Time required to vent air tank and standard sizes for compressed air tanks?

[Errant]

I have a situation where I need to purge propane from a PVC pipe, which vents to atmospheric pressure. I plan to hook up a compressed air tank to the pipe, and I have two questions.

I'd like to know, what are the equations for time required to vent an air tank? (assuming venting to atmospheric) Boyle's law can give me the necessary pressure/volume of the tank, but nothing about the time. The total volume of the PVC pipe is 12.2 ft^3.

Second question, does anyone have suggestions for where I can find a list of standard sizes for compressed air tanks? Preferrably something that is able to be carried (ie. <50lbs)

 

[redbeardthegiant]

1] DO NOT PURGE PROPANE WITH AIR !!! You are setting up a fuel/air bomb ! Use CO2 or N2. CO2 has the advantage that you get more pounds of CO2 per pound of tank, as it liquifies in the tank and it isn't just pressure making it dense.

2] Time to empty the tank depends on outlet flow rate. Also, what regulator you have, and how close to empty you need to go. Strictly speaking, it takes an infinite time to come to absolutely zero [gage] pressure, as the exit flow drops off in proportion to the tank pressure.

3] The gas vendor will be able to tell you how many SCF [standard cubic feet, ie, cubic feet at one atmosphere and room temp] are in the tank.

4] If the pipe is long and thin, you are not going to get much mixing in it, so when you start purging you are going to be blowing nearly pure propane out the end, which could pose a fire/explosion hazard [hope it is outdoors !]. However, if the pipe is short and fat you will get more mixing.

5] Blowing 5 pipe volumes of CO2 through the pipe, if it were perfectly mixed, will get you down to .67% propane. The equation is
Concentration of propane at the end of purging = e^ -[volume purged/pipe volume]

6] You should use a regulator. The gas vendor [Praxair, Air Liquide, Matheson, many others] should be able to rent you one. If not, a soda vendor could.
Otherwise you risk blowing that PVC pipe to bits with overpressure.

7] Keep the CO2 cylinder out of direct sunlight. It won't explode [though it can blow the rupture disc], but the pressure will go up.