API 650 Floating Roofs

Advance Tank has been in the above-ground welded steel storage tank industry for several decades. We regularly address API tank and design questions.  Many of these questions relate to API 650 Floating Roofs; how they work and what kind of accessories are most common.  We have put together this quick reference summary of floating roof styles and their accessories.

 

First, it is important to understand what a floating roof is and how it works inside of a tank. Floating roofs are designed to float on top of the product in the tank. The space between the tank shell and the floating roof is completely sealed with a floating roof seal which sits on the liquid within the tank.  The floating roof rises and falls with the level of the tank’s product.  In many cases the product inside the tank is petroleum based, like crude oil. There are support legs beneath the floating roof, which the roof rests on when the product level is low or emptied.  The purpose of a floating roof is to eliminate/minimize the vapors created by the product being stored to avoid emission loss and/or potential fire. The two main types of floating roofs designated by API 650 are external (Annex C) and internal (Annex H).

 

External Floating Roof (Annex C)

External Floating Roofs are used in open top tanks and serve as the tank’s primary roof.

STYLE

DESCRIPTION

API 650, twelfth edition, Addendum 2,  2016

Single Deck Pontoon The outer periphery of the roof consists of closed-top pontoon compartments, with the inner section of the roof constructed of a single deck without flotation means.

 

Double Deck The entire roof is constructed of closed-top flotation compartments.

 

 

 

Internal Floating Roof (Annex H)

Internal Floating Roofs are inside of a tank with a fixed roof.  The following summary shows each type of internal floating roof in order based on the cost.

 

STYLE

DESCRIPTION

API 650, twelfth edition, Addendum 2,  2016

EXPENSE

Metallic

(Skin & Pontoon-Aluminum)

 

Metallic internal floating roofs on floats have their deck above the liquid, supported by closed pontoon compartments for buoyancy. These roofs are not in full contact with the liquid surface and are typically constructed of aluminum alloys or stainless steel.

 

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Metallic Pan Metallic pan internal floating roofs has its peripheral rim above the liquid for buoyancy. These roofs are in full contact with the liquid surface and are typically constructed of steel.

 

$$
Metallic Open-Top Bulk-Headed

(Segmented Pan)

These internal floating roofs have peripheral open-top bulk-headed compartments for buoyancy.  Distributed open-top bulk-headed compartments shall be used as required. These roofs are in full contact with the liquid surface and are typically constructed of steel.

 

$$$
Metallic Pontoon

 

These internal floating roofs have peripheral closed-top bulk-headed compartments for buoyancy. Distributed open-top bulk-headed compartments shall be used as required.  These roofs are in full contact with the liquid surface and are typically constructed of steel.

 

$$$$
Metallic Double Deck Metallic double-deck internal floating roofs have continuous closed top and bottom decks which contain bulk-headed compartments for buoyancy. These roofs are in full contact with the liquid surface and are typically constructed of steel.

 

$$$$$
Sandwich-Panel/

Composite-Aluminum

 

Metallic sandwich-panel/composite internal floating roofs have metallic or composite material panel modules for buoyancy compartments. Panel modules may include a honeycomb or closed cell foam core; however, cell walls within the panel module are not considered “compartments” for purposes of inspection and design buoyancy requirements. These roofs are in full contact with the liquid surface and are typically constructed of aluminum alloys or Purchaser approved composite materials.

 

$$$$$
Hybrid (Aluminum/Stainless Steel)

 

Hybrid internal floating roofs shall, upon agreement between the Purchaser and Manufacturer, be a design combination of closed-top perimeter pontoon and open-top center compartments for buoyancy. These roofs are in full contact with the liquid surface and are typically constructed of steel.

 

$$$$$

 

ACCESSORIES

Each accessory serves a distinct purpose to support the success of the floating roof.  We have outlined each accessory based on what is minimally required by API 650, additional accessories for external and internal floating roofs, and optional accessories.

 

MINIMUM REQUIRED ACCESSORIES

Accessory

Description

Floating Roof Seal

(C.3.13 or H.4.4)

All floating roofs have a perimeter seal between the floating roof and the inside tank shell plates.  The Rim Space is the space between the tank shell and the floating roof outer rim plate which is normally 8”.

 

Support Legs All floating roofs, with the exception of some aluminum floating roofs, are supported off the tank bottom with pipe legs when there is no product present in the tank.  These legs are adjustable to two positions.  The high leg is usually 6’-6” high and intended for tank cleaning.  The low leg is usually 3’-6” and is intended for normal operation.

 

Deck Manhole

(1 minimum)

All floating roofs are to have a 24” manway in the deck plate to allow access to and from the tank bottom when the tank is empty and the floating roof is resting on its support legs.

 

Rim Vent

(1 minimum required)

A rim vent is installed in the outer rim to vent the air pressure in the rim space created when the tank is being emptied or filled.

 

Bleeder Vents

(2 minimum)

Bleeder vents are installed to allow air to flow when the tank is being emptied to avoid a vacuum being created while drawing out product in the leg supported position. To allow air to pass when filling and emptying the tank, the bleeder vent is set open 6” in both low and high leg positions.  The vent remains closed when the roof is floating.

 

Deck Drain Plugs

(2 minimum)

Deck drain plugs are provided in the deck to drain rain water off of the top of the deck during tank erection or to drain product off the top of the deck in the event the roof ever sinks.  The deck drain plug handle will always be extended to a vertical distance that is 24”, so that it is accessible if the deck is flooded.

 

Shell Manway Plugs Installed on all shell manways to keep the floating roof seal from being damaged when it comes down to the low operating level.
 

ADDITIONAL ACCESSORIES FOR EXTERNAL FLOATING ROOFS

Accessory

Description

Pontoon Hatch

 

Installed in each pontoon compartment to allow access for seal welding, weld testing and inspection purposes.  All pontoon hatches will be vented.

 

Primary Roof Drain Installed to drain rain water off the top of the deck.  The water runs to a center sump with a nozzle and check valve in the bottom of the sump. The sump has a drain hose or pipe assembly attached to the sump nozzle and carries the water down to a double flanged shell nozzle and out of the tank.

 

Emergency Deck Drains

(3 minimum)

Provided in a double deck floating roof to prevent sinking of the floating roof during severe rain fall.

 

Gaugers Platform and Rolling Roof Ladder Installed to allow access onto the floating roof from the top of the open top tank.
 

ADDITIONAL ACCESSORIES FOR INTERNAL FLOATING ROOF

Accessory

Description

Gauge/Anti-Rotation Pole All floating roofs have a means to keep the floating roof from rotating inside the tank and is referred to as an “anti-rotation” pole or cable. On internal floating roofs, it is normally a gauge pole ladder combination that also serves as access to the floating roof from the fixed tank roof.

 

Column Wells A column well and seal assembly are installed at each column.

 

Pontoon Hatch

 

Installed in each pontoon compartment to allow access for seal welding, weld testing and inspection purposes.  All pontoon hatches will be vented.
Center Roof Vent Center vent is installed in the fixed roof of the tank to allow air between the floating roof and the fixed roof tank to vent.

 

Periphery Roof Vents Installed in the fixed roof of the tank to allow air between the floating roof and the fixed roof to vent.

 

Gauge Pole Ladder Combination Installed to allow access onto the floating roof from the fixed roof. It is installed near the tank shell but far enough towards the center to fit the well seal assembly in a floating roof compartment and not interfere with the installation of the rim space seal.  The gauge pole ladder is accessed through a 36” x 36” roof hatch that is installed in the fixed roof.
 

OPTIONAL ACCESSORIES

Accessory

Description

Foam Systems A fire protection system.

 

Foam Dams May be included with the Foam System.  Can be bolted to the outer rim plate with the rim space seal or can be a fabricated plate ring with drain holes that is welded to the top of the deck or pontoon.

 

Mixers Side entry mixers.

 

Level indicators Attached directly to the floating roof or a float basket is installed so that the product level can be monitored to within 6” of the tank bottom.

 

Inlet diffusers Installed on the inside of the tank, attached to the inlet nozzle, to diffuse the product when being pumped into the tank.

 

 

Sources

API Standard 650, Welded Tanks for Oil Storage, Twelfth Edition, Addendum 2, 2016