Gap in double deck floating tank roof 1

[Oreco]

Hi,
We manufacture a cold tapping tool used for cutting holes on oil storage tank roof for mounting of tank cleaning nozzles used in automated tank cleaning.
We are developing a new tool that can be used for double deck floating roofs. We are therefore interested in knowing what the maximum gap between the internal and external tank roof can be or what the common gap is. Is there any API or other standard we can find this information?

Thank you in advance!

 

[IFRs]

API does not specify the maximum or minimum gap between the floating roof and the tank shell. 6" to 8" is probably what you will find on average. If you mean the gap between the fixed external roof and floating internal roofs, it depends on the liquid level and ranges from 60 feet or more to 3 feet or less. Double deck floating roofs are seldom seen under fixed roofs, so this may not be your real question. If you are asking what is the gap between the upper and lower decks of a double deck floating roof, that varies between 12" and 36" or so but I would be very concerned if you plan to cut a hole in the bottom deck as leaks there are very bad. Perhaps you could post a more detailed description of your process and question.

 

[Oreco]

Thank you very much for your quick response. I was asking about the gap between the upper and lower decks of a double deck floating roof, so thank you for the answer.
Yes we are fully aware about the risk of leaks when cutting a hole on the lower deck. The roof support legs are in maintenance mode and the oil level is in a lower level than the lower deck before the cutting begins. We have furthermore attached a trunk on the lower deck with some gaskets between the trunk and the lower deck.
We develop a fully automated non man entry tank cleaning equipment, you can read more about our process in our homepage

http://www.oreco.com.

Is there any reason why the gap varies between 12” and 36”. Is the gap chosen after some standard or is it calculated after the tank size?.

 

[IFRs]

The gap and gap variance depends a great deal on the manufacturer, tank size and loading. Cost is a major factor also. The floaters tend to be thicker at the outer edge for buoyancy and tend to get thinner towards the center so they will drain rainfall to a center location. There are often many compartments, braces and internal structures that you would want to avoid. The two and three way lap joints in the bottom plates are unlikely to be in the same locations as those on the to pdeck. I'm sure you want to pierce a single thickness of plate and remain at least 12" from any lap. At the bottom deck, a vapor tight connection is critical so the interior of the IFR does not exceed the LEL or be a future safety hazard. At the top deck, the connection should be liquid tight so rain does not flood the inside and vapor tight for similar reasons as the lower deck connection.