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.

This method depends upon the accuracy of the vessel’s tank ullage calibration tables for the larger ullages / smaller soundings in the cargo tank. If the tank calibration tables are accurate for this region of the cargo tanks, then this method will give added accuracy to the general method of calculating tank residues after discharge. The Outage (or Top gauging) method will tell you the measurement of the part of the tank that is NOT filled with liquid (the level that has been taken out, or the “ullage”). Because the exact top of the liquid is hard to find, the graduated plumb bob is lowered slowly from the reference point just until it comes into contact with the liquid. Tank Volume Calculator Oil Tanks BBL (US Oil); A tank volume calculator, also known as a tank size calculator, is a quick and easy way to convert the height, width and length of your tank into a volume format. Just make sure you are using the proper dimensions/dimension and appropriate sizes/size. The ullage measurement using the ullage tape and bob procedure is as follows: The approximate ullage value is obtained with the help of vessel ATG. At the top of the tank, the ullage hatch is opened and once the tape is grounded safely, the ullage tape and bob is lowered inside the tank until they touch the liquid. Tank Volume Calculator Oil Tanks BBL (US Oil); A tank volume calculator, also known as a tank size calculator, is a quick and easy way to convert the height, width and length of your tank into a volume format. Just make sure you are using the proper dimensions/dimension and appropriate sizes/size. 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

Tank Ullage Calculator Formula 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. Tank Ullage Calculator Estimate

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

Fuel Tank Ullage

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.

Tank Ullage Calculator Equation

• 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 Innage Vs Ullage

Reference: Rules of Thumb for Chemical Engineers, Carl Branan