Portable fill station

[bigchrisb]

I'm experimenting with some ideas on how to integrate some storage banks into my portable fill station. Let me start by describing my current configuration:

I have an old bauer utilus compressor, that gets mounted onto the bonnet of my 4x4 when I'm filling tanks, and stored in the back when not in use. Its belt driven straight off the 4*4's diesel motor, at idle. I have this set up with mounts for a continuous blending stick (of oxyhacker design), that I attach when wanting to do nitrox fills. I also have a disconnectable hyperfilter box, that I attach in line with the whip to provide O2 compatible air, for PP mixing (deco mixes above 40%).

This system works OK at the moment, and allows me to refill my tanks at the car. However, its not without its problems, as I have to lug the O2 around to the front of the vehicle to attach it each time (which limits me to O2 cylinders that I can easily lift and move), and refilling a number of twins can be a very slow exercise with the small compressor. I'm thinking of setting up an array of HP banks that I can leave filling whilst off diving, to be ready to decant from at the end of the dive, and wanted to sound off a few ideas here before implementing.

The system I'm thinking of consists of three large tanks, semi-permanntly attached and plumbed into the back floor of the 4*4. Note that I have already ripped the back seats out for another project, and there is plenty of available space, + heavy duty (constant load) springs in the rear, suitable for the additional weight. I'm thinking of installing 2 HP air banks, and a similar sized O2 cylinder. As the O2 cylinder is used for continuous blending, plumbing it in is easy, as it can all be done with LP hose. (note that cont blending would still only be done for individual fills, and only air (O2 compatible though) would be put in the banks). I'm more concerned about plumbing in the HP bank cylinders. This would involve running a stainless HP line from the front of the vehicle to the rear, which may represent a significant hazzard in a vehicle accident. To my mind, if the bank valves are isolated prior to moving the vehicle, and the HP line is purged, this should mitigate the risk. Please let me know if you can see an oversight with this issue!

To leave it running unsupervised, I'd need to install some solenoids on the coalescor drain, to autodrain the bulk of the condensate. Easy enough to do. I'm also tempted to righ up a high pressure cut-out to the vehicle ignition, so that it will turn off once a pre-set bank pressure is reached. Mainly to prevent the banks being overpressurised, and taking the vehicle with them!

The old utilus (utilii?) compressors were rated for continuous duty, so I cant forsee any major issues there. I did have some initial reservations about extended idling of the diesel leading to varnishing of the vylinder walls, however I have not experienced this to date.

So, after detailing my plan, I'm eager to see what suggestions come from the board as to potential weaknesses, or modificaitons people would suggest.

And if anyone is interested, if I decide to proceed, more than happy to upload a few photos of the progress!

Just a final comment - I work as an engineer, and enjoy projects like this for the project's sake, (and the convinience of truly portable fills). I.e, I'm not purley interested in the economic viability of the project, as I gain a lot of enjoyment from the construction itself.

 

[ScubaSarus]

How many 80cf 3000 PSI tanks can the compressor fill lets say in a 2 hours. This is just a curiosity question. I understand smaller compressors take a long time to fill a few tanks.

Or actually what is the volume of gas the tanks you are planning hold. I would love to see pictures. I thinks its a adventurous plan.

 

[bigchrisb]

How many 80cf 3000 PSI tanks can the compressor fill lets say in a 2 hours. This is just a curiosity question. I understand smaller compressors take a long time to fill a few tanks.

Or actually what is the volume of gas the tanks you are planning hold. I would love to see pictures. I thinks its a adventurous plan.

I deal primarily in metric units, so treat the imperial unit conversions in brackets as approximate at best.

The Utilus is producing about 90 LPM (about 3.3 cfm), at 1200 rpm. Filling a 12.2 litre steel tank to 232 bar, assuming some residual pressure (HP steel 100cf tank to 3500psi, with ~500 psi left in it) takes a bit under 30 minutes. Al80s take closer to 20 minutes, assuming 500 psi residual pressure. It's fine to fill a few tanks on a surface interval, and dosent take too much supervision. However, doing a number of fills becomes pretty tedious! Was thinking of banking in two 50 litre tanks at 300 bar (about 555 CF per cylinder @4500 psi). Would also allow me to fill the banks from standard single phase electricity when available.

 

[O2BBubbleFree]

To leave it running unsupervised, I'd need to install some solenoids on the coalescor drain, to autodrain the bulk of the condensate. Easy enough to do. I'm also tempted to righ up a high pressure cut-out to the vehicle ignition, so that it will turn off once a pre-set bank pressure is reached. Mainly to prevent the banks being overpressurised, and taking the vehicle with them!

I've never done anything like this, but would certainly be tempted to if I were in your situation!

And I would definitely encourage looking into the cut-off you are considering.

Here's a couple of other thoughts:

Could you factor in a burst disk vented to the outside world? Maybe on that HP line you're running from the back to the front? Rated somewhat lower than the burst disks on your banks?

If you have a ‘typical’ dive profile that you run, could you size your banks so that your system reaches max capacity about half-way through your surface interval? So that you are more likely to be present to monitor highest pressure, and still have time to do your fills before your next dive?

Personally, I think it would be better to have extended surface intervals rather than have the system reach max pressure unattended.

BTW, Pictures! We want Pictures! Lots of Pictures!

 

[PRL]

How do you intend to control exhaust conteminants from reaching your compressor intake when the wind direction changes? I'm not sure if a CO detector could be mounted to screen the intake air.

 

[bigchrisb]

Thanks, burst disks are a good suggestion, I'll give some thought to incorporating them into the system. Dive profiles are fairly variable, so I suspect that the bank sizing idea may not be viable, but certainly worth investigating. Perhaps I may be better off banking at somewhat below the banks rated pressure when automated,(such as 250 instead of 300 bar), and only top off to 50 bar when supervised? Another option would be to fill the banks to ~250, and use them for the bulk of the tank fill, and top tanks off straight from the compressor?

RE CO and exhaust - the engine exhaust is going through the standard vehicle exhaust system, which is emitted at the rear of the vehicle (about 5 meters away), and near the base of the vehicle. The air intake for the compressor is on a short snorkell hooked over the top of the bonnet, when the bonnet is open for filling. I have tested a few fills on a portable gas analyser from work, and found the CO levels to be slightly below ambient (in the 0.5 ppm range, whilst ambient is generally 0.7-1 ppm). I presume that the slight reduction is due to some being captured in the hyperfilter (14x and two grades of carbon). But a good suggestion nonetheless. I might do some tests with the exaust pipe deliberatley up-wind, and see what the results are. A CO cut-out, or addition of hopcalite in the filter array may be worthwhile. Thanks.

 

[oxyhacker]

Interesting setup! I think valves at each end would be all it would take to isolate the lines, don't thing purging would be necessary since the volume is so small the tanks probably represent about a 1000X the risk in the event of an accident.

However, I really don't think it is a good idea to run any compressor except one made specifically for it, with all the safety/auto features, unattended, especially when it's IC powered. If your autodrain fails, the pump can waterlock, and rip the cylinder right off the compressor. I've heard a few stories of this happening with homemade autodrains. If the wind changes you could get CO. And there are a few possible if not probably scenarios where something could go bang - you aren't thinking of doing continuous mixing unattended are you?

BTW the Utilus is one of my favorite compressors, but it is questionable how continuous duty a machine it really is, Bauer told me only the Utilus2, which is quite recent, is rated continuous duty, and I was warned my 1974 really shouldn't be asked to fill more than one storage bottle at a time.

When my Utilus was made, it came in a 230 and 300 bar version (or something like that). The only difference I can find, going through the parts sheets, is that the filtration systems were rated for the higher pressure on the 300 bar. But when I asked Bauer whether I could reset mine for the higher pressure, as long as I was already upgrading the filtration system, I was told no, don't do it, it's an old machine, and probably shouldn't have been sold in a 300 bar version in the first place.

 

[bigchrisb]

Interesting setup! I think valves at each end would be all it would take to isolate the lines, don't thing purging would be necessary since the volume is so small the tanks probably represent about a 1000X the risk in the event of an accident.

However, I really don't think it is a good idea to run any compressor except one made specifically for it, with all the safety/auto features, unattended, especially when it's IC powered. If your autodrain fails, the pump can waterlock, and rip the cylinder right off the compressor. I've heard a few stories of this happening with homemade autodrains. If the wind changes you could get CO. And there are a few possible if not probably scenarios where something could go bang - you aren't thinking of doing continuous mixing unattended are you?

BTW the Utilus is one of my favorite compressors, but it is questionable how continuous duty a machine it really is, Bauer told me only the Utilus2, which is quite recent, is rated continuous duty, and I was warned my 1974 really shouldn't be asked to fill more than one storage bottle at a time.

When my Utilus was made, it came in a 230 and 300 bar version (or something like that). The only difference I can find, going through the parts sheets, is that the filtration systems were rated for the higher pressure on the 300 bar. But when I asked Bauer whether I could reset mine for the higher pressure, as long as I was already upgrading the filtration system, I was told no, don't do it, it's an old machine, and probably shouldn't have been sold in a 300 bar version in the first place.

Thanks for the feedback Vance, some of the critique I was hoping for. Concur with the comment about the lines and volume, and agree that the cylinders are the biggest danger in the system. However, I'm usually driving around with upwards of 8 full scuba cylinders anyway, and can't see a great change in risk between larger storage cylinders and the scuba cylinders. If any of them rupture in a vehicle collision, trouble is likley. But the likleyhood of a rupture in collision is largely what tank standarda are about - such as the Australian Standard for pressure vessels, and your DOT regulations.

Certainly don't intend to do unattended contiunous mixing - I don't have THAT much of a deathwish! The other points you have raised are good to note, both the water and CO issues. I'll have to give some serious thought to the idea of unattended compression, and either overcome these obstacles, or scrap the idea, perhaps in favour of only filling banks from the electric drive when available.

My utilus is a 1973 300 bar version, and runs quite nicely. I dont know its full pedigree, but little wear is evident, which suggests that it has spent a large portion of its 30 years sitting idle. From my research, the only differences between the 200 and 300 bar versions were the filter and coalescor canisters - one has a "bullet" sort of shape to the canisters, whilst the other has a rounded classical HP cylinder profile. I'm also in a club that has has a utilis since the '70s, and routinley floggs the guts out of it for a few weeks straight each year. With the usual maintenance (oil, filters etc), its still running fine. But worth knowing, and taking on board. I doubt I would end up draining both banks, so might be ok. If I go ahead, I'll do some temperature monitoring and see how it goes.

 

[oxyhacker]

We would love to have some pictures of your rig, the hood-mount compressor in particular, for use in a book at some point.

Good to hear your 300 bar Utilus is holding up well. Bauer here tends to be very cautious in their advice, so one never knows whether to believe them or not. The Utilus runs very cool and quietly compared to other compressors of its output. I run mine mostly at 3300-3500 psi.

Two other things I'd want for untended or semi-untended operation would be a high temperature cut off (most Bauers have them but not all the portables) and a redundant relief valve (I had the Bauer one freeze up one day in very cold conditions, maybe not a problem where you are).

My utilus is a 1973 300 bar version, and runs quite nicely. I dont know its full pedigree, but little wear is evident, which suggests that it has spent a large portion of its 30 years sitting idle. From my research, the only differences between the 200 and 300 bar versions were the filter and coalescor canisters - one has a "bullet" sort of shape to the canisters, whilst the other has a rounded classical HP cylinder profile. I'm also in a club that has has a utilis since the '70s, and routinley floggs the guts out of it for a few weeks straight each year. With the usual maintenance (oil, filters etc), its still running fine. But worth knowing, and taking on board. I doubt I would end up draining both banks, so might be ok. If I go ahead, I'll do some temperature monitoring and see how it goes.

 

[Wayward Son]

Do you need to be able to fill the bank off this compressor while diving, or would simply filling the bank at home be enough? That way you wouldn't need an HP line running to the front, or at least not a permanent one, and could just fill the dive tanks from a whip while in the field. You could still mount a perm HP line in case you wanted to top it off in the field.