Sep 25, 2026
An atmospheric storage tank needs a vent sized for its worst credible inflow and outflow of gas, and the vacuum side is the one most often undersized. A standard API 650 tank is built to hold only a few inches of water column of pressure and about one inch of vacuum, so the margin between normal breathing and a buckled roof is small.
This note walks through how required venting is calculated under API 2000, how to turn a required flow into a vent that actually delivers it, and what to check in the field so a correctly sized vent keeps working. It closes with the tank data we need to check sizing for you.
Normal venting, emergency venting, and the limits you are protecting
API 2000 (7th edition, 2014), harmonized with ISO 28300, splits tank venting into two jobs. Normal venting handles everyday breathing: liquid moving in and out, and the vapor space heating and cooling. Emergency venting handles upset conditions such as an external fire, which drive large outward flows.
The split matters for vacuum. Emergency hatches and emergency relief valves relieve outward only, so every vacuum case is a normal venting case. The conservation vent or dedicated vacuum breaker has to carry it alone.
Per API 2000, the total normal venting requirement is the sum of the liquid movement and thermal requirements, unless the operations can never happen at the same time.
The vent is sized against what the tank can take:
| Tank design basis | Allowable vacuum | Allowable internal pressure |
|---|---|---|
| API 650, base standard (5.2.1) | 1 in. W.C. (0.25 kPa, about 0.036 psi) | Atmospheric, unless designed for more |
| API 650 with Annex V | Up to 1 psi (6.9 kPa), as specified | Per design |
| API 650 with Annex F | 1 in. W.C. unless Annex V also applies | Up to 2.5 psig (18 kPa) |
| Credible vacuum above 1 psi | Outside API 650 (ASME Section VIII) | Outside API 650 |
The tank nameplate or design data sheet shows which basis applies. If neither is available, assume the base standard: 1 in. W.C. of vacuum.
Inbreathing, part 1: pump-out
For every gallon pumped out, a gallon of air has to come in through the vent. The inbreathing requirement from liquid movement is simply the maximum emptying rate, converted to air flow:
Pump-out inbreathing (SCFH of air) = 8.02 × maximum pump-out rate (gpm)The 8.02 converts gallons per minute to cubic feet per hour. A 500 gpm transfer pump needs about 4,010 SCFH of air coming in.
Use the highest rate the tank can actually see, not the design rate of one pump:
- Two or more pumps that can run at once
- A pump upgrade or impeller change since the vent was specified
- Gravity drainage to a lower tank or a sump
- Truck or railcar loading that pulls faster than the in-plant transfer
This is the most common reason a vent that worked for years suddenly does not. The vent stayed the same; the pump-out rate did not.
Inbreathing, part 2: thermal cooling
A cold rain on a hot tank shrinks the vapor space fast, and on most tanks this is the larger of the two inbreathing loads. API 2000 7th edition (3.3.2.3) calculates it as:
Thermal inbreathing (SCFH of air) = 3.08 × C × Vtk^0.7 × RiV_tk is the tank volume in cubic feet (barrels × 5.618), R_i is the insulation factor (1.0 for an uninsulated tank), and C depends on latitude, vapor pressure, and average storage temperature:
| Latitude | Hexane-like vapor pressure, storage below 77°F | Hexane-like vapor pressure, storage 77°F or above | Higher than hexane or unknown |
|---|---|---|---|
| Below 42° | 4.0 | 6.5 | 6.5 |
| 42° to 58° | 3.0 | 5.0 | 5.0 |
| Above 58° | 2.5 | 4.0 | 4.0 |
All of Tennessee, Georgia, Alabama, and the Florida Panhandle sit below 42° latitude, so C is 4.0 or 6.5 across our territory. When the vapor pressure is unknown, use 6.5.
One caution for older tanks. Vents specified under the 5th edition method (now Annex A) often show much lower thermal inbreathing than the current equation, and the gap grows with tank size. A vent that met the standard when it was bought may fall short of the current edition.
Outbreathing: pump-in and thermal heating
The pressure side follows the same logic in reverse. Filling displaces vapor, and solar heating expands it.
Pump-in outbreathing, non-volatile liquids (SCFH) = 8.02 × maximum pump-in rate (gpm)Pump-in outbreathing, volatile liquids (SCFH) = 16.04 × maximum pump-in rate (gpm)Thermal outbreathing (SCFH) = 1.51 × Y × Vtk^0.9 × RiAPI 2000 treats a liquid as volatile above 5 kPa (about 0.73 psia) vapor pressure; volatile liquids get double the displacement flow. Y is 0.32 below 42° latitude, 0.25 from 42° to 58°, and 0.20 above 58°. Liquids that flash at storage conditions need an equilibrium calculation instead of these equations.
For most fixed-roof tanks, the pressure pallet is set by outbreathing and the vacuum pallet by inbreathing. Size and check each side separately.
Blanketed tanks and cases the equations miss
Nitrogen blanketing. The blanketing regulator is sized for the same inbreathing load (pump-out plus thermal), and it normally keeps the tank from ever reaching vacuum. The vacuum pallet is the backup, so size it for the full inbreathing load, not the leftover. If the regulator fails closed or the nitrogen supply is lost, the vacuum pallet carries the whole job. Set the vacuum pallet below the blanketing set point so it stays closed during normal regulation.
Regulator failing open. This is a pressure case, not a vacuum case. The pressure pallet or emergency vent has to pass the regulator’s full-open nitrogen flow.
Steam-out. Condensing steam pulls vacuum far faster than any normal breathing case, and the equations above do not cover it. Keep a large vent path open (a manway or a vent sized for the condensation rate) until the tank has cooled. Treat it as a written procedure step, not an assumption.
Other fast-cooling events. Cold liquid added to a hot tank, and vapor that absorbs into the liquid, also create vacuum outside the normal equations. If your process can do either, flag it when you send us your data.
From required flow to a vent that works
A vent is adequate only if it passes the required flow while the tank is still inside its limit. Three things eat into that margin: where the pallet starts to open, how much more pressure it needs to open fully, and the pressure drop of everything between the vent and the tank.
Set pressure is not opening pressure. A pallet starts to lift at its set pressure but reaches full flow only at a higher opening pressure. The difference is the valve’s overpressure characteristic, and it varies widely by design:
| Pallet design | Overpressure to full lift | Set point for a tank limited to 1.0 in. W.C. vacuum |
|---|---|---|
| Full-lift type | 10% | About 0.9 in. W.C. |
| Conventional | 40% | About 0.7 in. W.C. |
| Conventional | 100% | About 0.5 in. W.C. |
On a base-standard API 650 tank, this can decide the answer. Weight-loaded pallets have a practical minimum setting (0.8 in. W.C. on one common design), so a valve that needs 100% overpressure may not be settable low enough at all.
Read capacity at the opening pressure. Manufacturers’ certified flow charts show flow for a fully open valve against pressure. Enter the chart at the opening pressure, not the set pressure. Charts are for air at standard conditions, so outbreathing of hydrocarbon or chemical vapor needs a density correction.
Count every restriction in the path. The tank sees the vent’s opening pressure plus every pressure drop between them:
Pressure the tank sees = vent opening pressure + nozzle and piping pressure drop + flame arrester pressure drop + screen pressure drop, followed by a new line saying that total must stay at or below the tank's allowable pressure or vacuumAn in-line flame arrester is usually the largest of these, and its pressure drop rises as the element fouls. Size with the fouled condition in mind, not the clean one, and put arrester cleaning on a schedule. Any block valve under the vent should be full bore and locked or car-sealed open.
Worked example: vacuum side of a 40-foot tank
This tank needs about 44,000 SCFH of vacuum relief, and 89% of it comes from thermal cooling, not the pump.
| Input | Value |
|---|---|
| Tank | 40 ft diameter × 40 ft tall, fixed cone roof, uninsulated |
| Volume | 50,265 ft³ (about 8,950 bbl) |
| Design basis | API 650 base standard: 1.0 in. W.C. allowable vacuum |
| Location | Tennessee (below 42° latitude) |
| Product | Vapor pressure unknown, stored above 77°F: C = 6.5 |
| Maximum pump-out | 600 gpm |
| Step | Calculation | Result |
|---|---|---|
| Pump-out inbreathing | 8.02 × 600 gpm | 4,812 SCFH |
| Thermal inbreathing | 3.08 × 6.5 × 50,265^0.7 × 1.0 | 39,116 SCFH |
| Total required vacuum relief | Sum of the two | 43,928 SCFH (about 1,244 m³/h) |
Now check the vent against the 1.0 in. W.C. limit. If an in-line flame arrester costs 0.1 in. W.C. at this flow, the vent must be fully open by 0.9 in. W.C. A 10% full-lift pallet would be set near 0.82 in. W.C.; a 100% overpressure pallet would need a 0.45 in. W.C. setting, below what most weight-loaded pallets can reach.
Two lessons carry over to most tanks. First, a vent sized only for the pump will be several times too small. Second, on a base-standard tank the arrester drop and the pallet’s overpressure characteristic can matter as much as the vent’s nominal size.
The flame arrester drop above is illustrative. Actual values come from the arrester’s certified flow chart at the required flow.
Keeping a sized vent working
A correctly sized vent protects the tank only if it still moves and still flows. Check these at each inspection, under your site’s safe work procedures and the manufacturer’s maintenance instructions:
- Vacuum pallet confirmed free to move, not stuck, painted over, or corroded to the seat
- Screens clear of nests, debris, plastic, and ice
- Flame arrester element inspected and cleaned
- Any block valve under the vent full bore and locked or car-sealed open
- Set points on the nameplate match the documented design basis
- Current maximum pump-out rate compared to the rate the vent was sized for
- Blanketing regulator and nitrogen supply tested, where fitted
Re-check sizing whenever the tank changes service, a pump is upsized, a flame arrester is added, or the tank’s design basis is unknown.
Send us your tank data
For tanks in Tennessee and North Alabama, send the details below to sales@georgegrantco.com, and we will check your vent sizing against the current edition of API 2000. Partial data is fine; we will tell you what else we need.
| Item | Examples |
|---|---|
| Tank size | Diameter and height, or capacity in barrels or gallons |
| Design basis | API 650 base, Annex V, Annex F, or nameplate design pressure and vacuum |
| Location | Site city and state |
| Product | Name, vapor pressure if known, normal storage temperature |
| Maximum pump-in and pump-out | gpm, including pumps that can run together |
| Blanketing | Nitrogen or other gas, regulator set point |
| Vent line equipment | Existing vent make, model, size and set points; flame arrester; block valves; screens |
| Insulation | Thickness and coverage, if any |
| Special operations | Steam-out, hot/cold liquid mixing, absorbing vapors |
George Grant Company has represented process equipment manufacturers across Tennessee, Georgia, Alabama, and the Florida Panhandle since 1969. In Tennessee and North Alabama, we check vent sizing and supply the conservation vents, flame arresters, and emergency relief equipment, working directly with the manufacturer’s engineering team on selection.
References
Standards:
- API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks, 7th edition (March 2014), harmonized with ISO 28300
- API Standard 650, Welded Tanks for Oil Storage, section 5.2.1 and Annexes F and V