N2 requirement for piston blowing

[jentrak]

I would like to know the calculation for the N2 usage for a known volume of vessel using piston blowing in effect reducing the hydrocarbon level to less than 5%.

 

[TD2K]

What do you mean by the term 'piston blowing', is this the same as plug flow?

If you, you essentially determine the volume of the system and adjust for the pressure you are purging at.

 

[ApC2Kp]

jentrak,
Your problem is very similar to the case that faces propane tank commissioning where they want to reduce residual air to less than 5% for a stable propane flame. Their practice is to pressurize (with vapor) and then vent the new tank several times for diluting and exhausting the tank contents. Calculation of the diluted mixture is based on the fill pressure. Assuming that the tank begins at atmospheric 14.7 psia, then filling tank to 14.7 psig would dilute contents to 50% and after exhausting tank the residual contents would have 50% of original hydrocarbons in your case (50% air for the propane vapor case). If tank was pressurized to 30 psig, then the result would be 33% residual hydrocarbon. The usual practice for propane tank commissioning would be to pressurize to 45 to 50 psig of propane vapor, vent tank to atmosphere, and repeat two more times to result in a residual less than 3%.
If your tank is not designed for 50 psig pressure then you might make a calculation of an equivalent volume of N2 to purge the tank by continuous flow, approximately 9 times the tank volume, as a minimum. Continuous flow for purging out hydrocarbons will require some care as to vent location and N2 fill point. If the N2 fill is cold gas and vent point is high, then there might be better removal of the hydrocarbons with low point filling of N2. The fill and vent sequence is more efficient than the theoretical requirement for one time fill of 20 times tank volume to get dilution to 5% residual.
There are a number of safety concerns and possible air quality questions that would also need to be answered for the type of hydrocarbon vapors, size of the tank, and location with respect to surrounding residential population for potential environmental impact. Hopefully you don't have get into a vapor recovery procedure as for freon refrigeration system decommissioning.