Nitrox Partial Pressure Blending

[jeffkruse2000]

Give me a break. There are thousands of people doing this. If you keep the O2 under 40% you should be safe. How many of those incidents were caused by operator error? If your not careful then sure you can cause damage. Just keep the O2 lower than 40%. 40% is not a magic number it's just were most of us feel safe. I have pumped 40% but mostly pump around 32%.

There is risk in everything.

 

[wedivebc]

Give me a break. There are thousands of people doing this. If you keep the O2 under 40% you should be safe. How many of those incidents were caused by operator error? If your not careful then sure you can cause damage. Just keep the O2 lower than 40%. 40% is not a magic number it's just were most of us feel safe. I have pumped 40% but mostly pump around 32%.

There is risk in everything.

How many accidents do you know of involving pp blending? I know personally of 2 compressors exploding from continuous flow and 0 pp incidents and pp is used whenever greater than 40% is required so most of my mixes are higher than what some consider "safe" for compressors.

 

[boydski]

How many accidents do you know of involving pp blending? I know personally of 2 compressors exploding from continuous flow and 0 pp incidents and pp is used whenever greater than 40% is required so most of my mixes are higher than what some consider "safe" for compressors.

Hi Dave,

I know you don't mean to imply that PP blending is completely safe, and continuous blending is not, but that is what it sounded like. :06: I know Heather Choat had a bottle blow up in her hand a couple of years ago while adding oxygen and Cave Excursions had a recent incident where a deco bottle exploded with an Oxygen flash fire that killed one of the employees (although the accident was not a result of PP blending and happened before the oxygen was actually added). I'm sure there have been other incidents as well. Handling high pressure oxygen is not exactly safe, and technique is just as important as oxygen cleanlyness.

In no way refuting your statement about compressors blowing up while pumping 40%, it reminded me that I have also seen several compressors blow up. Most where simply pumping air or an inert gas such as nitrogen. I have never seen a compressor blow up due to combustion in the cylinder from oxygen, but have heard of it happening, and it is not something I really want to witness. :11:

 

[wedivebc]

Hi Dave,

I know you don't mean to imply that PP blending is completely safe, and continuous blending is not, but that is what it sounded like. :06: I know Heather Choat had a bottle blow up in her hand a couple of years ago while adding oxygen and Cave Excursions had a recent incident where a deco bottle exploded with an Oxygen flash fire that killed one of the employees (although the accident was not a result of PP blending and happened before the oxygen was actually added). I'm sure there have been other incidents as well. Handling high pressure oxygen is not exactly safe, and technique is just as important as oxygen cleanlyness.

In no way refuting your statement about compressors blowing up while pumping 40%, it reminded me that I have also seen several compressors blow up. Most where simply pumping air or an inert gas such as nitrogen. I have never seen a compressor blow up due to combustion in the cylinder from oxygen, but have heard of it happening, and it is not something I really want to witness. :11:

I am not saying either method is completely safe. I PMed Heather a year or so ago about details of her accident but I don't know her personally. I don't know any of the folks at Cave Excursions but I had heard about the tragedy. I personally know of 2 compressor accidents involving nitrox blending that is my only real level of experience with that. I have PP blended for years, using mostly mixes above 50%.
I just want to offer this little food for thought:
Fire needs an ignition source, fuel and oxygen to exist. Partial pressure plays as much a role in combustion as fraction of gas. Mostly the ignition temperature of a fuel goes down as the presence of oxygen goes up.
If I PP blend a tank to 40% I will add oxygen to 722 psi ( ~50 ata) then add air to 3000psi. The highest O2 exposure at the valve (the most likely ignition source, ask Heather) is 50 ata. If I continuous flow blend the 40% I inject up to 82 ata ([3000 psi/14.7+1] *0.4) of oxygen not only through a valve but through a reciprocating, oil filled, machine with multiple valves, pipes, and potential ignition souces (think adiabatic compression).
So what I am saying is although we treat 40% as if it is harmless from a combustion standpoint I know from mathmatics/physics and from personal experience that 40% should be given a lot more repect than we are giving it, especially in our compressors.

 

[triton94949]

BullSh**

I know of several compressors that have blown up doing exactly this.. One was definately making 36%, another was making 32%!!!!!

I also know of several fires on live aboards using a nitrox stick, which basically does the same thing.. These were making low mixtures as well..

Even with an oil free compressor like a RIX ther IS STILL a risk although its incredibly small..

I think I will stick to partial pressure blending. I can do that slowly, and the mix does not need to run through a compressor that way. I even get nervous whenever the booster is running. Oxygen is a powerful gas. This has been an interesting discussion, starting with letting your gasses cool long enough before topping up, and ending with blowing up your compressor or boat.

 

[boydski]

I just want to offer this little food for thought:
Fire needs an ignition source, fuel and oxygen to exist. Partial pressure plays as much a role in combustion as fraction of gas. Mostly the ignition temperature of a fuel goes down as the presence of oxygen goes up.
If I PP blend a tank to 40% I will add oxygen to 722 psi ( ~50 ata) then add air to 3000psi. The highest O2 exposure at the valve (the most likely ignition source, ask Heather) is 50 ata. If I continuous flow blend the 40% I inject up to 82 ata ([3000 psi/14.7+1] *0.4) of oxygen not only through a valve but through a reciprocating, oil filled, machine with multiple valves, pipes, and potential ignition souces (think adiabatic compression).
So what I am saying is although we treat 40% as if it is harmless from a combustion standpoint I know from mathmatics/physics and from personal experience that 40% should be given a lot more repect than we are giving it, especially in our compressors.

Very good point, and something I really hadn't considered. Most of the PP mixing I do is on top of Helium or high FO2 deco mixes, so the pressures are higher, and hence a little less safe than pp mixing standard Nitrox mixes.

Thanks for the explanation.

 

[jeffkruse2000]

What is your definition of compressor exploding?

Loosely, all compressors will eventually explode (loose a seal or something). I know of a lot of compressors that have blown heads/seals/rods/shafts/ect from just pumping air. It makes a loud noise some times with lots of gas hissing. Any one near by would say the compressor exploded.

Also, I would suspect if a compressor "exploded" from pumping Nitrox I would question the operator and make sure they didn't give the compressor a quick slug of 100% O2.

I just hate all the mis-information that goes on over this kind of stuff. One person has a compressor break while blending or the operator screws up and the next thing you know its all over the internet that continuous blending will explode your compressor.

 

[wedivebc]

What is your definition of compressor exploding?

Loosely, all compressors will eventually explode (loose a seal or something). I know of a lot of compressors that have blown heads/seals/rods/shafts/ect from just pumping air. It makes a loud noise some times with lots of gas hissing. Any one near by would say the compressor exploded.

Also, I would suspect if a compressor "exploded" from pumping Nitrox I would question the operator and make sure they didn't give the compressor a quick slug of 100% O2.

I just hate all the mis-information that goes on over this kind of stuff. One person has a compressor break while blending or the operator screws up and the next thing you know its all over the internet that continuous blending will explode your compressor.

How about a hole in a cinder block wall from the HP head flying off. I don't know what mix was being pumped but the operator, a very experienced trimix instructor, probably knew what he was doing.
So what mis-information are you referring to, my po2 calculations or are you denying the truth of my story?

 

[padiscubapro]

What is your definition of compressor exploding?

Loosely, all compressors will eventually explode (loose a seal or something). I know of a lot of compressors that have blown heads/seals/rods/shafts/ect from just pumping air. It makes a loud noise some times with lots of gas hissing. Any one near by would say the compressor exploded.

Also, I would suspect if a compressor "exploded" from pumping Nitrox I would question the operator and make sure they didn't give the compressor a quick slug of 100% O2.

I just hate all the mis-information that goes on over this kind of stuff. One person has a compressor break while blending or the operator screws up and the next thing you know its all over the internet that continuous blending will explode your compressor.

I have some pictures of a compressor that had an oxygen fire thats used in our gas blender class.. several parts of the compressor are missing.. plus a pile of parts that are [artially there.. there is clear evidence of a fire on the remainder of the compressor..

the risk for continuous blending is definately HIGHER tha PP blending if the proper procedures are followed for PP blending.. Heat is the major ignition source and compressors that people run "oxygen" though it run MUCH MUCH hotter than when air is being pumped..

Slower compressors are safer than compressors that run at higher rpms.. The motor on my 17cfm rix is on 600rpm if I remember correctly.. most bauer, makos and IR run much higer than that... The larger RIX compressors can also have a "water" jacket to cool the unit... The water jacket is usually normal antifreeze thats coupled to a heat exchanger.. I have seen systems that have been done this way where even after pumping gas for hours you can touch the last stage with a bare hand!

Its all a scale of relative safety... cntinuous blending can be safe when used with the right equipment... Oil lubricated compessors are not the right equipment... Sure alot of peope get away with it, but if you or your family is the unluck one to have an incident, then you are the only one to blame for going against manufacturer recommendations..

A perfect example of how stuff is twisted.. Bauer will ABSOLUTELY tell you do not put anything but air through their compressors, but their US istributor will pont you through a document thats not from bauer suggesting that its OK (for a nitrox stik) but Bauer itself will not support this use, and if anything happens you are SOL....

 

[pt40fathoms]

The simple rules to PP blending is go slow, and you get better with practice.

The "stick" has been criticized more and more lately by the compressor manufactures. They have been telling us two things, first the compressor will wear considerably faster with higher O2 blends running through them, and second the higher O2 blends greatly increase the possibility of a catastrophic failure of the high pressure cylinder. The high pressure cylinder is discussed mainly because it is there that the partial pressure of O2 in an enriched mixture will reach what they call a highly reactive state. Much more so then with the 21 percent O2 found in air. In a nutshell, they strongly discourage it.