A fixed-roof tank breathes every day. As temperature rises, vapor expands and pushes out through the vent; as it cools, air draws in. Get the vapor path wrong and you either collapse the roof (vacuum) or lift it (overpressure). For flammable liquids, that vent path also needs flame arrest and conservation control.
The two flows to size
Outbreathing has two components: thermal expansion of liquid and vapor, plus filling displacement. Inbreathing is emptying displacement plus thermal contraction. A common quick-check formula for thermal breathing is Q ≈ 0.096 × V^0.67 (m³/h) for moderate climates; the real calculation must include maximum daily temperature swing, solar gain, insulation and liquid flash point.
Component choices
PV vent (pressure-vacuum vent): set to tank design pressure, typically ±50 to ±100 mbar for fixed cone roofs
Conservation vent: combines P-V with vapor recovery connection
Flame arrestor: on the vent outlet for flammable service, rated to stable burning velocity
Emergency relief manhole: opens at a higher pressure if the PV vent is blocked
Vacuum rupture disc: protects against pump-out or steam condensation collapse
Where buyers make mistakes
Sizing only for fill rate and forgetting thermal breathing → vent chuffs all summer
Blanketing with nitrogen without raising the roof design pressure → roof deformation
Flame arrestor not inspected → carbon deposits block it, causing overpressure
Vent pipe routed downward or into a dead pocket → condensation and corrosion
Open vent on a volatile product → VOC loss and ignition risk
Blanketing note
Nitrogen blanketing suppresses oxygen and lowers fire hazard, but it turns the tank into a low-pressure system. The roof, seal and relief path must be designed for the blanket setpoint. ZN's edible oil and jet fuel tanks commonly use low-pressure blanket at 50–100 mbar; the same tanks have roof and seal rated accordingly.
FAQ snippet
"Does a water tank need a flame arrestor?" No. Flame arrestors apply to flammable vapor service. A fire water or potable tank needs a screened vent and insect proofing, not flame control.
Sizing the vapor path is a small line item with a large consequence: it is the difference between a tank that breathes quietly for 30 years and one whose roof is remembered as a safety incident.
