Vacuum vent valve requirement

[mfritzusa]

I have been asked to check the need for a vacuum vent valve on a pressurized liquid storage sphere. The sphere was designed for pressure and not full vacuum (but it will withstand a partial vac). The question is - does it need to be designed for full vac? I know the sphere diameter, metal thickness, etc. It holds a volatile hydrocarbon (butane) and I know the pump out rate (or at least the pump rating). If I know the local ambient conditions, etc., how can I tell if the material will vaporize quickly enough to prevent full vac conditions and thus not require a vac vent on the sphere? Or I guess a better way of asking would be will the design partial vac rating of the sphere be sufficient or do I need to keep the vac vent valve?

 

[don1980]

Check the vapor pressure of butane over the range of operating temperatures for this sphere. It could be that there's no risk of dropping below the MAWV of the vessel.

If you find that the butane vapor pressure is below the MAWV value, at the lowest possible liquid temperature, then you have a risk that needs to be addressed.

For flammability reasons, it's not a good idea to put a commom VRV on vessels like this. If there is a risk of excessive vacuum, then the best solution would be high integrity instrumentation (HIPS) that shuts off the pumps when the pressure drops below a minimum value.

 

[Latexman]

mfritxusa,

All good questions. Now you know why the smart thing to do is have it rated at FV if the pressure design makes it possible. The engineer that spec'd that one wasn't thinking ahead.

Good luck,
Latexman

 

[mfritzusa]

All good information. We inherited the system from another company and have no idea why they put an atmospheric vac vent on a butane storage sphere anyway. We intend to connect it (if its needed) to another source of "vapor". Hadn't thought about the vapor pressure angle. I was thinking more about solar incidence and rate of evaporation, etc., etc.

Thanx.

 

[Mikespell]

mfritzusa,

I have to do a high-level budget estimate for a project which includes a butane storage sphere. Can you help me with some general information? So far, I think the sphere will be 14m diameter X 38.1mm thick and will operate at 14 kg/cm2. Will I need any supplemental cooling systems with this sphere? Will I need a vapor recovery system?

Please note that this is not for design purposes, it is just to do a cost estimate.

Thanks for the help.

Mike

 

[gerhardl]

mfritzusa: to your original question: the answer is yes (should be for full vacuum) if the consequences by collapse are dangerous outlet of gas, explosion hazards etc.

Pump protection in itself would not be enough if the vessel could have a sudden pressure drop by gas condencing by sudden temperature change.

This has happened in vessels beeing cleaned by steam and rinsed immidiately afterwards in cold water.

If the vessel du not have a vacuum relief valve of acceptable size (all sizes available on market) an alternative is air inlet with bursting disc.

VRVs are the best solution for vacum relief, as they are 'one way' valves protecting against unwanted outlet.

Properly protected with vacuum relief it would be up to local laws and authorities if the vessel 'has to be' for full vaccum.

Sorry about the yes and no answer, but thats life....