Compare inbreathing and outbreathing capacity for tank vents, what drives each duty, and what to verify in supplier data before reviewing a candidate.
Inbreathing and outbreathing are two separate tank-vent capacity duties.
Inbreathing is inward flow used to address a vacuum-side condition; outbreathing is outward flow used to address a pressure-side condition.
For a defensible capacity review, establish each required duty from the tank’s operating and thermal cases, then compare it with the corresponding vacuum-side or pressure-side manufacturer performance data.
That distinction still applies when one pressure/vacuum vent performs both functions. One body can contain both pallets or relieving functions, but the tank does not acquire one interchangeable “P/V capacity.” The inward and outward duties can be driven by different operating cases, settings and flow conditions.
Inbreathing vs Outbreathing: What Changes in the Capacity Requirement?
The practical difference is the direction of the required flow and the tank condition being controlled. Inbreathing protects against an excessive vacuum tendency by admitting gas; outbreathing protects against an excessive pressure tendency by releasing gas or vapor.
| Capacity question | Inbreathing | Outbreathing |
|---|---|---|
| Flow direction | Into the tank | Out of the tank |
| Tank tendency | Falling internal pressure | Rising internal pressure |
| Vent function to verify | Vacuum side | Pressure side |
| Project requirement | Required inward flow | Required outward flow |
| Candidate evidence | Vacuum-side capacity data | Pressure-side capacity data |
A combined pressure/vacuum vent may provide both functions, but its two sides still need to be checked against different project duties. Industry application worksheets reflect this separation: they ask for total inbreathing and total outbreathing requirements independently rather than requesting one generic vent-capacity value.
Protectoseal’s conservation breather vent worksheet
is one example of this engineering practice.
This also separates required capacity from published or rated capacity. Required capacity originates with the tank and its operating cases. Published capacity describes how a candidate device performs under stated conditions. The capacity-review task is to determine whether the second covers the first on a comparable basis.
In practical terms:
required tank duty ≠ nominal connection size ≠ manufacturer capacity at stated conditions.
A larger nozzle or valve connection may permit a larger device, but connection size by itself does not establish flow capability at the required pressure or vacuum condition.
For broader system context, see ZOBAI’s
storage tank pressure and vacuum protection
application information.
What Determines the Inbreathing Capacity a Tank Needs?
Inbreathing is required when the vapor-space pressure tends to fall and gas must enter the tank to prevent an excessive vacuum condition. For tanks within its U.S. flammable-liquid scope, OSHA explicitly identifies filling or emptying and atmospheric temperature changes as normal-venting causes; the project still needs the applicable engineering method to convert those cases into a required flow.
One important operating cause is liquid withdrawal. As liquid leaves, additional vapor-space volume becomes available and the tank’s gas-management or venting system must accommodate that change. Atmospheric cooling can create a second inward-flow requirement as vapor contracts or, where applicable, condenses.
Within its specific U.S. flammable-liquid tank scope,
OSHA 29 CFR 1910.106
requires normal venting for pressure or vacuum created by filling, emptying and atmospheric temperature changes. That regulatory statement should not be used as a universal global sizing formula or as a substitute for the project-adopted venting standard.
A project-side inbreathing review commonly needs the maximum relevant withdrawal condition, the applicable thermal/cooling basis, the tank’s vacuum design or allowable basis, the proposed vacuum setting, and the way replacement gas enters the tank. A supplier worksheet that asks separately for tank design vacuum, vacuum setting, emptying rate, thermal inbreathing and total inbreathing is useful because it exposes missing inputs instead of hiding them inside a single nominal-size request.
The replacement-gas path also matters. In an atmospheric breathing arrangement, air may enter through the vacuum side of the vent. In a blanketed system, make-up gas may be supplied through a blanketing valve or regulator. Blanketing should only be credited to the extent allowed by the actual design basis; its presence does not automatically eliminate the need to review vacuum protection.
Supplier application documents commonly request tank design vacuum, vacuum setting, emptying rate, thermal inbreathing and total inbreathing as separate fields. Those inputs help establish the duty; they do not create a universal calculation formula.
For product-level questions specifically concerning vacuum protection, use the dedicated
vacuum safety valves
owner rather than treating this comparison article as a product catalogue.
What Determines the Outbreathing Capacity a Tank Needs?
Outbreathing addresses the opposite pressure tendency, but it is not simply an inbreathing calculation with the sign reversed. The pressure side has its own required flow, pressure setting and manufacturer performance evidence.
When liquid enters a tank, it displaces available vapor-space volume. Gas or vapor must leave as required by the operating condition. Warming of the tank and its contents can create another outward-flow contribution through vapor expansion and, where applicable under the selected method, additional vapor generation.
That normally requires the relevant filling condition, thermal basis, pressure design or allowable limit, pressure-side vent setting, and any fluid or process information required by the project’s chosen calculation method. If the stored liquid or operating condition changes, the project should not silently reuse an old outbreathing duty without confirming that the original assumptions still apply.
A pressure setting should not be confused with capacity. The setting describes the condition at which the pressure-side pallet or relieving element begins to respond; it does not state how much flow the tank requires or how much the vent can pass at the pressure used for rating.
For the same reason, maximum filling rate alone is not automatically a complete outbreathing specification. The applicable normal outward-flow sources must first be identified and combined according to the governing engineering basis.
Protectoseal’s application worksheet separates filling-related venting, thermal outbreathing, additional outbreathing and total outbreathing. Emerson’s current tank-management application data likewise separates filling and emptying, pressure and vacuum settings, maximum backpressure, and total outbreathing and inbreathing. See the
Protectoseal application worksheet
and
Emerson Enardo tank-management catalog.
That still leaves an important boundary:
not every outward-flow relief case is normal outbreathing.
Normal Venting vs Emergency Venting: Why the Duties Must Stay Separate
Normal venting and emergency venting are related to the same tank, but they are not the same engineering duty. A normal-breathing review answers transfer and thermal cases; an emergency review addresses separate abnormal scenarios defined by the applicable design basis.
ISO 28300:2008
covers both normal and emergency vapor venting for atmospheric and low-pressure storage tanks. ISO still lists the 2008 edition as the current published International Standard while a replacement draft is under development. The separation matters because completion of a normal inbreathing/outbreathing check should not be presented as evidence that a separate emergency case has also been resolved.
Commercial tank-protection practice reflects the same distinction. For example,
Protectoseal’s Series 7800 emergency pressure vent
is described as providing emergency relief capacity beyond the operating vent in relevant applications. That is evidence of separate duties, not a rule that every installation must use two separate physical devices.
Check the standard edition before using sizing tables or formulas
Edition-sensitive information deserves special care. API’s current Standards Plan lists
API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks
as Edition 8 dated August 2026. ISO continues to list ISO 28300:2008 as the current published ISO edition while its replacement is under development.
A project that relies on API 2000 or ISO 28300 should confirm the edition actually adopted by that project before reusing equations, factors, tables or worked examples from older online guides. API’s public material for the former seventh edition also emphasized that tank-venting requirements depend on many variables, reinforcing why a project-specific duty should not be reduced to a universal shortcut.
How to Compare Required Capacity with a Tank Vent’s Published Rating
Once required inbreathing and outbreathing duties have been established,
candidate capacity screening becomes a direction-by-direction evidence check. This is a necessary selection step, but it is not the whole suitability decision.
The question is not whether a catalogue contains a flow number that looks large enough. The question is whether the published performance corresponds to the correct direction, pressure or vacuum condition, reference basis and device configuration.
| Verification item | Inbreathing check | Outbreathing check | If unresolved |
|---|---|---|---|
| Required project duty | Required inward flow | Required outward flow | Keep the candidate unconfirmed |
| Manufacturer performance | Vacuum-side data | Pressure-side data | Request the missing data |
| Setting | Vacuum setting | Pressure setting | Clarify the comparison basis |
| Units / reference basis | Confirm compatibility | Confirm compatibility | Reconcile before comparing |
| Exact configuration | Confirm | Confirm | Hold the capacity review |
| Installed flow path | Check if relevant | Check if relevant | Engineering review |
Match the correct direction first
Pressure-side performance does not establish vacuum-side performance, and vice versa. Published vent literature may use separate flow curves for the two directions. Protectoseal, for example, notes that vacuum flow is represented on a separate curve in its
weight-loaded vent flow guidance.
Compare capacity at a meaningful operating condition
A set point by itself is not a capacity value. Protectoseal’s flow guidance explains that, at the set point of a weight-loaded vent, the pallet is still close to the seat and significant flow has not yet developed; flow increases as pressure or vacuum moves beyond that point and the pallet lifts. That is why a capacity check needs the manufacturer’s flow curve or table at the relevant rating condition, not merely a matching set pressure or vacuum setting.
The capacity review therefore needs performance data that can be related to the applicable project condition. Confirm the direction, set point, flowing pressure or vacuum basis, exact configuration and any reference conditions attached to the published capacity.
Normalize the basis before comparing numbers
A required-duty value and a supplier capacity value are useful only if their units and reference conditions can be reconciled. Do not compare two numbers simply because both are labeled as volumetric flow; first confirm whether they use compatible standard or reference conditions and whether the supplier’s data are for the intended service/configuration.
If the two values use different bases, obtain the necessary rating conditions or an engineering conversion basis before treating the comparison as valid.
Do not select from nominal size alone
Connection size is relevant to equipment configuration, but it does not by itself establish relieving capacity. Protectoseal’s flow guidance shows that size changes flow capability under the same tank-pressure conditions, while the flow curve also depends on set point and how far the pallet lifts. The selection question is therefore not “What size flange is this?” but “Can this exact vent configuration pass the required duty at the applicable pressure or vacuum condition?”
Manufacturer flow data—not the DN or NPS designation by itself—provide the evidence needed for the capacity comparison.
Include the installed flow path when it changes the comparison
A bare end-of-line capacity curve does not automatically describe a pipe-away or closed-header installation. Emerson’s application sheet asks for maximum backpressure and distinguishes vent-to-atmosphere from pipe-away configurations, which is a practical signal that the installed discharge arrangement belongs in the capacity review.
If the arrangement adds piping, a closed discharge path, a flame arrester or another significant restriction, review its effect where applicable rather than assigning an unsupported generic derating factor.
Capacity is necessary, but it is not the whole suitability check
Passing the inbreathing and outbreathing capacity checks does not establish material compatibility, leakage performance, corrosion resistance or installation suitability. Manufacturer application worksheets also ask for fluid, temperature, materials of construction, diaphragm or sealing materials, connection style, flame-arrester use and other options. Those checks belong beside—not inside—the capacity comparison.
The correct outcome when essential evidence is missing is not to guess. A candidate should remain unconfirmed for capacity until the required duty and applicable manufacturer performance can be compared on a defensible basis. Capacity confirmation alone should not be read as proof of complete product suitability.
What to Include in a Tank Vent RFQ for Both Flow Directions
A technically useful tank-vent RFQ should give the supplier enough information to review both duties without inferring critical project conditions.
Illustrative RFQ problem: “Quote one DN100 tank vent” identifies a connection size but does not establish whether the proposed device can handle either required flow direction. The corrective action is to submit the tank duty and the conditions under which the capacity must be demonstrated.
Tank and project basis
Depending on the project and applicable method, prepare the relevant:
- tank service and stored medium;
- tank capacity or geometry where needed;
- pressure and vacuum design or allowable limits;
- operating and design temperature information needed for the review;
- applicable project specification or venting standard;
- blanketing or vapor-management arrangement;
- material or sealing requirements that affect compatibility, where applicable.
Inbreathing duty
Keep the vacuum-side data explicit:
- required total inbreathing capacity;
- maximum withdrawal or emptying condition;
- applicable thermal inbreathing basis;
- vacuum setting;
- replacement-gas arrangement where relevant.
Outbreathing duty
Keep the pressure-side requirement separate:
- required total outbreathing capacity;
- maximum filling condition;
- applicable thermal outbreathing basis;
- pressure setting;
- any additional normal outward-flow source established by the project basis.
Installation configuration
Where relevant, also identify:
- vent-to-atmosphere versus pipe-away arrangement;
- inlet and outlet configuration;
- expected backpressure;
- flame or detonation arrester;
- other significant restrictions in the flow path.
These fields are not presented as a universal regulatory checklist. They reflect the types of engineering data that established vent manufacturers request when matching pressure/vacuum equipment to a tank duty. Protectoseal’s worksheet, for example, asks separately for tank design pressure and vacuum, fluid, temperature, backpressure, pressure setting, filling rate, thermal outbreathing, total outbreathing, vacuum setting, emptying rate, thermal inbreathing, total inbreathing, materials and vent design type.
Evidence to request from the supplier
The return package should allow the engineer to review the proposed configuration rather than rely on a model number alone. Relevant evidence may include:
- the exact candidate configuration;
- vacuum-side capacity data;
- pressure-side capacity data;
- applicable pressure and vacuum settings;
- flow curves or capacity tables and their rating basis;
- units and reference conditions;
- materials and sealing details needed for service compatibility;
- installation or configuration limitations that affect the comparison.
Frequently Asked Questions About Tank Vent Inbreathing and Outbreathing
What is the difference between inbreathing and outbreathing on a storage tank?
Inbreathing is gas flow into the tank when internal pressure tends to fall; outbreathing is gas or vapor flow out when internal pressure tends to rise. They are separate capacity duties and should be checked against the corresponding vacuum-side and pressure-side performance of the vent.
Is the tank filling rate the same as the required outbreathing capacity?
Not automatically. Filling can be an important normal outbreathing driver, but the applicable thermal contribution and any other project-defined normal outward-flow source also have to be considered under the chosen venting method.
Is the emptying rate the same as the required inbreathing capacity?
Not automatically. Emptying creates inward demand, but thermal cooling or condensation and the actual replacement-gas arrangement can also affect the required inbreathing duty.
Can I use the same capacity rating for both pressure and vacuum directions?
Do not assume so. Pressure-side and vacuum-side performance can be represented separately, and the correct manufacturer data should be checked for each direction at the applicable setting and rating condition.
How does backpressure affect a tank vent capacity review?
Backpressure is relevant when the outlet is piped away or connected to a header. Manufacturer application data may ask for maximum backpressure and distinguish pipe-away from vent-to-atmosphere service, so a bare end-of-line curve should not be reused for a modified discharge system without confirmation.
Does a nitrogen blanketing system eliminate the need for vacuum protection?
No automatic credit should be assumed. A blanketing system may supply make-up gas, but the project must define whether its capacity and availability are credited in the vacuum-protection basis and what independent protection remains necessary.
Does normal outbreathing capacity cover an external-fire case?
No. Normal transfer and thermal breathing should remain separate from an applicable emergency or fire-relief assessment. Emergency venting is a different engineering duty even when the hardware arrangement ultimately uses related tank openings or devices.
Which standards are relevant to tank inbreathing and outbreathing?
API Standard 2000 is a principal industry standard for venting atmospheric and low-pressure storage tanks; API lists Edition 8 in 2026. ISO currently lists ISO 28300:2008 as its published international standard while a replacement is under development. U.S. flammable-liquid installations may also be subject to OSHA 29 CFR 1910.106. The project should confirm the edition and jurisdiction actually adopted before using specific sizing tables, factors or formulas.
