Tank Purging Procedure with Nitrogen

[dmmarti3]

We recently purchased a dissolved oxygen meter, and I have found some interesting results when testing the DO during purging. Currently we use nitrogen to purge our lager tanks prior to transfer from the fermentation tanks. With our current method of purging (slowly from the top with N2) its taking near 2 days to get our 240bbl lager tanks to a level of reasonable DO.

Does anyone have experience with a proper purging technique using N2 that will reduce the time it takes to complete?

[mmussen]

I've never tried it with O2, and I don't have the meter to test it accurately but there was a article some years back that said that pressurizing to 15 psi, venting the tank, pressurizing again, venting again and then pressurizing was the way to go. The article sated that beyond this point there were serious diminishing returns. Maybe someone else on here still has the link to that - I found it here on probrewer.

[dick murton]

Assuming you continue to use nitrogen for top pressure gas after you have filled the tank, and then use N2 during emptying, the obvious thing to do would be to cold acid clean under pressure. This means that you only have to purge the tank completely after inspection / maintenance where the tank has been opened. The best way then would be to fill with water and then empty under N2 top pressure. Obviously the cost of water and effluent (if charged by volume) will prohibit doing this procedure too often. But if you are always using nitrogen or nitrogen / CO2 mix, or even CO2 only after this initial purging, then the water costs don't become too prohibitive. If you have a number of tanks to do at the same time, fill the first tank with water, and pump out into the next one whilst applying N2 / mixed / CO2 top pressure to the emptying vessel. Repeat as often as you have tanks.

[dmmarti3]

Thanks for the info guys. We will experiment a little bit and see if any of this helps us out!

[TL Services]

Just to echo dick murton's info about filling tanks with water and pushing out with N2 to purge air. Ideally it should be deaerated water to eliminate O2 coming out of solution as the N2 headspace increases.

[dick murton]

Hi Tim

I understand what you are saying, but in years of doing it this way, even with 1000 and 2000 hl BBTs the amount of oxygen leaching out has not found to be an issue. I think is probably because there is so little turbulence in the water that even though some flashes off, the gradient in the upper layers of the water is very stable in the few hours it takes to empty (in our case it would be circa 400 hl / hr flow rate). At some sites DAL was used, simply because that was the only water readily available, in others we used air saturated CIP system water to fill the tanks.

[TL Services]

I did say ideally..!

But seriously, DAL or not, the procedure of pushing water out with N2 is pretty-much guaranteed to result in a virtually O2-free tank, thus avoiding the possibility of pick-up during filling it.

[brewking69]

Does your dissolved oxygen meter read atmospheric oxygen? Just thought I would check that first!

What level of DO do you accept in your beer?

I have a brewer working on this for a few weeks and I will report back soon on the best way we have found.

One question you could answer for me - how do you go about venting the Nitrogen out of the workspace?

Thanks

[TL Services]

As air is ~80% N2, there is less issue with using N2 than CO2 for purging tanks, although adequate ventilation should always be ensured for safe working. As you are probably aware, the problem with CO2 is that it is heavier than air, so will sink to low level, meaning that should anyone suffering from CO2 asphyxiation is at greater risk if/when they collapse.

With some very simple maths, it is very easy to convert the dissolved reading on a DO2 meter to a gaseous percentage, if you're using it to monitor tank purge.

Typically a DO2 meter, set up for dissolved measurements, will read around 8ppm in air (ie. ~20% O2), so:

20% O2 is equivalent to 8ppm
2% O2 is equivalent to 0.8ppm (or 800ppb if your meter reads in ppb)
0.2% O2 is equivalent to 0.08ppm or 80ppb
etc.

I would suggest that if you're seeing levels of 100ppb or less during purge, then O2 pick-up will be minimal.

One point to note is that when measuring like this, it is better to have the meter's flow valve fully open and control a gentle gas flow from the sample tap. This keeps the pressure in the sample chamber at around atmospheric pressure and reduces the chance of pressure fluctuations.

[Greenbushguy]

N2 instead of CO2?

I'm not sure why you would use N2 as opposed to CO2 for purging tanks. N2 is nearly identical in weight to air/atmosphere/O2. It's terrible for purging tanks. If you need to purge the tanks of oxygen/atmosphere, use CO2 since it is MUCH heavier than both N2 and O2 and purge the tanks from the bottom. Doing it from the top takes for ever, especially if using N2 which is almost the same weight as the air you are trying to displace!

[TL Services]

There are several good reasons to use N2 rather than CO2 - safety and environmental concerns being two main ones.

Also, the method of filling a tank with water and pushing out with nitrogen to purge is more efficient than CO2 purging and doesn't require any monitoring to be certain that it has worked, so your DO2 meter can be used for its primary (and vital!) purpose of testing the beer.

[gitchegumee]

Another good reason...

We use N2 instead of CO2 to purge because it is many times cheaper! Besides that, N2 can be extracted from the air around us, as opposed to importing CO2. And I don't like to blow CO2 all over the place. Our N2 purging techniques are fantastic; we are very happy with the results.

[Greenbushguy]

I suppose if you are using the water fill and push method you would have good results then. Unfortunately I/we don't have vessels to reclaim the water we would use doing this so it would be a terrible waste on our end. And we already have a 6000lb CO2 tank on site that we get refilled at a very good rate. In our case CO2 is better because it stratifies well away from oxygen/air. So we just charge up the tanks and then dump the pressure back out via top stand pipe and we find we get very low oxygen readings off the tank via this method. Once we transfer the beer into said vessels we also find that we get good DO readings there as well off of the beer.