Internal rust & flash rust in steel tanks

[padiscubapro]

mer once bubbled...


So FredT,

Any suggestions on the right questions to ask a dive shop/fill station to make sure they are doing the right thing with the compressor and fill procedure.

I understand the physics but don't know the finer points of compressor operation... I'm the proud owner of new Faber steel tanks and I want to keep them happy.

Thanks,
Mer

I know there aren't many ANDI facilities in the US(so odds are there isn't one near your), but every ANDI facility must submit quarterly gas analysis reports to HQ and one of the requirements that must be met is a dewpoint of -50f (better than fredt stated) This takes care in operating the compressor(dumping the moisture at proper intervals either by an auto dump system or manually dumping), a large coalescer and a fairly constant backpressure (usually by a back pressure regulator(i have mine right at the output of the filters set at approxmately 2500 psi and a check valve before the filters to prevent them from dumping when the compressor is off or the auto dump is operating)....

 

[pescador775]

Yeah, -50 is achievable with 2500 psi backpressure, micronic filter and a dessicant cartridge. The Navy requires -60 for some aps. A run of the mill shop compressor should be able to do -40 dewpoint and this seems to be an acceptable level in the dive industry, except ANDI it seems.

 

[pescador775]

That's an old cowboy song from the 1920's. Ok, 0.5 gram per cubic meter is equivalent to 1.3 grams water pumped into a large steel tank. One and a third grams is about 1/4 teaspoon or 10 small drops. Sounds like -20 dewpoint. Quick question, anybody know what is the absolute humidity (gr/m^3) at saturation, standard temp and press? I know, look it up....

 

[pescador775]

I may have to disagree with brother Fred when he says that air coming from a condensator 'is saturated'. The statement is seemingly based on the fact that air will release water when compressed in the condensator and on the presumption that the remaining air remains at saturation level for that temp and press and therefore is saturated when exiting to the final filter. However, condensators aren't merely pressurized cans that 'squeeze' water out. They employ mechanical devices to deflect, jet, spin and strain water from the air. Some of these will remove water at atmospheric pressure, eg when the press in the condensator is near or just above atmospheric. I suspect the air routed to the final filter may have less relative humidity than saturation (less than 100%).

 

[FredT]

pescador775 once bubbled...
I may have to disagree with brother Fred when he says that air coming from a condensator 'is saturated'. The statement is seemingly based on the fact that air will release water when compressed in the condensator and on the presumption that the remaining air remains at saturation level for that temp and press and therefore is saturated when exiting to the final filter. However, condensators aren't merely pressurized cans that 'squeeze' water out. They employ mechanical devices to deflect, jet, spin and strain water from the air. Some of these will remove water at atmospheric pressure, eg when the press in the condensator is near or just above atmospheric. I suspect the air routed to the final filter may have less relative humidity than saturation (less than 100%).

A coalescer is a device to create an air stream free of micro droplets of both air and oil. They can not remove vapor phase moisture. because of the huge wet surface area of the coalescer element air passing over a coalescer will always contain 100% of the vapor it is capable of carrying at thattemperature and pressure. The dessicant or molecular seive drier element reduces the relative humidity in the output air at the output temp and pressure to below 15% if that filter is properly sized and maintained.

FT

 

[BlueDolphin]

Fred.... do you have anywhere to go to find some drawings for building a tumbler? Would very much like to tinker with putting one together, and find out what media to use, etc..

Thanks for all your great posts.

 

[Bubble Boy]

If you guys cant get -50 at your lds you better hope they are not doing anything for your local fire depts. Check out the NFPA requirements. Fill tanks at 300-600 psi/min and it cuts down on the adiabatic expansion

 

[pescador775]

Nice try, and it does give pause. You said:

A coalescer is a device to create an air stream free of micro droplets of both air and oil.
They can not remove vapor phase moisture. because of the huge wet surface area of the coalescer element air passing over a coalescer will always contain 100% of the vapor it is capable of carrying at thattemperature and pressure. The dessicant or molecular seive drier element reduces the relative humidity in the output air at the
output temp and pressure to below 15% if that filter is properly sized and maintained.

My point concerns the ability of the condensator to remove humidity by decreasing the velocity and increasing the surface area to which the air is exposed. This isn't just a mechanical 'straining' or scavenging of existing droplets as you state but, IMO, involves a process of condensation of gaseous water (humidity). It helps to have saturated air but I don't believe the air necessarily must be 'saturated' to condense out additional water. Remember, 'saturation' is defined as the maximum amount of water which can be held as a gas under specified conditions. Being twirled at high pressure does not meet this definition. To do this, to remove additional water from the air, some tricks such as venturi jets are needed. This is exactly what is used in modern condensators. It simply suggests to me that air leaving a condensator may be below saturation as defined for that pressure and temp.

 

[omar]

The relative humidity of a mass of air is dependent on the volume (mass of air) and temperature, not pressure. The coalescing filter is designed to intercept fine particles and liquid droplets not to reduce the relative humidity per se.

To remove water vapor you guys are overlooking the most basic and very effective method of removing moisture from the compressed air. Use an Aftercooler to cool the discharge from the compressor. It can be air or water cooled. By reducing the temperature of the compressed air you will dramatically reduce the water saturation.

In my case I have an air-cooled system that is very effective followed by the coalescing filter where the condensate is removed, a filter tower with desiccant fitted with a back pressure valve @ 2000 psig and micronic filter (outlet side). The hyperpure tower also has a backpressure valve and micronic filter as well.

 

[VTernovski]

I had my OMS 112 (LP) for about 2 yeas now, and when I brought it to the LDS for the visual, and asked to peek inside the tank, I saw some minor rust on the walls and on the bottom. The rust is not severe (no chips) but you can definitely see these brown blemishes and discoloration. They still passed the VIP, and explained that it is quite normal and it most often happens when you attempt to fill a tank right after the dive while it still wet, which only shoot small amount of water inside the tank. Should I be concerned with this rust, especially since this is a steel tank. Are those existing rust blemishes going to progressing and will the air go bad if stored for a few months?

I thought that OMS tanks have this anti-rust coating on the inside of tanks that prevent this type of rusting. This makes me not want to buy PST tanks since they don't even have that coating. So I could only imagine what that do to a PST tank.

:burnout: