Difference "Internal Floating Roof Tank", "Domed External Floating Roof Tank"

[Richard_V]

As I thought these two types of tank were the same, on this website they are presented as two different thing:
But this doesnt state the difference. Anybody who knows the difference?

Internal Floating Roof Tank
That tanks has both, a permanent fixed roof and a floating roof inside. There are two basic types of internal floating roof tanks:
tanks in which the fixed roof is supported by vertical columns within the tank
tanks with a selfsupporting fixed roof and no internal support columns
The fixed roof is not necessarily free of openings but does span the entire open plan area of the vessel. Fixed roof tanks that have been retrofitted to employ an internal floating roof are typically of the first type, while external floating roof tanks that have been converted to an internal floating roof tank typically have a self-supporting roof.
Tanks initially constructed with both a fixed roof and an internal floating roof may be of either type. An internal floating roof tank has both a permanently affixed roof and a roof that floats inside the tank on the liquid surface (contact deck) or is supported on pontoons several inches above the liquid surface (noncontact deck). The internal floating roof rises and falls with the liquid level.

Domed External Floating Roof Tank
Domed external floating roof tanks have the heavier type of deck used in external floating roof tanks as well as a fixed roof at the top of the shell like internal floating roof tanks. Domed external floating roof tanks usually result from retrofitting an external floating roof tank with a fixed roof.
As with the internal floating roof tanks, the function of the fixed roof is not to act as a vapor barrier, but to block the wind. The type of fixed roof most commonly used is a self supporting aluminum dome roof, which is of bolted construction. Like the internal floating roof tanks, these tanks are freely vented by circulation vents at the top of the fixed roof. The deck fittings and rim seals, however, are basically identical to those on external floating roof tanks.

 

[LittleInch]

They are fairly similar, but as your text explains, Internal floating roof tanks tend to be either designed that way from the start or have the internal roof retrofitted to an existing fixed roof tank. The floating roofs in this instance tend to be thinner and don't require any surface drains or design issues to do with rain, snow or wind loading.

Domed roofs with external floating roofs on the other hand tend generally to be initially designed as external floating roof, hence a more robust roof, allowing for rain, snow wind etc and then for some reason ( usually to do with water getting into the fluid) have a light weight roof added to remove the rain / snow / wind load off the roof.

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[racookpe1978]

As volatile "emissions" limits are more and more emphasized by the enviro's as they fight the energy businesses, the light outside roof becomes more valuable, right?

 

[LittleInch]

Not really no. The light weight roofs are not vapour retaining and usually free vented.

They tend to be used when either the roof drain is compromised and they get heavy rain / snow which sits on the roof and can sink it or have a high wind component ( though why this is a good idea for wind I'm not really sure).

The use of free vented tanks is generally not undertaken now for anything with a vapour pressure above about 10kPa, but then depends on the amount of cycles and "out breathing" as the tank fill up. Same thing when you're loading tankers ( road, rail or ship).

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IFRs]

LittleInch - I'm not sure where you are getting your information from - I must be mis-reading your post. Internal floating roofs are usually better in reducing emissions than the heavier ones, and the internal floating roofs are not freely vented. Converting an EFR to a Domed EFR immediatly reduces emissions because the wind effect goes away. Floating roof tanks are vented but the emissions from external floating roof tanks are generally much higher than for a domed EFR tank or an IFR tank. There is at least one aluminum internal floating roof design that has lower emissions than any steel internal or external floating roof ever made.

 

[LittleInch]

Floating roofs are much better at reducing emissions than free-vented tanks.

Internal floating roofs designed that way from the start can be lighter and have better seals, but what I meant was that the gap above the internal roof is free vented through the new fixed external roof.

Clearly if wind is an issue for an external roof then adding an external roof will reduce any emissions still further, but the key isn't the roof, but the type of primary and secondary seals used.

Floating roofs reduce vapour losses by >95% compared to fixed roof tanks. How much more IFRs improve this compared to EFR is able to be calculated using API 2517 and 2519, also the EPA issue various documents such as AP-42, but clearly we are working with small improvements.

The original OP just asked a fairly general basic question so I responded in the same general lines.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.