Employers (owners/operators and contractors), entry supervisors and qualified persons shall be aware of the poten- tial for failure or collapse of floating roofs including, but not limited to, excessive loading, improper leg pinning, stability, roof design, deterioration due to corrosion and seal design, and condition.
12.5.2 Excessive Loading
External floating roofs are typically designed for only 25 pounds per square foot, including both dead and live loads.
Internal floating roofs are designed to carry even less of a load.
12.5.2.1 Liquid on the Roof
A uniform layer of rain water on a floating roof, only 4 inches deep, provides a load of 25 psf. If the roof design includes the weight of the roof itself (dead load of 25 psf) plus a live load of 25 psf, it would not require much addi- tional liquid on the roof to increase the load significantly and cause collapse. To prevent an overload failure, entry supervi- sors shall not permit personnel to enter on top of or under a floating roof if a significant amount of liquid (water or prod- uct) is present.
12.5.2.2 Liquid in Pontoons or Compartments Entry supervisors shall assure that a qualified person checks pontoons and compartments for liquid product or water as the additional weight may cause spontaneous failure.
12.5.2.3 Repairs
Work on top of the roof may add excessive loads to the roof. In general, entry supervisors should not permit work to be conducted on top of and below the floating roof simulta- neously unless appropriate protection is provided.
12.5.3 Leg Pinning
Floating roofs whose legs are not pinned properly or are not pinned at the same relative position, will usually collapse upon landing. While a floating roof will tolerate some unpinned legs that carry no load, partial or total collapse will occur if too many legs are improperly pinned or misarranged in a particular configuration.
Figure 12-1—Example of Floating Roof Supports
Wiper seal Support leg pin Support leg sleeve Floating roof (pan)
Support leg (3 in. or 4 in. pipe)
Floating roof (pan)
Shell
Gauger's platform Anti-rotation pole 6 in. or 8 in. diameter pipe
Note 2
Note 1
Notes:
1. Lateral instability is the movement of the roof of about 6–12 in. due to the annular gap.
2. Rotation instability is the movement of the roof due to rotation about its axis of symmetry.
Figure 12-2—Examples of Floating Roof Support Legs
Applied force
Applied force
Support Leg in Pan Floating Roof
Support Leg in Pan Floating Roof
Support Leg in Pontoon or Double Deck Floating Roof
Low leg position High leg position
Support leg pin
Support leg (3 in. or 4 in. pipe) Support leg sleeve
Floating roof (pan)
Note: Deck is weak in bending and allows easy lateral deflection of the support leg.
Note: To deflect a leg in this section requires actual failure of the support leg by bending.
12.5.3.1 Pan Style Roof Legs
The legs on pan style roofs must be properly and securely pinned as they offer little or no bending resistance (e.g., the legs act as if they have a swivel joint at the attachment to the pan due to the flexibility of the single thickness deck).
12.5.3.2 Inspection
Before lowering the roof, a qualified person shall assure that legs are properly pinned and the steel cotter pins that hold the legs are not corroded.
12.5.4 Stability
The stability of the roof when it is landed depends upon the vertical support of the legs and the rotational resistance offered by the seals and the anti-rotation device.
12.5.4.1 Anti-Rotation Device
If the anti rotation pole or device is removed or fails (due to corrosion or weld failure), the roof will collapse with little or no external forces applied to it.
12.5.4.2 Repairs
Work on top of the roof may add excessive movement to the roof. Entry supervisors shall assure that back up systems are provided to stabilize the roof to prevent fail- ure in the event repairs require removing the anti rotation device for repairs, relocation, maintenance or for some other purpose.
12.5.4.3 Roof/Seal Combinations
Roof collapse may occur from the inherent instability of certain types of roof-seal combinations after the roof is landed.
1. The least stable combination of roof type and seal is a pan roof with a wiper type seal. The wiper type seal allows roof lateral movement and provides little or no resistance against rotation.
2. Other roof-seal combinations are less likely to fail.
12.5.5 Floating Roof Design
12.5.5.1 Annular Pontoon and Double Deck Roofs Floating roofs with annular pontoons or double decks are unlikely to collapse spontaneously by rotation, even without a guide pole.
12.5.5.2 Pan-Type Roofs
Employers (contractors) and entry supervisors shall be aware that pan type floating roofs are the least stable.
1. Internal pan type floating roofs with single wiper seals have a high potential for failure.
2. Some external pan and wiper seal floating roofs also have a high potential for failure.
12.5.6 Deterioration
Employers (owners/operators and contractors) and entry supervisors shall be aware that failures of floating roofs and legs from internal corrosion or welding cracks may occur.
12.5.6.1 Corrosion
Severe corrosion may occur in tanks that have been in cor- rosive service, including, but not limited to, crude oil, sour water and waste water tanks. Corrosion is not very likely to occur in refined product storage tanks. Legs pins should be checked for corrosion prior to setting legs and the roof deck, supports, pontoons and compartments checked for corrosion and cracking before lowering the roof.
12.5.6.2 Welds
Welds attaching the anti rotation devices or other critical components may have cracks. Some welding cracks are not readily noticed by visual inspection and will require a quali- fied person to use other approved means of identification.
12.5.7 Seals
Employers (contractors) and entry supervisors shall be aware that internal floating roof seals impact on the resistance of the roof to both lateral and rotational movement. Resis- tance to rotation and lateral movement is also reduced when seals are removed for repairs or replacement.
12.5.7.1 Shoe Seals
Shoe seals are designed to produce a positive outward radial force per unit of circumference that provides resistance of the roof to both lateral and rotational movement.
12.5.7.2 Log Seals
Log seals are basically large rings filled with vapor, liq- uid or polyurethane foam. New log seals provide resis- tance to both lateral and rotational stability. Qualified persons conducting inspections shall be aware that with service, these seals will eventually fail at the outer mem- brane and sag in sections around the circumference, losing all resistance.