Moisture in tanks; measuring humidity

[ScubaDemon]

Hello!

When I prepared my tanks for visual inspection, I found that there was some moisture in some of the tanks. I fill with a Coltrisub MCH16/ETS compressor (under 1000h, clean and well maintained), change the filter cartridges even more often than recommended, and never had a problem with moisture before.
Those tanks are never emptied completely, and our humidity here is quite low (average about 20-30%), even though the compressor station is located near the beach. The compressor has a well functioning auto-drain, and I cannot find any faults in the set-up.

What could cause moisture in tanks?

To avoid problems, I am thinking about monitoring the humidity of the compressed air electronically. Unfortunately I cannot find any components here. What systems are available, and what are the approx. costs?

 

[fire_diver]

Can't help with the second question....

Do you ever use a dive shop for air or just your own compressor?

What pressures are the filters seeing for most of the operation, IE are they only seeing 2-3000 psi at the very end? I ask, becuase the dwell time in the filters is very important to thier function.

 

[ScubaDemon]

Hey!
Thanks for the answer. I run a dive shop, so those are all heavily used tanks.
The filters are indeed seeing 2-230bar on a daily basis. I use double filtration, i.e. two filters in row. The first one is changed more often than required (at current ambient temperatures of up to 45°C every 5hrs!), and the second one (which is always in pristine condition, btw.) about every 50hrs. Accoring to manufacturer's tables, ONE filter should last about 15hrs at 45°C and 30% humidity. I change every 5hrs in the summer to be sure, and the filter material is always quite wet (second filter always 100% dry).

 

[fire_diver]

Sorry, let me be more specific. What percentage of the run time is the air passing through the filters in the upper range of pressure? 100psi air takes Y amount of time to pass through the filter media. 1000 psi takes Yx10, 2000psi takes Yx20 time to pass through.

So if you are filling tanks from completely empty, then the air is not being filtered properly.

Not saying that dwell time is your problem, but it could be.

Re-reading your last post....if your media is physically wet, it stopped working LONG ago, and is probably overloading the second filter. Double check your coalescers, timers and dumps, especially after the the last stages.

 

[ScubaDemon]

Thanks for sharing your expertise!

I will check the auto drain system and the timing again tomorrow. It should drain every 10min for 7seconds, as manufacturer's default settings. There is definately plenty of output though, so system should be working. I might manually drain in between, and see if this makes any difference.

When it is really hot (45°C), the filter is a bit wet after 5hrs. In the winter, it is much better. But having to change it even more often would be dull! I halfed the recommended intervalls already!

Tanks are normally never emptied completely, so most fills start at around 50bar. Nitrox and Tmx are the exception, as partial pressure blending requires an empty tank.

What would have to be changed, to make the filtration more effective?

 

[fire_diver]

Assuming all elements of the system are in order, I would recommend adding a storage bank system.

If you keep having problems, you might want to contact the manufacturer. Wet media after only 5 hours, sounds like something is wrong.

 

[CraigAClark]

Filters should be changed as a set. By changing the first filter, which is generally a dessecant only, you allow the "zone of mass transfer" to progress into the purification filter. Although it may look pristine, moisture may have contaminated it to the point of passing through to the recepticle. Have an air test done to varify the moisture content and I don't mean these on site tests that are notoriously inaccurate.

Craig

 

[veriqster]

From my understanding one of the best options is a setup with two identical filters set in series. A pressure maintaining valve (regulator) is installed AFTER the filters to maintain pressure / dwell time in the filters. In between filters is installed moisture indicator. When the moisture raises to 5/10 % you switch the filter cartridge from the downstream filter into the upstream and put a new on in the DOWNSTREAM filter. This way you always have a filter that is KNOWN to be good as a contingency. If you have a one way valve in between water separator and filters, make sure you have a way to unload the pressure in the filters when you need to change the cartridges , otherwise you will end up with a pressurized set of filters that you can't work on. I can make a drawing if this sounds too complicated.

 

[ScubaDemon]

That's all very helpul! Thanks a lot, guys!

@fire_diver: I have been looking for some bank cylinders for a while now. The problem is that proper tanks are rare here, and only Oxygen storage tanks with a work pressure of only 150bar are readily available. Tanks with higher work pressure are hard to get, but I keep trying ;-)

@veriqster: Filter swapping makes a lot of sense! Thanks for the idea. What kind of pressure maintaining valve would that be (could you maybe post a link to one)? As well, I cannot get an electronic moisture indicator here, so where would be a place to buy those? No need for drawings though, I complete get your point! Thanks a lot!

@craigaclark: What is curently installed is two identical filter units, both with dessecant molecular sieve and active carbon. Would it make sense to have only molecular sieve in the first filter, and only active carbon in the second? That would allow for some more moisture to be absorpted before entering the active carbon filter. We have regular air tests done, which have always been good. The moisture problem is a recent one.

I would be grateful for more equipment advice, maybe pictures of your filtration units, and links to manufacturers for electronic moisture meters, valves/regs, filters, etc.

Thanks guys, for the productive thread!!

 

[veriqster]

@veriqster: Filter swapping makes a lot of sense! Thanks for the idea. What kind of pressure maintaining valve would that be (could you maybe post a link to one)? As well, I cannot get an electronic moisture indicator here, so where would be a place to buy those? No need for drawings though, I complete get your point! Thanks a lot!

High pressure breathing air compressors, scuba air compressors, Scuba air
If you get an adjustable pressure regulator you can fidle around with it and see what pressure gets you the best results in terms of filter time, compressor load, etc.
I would use identical filters (desiccant, hopcalite, activated carbon), otherwise you can't swap filters upstream. If you want to save on chemicals look into buying them bulk and repacking the cartridges yourself. Regular "glass sight" moisture indicators are fine, once the disk changes colors you swap filters. The dry air coming out of the filter after the swap brings the disk back to initial color in minutes. Just make sure you get a "fresh" disk to begin with - they seem to have a shelf life of about 2 years.
P.S. don't worry about "missing" on the moisture indicator by 1-2 hours; that's why you have the seccond filter after it (assuming 1-2 hours is much less than your regular filter time for the upstream filer).