Nitrous Oxide Tank Install

[ggunn]

As I understand it, the critical pressure is the pressure required to keep the product liquid at the critical temperature. When the temperature goes above that, no amount of pressure will keep the product liquid and the volume will expand 440 times, causing catastrophic failure of the container.

Hmm... I'm skeptical. No amount of pressure? The pressure exerted by the substance when it reaches its critical temperature is infinite?

FWIW, I used to work for a specialty gas division of a major gas supplier. We had cylinders of nitrous oxide standing around on loading docks outside for days in the summertime and none of them ever blew up. They were standard (non vented) pressure cylinders, the same as the ones we kept O2, N2, H2, He, etc. in. Also, BTW, standard gas cylinders all have a burst disk type pressure relief valve that will vent the tank before the tank itself reaches its burst pressure.

 

[GoldDigger]

Hmm... I'm skeptical. No amount of pressure? The pressure exerted by the substance when it reaches its critical temperature is infinite?

FWIW, I used to work for a specialty gas division of a major gas supplier. We had cylinders of nitrous oxide standing around on loading docks outside for days in the summertime and none of them ever blew up. They were standard (non vented) pressure cylinders, the same as the ones we kept O2, N2, H2, He, etc. in. Also, BTW, standard gas cylinders all have a burst disk type pressure relief valve that will vent the tank before the tank itself reaches its burst pressure.

Your skepticism is well justified, and the correct physics and thermodynamics has been pieced out in other posts.
To sum up the key points from those posts:
1. The 440/1 volume expansion factor is for the vapor at atmospheric pressure and the liquid phase well below critical temperature.
2. When you near the critical temperature, the liquid density will be slightly lower and will be somewhat dependent on the pressure.
3. Above the critical temperature the molecules will not interact in the liquid state (I won't go into the technical definition for this) but can still be highly compressed by increasing pressure. As a result the density can be far higher than the density at atmospheric pressure.

In summary, there will be no sudden catastrophic expansion, but the pressure can still rise above the working pressure of the tank if the liquid volume is too high when filled, with insufficient vapor space for expansion. And that will vent the tank, either catastrophically or via a rupture disk.

In the same way a propane tank can rupture if overfilled at a temperature well below the highest ambient temperature it will be exposed to. The more it is overfilled, the lower the failure temperature.

 

[don_resqcapt19]

Your skepticism is well justified, and the correct physics and thermodynamics has been pieced out in other posts.
To sum up the key points from those posts:
1. The 440/1 volume expansion factor is for the vapor at atmospheric pressure and the liquid phase well below critical temperature.
2. When you near the critical temperature, the liquid density will be slightly lower and will be somewhat dependent on the pressure.
3. Above the critical temperature the molecules will not interact in the liquid state (I won't go into the technical definition for this) but can still be highly compressed by increasing pressure. As a result the density can be far higher than the density at atmospheric pressure.

In summary, there will be no sudden catastrophic expansion, but the pressure can still rise above the working pressure of the tank if the liquid volume is too high when filled, with insufficient vapor space for expansion. And that will vent the tank, either catastrophically or via a rupture disk.

In the same way a propane tank can rupture if overfilled at a temperature well below the highest ambient temperature it will be exposed to. The more it is overfilled, the lower the failure temperature.

Thanks, my only knowledge is what I remember from HazMat response training.

 

[kwired]

I guess what I don't understand is the difference in density between the liquid and the critical density. It is 48.21 pounds per cubic foot in the liquid state and 28.22 pounds per cubic foot at the critical point. To me that indicates a change in volume. How is this new volume contained in the original container?

I'll admit I don't really know the answer, but seems fairly obvious the ability of the container to hold the pressure is what is critical to whether the container ruptures, regardless of whether it contains a liquid or a gas. Liquid or gas it has to remain at same density if the container doesn't rupture.

 

[GeorgeB]

That's not how I understand the physics. As the temperature approaches the critical temperature, the gas phase density goes up (since the vapor pressure goes up and the gas phase density is proportional to that) and the liquid phase density goes down (the liquid expands with the rising temperature). At the critical temperature, those two density curves meet, at a density called the critical density. So at or above the critical temperature, there is no difference between gas phase and liquid phase, just one supercritical phase.

There is another compound, H2O, which has been very well analyzed at supercritical conditions. Virtually any fossil fueled power generation boiler and turbine operate in supercritical conditions, as an example, 3500 psi, 1000 degrees F.

From a boiler operators view, the most obviously visible situation is that there is no sight glass to determine liquid level. It's been many years since personal experience, but whether it is conventional, or just in jest with a then junior engineer, it was called "weam" taking parts of the words water and steam. As WWhitney says, "there is no difference between gas phase and liquid phase, just one supercritical phase."

Containers for moderately high pressure (say 1800-6000 psi) are not rare. Air, oxygen, nitrogen, hydrogen, and helium, among many others, are routinely stored and handled at these pressures.

 

[don_resqcapt19]

I think I took this thread way off track when I stated commenting on "cryogenic liquid" part of post #4. I don't think nitrous oxide is typically stored as a liquefied gas. In typical applications, I believe it is only stored as a compressed gas.

(of course my misconceptions on the physics of liquefied gasses did not help)

Thanks to those who set me straight.

 

[ggunn]

I think I took this thread way off track when I stated commenting on "cryogenic liquid" part of post #4. I don't think nitrous oxide is typically stored as a liquefied gas. In typical applications, I believe it is only stored as a compressed gas.

No, it is stored as a liquified gas, the same as is CO2, propane, etc. One way to tell is that except for thermal effects the pressure in the cylinder remains the same as the gas is released until all of the liquid phase has evaporated.

 

[ggunn]

No, it is stored as a liquified gas, the same as is CO2, propane, etc. One way to tell is that except for thermal effects the pressure in the cylinder remains the same as the gas is released until all of the liquid phase has evaporated.

A weird one is acetylene. Acetylene is similar, i.e., it will liquify if compressed at more than its vapor pressure, but it will also autopolymerize and in the process liberate a large amount of energy. Not good. The way it has to be stored is in a pressure vessel filled to the top with some sort of porous substrate saturated with acetone. As acetylene is pumped into the tank it dissolves in the acetone and is adsorbed from solution onto the large surface area of the porous substrate. When the valve is opened the process is reversed and acetylene is released.