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Inert Gas Purging Requirement Calculation

Rifka Aisyah Calculation

Purging is an activity used to remove oxygen in piping or vessels to certain concentration, for example 2%-mole. It uses inert gas, usually nitrogen, to do that.

In this post, I want to share you how to calculate inert gas for purging activity. There are three different calculation methods used.

Dilution Purging

In dilution purging method, the inert gas simply flows through the vessel and reduces the concentration of unwanted component. It is used for tanks, reactors, and other vessels. Figure below is used to determine requirement of inert gas.

Inert gas required for purging
Number of vessel-volumes of inert gas required

Let say, you want to purge a tank full of air (21%-mole oxygen) to 1%-mole oxygen. Then:

  • Ratio of initial to final concentration of oxygen is 21
  • From figure above, at ratio of 21, you need 3 vessel volume of inert gas

Pressure-cycle Purging

In this method, the vessel at 1 atm is alternately pressured with inert gas and vented. It is used for vessel that can withstand 30 psig (about 2 barg) or more, vessel with only one port, or vessels with coils or baffles inside. I also used this method for leak test in piping system.

The dilution ratio is (1/P)n. Where P is in atm and n is number of cycle.

The quantity of inert gas required for each cycle is P-1 vessel volumes.

Let say, you want to purge a vessel from 21%-mole oxygen to 1%-mole oxygen. You use inert gas to pressure to 5 atm.

  • Guess number of cycle. We guess 2 cycles for started.
  • (1/P)n = (1/5)2 = 0.04
  • 0.04 x 21%-mole = 0.84%-mole. The results is satisfied. If the result is bigger than requirement, try to increase number of cycle.
  • Purge requirement (P-1) = 5-1 = 4 vessel volumes

Vacuum-Cycle Purging

Based on its name, this method can only be used for vessels capable to withstand a vacuum. In this method, the vessel is alternately evacuated and fed with inert gas to 1 atm.

Concentration of unwanted component is reduced from C to CPn.

C = concentration (%)

P = pressure (atm)

n = number of cycles

The quantity of inert gas required for each cycle is 1-P vessel volumes.

Let say, you want to purge from 21%-mole oxygen to 1%-mole oxygen. You use vacuum-cycle purging to 0.5 atm.

  • C = 21%
  • CPn = 1%, (21%)(0.5)n = 1%. We get n = 4.4, so use 5 cycles
  • Purged gas required = 5 x (1-0.5) = 2.5 vessel volumes

Reference: Rules of Thumb for Chemical Engineers, Carl Branan

14 thoughts to “Inert Gas Purging Requirement Calculation”

    2. Hi Mba,
      Could you confirm that unit for N2 volume here is Standard condition.
      For example: volume system = 10 m3, if N2 volume =2x volume system, it means N2 volume = 20Sm3.
      Thanks.

      Reply

  3. Hello
    Thank you very much for your complete explanation.
    I used a lot.
    Can you tell me where this part is mentioned in the book?

    Reply

  5. Thank you for the great information! It appears there is a typo in the formula above under the Pressure Cycle Purging Section. (1-P)n = (1/5)2 = 0.04 Formula shows 1-P but then 1/P in the calculation. Maybe it should say (1/P)^2=(1/5)^2=0.04
    Cheers, Rob

    Reply

  6. Hello RIFKA AISYAH. Thank you so much to share information about this topic. I have some doubts:
    Metod 1 Dilution purging:
    1.- In your example (ratio of 21) we need 3 vessel volume. IF my pipeline volume is about 11 M3, that means that I need 11*3 = 33 Sm3 Of N2?
    2.- Ratio of Initial to final Condition is O2 Initial Concentration/ O2 Final Concentration.
    EXAMPLE: IF I Want to Purge a pipeline from 21% TO 2%. Ratio is 10.5? is that correct?

    Reply

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