Skip to main content

Inert Gas Consumption for Tank Blanketing

Inert gas is used to blanket certain fixed-roof tanks for safety. Nitrogen is a common inert gas used for the purpose. In addition to safety of storage tank, the benefit of tank blanketing includes improved product quality and equipment life cycle.

Why nitrogen is commonly used in tank blanketing?

Nitrogen has inert properties, wide availability, and relatively low cost at any economic efficiency. Other gases, such as carbon dioxide or argon, are also sometimes used for certain application.

Nitrogen reduces oxygen content in the vapor space of a storage tank of vessel, making it inert and eliminates the possibility of fire or explosion. It also decreases evaporation and protects the tank from structural corrosion damage caused by air and moisture. In addition, nitrogen blanketing is used to prevent air, moisture or other contaminants from entering the vapor space, causing product degradation or spoilage.

Element of Fire

Remember figure above? It is called fire triangle. It consists of oxygen, heat, and fuel. Removing one of these components will eliminate the possibility of fire. In tank blanketing, oxygen is the component that is controlled.

Nitrogen Consumption for Tank Blanketing

Volume of nitrogen for storage tank blanketing can be estimated by formulas and calculation. Essentially, nitrogen consumption for tank blanketing has two components: the nitrogen requirement by the throughput or total liquid discharge from the tank; and the nitrogen required by thermal breathing, or the rise and fall of the liquid level due to different temperature during day and night.

Average nitrogen consumption = nitrogen consumption due to working loss + nitrogen consumption due to thermal breathing

Nitrogen Consumption due to Working Loss (Material Discharged from Tank / Working Throughput)

Nitrogen requirement due to working loss is estimated from liquid annual throughput.

Monthly working loss = annual throughput (in barrel) x 5.615 (conversion from barrel to scf) / 12 (number of months in a year)

Nitrogen Consumption due to Daily Breathing Loss (Thermal Breathing)

To estimate nitrogen consumption due to daily breathing loss, we need the following data:

  • Tank diameter
  • Height of average vapor space (average outage)
  • Storage temperature
  • Daily temperature change

First, we need to calculate vapor volume by using the following equation:

V0 = π x D2 x Ho / 4

V0 = vapor volume (ft3)

D = tank diameter (ft)

Ho = average empty headspace/average vapor space/average outage (ft)

Secondly, we calculate daily breathing loss (DBL).

DBL = Vo {[(460 + Ts + Tdc / 2) / (460 + Ts + Δ – Tdc / 2)] – 1.0}

Ts = storage temperature (oF)

Tdc = daily temperature change (oF)

Δ = adjustment for differential between blanketing and pressure-relief setting (normally 2-4oF)

DBL = daily breathing loss (scf)

Finally, we calculation overall consumption which is addition of nitrogen consumption due to working loss and daily breathing loss.

Peak Consumption of Nitrogen for Tank Blanketing

Average monthly nitrogen consumption for tank blanketing can be relatively small compared to peak nitrogen consumption. Peak consumption can be larger due to fast thermal changes. Peak nitrogen requirement and frequency is important when selecting and sizing the nitrogen supply system.

Peak nitrogen consumption for tank capacity up to 840,000 gallon (20,000 barrel or 3180 m3) can be estimated by the following equation:

Maximum nitrogen flow rate (scfh) = 8.021 x pump out rate (in gpm) + 0.02382 x total tank capacity (in gallon)

8.021 is unit conversion factor from gpm to scfh.

0.02382 is a factor based on cooling an empty tank from a high of 120oF at a rate of change 100oR/h.

For tank larger than 840,000 gallons (20,000 barrel or 3180 m3), the peak usage can be estimated by the equation.

Maximum nitrogen flow rate (scfh) = 8.021 x pump out rate (in gpm) + nitrogen inbreathing requirement as per API 2000
For tanks larger than 840,000 gallons, peak nitrogen requirements due to thermal breathing are shown as per API standard no. 2000 for non-refrigerated, above ground, un-insulated tanks. Table below shows the requirement.

Example of Calculation Average Nitrogen Consumption for Tank Blanketing

A fixed-roof tank has the following specification.

Diameter = 128 ft

Height = 36 ft

Average empty headspace = 12 ft

Storage temperature (Ts) = 75oF

Daily temperature change (Tdc) = 15oF

Adjustment for differential between blanketing and pressure-relief setting (Δ) = 2oF (assumed)

Determine monthly inert gas usage?

Vapor volume = π x (128)2 x 12 / 4 = 463,243 ft3

Daily breathing loss = (155,000) x {[(460 + 75 + 15/2) / (460 + 75 + 2 – 15/2)] – 1.0} scf/d

Daily breathing loss = 3,791 scf/d x 30 days/month = 113,734 scf/month

Monthly working loss = 300,000 x 5.615 / 12 month =  140,375 scf/month

Thus, average inert gas consumption = 113,734 + 140,275 = 254,109 scf/month



[1] Branan, Carl, “Rule of Thumb for Chemical Engineers”, Elsevier, 2005

[2] Nitrogen blanketing to keep your product intact and your employees and equipment safe


One thought to “Inert Gas Consumption for Tank Blanketing”

Comments are closed.