What is the difference between working loss and breathing loss?

Working loss is the vapor expelled when liquid is added (vapor displaced by incoming liquid). Breathing loss — also called standing storage loss — is the vapor expelled by daily heating/cooling of the vapor space and barometric pressure swings, with no level change. Working loss scales with throughput; breathing loss scales with the vapor-space volume and daily temperature swing.

When is a tank subject to NSPS OOOOa/b/c?

A storage vessel containing crude oil, condensate, intermediate hydrocarbon liquids, or produced water is subject to NSPS OOOOa/b/c if the uncontrolled emissions exceed 6 ton/yr VOC (single-tank basis). The rule requires 95% control (typically VRU + flare) and monthly inspections.

How accurate are AP-42 tank emission estimates?

AP-42 estimates are intended to be screening-level — EPA gives them a quality rating of B (for fixed-roof) or D (for some EFR/IFR cases). For permit reporting they are accepted; for compliance demonstration, refer to a direct measurement program (Method 21 with continuous monitoring) or a more detailed model like TANKS 4.09D / WebFire.

Tank Emissions Calculator — AP-42 §7.1 Working & Breathing

Tank Physical

Tank Type

Diameter

Shell Height

Avg Liquid Height

Blank → half of shell height

Roof Type (Fixed only)

Shell Color / Condition

Liquid Properties

Liquid Type

API Gravity (crude/cond)

°API

RVP (Reid Vapor Pressure)

psi

Vapor MW (blank = auto)

lb/lbmol

Crude default 50, gasoline 64, condensate 60

Total HAP fraction (BTEX + n-hexane)

wt %

Crude typical 1–5%; condensate 3–8%

Methane fraction (vapor)

wt %

For GHG split — crude vapor 3–8% CH₄

Operating & Site

Annual Throughput

bbl/yr

Bulk Liquid Temperature

°F

Annual average bulk T (T_LA)

Daily Ambient ΔT

°F

Daily max − min ambient (annual avg)

Solar Insolation

Btu/ft²·day

US avg ~1,400–2,000; Midland ~1,800

Wind Speed (EFR only)

mph

Atmospheric Pressure

psia

📚 Learn the Theory

AP-42 equations, vapor space expansion factor, RVP-to-TVP conversion, OOOO applicability, control device payback.

Read Engineering Guide →

Key AP-42 Equations

Standing (breathing) loss:

L_S = 365 · V_V · W_V · K_E · K_S

Working loss:

L_W = 0.0010 · M_V · P_VA · Q · K_N · K_P

Vapor density:

W_V = (M_V · P_VA) / (R · T_LA)

Vapor space saturation factor:

K_S = 1 / (1 + 0.053 · P_VA · H_VO)

Turnover factor:

K_N = 1 (N ≤ 36) | (180 + N) / (6N) (N > 36)

Product factor K_P: crude oil = 0.75; all other organic liquids = 1.00 (AP-42 Eq. 7.1-35).
R = 10.731 psia·ft³/(lbmol·°R).

Frequently Asked Questions

What is AP-42 Chapter 7.1?＋

EPA's compendium of emission-factor equations for organic liquid storage tanks (fixed-roof, IFR, EFR). Output feeds Title V PTE and NSPS Kb / OOOOa/b/c applicability.

Working vs breathing loss?＋

Working = vapor displaced by added liquid (scales with throughput). Breathing = vapor pushed out by daily T and P swings (scales with vapor-space volume and ΔT). Total = sum.

When is NSPS OOOO triggered?＋

Single-tank uncontrolled VOC > 6 ton/yr → OOOOa/b/c controls required (95% capture, typically VRU + flare, monthly inspections).

Tank Emissions Calculator — AP-42 Working + Breathing Loss

Tank Physical

Liquid Properties

Operating & Site

📚 Learn the Theory

Key AP-42 Equations

Tank Emissions

Threshold Triggers

Recommendations

Frequently Asked Questions

Tank Emissions Calculator — AP-42 Working + Breathing Loss

Tank Physical

Liquid Properties

Operating & Site

📚 Learn the Theory

Key AP-42 Equations

Tank Emissions

Threshold Triggers

Recommendations

Frequently Asked Questions

Related Calculators

Tank Flashing

VRU Sizing

GHG Emissions

Potential to Emit