Opinions on 2 year old cylinder failed visual for pitting

[KDAD]

Please help educate some of us newbies here on this...

The industry in which I work we use desiccant to dry air used to calibrate equipment. When the desiccant is depleted, which is indicated by color, we change it. We can recharge the desiccant by baking it in an oven and reuse it. Doesn't cost a penny. Compressing air does convert moisture to water. That does need to be removed, preferably before compressing or through a water separator.

I suspect that the columns used in a compressor for tank fills are larger and/or multiple. Is this shop not changing desiccant when depleted?

 

[Yoyodyne-88]

When the desiccant is depleted, which is indicated by color, we change it. We can recharge the desiccant by baking it in an oven and reuse it. Doesn't cost a penny.

Yes. But high pressure from scuba air compressors also contains small amounts of oil from the compressor, so the dessicant will be contaminated with more than just water.

In fact if a filter system is overdue for changing, the breathing air can actually feel and taste oily in the mouth.

 

[Wookie]

Please help educate some of us newbies here on this...

The industry in which I work we use desiccant to dry air used to calibrate equipment. When the desiccant is depleted, which is indicated by color, we change it. We can recharge the desiccant by baking it in an oven and reuse it. Doesn't cost a penny. Compressing air does convert moisture to water. That does need to be removed, preferably before compressing or through a water separator.

I suspect that the columns used in a compressor for tank fills are larger and/or multiple. Is this shop not changing desiccant when depleted?

We throw our filter cartridges away, because as indicated above, it's contaminated with oil and other undesirables. A filter cartridge has a number of parts, desiccant as mentioned, a molecular sieve, Hopcalite to remove CO, carbon for the oil, and when you get too much water vapor, it reduces the efficiency of all of the before mentioned greatly. Most shops use a time method to change filters. My compressors are routinely in a 100 degree space, so the filter is good for 50 hours, I change it every 25. I have added an extra desiccant tower, so I now change both every 75. If I went to another dessicant tower, I could change every 150, and a third tower would take me to 300 I think. The towers are very expensive and only rated for so many pressure cycles.

 

[cool_hardware52]

A little water can make a difference. here is an article from Moisture In SCUBA & SCBA
It talks about moisture in a scuba tank. When we are talking PPM a drop represents a lot of moisture.

Understanding Dew Point & ppm moisture​

This topic is very complex; we will discuss just the basic parts. The term Dew Point is the temperature at which the gas can no longer hold the moisture in the evaporated state, but causes it to spontaneously condense onto surfaces or remains suspended as micro droplets. Moisture is expressed in three ways: Dew point (in degrees Fahrenheit or Celsius), parts per million by volume (ppmv) and mg/cubic liter.
If you have air that has a dew point of -65ºF at one atmosphere pressure, the water content will be 24ppmv, and the air is considered dry. If you then charge a SCUBA tank to 3100psig with that same air, the moisture content will still be 24 ppmv, but because the water and air molecules are now compressed together, it is easier for the water molecules to "stick" together and condense onto the cylinder walls. If you measure the dew point of the air in that cylinder at the 3100 psi pressure, you would discover that the dew point inside the cylinder is now +28 ºF (that dew point is called the Pressure Dew Point). That means that if the outside temperature becomes colder than +28ºF, the compressed air in the cylinder will condense onto the SCUBA flasks interior walls.

Yep, complex alright.

Your example of the Pressure dew point changing from -65F to +28F concerns the moisture introduced via the *compressed* air, not via a wet valve.

The fact remains that volume of water one can expect to introduce via "wet valve" is a tiny, tiny fraction of the volume of a scuba tank.

I want dry air, that's why I build a water cooled after cooler for my compressor and run a chiller to provide ~30F water to the after cooler. My bank tanks get cold when I fill them.

My compressor produces gas with a dew point below -95F. It's so dry it's off the scale....................


Tobin

 

[Keith.M]

We throw our filter cartridges away, because as indicated above, it's contaminated with oil and other undesirables. A filter cartridge has a number of parts, desiccant as mentioned, a molecular sieve, Hopcalite to remove CO, carbon for the oil, and when you get too much water vapor, it reduces the efficiency of all of the before mentioned greatly. Most shops use a time method to change filters. My compressors are routinely in a 100 degree space, so the filter is good for 50 hours, I change it every 25. I have added an extra desiccant tower, so I now change both every 75. If I went to another dessicant tower, I could change every 150, and a third tower would take me to 300 I think. The towers are very expensive and only rated for so many pressure cycles.

That is very interesting.

Are the towers in series or parallel? It would seem to me that if they are in series, that the first tower would need to be changed as often as you always had. The second would take much longer to need changing and the third even longer than that.

I could be wrong, but would be interested in knowing why.

 

[Wookie]

That is very interesting.

Are the towers in series or parallel? It would seem to me that if they are in series, that the first tower would need to be changed as often as you always had. The second would take much longer to need changing and the third even longer than that.

I could be wrong, but would be interested in knowing why.

Series, but not all of the cartridges are made the same. If you only have one cartridge, it has a molecular sieve, catalyst (Hopcalite), and carbon all in one filter. If you put 2 inline, the first one is all molecular sieve, and the second is the first one from before. Remember, I said moisture kills a cartridge, so having a dryer in front of the cartridge makes it last much longer.

 

[cool_hardware52]

That is very interesting.

Are the towers in series or parallel? It would seem to me that if they are in series, that the first tower would need to be changed as often as you always had. The second would take much longer to need changing and the third even longer than that.

I could be wrong, but would be interested in knowing why.

The typical breathing air filtration system will include, starting at the outlet of the high pressure stage of the compressor:

Some sort of air to air after cooler, essentially a coil of high pressure tubing that has ambient air blowing over it. The best you can hope for is the hot high pressure air is cooled to a temperature a bit above the temperature of the air entering the compressor.

A coalescing separator. This a mechanical separator that can remove condensed liquids, but not vapors. These may be stand alone or part of a filter cartridge.

A desiccant bed. The function of the desiccant is to absorb water vapor

A activated charcoal bed. The function of the AC is to absorb hydrocarbon vapors

A CO catalysis. This converts Carbon Monoxide to CO2. Oils in the compressor can oxidize and generate Carbon Monoxide

A priority valve that acts to maintain pressure in the entire filter assembly.

All of these are in series.

If you have a big, high CFM compressor this entire series "String" of absorption devices may be paralleled for greater flow, but that not typical.


I fabricated stainless steel, 6000 psi tube type heat water cooled exchangers for my compressor. These are not a typical "shelf" item.

The advantage is I can cool the gas to well below ambient before it enters the filters. That causes many of the vapors, water and hydrocarbons to condense and be removed by the coalescing separator *before* the gas ever impinges on any of the filter media.

That means my desiccant beds last a very long time, as they see very little moisture. Many desiccant cartridges have a color indicating strip on them that changes color as the bed is saturated. Mine basically never change.

I do however change my Activated charcoal cartridges on a set schedule as I cannot predict how much Hydro carbon they may have absorbed.

If you are a member of the Deco Stop you should search for an excellent article written by Ted Green concerning breathing air filtration.

Hope this helps.

Tobin

 

[KDAD]

We use Purafil Hypocalite and carbon in our air generators to remove impurities and the air passes through silica gel desiccant before it enters any of the scrubbers to remove the moisture. Some of the better generators have a cooling coil to help reduce the dew point.

It would seem advantageous to remove as much moisture as possible before the air enters the system.

 

[dberry]

Interesting thread about purifying compressed air. Although I haven't worked with a HP air compressor, I do work with inert atmosphere glove boxes which are constantly kept at <1-5ppm H2O by circulating through a fixed bed of 4A or 5A "molecular sieves" (zeolites). There is also a catalytic scrubber to keep the O2 levels < ca. 1ppm, but that's not relevant here My point is we regenerate them periodically by passing dry gas through the beds for a few hours while they're heated 200-300 deg C. The sieves also absorb small hydrocarbons, which are expelled with the water during the regeneration. We typically use the same molecular sieves for >10 years (hundreds of regeneration cycles), and they main reason to change is the pellets break down into dust.

I'm surprised no one regenerates the desiccant cartridges. Not in the interest of the cartridge suppliers, I suppose. And it's possible the desiccants used are less costly than the zeolites we use. Tobin is entirely correct that cooling the compressed air will greatly improve the efficiency of the desiccant and the ability of activated carbon to suck up the hydrocarbons.

 

[Spg01]

The OP can't possibly be the only guy with steel tanks getting fills at that shop. This kind of thing doesn't happen from a single bad fill. So either every other steel tank they fill is facing the same fate, or something is wrong with the "facts" presented. And since he probably would have heard about other people suffering the same fate, then there is something else wrong.

Or maybe I'm just too cynical.

That is why I was thinking it was something wrong with the cylinders. I have a few other single steel cylinders that I have been using for years and getting filled at that shop and used on boats, they got a tumble when needed. I was diving them 2 to 3 times a week as well and they have no pitting.

But a few years of diving these doubles 2 to 3 times a week and it failed a visual. I was hoping XS would handle it, but they just passed the buck.