# Gas flow from high to low pressure

1. Jul 15, 2016

### erjakel

Edit: Eh, sort of changed the question part way though, so if a mod could change the title to "Question regarding gas flow from high to low pressure".
Moderator edit: Done
I'm about 99% sure I already answered this question for myself but just wanted to make sure my logic was correct, so if anyone could help me out it would be much appreciated.

If an open vessel that was initially P=150atm of a pure gas is currently letting gas out and it closes at exactly 1atm, will any gasses or air from the surroundings some how mix with the gas in the vessel while its open? Also assuming surrounding atmosphere has P=1atm. I realize that gasses always flow from high to low pressure, but within this hypothetical opening, could any bit of other gases or air sneak into the tank?

Last edited by a moderator: Jul 16, 2016
2. Jul 16, 2016

### BvU

Yes. Gas molecules have a speed distribution (Maxwell-Boltzmann) that allows some of them to 'shoot into the hole' . It's a diffusion process (there is something of a mean free path length that works opposite). So it won't be much, but it's not excluded.

3. Jul 16, 2016

### Nidum

The flow velocity of the discharging gas will go from very high to very low as the tank pressure drops .

Probability of air getting back into tank is very low to begin with but will get higher as flow velocity decreases .

The real danger time is when pressures are coming near to being balanced and flow velocity is approaching zero .

Back flow of air can be controlled to some extent by suitable design and management of equipment . In particular by making discharge valve flow area relatively small and stopping the gas discharge when there is still a small positive pressure in tank .

The velocity of flow at any point in time during gas discharge can be estimated reasonably accurately for a real tank and valve . Perhaps someone else can relate flow velocity to diffusion back flow rates .

4. Jul 16, 2016

### Nidum

There are processes for ensuring that minimal back flow of air occurs .

In one process the gas is discharged into a vacuum chamber . Pumps maintain the vacuum and transfer the gas either to atmosphere or more commonly to recycling plant .

There is also a two stage process where the tank discharges from high to low pressure by simple venting and then pumps take over for the final purging right down to vacuum levels before new pure gas is reintroduced .

Some problems with high levels of air backflowing into gas tanks just occur because of human error . Most common one is having a warm tank with the discharge valve left slightly open . Tank cools down and air is sucked in .

Last edited: Jul 16, 2016