Expansion & materials
Expansion-vessel acceptance estimate
- where
- acceptance fraction (the calculator shows %)
- pre-charge pressure, gauge + atmospheric (bar)
- cold fill pressure, gauge + atmospheric (bar)
- maximum working pressure, gauge + atmospheric (bar)
- nominal vessel gas-space volume (L)
- expansion volume to accept (L)
Explain this equation
Estimates how much water an expansion vessel's gas space can take, and the nominal vessel size needed for a given expansion. Use it as a first check when choosing a vessel.
p_0,abs is the pre-charge, p_fill,abs the cold fill and p_max,abs the maximum working pressure, each as gauge plus atmospheric. a is the acceptance fraction, the share of vessel volume that takes water. V_e is the expansion volume.
Gas in the vessel follows Boyle's law: pressure × volume stays constant at constant temperature. Between fill and maximum pressure the gas shrinks by V × p_0,abs × (1/p_fill,abs − 1/p_max,abs). That shrinkage is the water accepted. Absolute pressures are essential because the gas law works on them.
Sense check: 1 bar pre-charge, 1.2 bar fill and 2.5 bar maximum gives about 34% acceptance, so 5 L of expansion needs about 15 L.
Watch out: no safety margin is added. This is not a final vessel selection.
Enter your values to see the result.
Good to know
Isothermal ideal-gas acceptance model only. No reserve allowance, diaphragm limits, glycol, product tolerances or relief-setting margin is selected automatically. This is not a final vessel selection.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.