Butane Sphere Vacuum Breaker 1

[highmarkr]

http://www.eng-tips.com/viewthread.cfm?qid=67046

I came across a closed thread about this issue and was wondering if anybody had some personal experience with this.

I am in a fairly cold climate, atleast as far as winter is concerned. On our butane spheres we have vacuum breakers set to 1.5 oz per square inch.

When it gets cold and the vapor pressure of the butane drops below atmospheric pressure, we inherently would be letting air into the tank. We now have two problems. We have incondensables in the tank, causing us to have a max limit on the level (atleast until it dissolves into the butane), otherwise we will pop the psv. We also have an oxidizer inside with the butane. Luckily the flammability limits of butane are fairly small and there is only a small temperature range where the butane is potentially flammable.

My path forward is to obtain the vacuum rating on the tanks, get vacuum breakers that are set for that limit, and to get lucky. I hope to have a high enough vacuum rating so as long as we don't have extreme record lows there is always enough vapor pressure to keep the breakers closed.

Does anyone have a different practice for this? From what I understand our current setup is a fairly normal practice, which in my mind works great in the US if your butane tanks are down south, not so much in the North.

FYI: A nitrogen or ng purge/venting system was considered, but because of the solubility in the butane, we are concerned of the consumption of the purge gas and the possible downstream issues.

Thanks for any input.

-Chad

 

[Latexman]

Obtain historical meteorlogical data for the location. You should be able to get many years of data. What is the lowest recorded temperature. IF the butane got to this temperature (which is theoretically impossible), is the vapor pressure of butane greater than the vessels vacuum rating? If yes, I don't see why vacuum breaker is needed. No need, right?

Good luck,
Latexman

 

[Chance17]

You may consider a tank heater and use butane to generate pad gas vapor. The heat source could be steam or electric.

 

[highmarkr]

The only reason to put vacuum breakers back on if the vacuum rating on the tank exceeds the vacuum the butane would pull would be the off chance the tanks is empty, closed up, and some how some condensable vapors were in the tank. Cheap investment so might as well. I have considered heating the tanks, but the tanks are enormous and it would cost a lot to heat. I still need to run the numbers though and it still may be a viable option.

Thanks for the replies,

-Chad

 

[Montemayor]

Chad:

The strongest geometric shape for a pressure vessel is – guess what? --- a sphere! You may be aware that the shape of extremely deep sea exploration vessels is a sphere. This is because the natural shape of a sphere allows for the strongest protection against external pressures – which also applies to internal vacuums.

In your case, you are fortunate in having a spherical storage vessel. The real smart and right thing that would apply to your application is to have your sphere rated for full vacuum (FV) operation, and not have to worry about winter conditions in the future. I am surprised that the original Data Sheet and Specifications for your sphere did not include FV rating. Nevertheless, you should have the original Data Sheet and/ fabrication drawings for the vessel and these should clearly indicate the pressure / vacuum ratings. I would be very surprised if you don’t have some degree of allowable vacuum rating for the tank – unless it is a used tank or one that was not originally designed for your specific application. I would look into this first. If that doesn’t work, I would look into modifying the tank for FV rating.

The last thing I would do is resort to heating the tank. Heating costs don’t bother me as much as the process hazards involved in maintaining the continuous, 24-hr heating while suffering increased heat transfer losses as the stored liquid butane’s temperature is increased and the external temperature drops. Here, I am assuming that insulating the tank is practically and economically out of the question.