TOOL GUIDE
Expansion vessel sizing
Estimate diaphragm vessel volume from fluid expansion, pressures and initial water reserve.
Purpose & scope
Estimate diaphragm vessel volume from fluid expansion, pressures and initial water reserve.
Ideal-gas model; no product selection or whole-building fill-pressure calculation. bar(g) is converted to bar(abs) internally.
Inputs
Use drawings, design conditions and manufacturer data. Units and example values follow the calculator; example values are not universal design requirements.
| Parameter | Value / interpretation |
|---|---|
| Total system fluid volume · L | Total system fluid volume, not expansion vessel volume. |
| Lowest design temperature · °C | Lowest fluid temperature within the design range. |
| Highest design temperature · °C | Highest fluid temperature within the design range. |
| Expansion input | Pure water — calculated from temperatures / Expansion fraction from fluid data |
| Fluid volume increase · % | Percentage volume increase from cold to hot; use the actual fluid data. |
| Gas pre-charge before filling · bar(g) | Gas pre-charge before water filling; enter gauge pressure bar(g). |
| Cold fill pressure · bar(g) | Cold fill gauge pressure at the vessel elevation, in bar(g). |
| Maximum allowed operating pressure · bar(g) | Maximum allowed operating gauge pressure at the vessel, in bar(g). |
| Atmospheric pressure · bar(abs) | Absolute atmospheric pressure used to convert vessel gas pressures. |
| Required initial water reserve · L | Required water reserve in the vessel at the cold condition. |
| Volume allowance · % | Allowance applied to the result; not a substitute for entering all losses. |
Steps
- Enter total system volume and cold/hot temperatures; select water or documented fluid expansion.
- Enter pressures at vessel elevation: pre-charge, cold fill, maximum operating and atmospheric.
- Enter reserve/allowance; read minimum volume and verify the actual vessel, relief valve and pressure/temperature ratings.
Worked example
This reproducible training example uses the values below. Select the appropriate mode and overwrite restored project values. Example rates, K values and equipment losses are assumptions, not mandatory design criteria.
Examples use a decimal point and no thousands separator.
All example inputs
| Parameter | Value / interpretation |
|---|---|
| Total system fluid volume | 1000 L |
| Lowest design temperature | 7 °C |
| Highest design temperature | 35 °C |
| Expansion input | Pure water — calculated from temperatures |
| Fluid volume increase | 1 % |
| Gas pre-charge before filling | 1 bar(g) |
| Cold fill pressure | 1.3 bar(g) |
| Maximum allowed operating pressure | 2.5 bar(g) |
| Atmospheric pressure | 1.01325 bar(abs) |
| Required initial water reserve | 3 L |
| Volume allowance | 10 % |
Expected results (rounded)
| Parameter | Value / interpretation |
|---|---|
| Expansion volume | 5.9 L |
| Minimum vessel volume | 23.13 L |
| Rounded reference volume | 24 L |
Small differences in displayed rounding are normal. A result that passes the entered limits does not establish suitability for the complete system.
Method & limitations
Ideal-gas model; no product selection or whole-building fill-pressure calculation. bar(g) is converted to bar(abs) internally.
Use absolute pressures: p = gauge pressure + atmospheric pressure. With isothermal ideal gas, incremental acceptance = p₀·(1/p cold − 1/p max). Initial water fraction = 1 − p₀/p cold.
Volume = max(expansion / incremental acceptance × allowance, reserve / initial water fraction). The vessel must accept the expansion and hold the initial reserve. Pure-water temperatures are limited to 4–90°C; use manufacturer expansion data for glycol or other fluids.
Pre-charge ≤ cold fill < maximum pressure. Maximum is the allowed operating pressure, not the relief setpoint without its operating allowance. Verify static pressure, pump location and vessel pressure/temperature limits. Not a pump-cycling accumulator calculation.
Common mistakes
Entering absolute pressure into gauge fields; confusing system/vessel volume; excessive pre-charge; requesting a reserve when pre-charge equals cold fill.
Projects & reports
- Choose or create an engineering project and enter your actual design inputs.
- Project inputs/results are autosaved in this browser. Where a saved-list panel is available, name and add the selected section or alternative to compare multiple options.
- Review the report before exporting PDF. In the print dialog select Save as PDF. Export JSON for backup and use Open file to restore it; PDF cannot restore project data.
Saved results do not automatically recalculate when another tool changes. Review any dependency warning, recalculate and update the affected results.
References & related tools
The tool implements the methods and limits stated above; linking a standard does not establish complete compliance with that standard.