Deck-Seam Loss Factors — Are We Using the Right Physical Model for Modern Full-Contact IFRs?

That comparison overlooks a more fundamental issue: the two roof concepts control emissions through different physical mechanisms. A non-contact roof contains vapour that has already formed beneath a deck suspended above the liquid. A full-contact roof is intended to remove the continuous vapour space beneath the deck and therefore reduce the area from which vapour can form. The importance of a seam is consequently not determined only by a loss factor per unit seam length; it also depends on the liquid area capable of feeding that seam, the actual roof geometry, and whether the installed roof is truly full-contact in service.

The project source material also reports modern panel-to-panel full-contact seam values around 0.0081 and 0.0063, far below the generic historical contact value. The purpose of this paper is not to declare standards “wrong”, nor to claim that every full-contact roof outperforms every non-contact roof. It is to ask a narrower and more important engineering question: whether a loss-factor framework developed from an earlier generation of roof technology is sufficiently representative when applied to modern full-contact designs.

Central thesis: A methodology can remain mathematically consistent while becoming physically less representative when the technology to which it is applied has changed.

1. The question behind the number

Emission calculations need standardised inputs. Deck-seam loss factors fulfil that role by expressing a loss rate per unit length of deck seam. The attraction is obvious: the factor is measurable, comparable and can be incorporated into a repeatable calculation. The difficulty begins when the factor is treated as if it were a complete measure of internal floating roof efficiency.

It is not. A seam is a pathway. Emissions through that pathway still depend on what vapour source exists beneath or adjacent to it. Two roofs can have the same total seam length and even the same measured seam factor while presenting very different evaporative conditions. If one roof has a continuous vapour space beneath almost the entire deck and another has liquid in direct contact with almost the entire underside of the deck, the physical source feeding the seam is different.

The central question is therefore not simply “what is the Kd value?” but “what physical situation does that Kd value represent?”

2. Two roof concepts, two emission mechanisms

2.1 Non-contact roofs: contain vapour after it has formed

In a conventional skin-and-pontoon or other non-contact IFR, the deck skin is supported above the product. A continuous or near-continuous vapour space exists beneath the deck. Product evaporates into that space until the local vapour approaches equilibrium. The roof reduces emissions by containing that vapour and limiting exchange with the tank vapour space above the roof.

When vapour escapes through seams or openings, the vapour space is replenished by further evaporation from the liquid surface below. The seam therefore connects to an evaporative source that may extend over a large area of the tank.

2.2 Full-contact roofs: reduce vapour formation at source

A true full-contact roof works differently. The deck is intended to remain in direct contact with the product over essentially the whole usable deck area. That removes the continuous vapour space beneath the roof. Vapour formation is then concentrated at local discontinuities: panel seams, fittings, penetrations, rim areas and any portions of the roof that are not actually in contact with the liquid.

This does not make a full-contact roof emission-free. It changes the geometry of the source.

3. What the historical data actually says

The source paper reproduces the following deck-seam test results from API MPMS Chapter 19.2 Appendix I:

Deck category	Test assembly	Reported Kd (lb-mol/ft-yr)
Non-contact	20-ft test tank	0.12
Non-contact	Large test cells – 2 seams per pan	0.053
Non-contact	Large test cells – 6 seams per pan	0.030
Non-contact	Small test cells – 5 seams per pan	0.011
Non-contact	Average reported	0.054
Contact	20-ft test tank	0.57
Contact	Large test cells – 6 seams per pan	0.13
Contact	Small test cells – 5 seams per pan	0.11
Contact	Private manufacturer test – 1 seam	0.12
Contact	Average reported	0.23

On these averages, the generic contact value is approximately 4.3 times the generic non-contact value. That result is valid as a summary of the referenced tests. It is not, by itself, proof that a complete contact roof emits more than a complete non-contact roof.

3.1 The historical context matters

The original paper places the development of deck-seam loss factors in the early 1990s, when non-contact aluminium IFRs were prevalent and modern panel-to-panel full-contact technology was still emerging. Key test work referenced in the source was undertaken between 1994 and 1996, followed by a testing-protocol report in 1999.

That history does not invalidate the work. It defines its technological context.

What this paper does not claim: There is no basis in the source material to attribute the historical factors to anti-competitive intent. The technical criticism stands without that claim.

4. Why a seam factor can mislead when the physical model changes

The problem is easiest to see by separating three quantities: (1) the amount of liquid surface able to generate vapour, (2) the path available for vapour to escape, and (3) the resistance or tightness of that path.

For a non-contact roof, the first quantity can approach the full tank surface area beneath the suspended deck. For a panel-to-panel full-contact roof, the first quantity is intended to be limited to very narrow seam interfaces and discrete fittings. The seam loss factor primarily characterises the third quantity. It cannot, on its own, represent the difference in the first quantity.

The original source paper illustrates this with a 36 m gasoline tank: the difference between two modern panel-to-panel seam factors of approximately 0.008 and 0.006 corresponds to only about 20 kg/year, whereas a comparable seam-factor difference applied to a non-contact configuration was calculated to have an effect roughly 90 times larger under similar assumptions.

5. “Full contact” must be a physical condition, not a label

Not every roof marketed or classified broadly as full-contact eliminates the vapour space to the same degree. Project design-review material includes an example in which substantial rim and structural areas were not in contact with the liquid; the review estimated the non-contact portion at approximately 17% and concluded that the roof should not be treated as full-contact for emission-calculation purposes.

A useful classification should therefore describe the mechanism: how much continuous vapour space exists beneath the deck, where exposed liquid remains, and how those exposed areas communicate with seams or fittings.

6. Technology has moved faster than the generic factor

The source paper reports later measured deck-seam loss factors of approximately 0.0081 for a panel-to-panel roof tested in 2006 and 0.0063 for a later design tested in 2022. Both values are far below the historical generic contact average of 0.23. Numerically, 0.23 is about 28 times 0.0081 and about 36 times 0.0063.

Those ratios should not be used carelessly: the tests were not necessarily identical in configuration, protocol, workmanship or boundary conditions. They nevertheless show that modern full-contact seam technology can occupy a performance range remote from the historical generic contact factor.

7. Laboratory Kd versus installed performance

A deck-seam test intentionally isolates the joint so that a repeatable loss factor can be measured. A tank installation is different. Thousands of metres of joints may be assembled under variable access, tolerances and field conditions. A design that needs perfect caulking to reproduce its certified value therefore has a different field-performance risk from a design in which sealing pressure is generated mechanically along the seam.

A laboratory Kd is best understood as an initial performance characteristic of a joint under defined conditions, not as a lifetime guarantee for the installed roof.

8. A more representative assessment framework

1. Classify the roof by physical mechanism. Is the deck genuinely full-contact, partially contact or non-contact in service?
2. Quantify non-contact and exposed liquid areas. Include rim zones, girders, wells and fittings.
3. Characterise the actual seam geometry. Panel-to-panel, girder-supported, gasketed, mechanically compressed, caulk-dependent or welded.
4. Use representative seam data. Where a tested Kd exists for the actual supplied joint, use it where the governing method permits.
5. Assess field reproducibility. Review tolerances, assembly, accessibility, workmanship sensitivity and inspection.
6. Treat rim and fittings separately. Deck seams are only one loss pathway.
7. Verify in service. Link design performance to commissioning, inspection and condition.

9. Implications for standards, BAT and procurement

Generic values are necessary defaults. The problem arises when a default becomes a proxy for technical merit after the underlying products have diverged from the population on which that default was based.

For BAT assessment and procurement, a purchaser should therefore ask whether the tested joint represents the supplied roof, whether the roof is genuinely full-contact over the relevant area, how the seam is sealed, how terminations are treated, how much non-contact area remains, and how the result can be verified after installation.

The strongest evidence chain is:

Representative test method -> representative joint -> constructible detail -> verified installation -> maintained in-service condition.

10. Conclusion

Deck-seam loss factors remain useful. The technical problem is not the existence of the factor; it is the assumption that a factor developed from one generation of roof technology can always be transferred without loss of meaning to another.

The historical dataset gives the contact-deck category a materially higher generic Kd than the non-contact category. That result can be valid for the tested seams and still be a poor basis for ranking complete roof concepts, because full-contact and non-contact roofs do not present the same vapour source to those seams.

Modern panel-to-panel technology further widens the gap between the historical generic category and current practice. The right response is not to dismiss established calculation methods. It is to use them with engineering context: roof architecture, actual contact area, seam geometry, representative test data, constructability and lifecycle condition should be considered together.

The question for the industry: Are we comparing modern full-contact IFRs with a model that represents how they actually control vapour – or with a legacy category that survives because it is easy to calculate?

Source notes

1. Robin Essenius, Deck seam loss factors and deck seam design for full contact roofs – Backgrounds and explanations, 1 June 2024.
2. API MPMS Chapter 19.2, Appendix I, Fourth Edition (2020), as reproduced and discussed in source 1.
3. Chicago Bridge & Iron Technical Services Company, Loss Factor Measurements of Internal Floating Roof Deck Seams Using the Weight Loss Test Method, Interim Reports 1-7, 1994-1996.
4. U.S. EPA, Emission Factor Documentation for AP-42 Section 7.1 Organic Liquid Storage Tanks, Final Report, September 2006.
5. Petrex, Inc., Test for Vapor Loss Through Clamp Bars Used on Petrex Internal Floating Roof System, March 5, 1984.
6. Chicago Bridge & Iron Company, Development of the Testing Protocol for the Measurement of Deck Seam Loss Factors Using the Fugitive Emission Test Method, Final Report, September 14, 1999.
7. EEMUA 213, Appendix A (2015 edition), cited in source 1.
8. Emission Zero project design-review material on full-contact classification, non-contact area and seam geometry.