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 CP**** ^{n}**.

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%
- CP
^{n}= 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

what is 0.84% mole in pressure cycle purging.

Hi,

0.84%-mole is final oxygen content after purging.

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.

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.

Hi, What is the reference of the curve?

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

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?

Hello Reza,

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

So glad the post helped you 🙂

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

Hi Pak Valentinus,

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

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

Thank you for your correction

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?

how to calculate inert gas pressure to remove oxygen from the particular atmosphere