Draining a tank too fast

[Jorbar1551]

the condensation inside your tank will become toxic and kill you

 

[Jorbar1551]

better idea...give the tank to me and i will drain it for you : ))

 

[Packhorse]

Condensation can't form in a pressurized environment.

Can you please elaborate. What about condensation forming at ambient pressure?
Dont get me wrong. Im sure in the context of a tank with relitivly high pressure and low humidity your statment is correct.

The guy at the LDS says that letting the air out too fast will cause internal condensation.
When I ask how can this be when the air is dry he never really gives me a satisfactory answer.
Now I understand that if you open up a valve and leave it open the tank will empty and cool. Condensation will form on the outside and if the valve is left open once the tank is empty moist air can enter and condensate on the inside. But at what point can the internal moisture condensate? How cold and moist will it have to get?
Would putting the tank in water help keep the tank warm enough to stop internal condensation?

 

[pescador775]

Packhorse, you are being abused by bad info from your source. There is absolutely no harm in fast draining a tank. Drop the pressure to 500 psi and let the tank dry off. When dry, blow a puff of air through the valve to clear it. You are done. Cold or hot, whatever your tank does is not harmful to the metal. Your steel tank will not be damaged by temps in the range of -40 to +800F. Below that, there is a slight possiblilty that the tank will be damaged by a physical blow or fast fill. Above that, there is a danger to the temper.

The dew point inside a 3000 psi tank is about 30F to 40F. Draining the tank causes the temp to approach the dew point but it is a moving target. At 500 psi, the dewpoint is approx -20F. Do you understand any of this? If so, you will stop worrying about nonsense told you by a dive shop guy.

 

[Packhorse]

Thanks I do understand and is what I assumed. I have a hard time believing anything someone tells me but cant explain the reasons behind their believe.
As for the dew point you gave what humidity are these at? A lower humidty would = a lower dew point correct?

 

[JeffG]

Thanks I do understand and is what I assumed. I have a hard time believing anything someone tells me but cant explain the reasons behind their believe.

Pretty much standard MO for shop monkeys.

 

[Packhorse]

Pretty much standard MO for shop monkeys.

I know but this is the shop owner/ head instructor

Anyone know where I can view charts on dew point vs pressure? or how to calculate it for diffrent pressures

 

[Packhorse]

Found a calculator
http://www.amsystems.co.uk/equivalents/calculate.html

Sorry about the metric but

@200bar dew point =-3degC Grade E air requirements
150 bar -6.4C
100 bar -11C
50 bar -18.5C
25 bar -25.6C
1 bar -54C = -64F

So assuming your tank is filled with air dry enough to meet the standards and you keep the tank above these temps then no condensation will form.

So next question is how cold will a tank get by dumping 50 bar, 100 bar or more quickly?

 

[pescador775]

Humidity is usually expressed relative to a temp and moisture content. The actual content is expressed as "specific" , "volumetric" or "absolute". The saturation level is expressed as "relative". Thus, a relative humidity of 100% is called "saturation" and is at that point equivalent to a dewpoint equal to the actual temp. For example, at 30F to approx 40F the relative humidity inside a 3000 psi tank is about 90-100%. The RH inside the same tank at the same temp at a pressure of 500 psi is about 20%. That is a guess but you understand it is quite low. Now, as to volumetric humidity, this is relavant because it allows us to calculate how much actual water vapor is present inside a tank at high pressure. It is on the order of 10 mg gaseous water vapor. Even should a part of this condense at very low temps it is not likely to do much damage to a steel tank. Figure on a little rust brown appearance inside the tank over a long period of exposure. If high PP O2 is present, some small amount of powder rust could form. Still, with the average good compressor set up to produce air with -60F dewpoint at 1 atm it seems unlikely that even such a small amount of rust might occur. I have not personally seen it. Most cases where rust is apparent the cause is likely the forcing of a water droplet which is present in the valve orifice. This is a matter of ordinary care but can occur on boats and some dive shops with water bath immersion. If the compressor is at fault it is because the filtration system does not contain molecular sieve. In fact, there is an existing British standard which allows a dewpoint of only about -40F. If this is the standard which is being applied then some powder rust is probable.

 

[pescador775]

The final temp of a draining tank will depend on several factors but will not approach dew point under average conditions. If you are dumping air at McMurdo Sound the valve will probably freeze solid before you can complete the dump. Once you enter the heated quarters you can complete the draining. Even if a mg of water should form it will immediately evaporate as the tank warms.

As to your calculator, it needs to be more sophisticated in interpretation. For example, in addition to other human factors, the humidity which may or may not intrude during visual inspection will have a material effect. This air, if quite humid, will raise the actual dew point by a measurable amount which will then decline with subsequent purging and refilling of the tank.

If you can't believe anyone why post here? Just had to ask. I don't believe much that a dive instructor or dive shop owner tells me. Here, everything is written so is easy to check details if necessary.

I hope this has been helpful. Good luck.