 # Inert Gas Purging Requirement 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.

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”

1. anjali says:

what is 0.84% mole in pressure cycle purging.

1. Rifka Aisyah says:

Hi,

0.84%-mole is final oxygen content after purging.

2. Maksim says:

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.

1. Rifka Aisyah says:

Hi,

Actually the book do not mention about at which condition the volume is expressed. But in my opinion, standard condition (T = 15 C, P = 1 atm) is reasonable.

2. Reihaneh Feiz says:

Hi, What is the reference of the curve?

1. Rifka Aisyah says:

Hi, the curve is taken from Rule of Thumbs for Chemical Engineers feom Carl Branan

3. Reza says:

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?

1. Rifka Aisyah says:

Hello Reza,

I used section 20 of the book (section 20 Safety), Equipment Purging Part.

So glad the post helped you 🙂

4. Valentinus Satrio says:

Hi Bu Rifka,

Such a wonderful article bu, may I ask a question. Can the above calculation be applied to pipeline purging? Thank you Bu Rifka

Regards,
Valentinus Satrio

1. Rifka Aisyah says:

Hi Pak Valentinus,

Yes, the calculation can be used to estimate purging requirement in pipeline.

5. Rob Feller says:

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

1. Rifka Aisyah says:

6. ISAAC says:
7. Karun Kumar says: