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12/19/11

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Steel Plate Engineering Data-Volume 1

Steel Tanks for Liquid Storage Revised Edition - 2011

The material presented in this publication is for general information only and should not be used without first securing competent advice with respect to its suitability for any given application. The publication of the material contained herein is not intended as a representation or warranty on the part of the Steel Market Development Institute—or of any other person named herein—that this information is suitable for any general or particular use or of freedom from infringement of any patents. Anyone making use of this information assumes all liability arising from such use.

Published by STEEL MARKET DEVELOPMENT INSTITUTE, A business unit of the American Iron and Steel Institute

In cooperation with and editorial collaboration by STEEL PLATE FABRICATORS ASSOCIATION, Div. of STI/SPFA

Acknowledgements

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cknowledgement is given to the important and valuable contribution made by members of the Steel Plate Fabricators Association in reviewing and updating the material for publication in this current edition. A special note of appreciation is given to Stephen W. Meier, P.E., S.E. of Tank Industry Consultants for his effort in updating this publication. The Steel Market Development Institute gratefully acknowledges the continued investment of its investor steel-producing companies in the steel pipe and tank markets.

Copyright Steel Market Development Institute 2011

STEEL MARKET DEVELOPMENT INSTITUTE 25 Massachusetts Avenue NW, Suite 800 Washington D.C. 20001 ii

Introduction

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he purpose of this publication is to provide a design reference for the usual design of tanks for liquid storage. Volume 1, "Steel Tanks for Liquid Storage,” deals with the design of flatbottom, cylindrical tanks for storage of liquids at essentially atmospheric pressure. Volume 2, "Useful Information on the Design of Plate Structures,” provides information to aid in design of such structures. For unusual applications, involving materials or liquids not covered within these pages, nor referenced herein, designers should consult more complete treatments of the subject material. Part I contains general information pertaining to carbon plate steels. This section is most helpful to readers who are not intimately familiar with steel industry terminology, practice and classification. Part II deals with the particular carbon steels applicable to tanks for liquid storage. Part III covers the design of carbon steel tanks for liquid storage. Part IV covers materials, design, and fabrication of stainless steel tanks for liquid storage.

Inquiries for further information on the design of steel tanks should be directed to: Steel Plate Fabricators Association Division of STI/SPFA 944 Donata Court Lake Zurich, IL 60047 www.steeltank.com iii

Contents

Part I Part II Part III Part IV

— — — —

Materials—General .............................................................................. 1 Materials—Carbon Steel Tanks for Liquid Storage......................... 7 Carbon Steel Tank Design................................................................. 11 Stainless Steel Tanks for Liquid Storage ......................................... 33

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Part I Materials—General Designation ost of the steel specifications referred to in this manual can be obtained from the American Society for Testing and Materials (ASTM). Each ASTM specification has a number such as A283, and within each specification there may be one or more grades or qualities. Thus an example of a proper reference would be “ASTM designation A283 grade C." In the interest of simplicity, such a reference will be abbreviated to "A283C." ASTM standards are issued periodically to report new specifications and changes to existing ones having a suffix indicating the year of issue such as "A283-C-03." Thus a summary such as is provided here may gradually become incomplete, and it is important that the designer of steel plate structures have the latest edition of ASTM standards available for reference.

certain elements increases in the liquid portion of the ingot. The resulting product, known as RIMMED STEEL, has marked differences in characteristics across the section and from top to bottom of the ingot. Control of the amount of gas evolved during solidification is accomplished by the addition of a deoxidizing agent, silicon being the most commonly used. If practically no gas evolved, the result is KILLED STEEL, so called because it lies quietly in the ingot. Killed steel is characterized by more uniform chemical composition and properties than other types. Although killed steel is a quality item, the end result is often not so specified by name, but rather by chemical analysis. Other deoxidizing elements are used, but in general, a specified minimum silicon content of 0.10% on heat analysis indicates that a steel is "fully killed." The term SEMIKILLED designates an intermediate type of steel in which a smaller amount of deoxidizer is added. Gas evolution is sufficiently reduced to prevent rimming action, but not sufficiently reduced to obtain the same degree of uniformity as attained in fully killed steels. This controlled evolution of gas during solidification tends to offset shrinkage, resulting in a higher yield of usable material from the ingot. As a practical matter, therefore, plates originating from ingots are usually furnished as semikilled steel unless a minimum silicon content of 0.10% on heat analysis is specified. The steels with which we are concerned are either continuous cast or cast into ingots. The ingots may be hot rolled to a convenient size for further processing, or they may be rolled directly into plates. The current practice is mostly to use continuous casting of the steel. The steel used for continuous casting is fully deoxidized.

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Definitions At least a nodding acquaintance with the terminology of the steel industry is essential to an understanding of steel specifications. This is especially true because, in common with many other industries, a number of shop and trade terms have become so thoroughly implanted in the language that they are used instead of more precise and descriptive technical terms. The following discussions may be of assistance. Steelmaking Processes Practically all steel is made by the electric furnace process or the basic oxygen process. ASTM specifications for the different steels specify which processes are permissible in each case. Types of Steel In most steelmaking processes, the principal chemical reaction is the combination of carbon and oxygen to form a gas. If the oxygen available for this reaction is not removed, the gaseous products continue to evolve during solidification in the ingot. Cooling and solidification progress from the outer rim of the ingot to the center, and during the solidification of the rim, the concentration of

Chemical Requirements A discussion of the effects of the many elements added to steels would involve a metallurgical treatise far beyond the scope of this work. However, certain elements are common to all steels, and it may be of help to briefly outline the effects of carbon, manganese, phosphorus, and sulfur on the properties of steel. 1

There are some exceptions to these rules in High Strength Low Alloy (HSLA) steels.

CARBON is the principal hardening element in steel, and as carbon increases, hardness increases. Tensile strength increases, and ductility, notch toughness and weldability generally decrease with increasing carbon content. MANGANESE contributes to strength and hardness, but to a lesser degree than carbon. Increasing the manganese content generally decreases ductility and weldability, but to a lesser degree than carbon. Because of the more moderate effects of manganese, carbon steels, which attain part of their strength through the addition of manganese, exhibit greater ductility and improved toughness than steels of similar strength achieved through the use of carbon alone. PHOSPHORUS. Phosphorus can result in noticeably higher yield strength and decreases in ductility, toughness, and weldability. In the steels under discussion here, it is generally kept below a limit of 0.04% on heat analysis. SULFUR decreases ductility, toughness, and weldability, and is generally kept below a limit of 0.05% on heat analysis. HEAT ANALYSIS is the term applied to the chemical analysis representative of a heat of steel and is the analysis reported to the purchaser. It is usually determined by analyzing, for such elements as have been specified, a test ingot sample obtained from the front or middle part of the heat during the pouring of the steel from the ladle. PRODUCT ANALYSIS is a supplementary chemical analysis of the steel in the semifinished or finished product form. It is not, as the term might imply, a duplicate determination to confirm a previous result.

Alloy Steel Steel is usually considered to be alloy when either: 1. A definite range or definite minimum quantity is required for any of the elements listed above in (1) under carbon steels, or 2. The maximum of the range for alloying elements exceeds one or more of the limits listed in (2) under carbon steels. Again, the HSLA steels demonstrate some exceptions to these general rules. High Strength Low Alloy Steels These steels, generally with specified yield point of 50 ksi or higher and containing small amounts of alloying elements, are often employed where high strength or light weight is desired. Mechanical Requirements Mechanical testing of steel plates includes tension, hardness, and toughness tests. The test specimens and the tests are described in ASTM specifications A6, A20, A370, and A673. From the tension tests are determined the TENSILE STRENGTH and YIELD POINT or YIELD STRENGTH, both of which are factors in selecting an allowable design stress, and the elongation over either a 2" or 8" gauge length. Elongation is a measure of ductility and workability. Toughness is a measure of ability to resist brittle fracture. Toughness tests are generally not required unless specified, and then usually because of a low service temperature and/or a relatively high design stress. Conditions under which impact tests are required or suggested will be discussed in connection with specific structures. A number of tests have been developed to demonstrate toughness, and each has its ardent proponents. The test most generally accepted currently, however, is the test using the Charpy V Notch specimen. Details of this specimen and method of testing can be found in ASTM-A370, "Mechanical Testing of Steel Products," and in A20 and A673. Briefly described, an impact test is a dynamic test in which a machined, notched specimen is struck and broken by a single blow in a specially designed testing machine. The energy expressed in foot-pounds required to break the

Carbon Steel Steel is usually considered to be carbon steel when: 1. No minimum content is specified or required for chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium, or any other element added to obtain desired alloying effect; 2. When the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, copper 0.60, silicon 0.60; 3. When the specified minimum for copper does not exceed 0.40%. 2

specimen is a measure of toughness. Toughness decreases at lower temperatures. Hence, when impact tests are required, they are usually performed near temperatures anticipated in service.

Referring to Figure 1-1, if the designer has selected a Charpy V Notch value of "x” ft.-lbs, as desirable under special service conditions, it will be noted that the steel illustrated would not be acceptable at temperatures lower than about +35F in the as-rolled condition. In the normalized condition, the same steel would be acceptable down to about -55F, and if quenched and tempered, to about -80F together with an increase in carbon, manganese, or other hardening elements. Note, however, that heat treatment adds to the cost and is indicated only when service conditions indicate the necessity for increased toughness and/or increased strength.

Grain Size Grain size is affected by both rolling practice and deoxidizing practice. For example, the use of aluminum as a deoxidizer tends to produce finer grains. Unless included in the ASTM specification, or unless otherwise specified, steels may be furnished to either coarse grain or fine grain practice at the producer's option. Fine grain steel is considered to have greater toughness than coarse grain steels. Heattreated fine grain steels will have greater toughness than as-rolled fine grain steels. The designer is concerned only with the question of under what conditions it is justifiable to pay the extra cost of specifying fine grain practice with or without heat treatment in order to obtain improved toughness. Guidelines will be discussed in later sections.

Classification of Steel Plates Plate steels are generally defined or classified in two ways. The first classification, which has already been discussed, is based on differences in chemical composition between CARBON STEELS, ALLOY STEELS and HIGH STRENGTH LOW ALLOY STEELS. The second classification is based primarily on the differences in extent of testing between STRUCTURAL QUALITY STEELS and PRESSURE VESSEL QUALITY STEELS1. It should not be construed that these terms limit the use of a particular steel. Pressure vessel steels are often used in structures other than pressure vessels. The distinction between structural and pressure vessel qualities is best understood by a comparison of the governing ASTM specifications. ASTM designation A6, General Requirements for Rolled Steel Plates for Structural Use, covers a group of common requirements and tolerances for the steels listed therein, the chemical composition and special requirements for which are outlined under separate specification numbers such as A36, A283, A514, etc. Similarly, ASTM designation A20, General Requirements for Steel Plates for Pressure Vessels, covers a group of common requirements and tolerances which apply to a list of about 35 steels, the chemical composition and special requirements for which are outlined under separate ASTM specification numbers.

Heat Treatment POST-WELD HEAT TREATMENT consists of heating the steel to a temperature between 1100F and 1250F, furnace cooling until the temperature has reduced to about 600F and then cooling in air. Residual stresses will be reduced by this procedure. NORMALIZING consists of heating the steel to between 1600F and 1700F, holding for a sufficient time to allow transformation, and cooling in air, primarily to affect grain refinement. QUENCHING consists of rapid cooling in a suitable medium from the normalizing temperature. This treatment hardens and strengthens the steel and is normally followed by tempering. TEMPERING consists of reheating the steel to a relatively low temperature (which varies with the particular steel and the properties desired). This temperature normally lies between 1000F and 1250F. Through the quenching and tempering treatment, many steels can attain excellent toughness, and at the same time high strength and good ductility. To illustrate the effect of heat treatment on toughness and strength, refer to Figure 1-1. The numerical values shown apply only to the specific steel described. For other steels, other values would apply, but the trends would be similar.

Pressure vessel quality steels were previously known as FLANGE and FIRE-BOX qualities, historically inherited terms used to define differences in the extent of testing, but which have no present-day significance insofar as the end use of the steel is concerned. 1

3

The arc is formed between the work to be welded and a metal wire which is called the electrode. The electrode may be consumable and add metal to the molten pool, or it may be nonconsumable and of a relatively inert metal, in which case no metal is added to the workpiece. In the welding of steel plate structures, we are concerned principally with five variations of arc welding: 1. Shielded metal arc process (SMAW) 2. Gas metal arc process (GMAW) 3. Flux-cored arc process (FCAW) 4. Electrogas or Electroslag welding 5. Submerged arc process (SAW)

Both A6 and A20 define tolerances for thickness, width, length, and flatness, but for the designer the important difference is in the quality of the finished product as influenced by the difference in the extent of testing. A general comparison of the two qualities follows: 1. Chemical Analysis—The requirements for phosphorus and sulfur are more stringent for pressure vessel quality than for structural quality. Both A6 and A20 require one analysis per heat plus the option of product analysis. Product analysis tolerances for structural steels are given in A6. 2. Testing for mechanical properties— a. In general, all specifications for structural quality require two tension tests per heat, size bracket and strength gradation. A6 specifies the general location of the specimens. b. In general, all specifications for pressure vessel quality require either one or two transverse tension tests, depending on heat treatment, from each plate as rolled2 (and as heattreated, if any). This affords a check on uniformity within a heat. Specification A20 also specifies the location from which the specimens are to be taken. 3. Repair of surface imperfections and the limitations on repair of surface imperfections are more restrictive in A20 than A6.

Shielded Metal Arc Welding In the early days of arc welding, the consumable electrode consisted of a bare wire. The pool of molten metal was exposed to and adversely affected by the gases in the atmosphere. It became obvious that to produce welds with adequate ductility, the molten metal must be protected or shielded from the atmosphere. This led to the development of the shielded metal arc process, in which the electrode is coated with materials that produce a gas as the electrode is consumed which shields the arc from the atmosphere. The coating also performs other functions, including the possible adding of alloying elements as well as slag-forming materials which float to the top and protect the metal during solidification and cooling. In practice, the process is limited primarily to manual manipulation of the electrode. Not too many years ago, this process was almost universally used for practically all welding. It is still widely used for position welding, i.e., welding other than in the down flat position. For the down flat position some of the later processes described below are much faster and hence less costly.

Welding Many plate structures are fabricated by welding. A brief discussion of welding processes follows. Welding consists of joining two pieces of metal by establishing a metallurgical bond between them. There are many different types of welding, but we are concerned only with arc welding. Arc welding is a fusion process in which the bond between the metals is produced by reducing the surfaces to be joined to a liquid state and then allowing the liquid to solidify. The heat required to reduce the metal to liquid state is produced by an electric arc.

Gas Metal Arc Welding In the gas-shielded arc welding process, the molten pool of metal is protected by an externally supplied gas, or gas mixture, fed through the electrode holder rather than by decomposition of the electrode coating. The electrode is a continuous filler-metal (consumable) bare wire and the gases used include helium, argon, and carbon dioxide. In some cases, a tubular electrode is used to

2 The term “Plate as rolled” refers to the unit plate rolled from a slab or directly from an ingot in relation to the number and location of specimens, not to its condition.

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Weldability It will be observed from the above that all arc welding processes result in rapid heating of the parent metal near the joint to a very high temperature followed by chilling as the relatively large mass of parent plate conducts heat away from the heat-affected zone. This rapid cooling of the weld metal and heataffected zone causes local shrinkage relative to the parent plate and resultant residual stresses. Depending on the chemical composition of the steel, plate thickness and external conditions, special welding precautions may be indicated. In very cold weather, or in the case of a highly hardenable material, preheating a band on either side of the joint will slow down the cooling rate. In some cases post-heat or stress relief as described earlier in this section is employed to reduce residual stresses to a level approaching the yield strength of the material at the post heat temperature. With respect to chemical composition, carbon is the single most important element because of its contribution to hardness, with other elements contributing to hardness but to lesser degrees. It is beyond our scope to provide a definitive discussion on when special welding precautions are indicated. In general, the necessity is dictated on the basis of practical experience or test programs.

facilitate the addition of fluxes or addition of alloys and slag-forming materials. Some methods of this process are called MIG and CO2 welding. The gas-shielded process lends itself to high rates of deposition and high welding speeds. It can be used manually, semi-automatically, or automatically. Flux-Cored Arc Welding This is an arc welding process wherein coalescence is produced by heating with an arc between a continuous filler-material (consumable) electrode and the work. Shielding is obtained from a flux contained within the electrode. Additional shielding may or may not be obtained from an externally supplied gas or gas mixture. Electrogas or Electroslag Welding This process is a method of gas metal arc welding or flux-cored arc welding wherein molding shoes confine the molten weld metal for vertical position welding. Submerged Arc Welding Submerged arc welding is essentially an automatic process, although semi-automatic applications have been used. The arc between a bare electrode and the work is covered and shielded by a blanket of granular, fusible material deposited on the work ahead of the electrode as it moves relative to the work. Filler metal is obtained either from the electrode or a supplementary welding rod. The fusible shielding material is known as melt or flux. In submerged arc welding, there is no visible evidence of the arc. The tip of the electrode and the molten weld pool are completely covered by the flux throughout the actual welding operation. High welding speeds are achieved. It will be obvious that the necessity of depositing a granular flux ahead of the electrode lends itself best to welding on work in the down flat position. Nevertheless, ingenious devices have been developed for keeping flux in place, so that the process has been applied to almost all positions except overhead welding.

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Figure 1-1 Typical Effect of Heat Treatment on Notch Toughness Of a Fine-Grained C-Mn-Si Steel (1 Inch Thickness)

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Part II Materials—Carbon Steel Tanks For Liquid Storage Introduction he intent of this publication is to provide information that may be useful in the design of flat-bottom, vertical cylindrical tanks for the storage of liquids at essentially atmospheric pressure. Considerable attention has been directed to tanks storing petroleum-based liquids or water, which constitute most of the tanks built. However, suggestions have been included for storage of liquids meriting special attention, such as acid storage tanks. There are two principal standards in general use in the U.S.: the American Petroleum Institute (API) covering welded steel tanks for petroleum storage, and the American Water Works Association (AWWA) covering tanks for water storage. The abbreviations API and AWWA will be used for the sake of convenience. While API has developed and maintains numerous standards related to the construction, operation and inspection of tanks, the API 650 Standard, Welded Tanks for Oil Storage and the API 620 Standard, Design and Construction of Large, Welded, LowPressure Storage Tanks are the commonly used tank design basis standards. AWWA has also developed and maintains numerous tank-related standards for concrete and steel tanks, included bolted and welded. The most commonly applied and used in this publication is AWWA D100- Welded Carbon Steel Tanks for Water Storage. Both API and AWWA permit the use of a relatively large number of different steel plate materials. In addition, the basic API Standard 650 and AWWA Standard D100 Section 14 provide refined design, construction and inspection rules for tanks designed at higher stresses in which the selection of steel is intimately related to stress level, thickness and service temperature, as well as the type and degree of inspection. As a result, knowledge of available materials and their limitations is equally as important as familiarity with design principles. Useful information concerning plate steel in general has been covered in Part I. It is the purpose of this section to assist in the selection of the proper steel or steels in the construction of tanks for liquid storage.

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Factors Affecting Selection of Steel Plate As you will learn in more detail in Part III of the publication, both the AWWA and the API offer optional methods of shell design. The AWWA basic procedures apply simplified rules which use conservative allowable stress levels. The optional design methods are based on refined procedures that take into account plate grade, service temperature, thickness and higher standards of inspection. It will be obvious that inasmuch as the simplified design provisions of both standards allow identical design stresses for any of the permissible steels, economic considerations will lead to the selection of the least expensive steel that will be satisfactory for the intended service. Steel selection is not so simple and straightforward in the case of tanks built in accordance with either the API or the AWWA Section 14 design provisions. Unstressed portions of such tanks, including bottoms and roofs, will probably be furnished as A36 unless the purchaser specifies otherwise. The selection of material for the shell demands further attention. The design provisions AWWA Section 14 resulted from a desire to utilize newer and improved steels and modern welding and inspection techniques to build tanks of higher quality. The use of higher stresses demanded attention to other properties of steel, primarily toughness. An exhaustive discussion of toughness is beyond the scope of this work, but it can be pointed out that as the stress level increases and temperature decreases, toughness becomes more important. At the stress level existing in API and AWWA basic design criteria tanks, experience has demonstrated that the steels used in combination with the specific welding and inspection rules have proven adequate for the service temperatures involved. Operating at the field of higher stress levels of the optional design methods requires steels having greater toughness. Thanks to research in metals, such steels are readily available. A number of factors enter into making a proper selection. For example, for any given steel, toughness generally decreases as thickness increases. The 7

Although both the API and AWWA standards permit the ordering of plates for certain parts of the tank on a weight rather than thickness basis, there is no longer any economic advantage in doing so.

toughness of carbon steels is improved if part of the hardness and strength is obtained by a higher manganese content and lower carbon at the same strength level. Fine-grained steels exhibit greater toughness than coarse-grained steels; this can be accomplished in the deoxidizing process, and in heat treatment. Thus as thickness increases and service temperature decreases, more stringent attention must be paid to toughness from the standpoint of materials selection and fabrication. The steels permitted by API and AWWA Section 14 for use at these higher stress levels have statistically demonstrated adequate toughness for the thickness and temperature ranges shown. The API standard includes an Impact Exemption chart which establishes requirements for impact testing, based on thickness, temperature and type of material. In the final analysis, the goal is to design the most cost-effective but acceptable tank for a given set of conditions. API and AWWA rules permitting higher design stresses afford a fairly wide selection of steels and stress levels from which to choose. A definitive treatment of economics is beyond the scope of this work. Basically, the factors involved are: 1. Cost of material 2. Weight of material as it affects freight and handling 3. Fabrication, erection and welding costs 4. Inspection and QA/QC costs None of these factors is necessarily conclusive in itself. In any given case, the lightest weight or lowest material cost may or may not be the least expensive overall depending on the relative importance of the factors listed above. The tank fabricator is usually in the best position to judge which steel or combination of steels will permit construction of the most economical, safe tank based on current market conditions. It is generally unwise to specify a more expensive steel than can be justified by the application. There are material costs not associated with quality. The cost of plates will vary according to both width and thickness, and from this consideration tank shell plate approximately 8' wide will commonly be used. Particular situations may dictate the use of wider or narrower plates for all or part of a tank shell. Plate widths of 10 ft to 12 ft are not uncommon.

The Future To this point, only those steels specifically permitted by API or AWWA have been discussed. Other steels have been used to a minor extent by those thoroughly familiar with the problems involved. Among these are the materials referred to in Part I as high strength low alloy steels, manufactured either as proprietary, trade-named steels, or to ASTM specifications. Some of these steels offer the additional attraction of improved atmospheric corrosion resistance, thus eliminating the necessity for painting outside surfaces. As is the case with all high-strength materials, the designer and user must assure themselves that factors other than strength (toughness for example) are properly allowed for in design and construction. For obvious reasons, construction codes often lag behind technical progress. The extensive research facilities of individual steel producers are constantly searching for ways to better serve the needs of our modern economy. But before any construction standard such as those of API and AWWA can accept and permit a new material, it must have been established that it is suitable for the structure in which it will be used. Usually, but not always, acceptance by API and AWWA implies prior acceptance by ASTM. Primarily this is because ASTM specifications clearly delineate the materials to be furnished, whereas any departure from ASTM requires that the standards involved spell out the requirements in corresponding detail. New ASTM steels may or may not eventually find their way into the construction standards, depending on economics and the proven properties of the materials. It should be left to those who have acquired the necessary experience in tank design and construction to pioneer in the use of materials not approved by API or AWWA. The designer, the user, and the fabricator assume added responsibilities in working outside of recognized industry standards. On the other hand, such pioneering by qualified organizations in the past led to the progress represented by the refined procedures of Section 14 of AWWA D100 and API-650. 8

As in the case of steels already approved by API and AWWA, time and experience will eventually lead to recognition of the steel or combination of steels that will yield the highest quality tank at least cost.

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Part III Carbon Steel Tank Design excellent. Very few tank failures have been recorded under even abnormal conditions and properly maintained steel tanks have endured long past their original design lives. Before applying them to tanks storing liquids other than water or oil, the designer should consider which philosophy best fits his circumstances. In either case the design standards provide minimum requirements for safe construction and should not be construed as a design manual covering all possible service conditions.

Introduction art III will consider the design of flat bottom, vertical, cylindrical, carbon steel tanks for the storage of liquids at essentially atmospheric pressure and near ambient temperatures. Practically all tanks in the United States within the scope of this part are constructed in accordance with API 650 covering welded steel tanks for oil storage or AWWA D100 covering welded steel tanks for water storage. Tanks of other shapes and subject to gas pressure in addition to liquid head, and tanks subject to extreme low or high temperatures present radically different problems. Consult ASME Section VIII, API 650 Appendices F and M, and API 620 for further information. API 650 and AWWA D100 contain detailed minimum requirements covering inspection. Any attempt to summarize the inspection requirements of either standard would be voluminous and dangerously misleading. It will be the purpose of Part III to discuss only those portions necessary to understand the various design bases. Anyone concerned with fabrication, erection, or inspection must obtain copies of the complete standards. There are basic differences between the standards of API and AWWA. API 650 is an industry standard especially designed to fit the needs of the petroleum industry. The petroleum tank is usually located in isolated areas, or in areas zoned for industry where the probable consequences of mishap are limited to the owner’s property. The owner is conscious of safety, environmental concerns and potential losses in his operations, and will adjust the minimum requirements to suit more severe service conditions. AWWA D100 is a public standard to be used for the storage of water. The water storage tank is often located in the midst of a heavily populated area, often on the highest elevation available. The consequence of catastrophic mishap could not be tolerated in the public interest. The API 650 and AWWA D100 standards have been in existence for many decades (since the 1930s). The performance of the tank population throughout the U.S. has been

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General Design Formula for Tank Shells Membrane shell theory, as it applies to cylindrical tanks of large diameter, is elementary and needs no explanation here. Starting with the basic premise that circumferential load in a cylinder equals the pressure times the radius, then expressing H and D in feet for convenience, the circumferential load at any level in a vertical cylinder containing water weighing 62.4 #/ft3, can be expressed as: T = 2.6 HD where T

=

H

=

D

=

(3-1)

the circumferential load per inch of shell height depth in feet below maximum liquid level tank diameter in feet

Then the minimum design thickness can be expressed as:

t (inches) = where

G

=

S E C

= = =

2.6 HDG +C SE

(3-2)

contained liquid specific gravity allowable design stress in psi joint factor corrosion allowance in inches

Obviously the ideal situation would be to vary the thickness uniformly from bottom to top, but since steel plates are rolled to a uniform thickness, any given course of plates is uniform throughout its width. Thus a course designed for the stress at its lower edge will have excess thickness at the top, which will help carry part 11

asymmetric loading criteria depending on the design standard applied.

of the load in the lower portion of the course above. API takes advantage of this and designs each course of plates for the stress existing one foot above the bottom of the course in question. AWWA designs on the basis of stress existing at the lower edge of each course. Application of other methods of shell design, such as the variable point method, is permitted and explained in API 650.

Live—The minimum roof live load shall be 15 psf to account for future maintenance loads and possible accidental vacuum. Seismic—Because of their flexibility and ductility, flat-bottomed cylindrical steel tanks have had an excellent safety record in earthquakes. Steel has the ability to absorb large amounts of energy without fracture. Prior to the Alaskan earthquake of 1964, flat bottom ground storage tanks had an excellent record of surviving western hemisphere earthquakes with essentially no effects other than broken pipe connections or minor buckles. In the Alaskan quake, the horizontal oscillations of the tank contents caused vertical shell stresses of sufficient magnitude to permanently deform the shell in a peripheral accordion-like buckle near the bottom (exaggerated elephant foot buckling). But again, the properties of steel were sufficient to accommodate this deformation without fracture of the shell plates.4 AWWA D100 and API 650 contain recommendations for the seismic design of tanks. The seismic design requirements in both API and AWWA were recently updated to follow the ASCE 7-05 and IBC 2006 requirements.

Loads To Be Considered As outlined in the preceding section, the thickness of the shell is determined by the weight of the product stored. However, there are other loads or forces which a tank may have to resist and which are common to both oil and water tanks. Wind—Historically for tank design, wind pressure has been assumed to be 30 psf on vertical plane surfaces which, when applying shape factors of 0.6 and 0.5 respectively, becomes 18 psf on the projected area of a cylindrical surface, and 15 psf on the projected area of a cone or surface of double curvature as in the case of tank roofs. These loads are considered to be the pressure caused by a wind velocity of 100 MPH. For higher or lower wind velocity, these loads are increased or decreased in proportion to the square of the velocity ratio, (V / 100) 2 , where V is expected wind velocity expressed in miles per hour. In recent years, the ASCE 7 has been the basis of loads for the U.S. buildings codes. This document is more advanced and includes effects of escalation of wind speed with height, increased wind speed along coastal regions and other factors not considered in the original simplified approach of the tank standards. This newer method was adopted; but, for AWWA the historical wind pressures were retained as minimum design pressures.

Negative Pressure—(such as partial vacuum)— Most tanks of this nature at some time will be subject to a negative pressure (partial vacuum) by design or otherwise. Approximately one-half oz. per square inch negative pressure is built into the shell stability formulae in AWWA D100 and API 650. AWWA D100 tanks are not usually designed for negative pressure but negative pressure due to the evacuation of water is considered in the venting requirements. Occasionally API 650 tanks are specified to resist a certain negative pressure, usually expressed in inches of water column. To meet these requirements the shell and roof must be designed to resist the specified negative pressure. See API 650 Appendix V for the current design methods applicable to flat bottom tanks for external pressure.

Snow—Snow load is assumed to be 25 psf on the horizontal projected area of the roof that has a slope of 30 degrees or less with the horizontal plane. If the roof slope is greater than 30 degrees, then the snow load may be zero. Snow loads reduction may also be made in regions where the lowest one day mean temperature is 5F or warmer. Fixed steel roofs on tanks are not usually designed for nonsymmetrical loads but if such load conditions are anticipated, these should be considered by the designer. Aluminum geodesic dome roofs may have an 12

should remain flexible to facilitate plate seams, nozzles and other interferences. For example, for a shell plate that is 10π feet long, it would be advantageous to use three anchors per plate and space the anchors at approximately 10.5 feet. Obviously the anchor bolt circle must be larger than the tank diameter, but care should be taken so interference will not occur between the anchor bolts and foundation reinforcing. Volume 2 Part V provides design rules for anchor bolt chairs.

Top and Intermediate Wind Girders Open top tanks require stiffening rings at or near the top of the shell to resist distortion or buckling due to wind. These stiffening rings are referred to as wind girders. In addition, some tank shells of open top and fixed roof tanks require intermediate wind girders to prevent buckling due to wind. API 650 and AWWA D100 provide differing design requirements for intermediate wind girders and are explained in the examples of Appendix A. The formula for maximum height of unstiffened shell is based on the MODIFIED MODEL BASIN FORMULA for the critical uniform external pressure on thin-wall tubes free from end loadings.

Corrosion Allowance As a minimum for all tanks, bottom plates should be l/4" in thickness and lap welded top side only. If corrosion allowance is required for bottom plates, the as-furnished thickness (including corrosion allowance) should be specified. The thickness of annular ring or sketch plates beneath the tank shell may be required to be thicker than the remainder of the bottom plates and any corrosion allowance should be specified as applicable to the calculated thickness or the minimum thickness. API 650 and AWWA D100 specify minimum shell plate thicknesses based on tank diameter for construction purposes. If corrosion allowance is necessary, it should be added in accordance with the respective standard. A required minimum above those stated in the standards may also be specified, but it should be made clear if this minimum includes the necessary corrosion allowance. As a minimum for all tanks, roof plates should be 3/16" in thickness and lap welded top side only. If corrosion allowance is necessary, it should be added in accordance with the respective standard. A required minimum greater than 3/16" in thickness may be specified, but it should be made clear if this minimum includes the necessary corrosion allowance. If corrosion allowance is necessary for roofsupporting structural members, it should be added in accordance with the respective standard. If a corrosion allowance requirement different from the standards is necessary, it should be made clear what parts of the structure require the additional thickness (flange or web, one side or both sides) and/or the minimum thickness necessary.

Anchor Bolts The normal proportions of petroleum tanks are such (diameter greater than height) that anchor bolts are rarely needed. It is quite common, however, for the height of water tanks to be greater than the diameter. There is a limit beyond which there is danger that any empty tank will overturn when subjected to the maximum wind velocity. As a good rule of thumb, if C in the following formula exceeds 0.66, anchor bolts are required:

2M where (3-3) dw overturning moment due to wind, ft. lb. diameter of shell in feet weight of shell and portion of roof supported by shell, lb. C=

M

=

d

=

w =

4M W Design tension load per bolt = ND − N

(3-4)

Where M and w are as above and N = number of anchor bolts D = diameter of anchor bolt circle, feet The diameter of the anchor bolts shall be determined by an allowable stress of 15000 psi on the net section at the root of the thread with appropriate stress increase for wind or earthquake loading. Because of proportionately large loss of section by corrosion on small areas, it is recommended that no anchor bolt be less than 1.25" in diameter. Maximum desirable spacing of anchors as suggested by API 650 and AWWA D100 is 10'0.This spacing is a matter of judgment and 13

API Standard 650

construction difficulties in order to perform the work in a safe manner.

General The following information is based on API 650. Anyone dealing with tanks should obtain a copy of the complete standard.

Top Angle Except for open-top tanks and the special requirements applying to self-supporting roofs, tank shells shall be provided with top angles of not less than the following sizes:

Shell Design API requires that all joints between shell plates shall be butt-welded. Lap joints are permitted only in the roof and bottom and in attaching the top angle to the shell. API 650 offers optional shell design procedures. The refined design procedures permit higher design stresses in return for a more refined engineering design, more rigorous inspection, and the use of shell plate steels which demonstrate improved toughness. The probability of detrimental notches is higher at discontinuities such as shell penetrations. The basic requirements pertaining to welding, stress relief, and inspection relative to the design procedures are important. Tank shells designed in accordance with refined procedures will be thinner than the simplified procedure, and thus will have reduced resistance to buckling under wind load when empty. The shell may or may not need to be stiffened, but must be checked. This is discussed in the section on wind girders.

Tank Diameter 35 feet and less over 35 to 60 ft. incl. over 60 feet

Roofs The selection of roof type depends on many factors. In the oil industry, many roofs are selected to minimize evaporation losses. Inasmuch as the ordinary oil tank is designed to withstand pressures only slightly above atmospheric, it must be vented against pressure and vacuum. The space above the liquid is filled with an air-vapor mixture. When a nearly empty tank is filled with liquid, this air-vapor mixture expands in the heat of the day and the resulting increase in pressure causes venting. During the cool of the night, the remaining airvapor mixture contracts, more fresh air is drawn in, more vapor evaporates to saturate the air-vapor mixture, and the next day the cycle is repeated. Either the loss of valuable "light ends" to the atmosphere from filling, or the breathing loss due to the expansion contraction cycle, is a very substantial loss and has led to the development of many roof types designed to minimize such losses. The floating roof is probably the most popular of all conservation devices and is included as Appendices to API Standard 650. The principle of the floating roof is simple. It floats on the liquid surface; therefore, there is no vapor either to be expelled on filling or to expand or contract from day to night. Inasmuch as all such conservation devices are represented by proprietary and often patented designs, they are beyond the scope of this discussion, which will be limited to the fixed roofs covered by API Standards. API 650 provides rules for the design of several types of fixed roofs. The most common fixed roof is the column-supported cone roof, except for relatively small diameters where the added cost of a self-supporting roof is more than offset by saving the cost of structural framing. The dividing line cannot be accurately defined

Bottoms Tank bottoms are usually lap welded plates having a minimum nominal thickness of 1/4". After trimming, bottom plates shall extend a minimum of 1 inch beyond the outside edge of the weld attaching the bottom to the shell plates. The attachment weld shall be a continuous fillet inside and out as shown in the following table of sizes: Maximum t of Shell Plate Inches 3/16 over 3/16 to 3/4 over 3/4 to 1-1/4 over 1-1/4 to 1-3/4

Minimum Size of Top Angle 2 x 2 x 3/16 2 x 2 x 1/4 3 x 3 x 3/8

Minimum Size of Fillet Weld* Inches 3/16 1/4 5/16 3/8

* Maximum size Fillet 1/2"

Butt-welded bottoms are permissible, but because of cost, are seldom used except in special services. Butt-welded bottoms are usually welded from the top side only using backing strips attached to the underside. Welding from both sides presents significant 14

because different practices and available equipment may affect the decision in any given case. If economy is the only consideration, the purchaser would be well advised to specify the size of tank and let the manufacturer decide whether or not to use a self-supporting roof. A self-supporting roof is sometimes desirable for special service conditions such as an internal floating roof, or where cleanliness and ease of cleaning are especially important. All roofs and supporting structures shall be designed to support dead load plus a live load of not less than 15 psf. Roof plates shall have a minimum nominal thickness of 3/16 inch. Structural members shall have a minimum thickness of 0.17 inch. Roof plates shall be attached to the top angle with a continuous fillet weld on the top side only: 1. If the continuous fillet weld between the roof plates and the top angle does not exceed 3/16 inch and the slope of the roof at the top-angle attachment does not exceed 2 inches in 12 inches, and when the cross-sectional area of the roof-to-shell junction does not exceed A=

W 201,000 tan θ

connection for the device and the drawings should reflect the need for such a device to be supplied by the customer. The top angle may be smaller than previously noted when a frangible joint is specified. 3. Tanks less than 50 ft. diameter may not be considered to have frangible roof joints even when the provisions of item 1 are satisfied. Supported Cone Roofs — Supported cone roofs are usually lap welded from the top side only with continuous full fillet welds. Plates shall not be attached to supporting members, and shall be attached to the top angle by a continuous 3/16" fillet weld or smaller on the top side if specified by purchaser. The usual slope of supported cone roofs is 3/4" in 12". Increased slopes should be used with caution. The columns transmit their loads directly to the supporting soil through bases resting on but not attached to the bottom plates. Some differential settlement can be expected. A relatively flat roof will follow such variations without difficulty. As pitch increases, a cone acquires stiffness, and instead of smoothly following a revised contour, unsightly local buckles may develop. In general, slopes exceeding 1-1/2" in 12" may be undesirable. Rafters in direct contact with the roof plates may be considered to receive adequate lateral support from friction, but this does not apply to truss chord members, rafters deeper than 15", or roof slopes greater than 2" in 12". Rafters are spaced so that, in the outer ring, their centers are not more than 6.28 feet apart at the shell. Spacing on inner rings does not exceed 5.5 feet. All parts of the supporting structure shall be so proportioned that the sum of the maximum calculated stresses shall not exceed the allowable stresses as stated in the appropriate section of API 650.

(3-5)

where W =

total weight of the shell and roof framing supported by the shell in pounds θ = angle between the roof and a horizontal plane at the roofto-shell juncture in degrees, the joint may be considered to be frangible and, in case of excessive internal pressure, will fail before failure occurs in the tank shell joints or the shell-to-bottom joint. Failure of the roof-to-shell joint is usually initiated by buckling of the top angle and followed by tearing of the 3/16 inch continuous weld at the periphery of the roof plates. 2. Where the weld size exceeds 3/16 inch, or where the slope of the roof at the topangle attachment is greater than 2 inches in 12 inches, or when the cross-sectional area of the roof-to-shell junction exceeds the value calculated per Equation 3-5, or where fillet welding from both sides is specified, emergency venting devices in accordance with API Standard 2000 shall be provided by the purchaser. The manufacturer shall provide a suitable tank

Self-Supporting Roofs — Self-supporting cone, dome or umbrella roofs shall conform to the appropriate requirements of API 650 unless otherwise specified by the purchaser.

15

such storage are referred Appendices R and Q.

Accessories API 650 contains specific designs for approved accessories which include all dimensions, thicknesses, and welding details. For all cases, OSHA requirements must be satisfied. No details are shown, but specifications are included for stairways, walkways and platforms. All such structures are designed to support a moving concentrated load of 1000 Ibs. and the handrail shall be capable of withstanding a load of 200 lbs. applied in any direction at any point on the top rail. Normally all pipe connections enter the tank through the lower part of the shell. Historically tank diameters and design stress levels have been such that the elastic movement of the tank shell under load has not been difficult to accommodate. With the trend to larger tanks and higher stresses, the elastic movement of the shell can become an important factor. Steel being an elastic material, the tank shell increases in diameter when subjected to internal pressure. The flat bottom acts as a diaphragm and restrains outward movement of the shell. As a result, the shell is greater in diameter several feet above the bottom than at the bottom. Openings near the bottom of the tank shell will tend to rotate with vertical bending of the shell under hydrostatic loading. Shell openings in this area, having attached piping or other external loads, should be reinforced not only for the static conditions but also for any loads imposed on the shell connections by the restraint of the attached piping to the shell rotations. Preferably the external loads should be minimized or the shell connections relocated outside the rotation area.

to

API

620

Molasses Tanks — Molasses presents no unusual problems other than the fact that its specific gravity is about 1.48, and the shell design must, of course, take this into account. It is quite common to require such tanks to be built in accordance with API 650. It must be remembered that the API Appendix A design stress of 21,000 psi at 85% joint factor is predicated on the tank being full of water during test, and that the actual stress in petroleum service is usually considerably less. Because molasses is heavier than water, the full design stress is present in service. Thus if the designer is depending on the long and successful record of tanks designed in accordance with API 650 Appendix A design, it would be more consistent with the true situation to use a somewhat lower design stress. On the other hand, on tanks built to the basic design of API 650 this difference between usual petroleum service stress and design stress does not exist. However, the addition of a corrosion allowance is required when warranted by service conditions. Acid and Caustic Tanks — To attempt a comprehensive discussion of the subject of storing acids and caustic solutions is beyond the scope of this work. While stainless steel or other high alloy materials are often required, some acids and caustic solutions can be stored successfully in carbon steel tanks, and the following discussion will be limited to such application. In the absence of personal experience, information concerning the corrosive properties of many common solutions can be found in chemistry and chemical engineers' handbooks or in the publications of the National Association of Corrosion Engineers. However, it should be noted that very small differences in content (such as slight impurities) or conditions can influence the corrosive effect of many chemicals. As an example, concentrated sulfuric acid does not attack carbon steel, whereas dilute sulfuric acid is extremely corrosive. Thus concentrated sulfuric acid can often be safely stored in carbon steel tanks provided proper precautions are taken to cope with dilute acid that may form in the upper portions of the tank

Tanks Other Than for Oil or Water There are many applications for steel tanks other than the storage of oil or water. Since most such applications are industrial in nature for which no industry standard has been developed, it is quite common to use API Standard 650 as a basis for design and construction. This is a logical approach provided that problems peculiar to the contents stored are taken into account. Tanks designed to store liquefied gases at or near atmospheric pressure are beyond the scope of this document. However, those interested in 16

when acid fumes and water condensation meet in the vapor space. Thus one fundamental requirement for an acid tank is that the interior of the tank be smooth without crevices or pockets where dilute acid condensation can collect. Selfsupporting roofs are good practice. If the design of the roof or size of tank requires structural stiffeners, it is desirable that they be placed on the outside. If the roof is lap welded, it should be welded underneath as well as the top. The connection of the roof to the shell should eliminate any pocket which might exist at the top of a standard API tank. When using Appendix A design basis of API 650, a lower design stress should be considered for the same reasons as given under "Molasses Tanks." The tank user should specify the amount of corrosion allowance, if any is required, for his particular purpose. In the case of carbon steel tanks storing caustic solutions, both the concentration and temperature are important. Carbon steel tanks should not be used if the combination of concentration and temperature exceeds the following values and may in some cases be unsatisfactory below these limits: 50% and 120F 25% and 150F 5% and 200F It is most important to make sure that the specified design conditions are not exceeded in service. Automatic temperature controls are recommended. In addition to ordinary corrosion, the principal problem in caustic tanks is one referred to as "caustic embrittlement" or "stress corrosion cracking." In the presence of high local stresses this type of corrosion can rapidly result in cracks and leaks. Local stress concentrations approaching the yield point can exist at shell penetrations, in the vicinity of welds and at other details. In caustic service these are the points where stress corrosion cracking can occur. Thus, in the case of caustic storage tanks, all fittings penetrating the shell or bottom, or any permanent attachments welded to the interior surface thereof, should be installed in a plate in the shop and the entire assembly thermally stress relieved. Essentially, this leaves only main seam welding to be performed in the field. Self-supporting roofs without structural members immersed in the tank contents are advisable. It is not necessary, however, to

eliminate crevices and pockets as is recommended for acid tanks. For caustic tanks, a standard API roof is acceptable. Certain additional precautions in welding should be taken for both acid and caustic tanks. Lap welds in the bottom and the inside bottomto-shell fillet should be made in at least two passes. Since the bottom-to-shell weld usually consists of a fillet inside and out, it is advisable to provide a water stop (complete penetration) at each vertical shell joint so that if a leak does occur in the inside fillet, channeling will be limited to one plate length. All other shell joints should be designed for complete penetration and fusion. The inside passes should be made first. The later welding of outside passes will partially heat treat and reduce residual stresses in the inside weld. If anticipated corrosion indicates a bottom plate thickness greater than 3/8", the bottom should be butt welded and the same sequence followed; i.e. weld the inside passes first. Inasmuch as all welds create locally high residual stresses, all brackets, welding lugs, etc. should be kept to a minimum, be located on the outside, and attached with small-diameter electrodes to limit the heat input and consequently the effect on the inside surface. When the corrosive attack is considered sufficiently severe to admit the possibility of local penetration, but not severe enough to warrant the expense of high alloy or clad steel plates, the tank is sometimes supported on a structural grillage to permit inspection from the underside.

AWWA Standard D100 General The following information is based on the AWWA Standard D100. Anyone dealing with tanks should obtain a copy of the complete standard. With the exception of shells, roofs and accessories, many of the comments made in connection with API tanks also apply to AWWA tanks and will not be repeated here in detail. Bottoms may be either lap or butt-welded with a minimum thickness of 1/4 inch. AWWA does not specify top angle sizes, but the rules of API represent good practice.

17

Shell Design AWWA D100 offers two different design bases – the standard or basic design and the alternate design basis as outlined in Section 14. The alternate design basis permits higher design stresses in return for a more refined engineering design, more rigorous inspection, and the use of shell plate steels with improved toughness. AWWA D100 Section 14 includes steels of significantly higher strength levels and correspondingly higher design stress levels. This introduces new design problems. For example, for A517 steels, the permissible design stress of 38,333 psi will result in reaching the minimum required nominal thickness several courses below the tank top. It would be uneconomical to continue the relatively expensive steel into courses of plates not determined by stress. The obvious answer is to use less expensive steels in the upper rings. To govern this transition, Section 14 adds the following requirements:

the high water level will extend up into the roof itself. The resultant upward pressure on the roof is resisted by the combination of the roof dead load and the weld joint between the roof and shell. AWWA requires that for all roof plate surfaces in contact with water, the minimum metal thickness shall be 1/4". Roof plate surfaces not in contact with water may be 3/16". As applied to rolled shapes for roof framing, the foregoing minimum thicknesses shall apply to the mean thickness of the flanges regardless of web thickness. Roof supports or stiffeners, if used, shall be in accordance with current specifications of the American Institute of Steel Construction covering structural steel for buildings, with the following exceptions: 1. Roof plates are considered to provide the necessary lateral support by friction between roof plates and rafters to eliminate reduction in the basic allowable compressive stress, except where trusses and open web joists are used for rafters, or rafters having nominal depth greater than 15 in. or rafters having a slope greater than 2” in 12.” 2. The roof, rafter and purlin depth may be

“In the interest of economy, upper courses may be of weaker material than used in the lower courses of shell plates, but in no instance shall the calculated stress at the bottom of any course be greater than permitted for the material in that course. A plate course may be thicker than the course below it provided the extra thickness is not used in any stress or wind stability calculation.”

Compliance with this requirement will probably result in the course or courses immediately below the transition point being somewhat heavier than required by stress. Using a steel of intermediate strength level as a transition between A517 steel and carbon steel may help the situation. In any event, the use of two or more steels will result in plates of the same thickness made of different steels. Careful attention to plain marking for positive identification becomes very important. Consideration might be given to varying plate widths for different materials of the same thickness to aid in identification in the event that markings are lost.

less than

fb 600,000

times the span length in inches where f b is the maximum bending stress in psi, providing slope of the roof is ¾” to 12” or greater. 3. The maximum slenderness ratio (L / r ) for roof support columns shall be 175. 4. Roof support columns shall be designed as secondary members. 5. Roof trusses, if any, shall be placed above the maximum water level in climates where ice may form. Roof trusses are not recommended due to the high degree of maintenance required over the life of the tank roof. 6. Roof rafters and connections shall be placed above maximum water level.

Roofs Whereas oil tanks are strictly utilitarian, a pleasing appearance is often an important consideration in the case of water tanks. Since the roof line has an important effect on appearance, this striving for beauty has led to a wide variety of roof designs. Often a self-supporting roof, such as an ellipsoid, will extend a considerable distance above the cylindrical portion of the shell, and 18

Accessories AWWA does not provide detailed designs of tank fittings and accessories, but specifies the following: 1. Compliance with OSHA and other regulations. 2. Two manholes shall be provided in the first ring of the tank shell. Manholes shall be either a 24" diameter or at least 18" x 22" when elliptical manholes are used. 30-inch diameter manholes are often recommended for safe recovery of personnel. 3. The purchaser shall specify pipe connections, sizes, and locations. Due to freezing hazard these connections are normally made through the tank bottom and as near to the shell as practical. A concrete valve box may be provided to permit access to piping. This valve box must be designed as a part of the ringwall. 4. If a removable silt stop is required, it shall be at least 4" high. If not required, then the connecting pipe shall extend at least 4" above the tank bottom. 5. The purchaser shall specify the overflow size and type. If an overflow to ground is required, it should be brought down the outside of the tank and discharged onto a splash block or other appropriate drainage structure with an air break. Inside overflows are not recommended. They are easily damaged by ice, and a failure in the overflow will empty the tank to the level of the break. 6. An outside vertical ladder shall begin 8 feet (or as specified) above the tank bottom and afford access to the roof. Need for access to AWWA tanks is infrequent and a conscious effort is made to render access difficult for unauthorized personnel. 7. The contractor shall provide access to the roof hatches and vents. The access must be reached from the outside tank ladder and fulfill the AWWA D100 requirements consistent with the roof slope or as specified by the purchaser. 8. A roof door or hatch whose least dimensions are 24" x 15", with a curb 4" high, provided with a hinged door and clasp for locking shall be placed near the outside tank ladder. A second opening of

9.

at least 20" in diameter and with a 4" neck must be provided near the center of the tank. Additional openings may be required for ventilation during painting. Adequate venting shall be provided to accommodate the maximum filling and emptying rates. A frost resistance pressure–vacuum relief device is required. These rates should be specified by the purchaser.

Venting for outflow (partial vacuum condition) is based upon the unrestricted vent area and the pressure differential that can safely be allowed between the outside and inside of the tank. This differential is established by quantifying the strength of the roof and shell above and beyond other structural requirements; for example, the margin of extra strength of the shell against buckling with respect to the design wind load. Venting for inflow (pressure condition) is again based upon the restricted vent area and the pressure differential that can safely be allowed before lifting the roof plates. For example, if 3/16" roof plates are used, the pressure differential would be 7.65 psf, 0.053 psi, or 1.47 inches water column. If the differential is limited to the weight of the roof, the shell/roof juncture does not become involved. The overstress in the shell would be minimal. The equation for outflow vent capacity is:

⎡⎛ P ⎞0.286 ⎤ Q = 0.5 A × 110 × T × ⎢⎜ a ⎟ − 1⎥ ⎥⎦ ⎢⎣⎝ Pi ⎠

1/ 2

(3-6)

where Q =

vent capacity in cubic feet per second A = minimum clear vent open area in square feet T = air temperature in degrees Rankine Pa = atmospheric pressure in psia Pi = pressure in tank during withdrawal in psia The equation for inflow vent capacity is: 1/ 2

0.286 ⎧⎪ ⎡ ⎤ ⎫⎪ 6 ⎛ Pi ⎞ Q = 0.5 A⎨6.25 × 10 ⎢⎜⎜ ⎟⎟ − 1⎥ ⎬ ⎢⎣⎝ Pa ⎠ ⎥⎦ ⎪⎭ ⎪⎩

19

(3-7)

APPENDIX A The following design example covers the AWWA D100 tank. Calculate shell thickness using the basic equation:

Design Example For typical examples of tank design consider two tanks 150 feet in diameter by 40 feet nominal height with flat cone supported roofs. Consider one tank per AWWA D100 and the other tank per API 650. See Figure 3A-1 for tank dimensions. These examples are for illustration only and are not to be used for an actual design or construction. Design of similar tanks should be accomplished by competent people experienced in the design of like structures and the use of applicable standards. For the AWWA tank consider Section 14, shell design and a site with mapped seismic ground motion values per ASCE 7-05 and AWWA D100-05 values of Ss = 0.5 and S1 = 0.15. Assume the Seismic Use Group is III. The Site Classification is “C” and TL = 8 sec. For the API 650 tank consider the standard, shell design by the variable point method, 1/16 inch corrosion allowance on the shell only. The seismic design procedures for API 650, API 620 and AWWA D100 are similar and are not repeated. Consider design metal temperature (DMT) of 20°F, standard 100 mph wind loads, standard 25 PSF roof loads, a maximum liquid content height of 39'-6,” and a design specific gravity of 1.0 for both tanks. The economics of plate selection with respect to width and grade and structural selection will differ with location and construction capabilities. Factors to consider are plate width and grade availability in a particular locality and structural rolling schedules. Also the availability of plate and structural stock in a particular locality will sometimes influence the selection of material. Further discussion of material selection will be beyond the scope of this document.

t=

2.6h p DG sE

= 0.1547"

(3-8)

All nomenclature in the above and following equations is defined in the AWWA D100 standard. Notice that hp in the above equation is the full liquid height above the design point rather than h - 1 as used in API 650. The calculation for ring five (top ring) is:

t5 =

2.6 × 7.66 × 150 × 1.0 = 0.1547" 19,330 × 1.0

The thicknesses for the remaining rings calculate: hp =15.63’ hp= 23.58’ hp= 31.54’ hp= 39.50’

S=19,330 psi S=23,330 psi S=23,330 psi S=23,330 psi

t4=0.3152” t3=0.3942” t2=0.5273” t1=0.6603”

using A36 steel for rings 4 and 5 and A573 GR70 for rings 1, 2 and 3. Ring 5 will be increased to 0.3125” because of minimum thickness requirements in AWWA D100. Shell stability is calculated using the basic equation. H=

10.625 × 106 × t

(3-9)

Pw (D / t )1.5

The calculation for ring five (top ring) is:

H5 =

10.625 × 10 6 × 0.3125 18 × (150 / 0.3125 )1.5

= 17.54' > 7.96'

For each ring the “h” calculated is compared to the actual height of shell above the design point. When “h” calculates less than the height of shell above, the shell is unstable. This may be corrected by thickening the shell or adding a stiffening ring. For this example we will consider only thickening the shell. h4 = 17.73’ > 15.92’ h3 = 21.76’ < 23.87’ Recalculate the thickness of ring 3 by using a lower strength steel (A36). t3 = 0.4758” 20

Using an inner support radius of 2.38 ft, which is dependent upon the method of supporting the inner rafters, the maximum design length of the inner rafters is 39.33 ft, as indicated in Figure 3A-2. The maximum design moment calculates to be 27,580 ft-Ibs. Using an AISC allowable stress of 0.66 × Fy, a section modulus of 13.93 in3 is required. A W12 × 14 section with a section modulus of 14.9 in3 is chosen. See Figure 3A-3 for a typical rafter loading. The maximum design length for the outer rafters is 35.33 ft, as indicated in Figure 3A-2. The maximum design moment calculates to be 27,890 ft-lbs. A section modulus of 14.09 in3 is required and again we will choose a W12 × 14 section. The rafter reactions are placed on the girder at the locations as determined by the roof framing layout. The outer rafter reactions are 3480 Ibs.; the inner rafter reactions are 2840 lbs.; and the girder design length is 29.07 ft. The maximum design moment calculates to be 150,440 ft-lbs. Again using AISC allowable stresses, a section modulus of 75.98 in3 is required. AW18 × 46 sections with a section modulus of 78.80 in3 is chosen. See Figure 3A-4 for a typical girder loading. For the center column a design load of 74,900 lbs. is calculated from the accumulated reactions of the inner rafters. Using AISC design procedures an allowable compressive stress is determined based upon the unsupported column length of 486.5 inches and a calculated slenderness ratio of 131. A 10" diameter schedule 20 pipe will meet the design criteria. See Figure 3A-5 for typical center column detail. For the outer columns we have chosen an 8” diameter schedule 20 pipe based upon a design load of 41,400 lbs, an unsupported column length of 470.6 inches, and a slenderness ratio of 159; using the same design criteria as the center column. See Figure 3A-6 for a typical outer column detail. Seismic design requirements are given in Section 13 of AWWA D100-05 and follow ASCE 7-05. The weights of the tank and liquid are computed to be: Weight of the product = 43,556,000 lbs Weight of the tank’s shell = 340,000 lbs Weight of the roof/framing = 354,000 lbs Weight of roof acting on shell = 205,000 lbs

Recalculate: h3 = 26.37’ > 23.87’ The shell is now stable above ring 3; continuing; h2 = 34.10’ > 31.83’ h1 = 45.67’ > 39.79’ The entire shell is now stable for a design wind velocity of 100 mph. See Table 3A-1 for shell thicknesses before and after minimum thickness and wind stability adjustments. For 90 mph wind load, minimum design loads are 18 psf on projected areas of cylindrical surfaces (shell) and 15 psf on projected areas of double curved surfaces (roof). Based upon the tank geometry and the design loading, the wind shear is calculated: Shell = 150 × 40.04 × 18 = 108,113 lbs Roof = 150 × 4.69 × 0.5 × 15 = 5,273 lbs Total = 113,386 lbs The minimum required coefficient of friction against sliding is: Wind Shear = 113,386 = 0.154 Tank Weight 734,250

(3-10)

This coefficient is well below established values which range as high as 0.4 to 0.5. The wind moment at the base of the shell is calculated: Shell = 108,113× 20.02 = 2,164,421 ft-lbs. Roof = 5,273 × 41.60 = 219,357 Total = 2,383,778 ft-lbs. The ratio, C = 2M/dw, calculates to be 0.076 < 0.666; therefore, no anchors are required to resist overturning due to wind. Roof framing concepts, layout and detail vary among tank designers and suppliers. Rafter spacing is dependent upon roof loading and plate thickness. For reasons of plate strength and construction a maximum rafter spacing of approximately 7.00 feet is desirable. For this example consider nine girders and outer columns, 36 inner rafters and 72 outer rafters (see Figure 3A-2). The outer columns will be located on a 42'-6" radius. The rafter spacing is 6.54 feet at the shell and 6.92 feet at the girder. Consider 25 psf snow load and 7.65 psf (3/16" roof plate) dead load. 21

Sc = 416 psi < 4570 psi

Weight of tank bottom = 181,000 lbs Center of gravity of roof above shell = 3 ft Center of gravity of shell above fdn = 16.7 ft H/D ratio = 0.263

OK

The additional hydrodynamic hoop stresses are calculated by Section 13 (Equations 13-43 through 13-46). These hydrodynamic hoop stresses are added directly to the hydrostatic hoop stresses.

Given Ss = 0.5 and S1 = 0.15. From Table 26 and 27 of AWWA D100 and Site class C, Fa and Fv can be determined as Fa = 1.2 and Fv = 1.65. Substituting into the equations using k=1.5, U = 2/3, I=1.5 Ri= 2.5 for self-anchored and Rc = 1,5, the values for Ai and Ac are computed as:

Hydrodynamic Hoop Stresses Y

Ai = 0.172 Ac = 0.022 and Av = 0.056 The sloshing period, Tc is 8.18 sec. Using Section 13.5, the impulsive weight, Wi, and convective weight, Wc, are calculated:

Y/D

Eqn 13-43

Eqn 13-44

1

7.5 .050

790

692

2

15.5 .103

1450

3

23.5 .157

4 5

Eqn Ni 13-45 5388

Eqn 13-46 Nc

790

432

1378

5388 1450

384

1922

2009

5388 1922

351

31.5 .210

2207

2586

5388 2207

332

39.5 .263

2303

3108

5388 2303

326

The addition of the stresses is usually done on a force/per unit length of circumference basis. In the longitudinal or vertical membrane (phi) direction, the forces from dead load, snow load, live load, and overturning are added together in the appropriate load combinations and compared to the applicable allowable stress. Similarly, in the circumferential (hoop or theta) membrane direction the hydrostatic and hydrodynamic forces are added. The results of this calculation are summarized in the table on page 23. Finally, the freeboard must be evaluated per Section 13. Since this was identified as a SUG III tank, and the sloshing period, Tc > 4 sec. Af = 0.03g, and the calculated wave height is 2.2 ft. The freeboard provided is 0 ft (0.5 ft from roof plate was there for rafter ends to project into the tank). Thus, the design must be modified. Either the liquid level must be reduced or the shell height increased by 2.2 ft to provide the required freeboard.

Wi = 13,208,000 lbs Wc = 28,423,000 lbs Similarly, substituting into the equations for the moment arms of the lateral forces, Xi, Xc can be computed: Xi = 14.81 ft Xc = 21.16 ft Substituting into Equation 13-23 of AWWA D100, the ringwall moment is Ms = 37,354,000 ft lbs Using this value of Ms in Equation 13-36, calculate the “J” ratio. Less than 0.785, so there is no net uplift for the design overturning moment and the tank is self-anchored if the maximum shell compression calculated by Equation 13-39 is met. Substituting into Equation 13-39,

22

Summary of Shell Stresses (AWWA) Ring No Material Design allowable tensile Ht of Ring Thickness of Ring Y RoofDL, including equipment Shell Weight Cummulative DL, Self Weight Nphi DL(metal) Nphi , LL Nphi HEQ, impulsive Nphi HEQ, convective Nphi HEQ (direct sum) Nphi HEQ (srss) Nphi VEQ Ntheta, hydrostatic Ntheta HEQ, Nimpulsive Ntheta HEQ, Nconvective Ntheta, HEQ (direct sum) Ntheta VEQ, Ntheta x EQ vert factor Ntheta EQ (srss) Ntheta total Check Load Combinations DL+LL Nphi Sphi Sphi allowable

Roof psi ft inches ft lbs/in kips lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in lbs/in

Top A36M 19330 8 0.3125 7.5 36.3 48.1 44.8 44.8 0.0 3.4 2.6 5.9 4.2 2.5 2925 790 432 1221 165 915 3840

36.25 36.3 36.3 0.0 0.5 0.0 0.5 0.5 2.0

Top lbs/in psi psi

Ntheta Stheta Stheta allowable

lbs/in psi psi

Nphi Sphi Sphi allowable

lbs/in psi psi

Ntheta Stheta Stheta allowable

lbs/in psi psi

36.3 116 611 OK

44.8 143 611

2 3 4 5 A36M A573Gr70A573Gr70A573Gr70 19330 23330 23330 23330 8 8 8 8 0.3152 0.3942 0.5273 0.6604 15.5 23.5 31.5 39.5 36.3 36.3 36.3 36.3 48.5 60.6 81.1 101.6 53.3 64.0 78.4 96.4 53.3 64.0 78.4 96.4 0.0 0.0 0.0 0.0 12.9 34.5 73.5 135.4 11.0 25.3 45.5 71.6 23.9 59.9 119.1 207.0 17.0 42.8 86.5 153.2 3.0 3.6 4.4 5.4 6045 9165 12285 15405 1450 1922 2207 2303 384 351 332 326 1834 2274 2539 2629 340 516 691 867 1538 2021 2336 2482 7583 11186 14621 17887 2 3 4 5

OK 2925 9360 19330 OK

53.3 169 617 OK 6045 19178 19330 OK

64.0 162 774 OK 9165 23250 23330 OK

78.4 149 1043 OK 12285 23298 23330 OK

96.4 146 1319 OK 15405 23327 23330 OK

49.0 157 2096 OK 3840 12288 25773 OK

70.4 223 2312 OK 7583 24058 25773 OK

106.9 271 2895 OK 11186 28376 31107 OK

164.9 313 3750 OK 14621 27728 31107 OK

249.6 378 4570 OK 17887 27086 31107 OK

DL+EQ (srss, default AWWA) 36.8 118 815 OK

23

APPENDIX B - TANK FOUNDATIONS Soils Investigation The subgrade of a potential tank site must be capable of supporting the weight of the tank and contained fluid. A qualified geotechnical engineer should be retained to conduct the subsurface exploration and to make specific recommendations concerning: the type of foundation required, anticipated settlements, allowable soil bearing and specific construction requirements. The ultimate soil bearing capacity should be determined using sound principles of geotechnical engineering. The following minimum factors of safety should be applied to the ultimate bearing capacity when determining the allowable soil bearing: 1. A factor of safety of 3.0 for normal operating conditions. 2. A factor of safety of 2.25 during hydrotest. 3. A factor of safety of 2.25 for operating conditions plus the maximum effect of wind or seismic forces. An allowable soil bearing based solely on the above factors of safety may result in excessive total settlements. If required, these factors of safety should be increased in order to limit the anticipated total settlements to acceptable values. Factors of safety larger than the above minimums are also required by certain codes and standards, such as AWWA D100.

24

3/16” ROOF PLATE LAP WELDED TOP SIDE ONLY

¼” BOTTOM PLATE LAP WELDED TOP SIDE ONLY

Figure 3A-1 – Flat Bottom Tank

Table 3A-1 – Shell Plate Thicknesses a.) CALCULATED SHELL THICKNESSES FROM STATIC HEAD ONLY (AWWA DESIGN) RING # 5 4 3 2 1

THICKNESS 0.1547” 0.3152” 0.3942” 0.5273” 0.6603”

b.) ADJUSTED FINAL THICKNESSES FOR STATIC HEAD AND WIND STABILITY (AWWA DESIGN) RING# 5 4 3 2 1

MATERIAL A36 A36 A573GR70 A573GR70 A573GR70

25

THICKNESS 0.3125” 0.3152” 0.4758” 0.5273” 0.6603”

MATERIAL A36 A36 A36 A573GR70 A573GR70

Figure 3A-2 – Framing Layout -- AWWA

Figure 3A-3 – Typical Rafter Loading

Figure 3A-4 – Typical Girder Loading 26

Figure 3A-5 ⎯ Typical Center Column 27

Figure 3A-6 ⎯ Typical Outer Column

VARIABLE POINT DESIGN: API 650 RING NO. 1 DESIGN: D = 150.000 H = 39.500 G = 1.000 S = 28000. CA = 0.0625 Td = 2.6*D*(H - 1) * G/S + CA = 0.5362 + CA = 0.5987 T1d = [1.06- (0.463*D/H)* SQRT(H*G/S)] *2.6*D*H*G/S + CA = T1d = 0.5469 + CA = 0.6094 HYDROTEST: D = 150.000 H = 39.500 G = 1.000 S = 30000. TT= 2.6*D* (H-1) * G/S = 0.5005 T1T = [1.06-(0.463* D/H) * SQRT(H* G/S)] * 2.6* D* H* G/S= 0.5115 USE: 0.599 IN. A573 70 L/H = SQRT(6.0* D* T)/H3 = 0.5929