When your plant asks you to size a breather valve, the hardest part is rarely the arithmetic. The formulas behind API 2000 are public, and our companion guide walks you through every one of them. The part most engineers stall on is the second step: once you know your required venting capacity, how do you turn that number into the right valve — the right model, orifice, set pressure, and flame-arrester combination — for your tank? This article focuses on exactly that: moving from a calculated capacity to a correctly specified ZhenChao breather valve.
Need the full calculation worksheet first?
If you have not yet worked out your inbreathing and outbreathing demand, open our API 2000 Calculation: Step-by-Step Tank Vent Sizing Guide for every formula, the climate and product factors, and a fully worked tank example. Then come back here to choose the valve.
Every fixed-roof storage tank “breathes.” Solar heating expands vapour and pushes it out; cooling draws air back in; pumps move liquid in and out. If that air and vapour cannot move freely, pressure builds — and a tank is not built to fight its own breathing. An undersized valve lets vacuum collapse the roof or overpressure lift it off. An oversized valve wastes budget and, worse, breathes too freely, losing product and venting vapour to the atmosphere. Getting the sizing right is the difference between a tank that runs for decades and one that becomes a safety incident.
Before you pick a valve, be clear about the two flows it must pass. Each produces a required capacity, normally expressed in Nm³/h of air-equivalent:
1. Thermal outbreathing (vapour out) — daytime heating boils off vapour that must escape.
2. Thermal inbreathing (air in) — nighttime cooling pulls air back in to replace contracting vapour.
3. Liquid inbreathing / outbreathing (pump transfer) — filling displaces vapour (out); emptying draws air in (in). This usually dominates for active tanks.
Your valve’s job is to pass the larger of the inbreathing and outbreathing demands — plus any fire-case emergency flow if your design requires it — without exceeding the tank’s mechanical pressure limits. Hold that “required capacity” number in mind, because the rest of this guide is about matching a valve to it.
Three standards frame breather valve sizing, and you should know which one governs your project. API 2000 is the default for atmospheric and low-pressure petroleum storage tanks. ISO 28300 is the international equivalent and is often specified for projects outside North America. EN 14591 covers venting for mines and, by extension, some European process applications. They share the same logic — quantify normal and emergency flows, then select a device — but differ in factors and units. Pick the one your client or authority asks for, then size consistently within it.
In practice you gather four inputs: your tank diameter and wetted area, your maximum liquid level change rate, the temperature swing (ΔT) at your site, and whether the tank is insulated or painted. API 2000 then gives you a thermal inbreathing demand, a thermal outbreathing demand, and a pump/liquid-transfer demand — all in air-equivalent flow. The controlling number is the largest of these for each direction.
For the complete formula set, the factor tables, and a tank worked end-to-end, see our API 2000 Step-by-Step Tank Vent Sizing Guide. The section below is where this article deliberately goes further than that guide — turning your capacity into a real valve.
You now have a required capacity in Nm³/h for inbreathing and for outbreathing. Here is how you convert that into a valve specification.
A breather valve is rated by the flow its orifice passes at its set pressure. Choose an orifice whose rated capacity comfortably exceeds your controlling demand — a typical engineering margin is 10–25% so you are never running the valve at its limit. Do not select on the model family alone; two valves with the same name can carry very different flows depending on orifice diameter. Read the certified flow curve, not the brochure headline.
Your set pressure must sit safely below the tank’s mechanical design pressure and above normal operating pressure, so the valve only cracks when it must. Vacuum setting is set negative, again within the tank’s rated vacuum. The goal is a valve that stays closed during normal operation (minimising product loss and vapour emission) but opens the instant the tank needs to breathe. When in doubt, keep the set point as close to the tank limit as your safety factor allows.
A pressure/vacuum relief valve (PVRV) is a single device with two seats — one opens outward under pressure, one opens inward under vacuum. You do not need separate valves for inbreathing and outbreathing; you need one valve sized for the larger demand in each direction. That is why the capacity check above is done per-direction before you specify.
If the tank holds a flammable liquid and sits where an ignition source is plausible, combine the breather valve with a flame arrester. The arrester stops a flashback from reaching the tank vapour space while the valve manages pressure. Size the arrester for the same flow so it does not become the new bottleneck.
Finally, match the valve’s flange (DN) to your tank nozzle, confirm the height and any foul-air pocket are addressed, and keep the inlet path short and unrestricted. A perfectly sized orifice loses its benefit if the inlet is throttled by piping.
Not sure which orifice and set pressure fit your tank? Send us your tank data — diameter, liquid, ΔT, and pump rates — and our engineers will return an exact model and orifice recommendation.
Get a Free Sizing QuoteWith your capacity and duty known, match the model to the service. The table below is a starting point; ZhenChao builds each valve to the orifice and set pressure your calculation demands.
| Model | Typical duty & capacity range | Key feature | Best for |
|---|---|---|---|
| ZCBV-I | Standard atmospheric tanks, general venting | Simple, cost-effective PVRV | Storage of non-flash, stable liquids |
| ZXBV-I + ZGB | Tanks with flammable vapour, near ignition sources | Integrated flame arrester | Fuel, solvent, API / ATEX zones |
| ZCHX-IZ | Higher set pressures, aggressive service | Robust body with flame elements | Pressurised or corrosive tanks |
Need the certified flow data to confirm a match? Download our breather valve specification sheet (PDF) for rated capacities, materials, and flange options across the range.
Yes. A PVRV has two independent seats in one body — the pressure seat opens outward, the vacuum seat opens inward. You size each seat for its own controlling demand; you do not need two separate devices.
Add one whenever the tank holds a flammable liquid and an ignition source is credible nearby (tank farms, refineries, solvent storage). The arrester blocks flame propagation into the tank while the valve controls pressure. Size it for the same flow as the valve.
API 2000 is the norm for petroleum storage tanks; ISO 28300 is its international counterpart; EN 14591 applies to certain European and mining venting cases. All follow the same “quantify flows, then select” logic but differ in factors and units — use the one your project specifies.
Set them inside the tank’s mechanical limits: set pressure below the tank design pressure and above normal operating pressure, vacuum setting correspondingly negative. Keeping the set point close to the limit reduces needless vapour loss while still protecting the tank.
Under-sizing can collapse or lift the roof and vent flammable vapour — a safety event. Over-sizing wastes money and breathes too freely, increasing product loss and emissions. Correct sizing lands between: open only when needed, pass the full demand when it is.
Read the tank nozzle’s DN and flange rating, then select a valve with a matching inlet. If the required orifice is larger than the existing nozzle, you either re-nozzle the tank or step up to a valve sized for the available inlet with the right flow curve — our engineers can confirm which is cheaper for your case.
• API 2000 Calculation: Step-by-Step Tank Vent Sizing Guide — the full formulas, factor tables, and worked example (read this before sizing).
This guide is prepared by ZhenChao, a manufacturer with 18+ years of experience in tank protection valves. Our breather valves are supplied to storage and process projects across the Middle East, Central Asia, and beyond, built to ATEX and ISO requirements, and backed by an 18-month warranty with factory-direct engineering support. For certified capacity and material data, see the breather valve specification sheet (PDF).
Let’s size your breather valve correctly — the first time.
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