Tank pressure for storing. Nitrox only.

[Johanan]

I have two questions about tanks:

1) Someone told me, it is not good to store cylinders fully pressurized for prolonged periods, as afterwards they may fail to pass the hydrostatic tests. Is that true? How serious? What would be then an acceptable pressure?

2) Why was I taught in the courses that oxygen clean cylinders and regulators should not be used with air? How can air contaminate the equipment? Does that mean that generally the dive shops use higher standard compressors for Nitrox than for air? Does air normally contain particles of lubricants or suchlike?

 

[Tywoky]

Oxy clean cylinders should not be filled from a regular compressor. Partial pressure filling of O2 requires double filter on the compressor.

 

[tbone1004]

1. aluminum tanks shouldn't be stored full. Luxfer does not recommend storing long term over 300psi. Steel tanks don't really care, though I try not to leave them really full. Somewhere around 100bar/1500psi give or take

2. if the compressor is an oxygen clean compressor, it doesn't matter. If you are PP filling nitrox, you have to fill it with air to top it off, so whoever said does not understand what they are talking about and are perpetuating myths in the industry. You should not fill them with "any" air, because most compressors are not in fact O2 clean. So, sort of is the answer. I wouldn't take an O2 clean cylinder to somewhere filling with beat up oiled compressors and then dump it and fill with pure O2, that would be very risky, however if the shop PP blends nitrox and you just want air, then there is no reason not to.

 

[Johanan]

Thanks, tbone1004, very concise and informative!

 

[kelemvor]

Interesting subject. I'll find out next spring when my steels go for their first hydro. I've always kept my tanks full. More often than not, overfilled. Except when dumped for a pp blend or to change a valve or to vip/clean, my tanks have never even seen pressures below 300psi.

I can say from experience that some shops who fill air do not filter to the same level as those that fill nitrox or at least blend. I've come back from both West Palm Beach and Key Largo to find "stuff" in the bottom of my tanks. I've never had a blend in either of those areas so they were just cases of the local shop topping off my tanks with air.

 

[scubadiver888]

2. if the compressor is an oxygen clean compressor, it doesn't matter. If you are PP filling nitrox, you have to fill it with air to top it off, so whoever said does not understand what they are talking about and are perpetuating myths in the industry. You should not fill them with "any" air, because most compressors are not in fact O2 clean. So, sort of is the answer. I wouldn't take an O2 clean cylinder to somewhere filling with beat up oiled compressors and then dump it and fill with pure O2, that would be very risky, however if the shop PP blends nitrox and you just want air, then there is no reason not to.

One thing to watch for, if the shop uses the SAME compressor to do partial pressure nitrox fill as it does for air then there is no reason to not use it on 02 clean tanks. However, there are some shops in my area which have two systems. One is for nitrox and the other is for air. If you want an 'air' fill from the nitrox system you have to get an EAN21 fill which costs more than an air fill.

 

[tbone1004]

why on earth would they do that? That is idiotic

 

[scubadiver888]

why on earth would they do that? That is idiotic

The shop I'm thinking of has enough business than both systems are running all day long. They actually employee two guys, one for air and one for Nitrox/Trimix.

 

[tbone1004]

that is justified. Very rare you see two compressors running all day. Do they not have large banks? In cave country most of the shops have two compressors, but run one at a time and just fill the banks up for standard mixes.

 

[Jax]

I cannot find my source - I believe it was a DAN article.

You should store aluminum tanks full. If a fire happens, the full tank will blow its burst disk and therefore no problem.

If the tank is less full, say half, the tank has time to heat up and may get warm enough from the flames to burst and fragment before the burst disk goes.

Anyone else remember this?