downstream fittings change.

How FluidFlow helps

FluidFlow models gravity and elevation-driven liquid systems as connected steady-state networks. You place the bounding boundaries — typically the Tank or Vessel Reservoir and Atmospheric Ends boundaries, or a fixed Pressure boundary — at their true elevations, connect the route, assign pipe and fluid data, and solve for the flow that the available net head will sustain. The same model gives you the TDH at any target duty when you add a pump later.

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