Floating-Roof Tanks Design and Operation

Floating-Roof Tanks Design And Operation

In The Petroleum Industry

Prepared by: Terry A. Gallagher

Chicago Bridge & Iron Company

Presented at: Independent Liquid Terminals Association (ILTA) 20th Annual International Operating Conference

Bulk Liquid Transfer and Aboveground Storage Tank Terminals June 12 15, 2000 Houston, Texas

CB&I Technical Publication Number: CBT - 5650

Abstract

The floating-roof tank has been the most widely used method of storage of volatile petroleum products since the first demonstration by Chicago Bridge & Iron Company

(CB&I) in 1923. There have been many changes and design improvements to that first pan-style-floating roof.

A floating roof is a complex structure. It must be designed to remain buoyant even when exposed to combined loads from varying process, weather and product

conditions. There is a continued demand for improved floating-roof tanks to store a

Table of Contents1.0 Introduction 12.0 Why Use a Floating Roof? 22.1 Product Evaporative Loss Control 22.2 Safety 4 3.0 Floating Roof Applications 53.1 Floating-Roof Design 53.2 Internal / External Floating-Roof Design 6

3.2.1 Single-Deck Roofs 73.2.2 Double-Deck Roofs 103.2.3 Internal Floating-Roof Options 113.2.4 Light-Weight Internal Floating-Roofs 124.0 Floating-Roof Selection 134.1 Product Considerations 14

4.1.1 Product Composition 14

4.1.2Product Specific Gravity and True Vapor

Pressure 14

4.1.3 Heavy, Waxy Crude Products 154.2 Process Considerations 16

5.0 Technical Developments-Why Are They Important

16

6.0 Maintenance and Life Cycle Cost Control 177.0 Conclusions 19

Sayfa 1 / 15Chicago Bridge & Iron Company / Aboveground Storage Systems

22.09.2002http://www.chicago-bridge.com/designop.html

wide range of petroleum and petrochemical products in compliance with state and federal environmental regulations. Floating roofs are used in open top tanks (EFRT),

inside tanks with fixed roofs (IFRT), or in tanks that are totally closed where no product evaporative losses are permitted for release to the atmosphere. This very special type

of installation is referred to as a zero emission storage tank (ZEST). Products that might have been stored in basic fixed roof tanks must now utilize a floating roof to limit

evaporative emissions to the atmosphere. High vapor pressure condensate service and blended heavy crude oils also present new design challenges to the floating roof

tank industry.

This paper will review the most prominent styles of floating roofs from 1923 to the present. Design and operating limits for current day floating-roof structures are

presented. New trends in environmental regulations and the potential impact on the design and operation of floating-roof tanks will be presented. Current maintenance

practices and the effect on Life Cycle Cost Management of the storage system are also reviewed.

1.0 Introduction

The floating-roof tank is the most widely used storage method to safely control product evaporative emissions from volatile petroleum products. Literally thousands of floating-roofs are in operation throughout the world, some having been in continuous operation

since first constructed over 50 years ago. Many floating-roof tanks far exceed the service life of the owners, operators and maintenance personnel responsible for safe keeping of many refinery and terminal operations. Herein lies a basic problem we will address in this paper - the ability to maintain a working knowledge of the floating-roof

tank, associated equipment and operating systems common to the petroleum industry. This paper presents an introduction into the design of the most prominent styles of

floating roofs from 1923 to the present. What are the important features of each type of floating roof and why is one design better suited to a certain set of operating

conditions? Practical operating and maintenance practices are reviewed. Proper selection of the floating-roof type can reduce the potential risk of a problem during

every day operations.

Development and improvement of the floating-roof has generally been in response to changing owner requirements, recognized design codes or federal and state

environmental regulations. The floating roof was originally developed for use in open top tanks (EFRT). As product, weather and environmental concerns became more of

an issue, floating-roof designs were adapted for use in a tank with a fixed cover (IFRT). The most advanced type of installation is used in applications that require absolute

control of all emissions from the storage tank. A ZEST installation contains all product vapors while maintaining the tank within safe operating limits.

Developments of new equipment details have reduced the overall environmental impact from a floating-roof tank. Recent equipment developments and why they are important from a regulatory compliance standpoint are presented. A basic life cycle

cost analysis of typical floating roof tanks is presented. Floating -roof tanks remain the safest, most cost effective method of storing petroleum and many petrochemical

products.

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2.0 Why Use a Floating Roof?

The basic reasons to use a floating-roof have not changed in over 75 years. Safety, effectiveness and economy are the reasons floating roof tanks remain the worldwide

"standard" for storage of volatile petroleum and chemical products. Although significant



advancements have been made since the first successful tests in 1923, the basic principles remain unchanged. Floating roofs should be designed for full liquid contact in order to minimize evaporation and reduce product side corrosion. Another benefit from reduced evaporation is improved fire safety. Each floating roof must include features

designed to accept the full range of roof movement, keep product emissions to a minimum, and provide an extended service life with minimal maintenance.

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2.1 Product Evaporative Loss Control

Product evaporative loss is not only a significant environmental and economic concern to an owner but is a major consideration when improving storage safety. Four

conditions that affect product evaporation from a storage tank are:

1. Liquid temperature 2. Vapor space above liquid

3. Ventilation of the vapor space 4. Available liquid surface area

In general, for any liquid stored in a container with an open vapor space above the liquid surface, a portion of that liquid will evaporate and the vapor migrate past the liquid surface to the vapor space. The process will continue until the vapor space becomes saturated with product vapor. Once equilibrium is established no further evaporation will occur unless one of the above conditions are altered. Saturation

conditions are specific to the product stored (liquid composition) and to its temperature. Since petroleum product is generally a mixture of different hydrocarbons the

evaporation process is slightly more complicated. Lighter, more volatile hydrocarbons (those with the highest vapor pressure) will evaporate first. The process will continue until saturation conditions for the specific hydrocarbon mixture is achieved. Saturation properties for any known hydrocarbon mixture may be calculated and used to predict

evaporation rates at different storage conditions. How will each condition affect the loss control performance of a storage tank? Consider a conventional Fixed Roof Tank

(FRT) shown in Figure 1 and an EFRT shown in Figure 2.

When a volatile product is stored in a freely ventilated fixed-roof tank (Figure 1), the concentration of volatile vapors in the vapor space will vary depending on tank

operating conditions. During holding periods, when no liquid is added or removed the vapor space will come to equilibrium based on product temperature and vapor

pressure. Emissions during holding are generated by the vapor space breathing process. As a result of daily ambient heating and cooling processes, the air-vapor

mixture in the vapor space expands and contracts. During the daily heating process, some of the air-vapor mixture is expelled from the tank, resulting in evaporative

emissions. During product cooling, air is drawn into the vapor space that helps to dilute the concentration. This initiates further evaporation that continues until the space again

reaches equilibrium. Emissions of volatile products due to breathing are a naturally occurring process wherever there is a defined vapor space[1][2].

Normal tank filling and send out operations also effect the vapor space of a fixed-roof tank. When product is removed from the tank air is drawn into the vapor space. Unless



the tank is completely emptied, the air in the new, larger vapor space will become saturated with product vapors. During the holding period before the next tank filling

operation, evaporative breathing losses will increase due to the increased volume of the vapor space. When product is added to the tank, the increasing liquid volume

displaces the air -vapor mixture through the tank vent, resulting in significant evaporative emissions. Tank filling operations produce more evaporative emissions

than any other tank operation in a fixed-roof tank.

If the same volatile product is stored in an EFRT (Figure 2), an analysis of evaporative emissions will produce significantly different results. Addition of a welded-steel floating

roof will significantly reduce evaporative emissions and, as a result, greatly improve operating safety. Evaporative emissions are reduced by covering as much of the liquid surface as possible without effecting the mechanical operation of the floating roof. In general the floating roof covers the entire liquid surface except for a small perimeter

rim space. Under normal floating conditions, there should be no vapor space underneath a properly designed floating roof.

Evaporative emissions, although reduced in excess of 98%, can not be entirely eliminated. Evaporation and associated product losses will still occur from the rim

space, from standard roof deck fittings, from product which remains on the tank shell, and from tank operations that require the tank to be emptied and the floating roof

landed on its supports for maintenance purposes.

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2.2 Safety

Crude and refined petroleum products are volatile in nature and will readily evaporate at normal storage and handling conditions producing vapors that are combustible

(explosive) over a range of concentrations with air. It has been shown that the addition of a welded-steel floating roof to an open top tank can reduce the evaporative

emissions by more than 98%[2]. Properly designed, the floating roof, floating-roof seals and floating roof deck fittings can control the quantity and release of product emissions

to the environment. Operating experience and independent studies of the air flow in and around an external floating-roof tank have shown that the risk of a combustible mixture of product vapors and air existing above a floating roof can be effectively

controlled during normal tank operations. General operating guidelines and suggested safe operating limits of floating-roof tanks are covered in Section 5.

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3.0 Floating Roof Applications

The use of a floating-roof tank is a preferred method of storage for any petroleum liquid with a true vapor pressure (TVP) greater than or equal to 1.5 psia (78 mm Hg) but not

greater than 11.1 psia (570 mm Hg). This criteria is documented by the US Environmental Protection Agency (USEPA) in 40CFR60.110, Subparts K, Ka, and Kb.

Local regulatory agencies may follow these guidelines or may elect to follow more stringent criteria. As the vapor pressure increases the evaporative emissions will

increase proportionally. For more information on evaporative emissions from floating-roof tanks refer to the American Petroleum Institute (API) publication "Evaporative Loss

From Floating -Roof Tanks," Chapter 19.2 of the Manual of Petroleum Measurement Standards [2].

Floating-roof tank configurations most commonly used are the external floating-roof tank (EFRT) and the internal floating-roof tank (IFRT). Special applications that permit no emissions to the atmosphere may use a modified internal floating -roof tank design the zero emission storage tank (ZEST). Factors that should be considered when selecting the tank configuration are product characteristics, site weather conditions,

process operating requirements, storage capacity and required throughput. The largest capacity floating-roof tanks are generally EFR designs, are used primarily for storage

of refinery feedstock and may have capacities up to 1.5MMBbl or more in a single tank.



The IFR options may be used when product quality must be tightly controlled, where tighter control is warranted due to personnel exposure limitations or in locations where

weather conditions may warrant the added protection provided by a fixed roof.

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3.1 Floating-Roof Design

A floating-roof design can range from a very simple pan roof to the complicated structure of a double deck floating-roof. What design is selected depends on many of

the same factors used previously to select the tank configuration product characteristics, site weather conditions, system operating requirements, storage

capacity and required through-put. The first floating-roof designs were stiffened pans similar to that shown in Figure 3. In 1923 when the first floating-roof was tested the only application was as an external floating-roof. The roof deck was sloped to the

center for water drainage and to permit vapors to pass from underneath the deck to the perimeter rim space. The need for improved rim seals, roof drains, manual and

automatic bleeder vents, rolling ladders, and other tank equipment has never stopped.

As with any new concept, the evolution of the floating-roof has included many different designs. Some have worked very well while others have quickly failed when exposed to the rigors of normal floating roof operations. Eventually minimum requirements for

the design and construction of EFRTs and IFRTs were developed and documented in Appendices C and H of API Standard 650, Welded Steel Tanks for Oil Storage[3].

Similar requirements are included in British Standard BS2654[4].

Floating roofs are complex structures to design, analyze and construct. Even with advanced finite element analysis techniques, design of a floating-roof structure may still require a rigorous analysis. Many designs were developed before the computer. Final designs were verified by full-scale tests that included strain gage and deflection measurements while the roof was subjected to the design loading conditions specified

in API 650 Appendix C. Designs may now be completed using computer analysis programs developed and verified with data obtained from the original field test

programs.

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3.2 Internal / External Floating-Roof Design

Design conditions for an external floating roof differ significantly from those used to design an internal floating roof. A major difference is the added requirement that the

external floating roof design must accommodate loads due to varying weather conditions. Other differences include floating roof access and the design of product emission control hardware. However, many of the basic design principles remain

unchanged.

l Floating roof must be designed to remain buoyant (floating) on liquid with a specific gravity of 0.7 under specified design operating conditions.

l Floating roofs should maintain full liquid contact to minimize evaporation and reduce product side corrosion.

l Floating roof and its accessories must be designed to accept the full range of roof movements during unattended operation from low operating level to the



maximum design liquid level. l Floating roof should include features that keep product emissions to a minimum. l Floating roof should include features designed to provide an extended service

life with minimal maintenance. API 650, Appendix C also requires that an external floating roof must have sufficient buoyancy to remain floating on liquid

with a specific gravity of 0.7 when loaded as described below. l The single-deck roof drain is inoperative and roof exposed to 10" (250mm)

rainfall in 24 hours. Double-deck roofs may be designed for a lesser amount provided the roof is equipped with emergency roof drains to keep water

accumulation to a lesser volume than the roof will support. l The single-deck and any two adjacent pontoon compartments punctured in

single-deck pontoon roofs and any two adjacent compartments punctured in a double-deck roof. It is assumed that no live load or water load exists for either

case.

The physical size and number of pontoons are adjusted depending on the overall diameter of the floating roof. During the early design years, field tests were completed

on various sizes of external floating roofs to demonstrate that the final design configuration would meet all API loading conditions. Experience has shown that full scale testing is the best method available to learn how one roof design will perform

versus another.

Two different types of external floating roof will be presented the single (or center) deck floating roof and the double-deck floating roof. Variations of these designs used as internal floating roofs and several alternative internal floating roof designs are also

presented.

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3.2.1 Single-Deck Roofs

Single-deck floating roofs include any welded steel floating roof comprised of a single center deck surrounded by a series of liquid-tight pontoons. Many different versions of

a single-deck roof were developed and used since the first external pan roof was introduced in 1923. A brief review of some of the more prominent designs will help

demonstrate what design features are most important.

Steel Pan External Floater (1923 1927) As with any new concept, the pan external floating roof did have its problems. A major issue that encouraged a new design was excessive product evaporation due to product heating under the deck

plates. Also since there was no positive buoyancy provided with the pan roof, excessive rainfall or any product leaks resulting from poor riveting or welding could sink the roof. This was the first full contact floating roof and first use of rim vents.

Day Type Floating Roof (1929 1950) This design answered many of the previous problems and proved to be a very successful design. Day Type roofs may still be found in service today, some having survived for over 55 years. This design is characterized by large sloped pontoon with an octagonal inner rim. With the pontoon covering more than 50% of the available roof area, sufficient buoyancy is provided so that the deck could be equipped with open emergency overflow drains. The pontoon bottom was

sloped up to the membrane-like center deck. Product vapors produced during the heat of the day would be contained under the deck by the pontoon ring until the cooler

temperatures of the evening would condense these vapors. The Day Type roof was the first design to effectively control product evaporative losses during all tank operations.

The only down side to this design was the added cost associated with the large pontoon.



Figure 4 Day Type floating roof during repairs in 1994

High Deck Floating Roof (1940 1950) This design was developed as a cost-effective alternative to the Day Type roof. A review of the design shows that it included design concepts from each of the three previous designs. The center deck was sloped

towards a large center pontoon with additional support provided by an outer ring of pontoons. Consequently there were areas under the center deck that were always

exposed to product vapors which caused an increase in product side corrosion of the center deck. The size of the center pontoon also interfered with deck drainage

increasing the risk for stability problems. Similar to the Day Type roof, examples of the high deck design can still be found in service today. However many of these roofs have

been replaced due to repeated corrosion and drainage problems.

Horton Clear Deck Floating Roof (1946 1950) The Clear Deck roof was developed as an alternative to the high deck roof design. This design did provide

improved drainage however the vapor-mounted design significantly increased corrosion problems. Not many of this roof design were ever built.

LODEK, Center-Weighted, and Everflote Floating Roofs Some other single-deck floating roof designs were the LODEK, the Center-Weighted Pontoon, and the

Everflote roof. The LODEK roof is similar to the high deck roof in that it includes a relatively large center pontoon except that the pontoon configuration is to keep the center deck in full contact with the product. Although product side corrosion was

reduced there were typically drainage problems around the center pontoon.

The Center-Weighted roof is effectively a pan roof with small annular pontoons and a center deck-mounted container filled with sand. Sufficient weight is added to establish a slope to the drain sump located on one side of the center weight. Original designs

experienced the same previous drainage problems. Later versions of this design relocated the weights to underneath the deck in an attempt to reduce drainage

problems.

The last type of single-deck roof to discuss is the Everflote design. This roof included annular pontoons plus multiple rows of half-round buoyancy chambers welded to the

center deck plates. This design also experienced drainage problems and surface corrosion due to water ponding around many of the buoyancy chambers.

Horton Type 5 Pontoon Floating Roof (1954 ) The Type 5 roof represented the first design that was optimized based on specific API design criteria[3]. The design

provides a full contact, single-deck roof with the pontoon size and configuration proportioned to support a 25 Lb/Ft2 snow load and a rain load equivalent to 10" of

rainfall over the area of the deck. Differences between the new Type 5 roof and the older Day Type design is the smaller pontoon size and corresponding increase in

center deck diameter. The increase in center deck improved the water storage capacity but eliminated the use of open emergency drains and magnified problems of

intermittent water puddles on the single deck after a rain. Consistent product loss control performance, excellent mechanical stability and a cost effective design made the Type 5 roof the most widely used external floating roof by the petroleum industry.



Figure 5 Large Diameter Type 5 Floating Roof during construction

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3.2.2 Double-Deck Roofs

The Horton Double-Deck Floating Roof was first introduced in 1946. The double-deck roof offered all of the good design features but at an added cost. The design provides excellent primary drainage while inexpensive emergency drains limit the total water load to the roof. The double-deck roof introduced the automatic bleeder vent to the

industry, eliminating the need for an operator to physically open or close a manual vent when the roof is floated off its supports or landed and the tank emptied. A typical

double-deck configuration is shown in Figure 6 below.

Figure 6 Horton Double Deck Floating Roof

Although the structure appears very stiff the center bays provide sufficient flexibility to contain some quantity of product vapors similar to the Type 5 roof. The amount of vapor produced is less than with a single-deck roof since the trapped volume of air

between the upper and lower decks acts as an insulating layer to the product. A properly designed double deck roof can provide improved stability with high vapor pressure products. Deck distortion is minimized with the double deck design even if vapor is captured beneath the bottom deck. Drainage characteristics are effectively

unchanged. Design and construction details used in double-deck floating roofs are very important. As with the variations in designs presented for single -deck roofs, there are different manufacturer details that can make a difference in the mechanical stability

and overall performance of a double-deck floating roof. Details can make a difference.

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3.2.3 Internal Floating Roof Options

The design and operation of an internal floating-roof tank can be considerably different from a similar application using an external floating-roof tank. In general the design guidelines for external floating-roofs also apply to the internal floating-roof. Specific

differences in design criteria are presented in API 650 Appendix H Internal Floating-Roofs[3].

Because many of the design conditions are more controlled inside a fixed roof tank,



more design options are available with the internal floating-roof. Ambient conditions, wind, rain, snow, blowing sand and the associated loading conditions no longer apply.

For this reason internal floating roofs are not only available in conventional welded steel construction but may be constructed from light weight aluminum or even

composite fiberglass construction. Specific operating recommendations for lightweight floating roofs will be presented in Section 3.2.4.

External floating-roofs can (and have) been used as internal floating-roofs. In general this occurs when a decision has been made to add a fixed roof to an existing external floating-roof tank. For new construction the internal floating roof is designed as such and will have slight differences in construction details in response to reduced loading

conditions. The most common internal floating-roof designs are presented in the following sections.

Open Pan Design Generally considered the lowest cost and most widely used welded-steel roof for internal applications. This type of roof is well suited for products with low vapor pressures or at least for products where the vapor pressure does not vary much with ambient conditions. A pan roof should not be selected if excessively

high fill rates are predicted, if extensive blending operations are expected or if there is a possibility of product vapors collecting underneath the deck. Any of the above

conditions may lead to an unbalanced floating condition where product liquid can overflow on to the deck, ultimately sinking the roof.

Bulk-headed Pan Design This design provides additional stability over a basic open pan design. The outer bay is divided into open top compartments by bulkheads

located between the inner and outer rims. This roof will still respond in a similar manner to the conditions described for the basic pan roof but will provide improved

floatation characteristics if product should overflow onto the roof.

Pontoon Roof Design Several pontoon floating-roof configurations are commonly used. The designs use either a flat bottom pontoon or a modified Type 5 pontoon roof. The pontoon roof can provide improved floatation characteristics over a standard pan

or bulk-headed pan since the outer ring of pontoons are fully covered and can be made liquid tight. Should product overflow on to the roof deck, it will collect on the center

deck resulting in a more uniform displacement of the roof. Although more stable than the basic pan roof design, a flat pontoon may still become unstable if product vapors

are produced underneath the deck.

Double-Deck Roof Design Double -deck designs are rarely used for internal floating-roof applications. Double-deck designs, however, are in operation as internal floating roofs. Generally these are cases in which a fixed roof has been added to an

existing external floating -roof tank.

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3.2.4 Light-Weight Internal Floating-Roofs

Lightweight internal floating roofs were developed as an alternative to welded-steel construction, especially for situations where an owner wanted to add an internal

floating roof to an existing fixed roof tank. In the United States, the most common lightweight internal floating roof is constructed from aluminum. There are also

lightweight roof designs that use fiberglass materials. A fiberglass roof is generally a full contact design and has been used in some overseas locations for chemical storage

applications where metal materials are not compatible with the stored products.

Lightweight aluminum roofs are available in two basic configurations. They are either designed for full liquid contact or as a non-contact (vapor mounted) roof. Non- contact

designs generally include a lightweight aluminum deck supported by a structural framework with a series of tubular pontoons. A typical pontoon configuration will form a

vapor space approximately 6" deep below the entire floating roof. The framework includes an outer rim that must extend into the product to seal off the large vapor

space underneath the bolted aluminum decking. See Figure 7.



Figure 7 Vapor Mounted Aluminum Internal Floater

Full contact aluminum internal floating roofs are generally constructed from a series of panels that secured by an aluminum framework and surrounded by an outer structural rim. The panels can be either expanded aluminum honeycomb or may be a foam core

that is sandwiched between two layers of aluminum sheet. As with welded steel floating-roofs there are numerous versions of both contact and non-contact designs available. Joint and seam details are an important feature of any lightweight bolted

floating roof. Tests sponsored by the API have shown that all bolted seams provide a leak path for vapors to escape to the tank vapor space above the floating roof, leading

to increased product losses when compared to a welded structure[2].

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4.0 Floating-Roof Selection

Petroleum storage tanks serve many purposes in a refinery, marine terminal or pipeline terminal environment. Tank operations, the type and quality of product being stored,

and site specific conditions must be considered when you are selecting a floating-roof design. The way a tank is operated can effect future maintenance required for the tank,

the floating-roof and its associated equipment. Proper selection of the floating-roof design and associated equipment can ensure an excellent return on the investment or

it can provide a costly maintenance headache for many years.

Mechanical stability of the floating roof is probably the most important parameter of any floating roof. Any load acting on the floating roof that is not balanced will force the roof to float over to one side of the tank or in the most severe cases; the roof will tilt due to unbalanced loads. When a roof is operated under unbalanced loads there is increased

risk that conditions could develop that would sink the roof.

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4.1 Product Considerations

Floating-roof storage tanks are used to store everything from the heaviest crude oil imaginable to light, extremely volatile refined liquids produced in the refinery. The main product factors that must be reviewed are product composition and chemical stability, product specific gravity, product true vapor pressure at storage conditions, and with

crude oils the product viscosity and wax content must also be considered. Most recently there are new crude oils that are modified at the well or mine site to ensure the

end product can be safely transported to the refinery for processing. There are increased risks associated with the storage and handling of these modified or blended

crude products.

All product characteristics can be effected by site ambient conditions. It is important that variations in ambient temperature be considered when reviewing the potential

design impact on product true vapor pressure and product viscosity. How does each factor impact the selection of the floating roof?

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4.1.1 Product Composition

Product composition is important because of the potential effects certain chemicals will have on the service life of some of the equipment associated with the floating roof. The

greatest potential for a problem is with non-metallic materials used with the floating-roof perimeter seals. Some instrumentation may be effected depending on installation details. There are chemicals that can destroy commonly used seal materials after only

a few weeks exposure. Optional materials are available that can be used with all known products.

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4.1.2 Product Specific Gravity and True vapor Pressure

In most cases floating-roofs are designed for a specific gravity of 0.7. It is unusual to encounter a product with a lighter specific gravity but it can happen. It is important that the roof designer know what the minimum specific gravity is since this will determine

how far into the product a given roof will float. The distance from the product surface to the top of the floating roof deck is one measure of overall roof stability. This information

also is used to check the relative position of the rim seal with respect to the liquid surface. For example if a mechanical shoe seal is used it is suggested that the shoe extend into the product approximately 4" to ensure the shoe maintains a liquid seal

over all expected roof movements.

Product true vapor pressure (TVP) is the single most critical design parameter when selecting the type of floating roof. Most current environmental regulations limit the product TVP to less than 11.1 Psia. As the TVP increases above 11 12 Psia, daily heating of the product underneath a center deck will produce sufficient vapors to

balloon the deck. It is common for these vapors to condense during the cooler evening hours, allowing the roof to resume a normal flat shape. It must be emphasized,

however, that if the tank is in an area of significant rainfall, ballooning of a single deck roof may not permit normal water drainage to the primary roof drain. An unbalanced

load can quickly be developed that can sink the floating roof.

A secondary consideration with high vapor pressure products is that with increasing TVP the overall effectiveness of any floating roof design is reduced. More evaporation

will occur and this vapor will escape to the atmosphere above the floating roof. Air pollution and the risk for a fire is increased under these conditions.

If the TVP is going to be high, approaching 11.1 Psia or greater, consideration should be given to using a double-deck floating roof. A double deck will maintain its ability to drain water while containing some amount of product vapor. As noted previously, the double-deck roof can help reduce vapors from product heating due to the insulating

effect the design provides to the product surface. Properly designed, the double-deck floating-roof can be the most stable design available.

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4.1.3 Heavy, Waxy Crude Products

Heavy crude oil is characterized by higher pour point temperature, potentially high viscosity and tendency for a higher wax content. Unless these products are maintained

at storage conditions that limit these conditions, excessive amounts of product (clingage) can remain on the inside surface of the tank shell. Unfortunately there is no

100% fault-free answer to this problem. Heavy, waxy crude products are messy to handle, store and transport. There are methods that when followed can help to reduce

or control the problem.

The best option is to heat the product and to use wax or product scrapers that are mounted to the lower portion of the primary rim seal. Reducing the amount of clingage

is critical if the problem is to be controlled. Older mechanical shoe seals included a trough designed to reduce the amount of product flowing down the shell and onto the



top of the floating roof. The increased use of a secondary seal renders the trough useless since product will flow down the secondary seal and on to the roof deck.

Improvements have been made to wax scraper designs and to secondary seal tip designs that can help reduce the problem.

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4.2 Process Considerations

Process conditions upstream of the tank farm should be considered when selecting the type of floating roof. Process flow rates and knowing whether there are any process upset conditions that might result in large quantities of vapor to be mixed with the

product fill stream must be identified. Each of these conditions can be addressed in the roof selection process and during detailed design of the storage tank. If the application is an internal floating roof and there is a possibility that significant quantities of vapor may be included with the fill stream, a pan or lightweight floating roof should not be

considered. The minimum suggested roof design would be a pontoon single deck roof or in extreme cases a double-deck roof equipped with surface discretionary vents

might be required.

If a process upset will result in unusually hot product being sent to the tank, adequate checks must be made to determine if product mixing could produce significant

quantities of flash vapor during filling operations. If the conditions can be identified, the designer may be able to reduce the risk of a problem by adding extra deck vents.

Some terminal operations will offload products from ships or barges and then clear the fill lines from the dock to the storage tanks. One location had repeatedly sunk three internal floating roofs over several years time. Further investigation determined that after every ship/barge load of product had been transferred the fill lines had been

purge to storage by forcing the product through the pipeline with nitrogen. This is not good practice and should be avoided whenever possible. If a line must be cleared it

should be done in a controlled manner, monitoring pipeline conditions and tank conditions during the process.

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5.0 Technical Developments Why Are They Important

Floating roof tanks have been in use for over 75 years. Refinery operations change, processes and products change and most importantly people responsible for tank operations change. Floating-roof storage tanks and associated equipment must be

kept up to date and a working knowledge of the equipment maintained by refinery and terminal operators.

There is always a need for more efficient product storage. Conventional floating-roof tanks operate with a low landing position of approximately 3Ft (1M) and require 5 Ft (1.5M) or more vertical clearance for the floating roof. Roof/tank designs and new

equipment options are available that would permit a floating roof to land almost on the tank bottom, increasing the storage and operating efficiency of the system [5]. Some of

these new options will not be accepted without further development and testing.

Environmental regulations continue to be a driving force for change in the petroleum industry. For example, the US EPA in the January 14, 2000 copy of the Federal

Register announced a regulation that will require industry action. The announcement reaffirms that slotted guide poles for petroleum and volatile organic liquid storage tanks are subject to the "no visible gap" clause of NSPS requirements 40 CFR 60 Subparts Ka and Kb. The opening in the storage tank roof through which the guide pole passes as well as the slots in the guide pole constitute "visible gaps" that must "be maintained

in a closed position at all times except when the device is in actual use" [5].

It is expected that there will be a need to modify many of the external floating roof tanks currently in operation in US refinery and terminal facilities over the next few



years. New equipment details such as the low loss slotted guide pole are available. They have been in operation for several years and have proven performance records. In many cases these details can be implemented economically and without removing

the tank from product service. Floating-roof tanks utilize a significant amount of mechanical equipment to ensure safe and efficient tank operation. Floating-roof

primary and secondary seals, adjustable roof supports, mixers, drains, swing-lines, special fitting details and rolling ladders all must be designed to operate with the

floating-roof without compromising the integrity of the storage tank. Rolling ladders, roof drains, fire foam systems all must be designed and installed so that the floating-

roof remains in a balanced operating condition.

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6.0 Maintenance and Life Cycle Cost Control

By now it is clear that there are many options available to an owner when they must decide to modify an existing tank with a new floating roof or start from the ground up and select, specify and build a new floating-roof storage tank. Floating-roof tanks are not maintenance free. However with proper selection of the floating-roof design and

use of well designed rim seals, fittings and related equipment the new floating-roof can provide many years of trouble free operation. A floating-roof tank is truly a long-term

investment when you consider that many of the tanks built over 45 years ago are still in operation today. As an owner you should consider that some of the capital spent on a new installation is used as insurance against future maintenance costs. Extra attention to roof construction details, rim seal construction, roof drains, rolling ladders and deck

fittings will pay dividends for many years.

Periodic visual inspections of the floating roof will help eliminate surprise maintenance. It is suggested that any time an operator is on the gagers platform they should make

the following general observations of the floating roof.

Roof Position

l Is it floating in a generally flat configuration? l Is the roof centrally located within the confines of the tank shell? Is rim space

equal around roof?

General Roof Conditions

l Is there any visible indication of product on the deck? l Are there any indications of unusual water ponding on the deck?

l Is the center deck ballooned due to non-condensable vapors or high vapor pressure product?

l Are all pontoon manhole covers in place? l Are shunts contacting the tank shell above the secondary seal?

l Is the drain free of trash? l Are roof supports / bleeder vents locked in the correct location?

l Are foam chambers free of birds nests?

Whenever access to the deck is permitted a closer visual inspection can be made as deemed necessary from the previous inspection.

l Is the secondary seal tip in contact with the shell? l Is the guide pole sliding cover and wiper seal in good condition?

l Is there any indication that the rolling ladder wheels are not freely turning? Look for unusual scrape marks on the ladder wheels or runway.

Planned maintenance is generally completed at a lower cost than if there is an



emergency situation that requires taking a tank off line for a number of hours or even days. Simple, periodic visual inspections will not catch all problems but they should

reduce the surprises. Visual inspections of an external floating-roof tank are considerably easier than if you are dealing with an internal floating roof. However it is

very important to be able to check an internal floating roof to verify that the deck is operating in a stable condition.

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7.0 Conclusions

The floating-roof tank remains the most widely used method of storage of volatile petroleum products. Evolution of the floating-roof tank has involved the combined

efforts of engineers, contractors, and numerous operators and maintenance personnel who have had to live with the end product the last 77 years. Development of new floating-roof designs and equipment have improved the operating efficiency and

reduced the risk of operation to a practical minimum level.

Complex structures such as the floating-roof tank do have design and operating limitations. When selecting a floating-roof design for a new storage application

remember the following:

l Know the product or range of product conditions including temperature, true vapor pressure, and viscosity (crude oils).

l Know where the product is produced and if process upsets can be a factor in the operation of the floating-roof tank.

l Know the site weather conditions and understand how these may effect operation of the floating-roof.

l Design and installation details of equipment used with a floating-roof can effect its operation, service life, and maintenance requirements.

l The first cost of a floating-roof storage tank is important but does not represent the true cost of storage. A life cycle cost analysis represents the true cost of operation and should be a primary consideration when selecting the type of

storage system best suited to meet your needs.

When properly designed and constructed a floating-roof tank should provide 15, 20 or even 30 years of service with a minimum level of preventative maintenance. The floating-roof tank remains the cost-effective storage method for volatile petroleum

liquids.

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REFERENCES

1. American Petroleum Institute, October 1991, "Evaporative Loss From Fixed-Roof Tanks," API Manual of Petroleum Measurement Standards, Chapter 19.1,

(API Publication 2518), First Edition, Washington, D.C. 2. American Petroleum Institute, April 1997, "Evaporative Loss From Floating-Roof

Tanks," API Manual of Petroleum Measurement Standards, Chapter 19.2, (Combined API Publications 2517 and 2519), First Edition, Washington, D.C.

3. American Petroleum Institute, May 1993, "Welded Steel Tanks for Oil Storage," API Standard 650, Tenth Edition, Washington, D.C.

4. British Standards Institute, September 29,1989, "Manufacture of Vertical Steel Welded Non-Refrigerated Storage Tanks With Butt -Welded Shells for the

Petroleum Industry," BS2654, London, England. 5. Gallagher,T.A., Laverman, R.J. and Desjardins, C.R., "Floating-Roof Tank Heel

Reduction Options and Heel Turnover Emissions," Paper presented at the International Pipeline Conference(IPC98), Calgary, Alberta, Canada (CB&I

Technical Publication Number CBT-5648).



Floating-Roof Tanks Design and Operation

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