Gas Blending 1

[Xenobius]

Hi,

I am new here and I have a small question regarding gas blending. In the scuba diving industry, we can mix several different gases together to get the right mix for the right depth / dive. The most common gases are Oxygen, Helium and Nitrogen. There are many formulas which a gas blender must know in order to be able to mix these gases to the right percentage however my question has nothing to do with this.

Here's the problem. The gas company gives you 50ltrs of gas @ 200bar pressure. However as soon as you connect it to a small 15ltr tank, immediately there is less than 200bar. Eventually those 50ltrs @ 200 bar (thus originally having 10,000 litres of gas) will go down to lets say 50ltrs @ 45bar which means that you still have 2250 litres in that tank but if you want to say put 100bars in a 15ltr tank, you cannot get more then 45bar because the 50ltr tank only has 45bars left even if it actually has 2250 litres left in there...

So the question in short, what do you use to put 50ltr@45bar into 15ltr@150bar. I assume I need something to capture the air from the 50ltr tank, pressurise it and pump it into the smaller tank at higher pressure... I also would like to build it myself so I'd appreciate if you could help out. It should be a fun project provided I use enough thickness for my compression chamber...

Thanks for listening
X

 

[zeusfaber]

Try googling "Breathing gas booster".

The big name in the business is "Haskel".

If you're thinking of compressing oxygen to 200 bar in a home-built machine, how comfortable are you with the idea of getting your eyebrows burnt off (possibly right to the back of your eyesockets)?

A.

 

[zeusfaber]

Sorry - last post wasn't meant to sound so harsh.

You're usually looking at two cylinders in tandem. One acts as a compressed air engine and this drives the other which compresses your gas.

Seals and bearings on the booster stage either need to be water lubricated (in which case, you will need some serious downstream drying and filtration) or dry-running (which is beginning to get challenging in your pressure range).

Boosting oxygen to high pressures still scares me though.

A

 

[Xenobius]

I think you're right. No its OK sometimes there is no better way to say things. I'd rather buy the thing than to do it myself.
Oxygen however is the cheapest to buy at €20 per 50ltr. Whats expensive is the Helium at €210 per 50ltr and so making the best use of helium is the most important.

Thanks for the technical names and company. I now know what to look for.

Cheers

 

[rotw]

Xenobius, I don't know about your industry but I have a question.

When you get your system ready (either by buying it or building it yourself) do you need to apply for certification of the equipment by an authority or local regulation specific to your industry before use of the equipment ?

 

[Xenobius]

Hi,

Well you need to have gas blending certification that's for sure and you need to have an analyser so that it gives you exactly the percentage of the mix that you have because ultimately its not the formulas and the equipment but what's inside your tank. A scuba computer needs to be set-up before a dive with the percentages of the mix as it needs to calculate several factors such as the time you can stay at a certain depth and the time you need to wait before completely surfacing.

As for the equipment I am not aware of any certification needs but the equipment must not alter the quality of air by for example contaminating it with water or oil for example as breathing it at extreme pressures will have a very bad effect on the human body. Maintenance is the key. Most of the time, whenever helium is used in a dive, it is usually (rarely the case when it is not) used for deep dives beyond 60 metres. At 60 metres deep your body is under 7 bars of pressure making the mixture in your tank 7x more dense, making you loose 7x more air and having 7x more effect. (btw you can actually go down with a mixture made up of oxygen and helium only but apart from having to be very experienced to use it, you must be very rich )

X

 

[rotw]

Thanks for the info.
With regard to equipment, my guess is that it does not need certification because it will supply gas to some sort if storage reservoir or tank and then from there to the human body. Is that right?
So if equipment fails, say mechanical element breaks, this happens offline.
But when people get into scuba diving the whole system including storage and supply to human body must be a rely upon and sort of certified or not?
In that sense I mean the storage tank must be certified and instrumentation must have some sort of safety integrated level certification like SIL or whatsoever?
Am I right because I am just guessing being ignorant of these systems
My questions are out of curiosity.

 

[Xenobius]

Yes I will answer as far as I know. Where I am in doubt I will clearly state it.

You are correct that if anything should break, it will happen on shore where (unless being too close to it) nothing will happen however there is always a risk of using contaminated air. In fact when we fill our tanks, we always use the same supplier as we trust him with the air he pumps for us. I mean most dives use normal compressed air (21% oxygen and 79% nitrogen) in which case all you need is a good clean filter and an oil-less compressor.

When you use oxygen for example our local supplier provides this with a certification as well as the Helium when it is imported. What you do with the gas is entirely up to you and so you must be responsible not to use dirty filters and other equipment which might not be fit for breathing. Especially if you intend to supply gas to other fellow divers.

Those who use a re-breather for example (a very complex machine which reuses the exhaled air, cleans it and gives it back to you), have the option to analyse how good the air is and those who use tanks can still buy a gas-quality analyzer but once again, you must be responsible if not for yourself, for the others. As a rule of thumb me and my fellow divers always use the same suppliers to ensure good air quality. If not mistaken most filling stations stop working when they detect that a filter needs cleaning for example.. not sure depends what you buy I guess..

 

[zeusfaber]

Exactly what the rules are depends where you are. Here's my interpretation of how things stand in the UK.

The fixed installation is (probably, though just possibly just small enough to escape) subject to the Pressure System Safety Regulations. These mean that the people who design, install, operate and examine it all need to be a "Competent Person" - and that the examination needs to be carried out in accordance with a written scheme of examination devised by a Competent Person. The definition of "Competent Person" is left a bit open but, if there's any dispute, the courts will probably take a close look at what standards are available and what the rest of the industry considers to be Good Practice. What this means in practice is that there is no central inspectorate who have to certify your installation - but you still need to demonstrate that the whole thing is safe (and deliberately so).

The situation with diving air quality is similar, but not quite the same. Unless you are compressing your own air for your own recreational use, the Diving at Work Regulations come into play. These require the air to be safe and suitable and to be tested by a Competent Person to a scheme devised by a Competent Person. The difference here is that the Health and Safety Executive Diving Inspectorate publishes a Accepted Code of Practice. Although this isn't mandatory, if you follow it, the courts will (probably) accept that you have discharged your duties. The ACOP specifies a purity standard (BS EN 12021 - a somewhat harder standard to meet than the vanilla EN 12021) and a test frequency - but is pretty much silent on the subject of what constitutes a competent person and what test methods they should use. This leads to significant variation in practice, with some operators testing their own air with Draeger Tubes and some sending samples off to a UKAS-accredited independent lab for analysis by FTIR (or some other broadband spectrographic technique) - and a number of variants in between.

The situation is different again for the bought-in components which go to make up the filling installation. These are covered by the Pressure Equipment Safety Regulations, which are a whole lot more prescriptive when it comes to certification of design, manufacture and testing. The same regulations apply in even more prescriptive form for the equipment which is being filled (since these are transportable pressure equipment) - with a requirement for new equipment to be pi marked as conforming to an appropriate EU standard and tested to methods, periodicities and criteria defined in an appropriate EU standard, by a body accredited (perhaps indirectly) by UKAS.

What you find in real life: The vast majority of diving air providers use oil-lubricated compressors with most of the resulting oil mist knocked out in the water separators. Some then have a coalescer (a few even have desiccant driers) and there's usually a multiple media filter tower or two in there too (I say usually, because I did operate a machine for a few years that turned out to rely on a column of loose-packed, second hand 4A molecular sieve for everything). Filters are usually changed on a running-hours basis (unless too many people complain that they can taste the oil first). In-line monitoring of gas purity (as opposed to oxygen content) is possible for a limited range of contaminants (usually moisture, CO2, CO and generic VOCs), but is only just beginning to make any inroads into the market at all. In most cases, you're relying on a test certificate that will have been issued by "somebody" sometime during the last three months using a test method that they consider "suitable". There is a real problem with several of the test methods in use being quite poor at detecting some of the compressor oils in common use. With standards having tightened in recent years, CO2 content is becoming a common cause of test failures. All this happens out the back: The charging panel on view to the customers will probably look very neat and shiny.

The portable test gear bought by techie divers focuses on the basic composition of the mix (proportions of O2, N2 and He), but not on the trace contaminants that define purity.

So does this mean we're all going to die straight away? Or is it just that we aspire to the very best safety standards, can't always afford our aspirations and generally get away with it because the standards have such large safety margins built in?

A.

 

[rotw]

Great post zeusfaber. Thanks.

My guess is that when divers start diving, it is vital that they rely on the installation located at the surface. That means the pressure safety regulations have to be fullfilled for protection and safety of personal and divers during equipment operation but also the air supply equipment has to be 100% available at the time when it is required to be operating (possibly the equipment is vital and has to be spared / made redundant ?).

So beside pressure vessels safety issues, my guess is that all the sub-components of the air supply system shall be certified and made 100% reliable e.g. including valves, orifices, gauges if any. Indeed failure of any sub components during operation (air supply to diver) may result in death, or health issues if I am correct.

Again I'am not expert so this is only speculative about how I figure out things might be.

 

[Xenobius]

Hmmph some very good points there! It depends where you are. So far so good

 

[zeusfaber]

100% available at point of use would be nice - but is really asking the impossible. All diving BA marketed in this part of the world has to be CE marked (with all that entails); even so, personal experience puts the main system availability at about 99.8% (per dive, counting only failures which forced me to chop to a different air source in a hurry). As rotaryw speculates, redundancy is important. I like to get that from a completely independent rig (bottle and valves) strapped firmly to my own body - but lots of people seem to think that blagging a bit of gas off your buddy is going to be good enough.

A.

 

[Xenobius]

hehehe you're right. I know of someone who goes to 68metres with just 1 15ltr... its insane. We use twinsets with 2 additional deco bottles and yes another one hanging on a rope just in case the 4 bottles attached to your body are not enough or something happens to them. We train to deploy DSMB, change deco bottles and the like without a mask. Remove and replace equipment without mask and worse of all ... a blacked out mask with 2 holes to leak water...

safety first !! never enough training

 

[berkshire]

Xenobius (Marine/Ocean),
The mechanism for charging tanks without a compressor, is a decanting line, It is usually best to have at least 6 tanks in the line.
Step one, fill your small cylinder from the first big one with the lowest pressure, let your pressures equalize, shut off first big tank, crack open second big tank, let pressures equalize. (With new tanks sometimes you can get sufficient pressure from just a couple.).
As your tanks empty out, you start at say 20 or 40 bars, then to 100 bars, then 150,175, 185, then top off on the last tank of the cascade.
When you cannot get a good full pressure from the last tank , you put a new one on, and sent the lowest pressure tank back for refill.
Getting good use out of a cascade depends on how low the pressures on your " empty" tanks are.
B.E.



You are judged not by what you know, but by what you can do.

 

[berkshire]

Xenobius (Marine/Ocean),
If you do want to suck that last bottle dry, then look at

http://www.hydratron.com/media/html/gas_pumps.htm

B.E.

You are judged not by what you know, but by what you can do.

 

[Xenobius]

berkshire (Aeronautics),

Thanks a lot for your tips. Much appreciated.
That's the thing however that a tank costs 50eur to rent and 200eur to refil. Having 6 tanks costs 1500eur which is beyond resonable for some diving. That's why I was considering a pump but seeing how much they cost, I think its not feasible as well.

Thanks once again

 

[berkshire]

Xenobius (Marine/Ocean)
If having a decanting line in house is not feasible, why not get your gas supplier to fill your small tanks, It may mean having a greater number of tanks so that you have tanks available for your job. This of course assumes that your gas supplier will fill customer supplied tanks.
We have the same problem in the aircraft industry, where pilots need 1500 litre or 750 litre oxygen tanks filled at airports. We either use a decanting line, or a bottle exchange.
B.E.

You are judged not by what you know, but by what you can do.