Circulation vent design- API 650 Internal Floating Roof Storage Tank

[Wazobia]

Hi guys

I had posted this topic in another thread, but was advised to post over here instead.

Basically I'm involved in a project for a fuel storage facility.

We are designing tanks both fixed roof tanks, and internal floating decks, and I am sizing vents for these tanks.

The requirements and calculations for fixed roof tanks are quite detailed in API 2000, however it explicitly states that it doesn't cover floating decks.

After going through API 650, I came across appendix H which mentions circulation vents at the top of the fixed roof (geodesic). It goes into some detail about maximum spacings between vents, minimum total open area of vents as a ratio of tank diameter, and minimum area of an open vent at the centre of the fixed roof.

My question is if there is a definite calculation for the sizes of these vents, as opposed to minimum and maximum values, since I apparently cannot use API 2000? My background over the past few years was in upstream oil & gas design, and I have hardly ever come across relieving a non pressure vessel. It's been a real learning process, however I still need some expert advice

Any help would be greatly appreciated. Thanks.

Some more information - The floating roof tanks are 60,000 m3 capacity, with a diameter of 60 m. The fuels are petrol and naphtha.

The original thread is at thread124-317137

 

[JStephen]

The "regular vent" on a tank is just to let air in and out as the level moves up and down or the tank heats and cools. Usually, this will work out to a fairly small vent if actually sized per flow charts.

The circulation vents are intended to keep the air/product vapor mixture in the tank from being an explosive or combustible mixture. The minimum sizes given should be much much larger than what is required to vent the tank per the "normal venting" requirements. The idea is that even if the product level is static and the temperature doesn't change, there should still be some air moving through the tank.

As confirmation, you might look up the flow charts for some of the "free vents", calculate what vent area is required based on that sizing, and see how it compares to the App. H requirements.

 

[IFRs]

JStephen is quite correct (and succinct), as usual. Calculations based on pressure and flow are not needed, nor will they teach you anything. API 650 Appendix H says that you must have a minimum number of vents with a minimum amount of open area. This determination does not have a basis in calculation. Specifically, it says you need one vent every 32 feet of circumference maximum and a total vent open area of at least 0.20 square feet per foot of tank diameter. Most floating roof vendors make their own vents and typically make them with 2 square feet of open area. If you email me at 'davidr at fast dot net' I can send you drawings.

 

[Wazobia]

JStephen, thanks for the explanation of the differences in terms. I have a much clearer view now. I'll certainly check out the difference in vent area between the two.

IFRS, thanks for the email address. I'll send you an email in a bit.