PLEASE HELP: Atmospheric tank relief capacity

[Subaltern]

I would like to calculate the relief capacity for an atmospheric condensate storage tank. I would appreciate if you can give general short cuts, methods, tips, useful formulas and other info that would be helpful to calculate the required relief capacity for this tank. A vendor I communicated with on this matter has forwarded me 750000 SCFH capacity with the information I supplied.

INFORMATION

Diameter: 35 feet
height: 16 feet
MAWP: Atmospheric
Design Pressure: Atmospheric
Maximum Operating Temperature: 108 F
Design Temperature: 212F

CONTENT (mass %)
Ammonia: 4%
Carbon dioxide: 5%
Urea: 0.5%
Water: 90.5%

Flow Information

Pump-out: 350 US gallons per minute on the high end.

The flows that are coming into the tank are not measured and are coming from different exchangers and overflow returns.

 

[ggordil]

You need to use API Standard 2000 "Venting Atmospheric and Low Pressure Storage Tanks," to find the required relief capacity.

 

[Subaltern]

A discription of the standard which I saw on the internet says

This standard covers the normal and emergency vapor venting requirements for above ground liquid petroleum or petroleum products storage tanks, and above ground and underground refrigerated storage tanks designed for operation at pressures from vacuum through 15 pounds per square inch gauge (1.034 bar gauge).[/]b

Does this standard apply to regular water condensate tanks with small percentage of dissolved ammonia, co2 and urea?

 

[ggordil]

Based on your description, I would say no. However, I have used it in the past for tanks containing water and other non-hydrocarbons. I am not aware of any other method for calculating the required relief rates, that is as straight forward.

I guess that you could use the pump in rate to calculate the amount of vapor displaced by the incoming liquid for outbreathing relief.

For vacuum relief, you could calculate the volumetric flow out of the tank and convert to a mass of air.

I don't know how you can calculate the relief required for thermal changes or external fire without API 2000. Maybe someone else has some insight.

 

[Subaltern]

Thanks ggordil. I'll wait if anyone else can help on this.

 

[tickle]

http://www.protectoseal.com

(the website is worth a star)

Would need to include any vapour that is flashed (try Hysys)

 

[psafety]

I determine a pressure and a vacuum allowable based on ASME &/or API tank equations. This allows for a delta P to atmosphere.

Overpressure protection:
I use the sum of all inflow mass the overflow is sized based on height of liquid above centerline. I take the mass to equivalent air to size the vent.

Vacuum protection: All outflow possible pumping at installed bhp + draining simultaneously. If steam is a potential, condensation on the walls of an empty vessel. Hot/cold wash vacuum due to btuh change.

 

[zerosum]

It sounds like your tank was possibly built to API 650. That should be confirmed first. Appendix F of that standard tells you how to calculate the max allowable pressure as <psafety> alludes to above. Also somewhere in either API 650 or API 2000 is a vacuum allowable pressure (not much). You will need those figures, plus the inflow and outflow due to pumping and breathing to calculate your relieving capacities required. Manufacturers' data will size the relieving devices. Since your max design temp is 212 deg F, hopefully you won't have to contend with condensing steam.

I don't think there are any shortcuts except using the appropriate standards.

 

[zerosum]

Inadvertant condensing of any vapor above the liquid can be catastophic and the possibility must be carefully safeguarded against. Rainshowers on a hot roof can be a big (think junk-the-tank) problem. Again, re-iterating <psafety's> warnings.