Inspection Practices for Atmospheric and Low-pressure Storage Tanks PDF (API RP 575, 2020)

Inspection Practices for Atmospheric and Low-pressure Storage Tanks API RECOMMENDED PRACTICE 575 FOURTH EDITION, JULY 2020 https://t.me/MyAPI https://t.me/MyInspectors Special Notes API publications necessarily address problems of a general nature. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed. The use of API publications is voluntary. In some cases, third parties or authorities having jurisdiction may choose to incorporate API standards by reference and may mandate compliance. Neither API nor any of API's employees, subcontractors, consultants, committees, or other assignees make any warranty or representation, either express or implied, with respect to the accuracy, completeness, or usefulness of the information contained herein, or assume any liability or responsibility for any use, or the results of such use, of any information or process disclosed in this publication. Neither API nor any of API’s employees, subcontractors, consultants, or other assignees represent that use of this publication would not infringe upon privately owned rights. API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any authorities having jurisdiction with which this publication may conflict. API publications are published to facilitate the broad availability of proven, sound engineering and operating practices. These publications are not intended to obviate the need for applying sound engineering judgment regarding when and where these publications should be utilized. The formulation and publication of API publications is not intended in any way to inhibit anyone from using any other practices. Any manufacturer marking equipment or materials in conformance with the marking requirements of an API standard is solely responsible for complying with all the applicable requirements of that standard. API does not represent, warrant, or guarantee that such products do in fact conform to the applicable API standard. All rights reserved. No part of this work may be reproduced, translated, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher. Contact the Publisher, API Publishing Services, 200 Massachusetts Avenue, NW, Suite 1100, Washington, DC 20001. Copyright © 2020 American Petroleum Institute https://t.me/MyAPI https://t.me/MyInspectors Foreword Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anything contained in the publication be construed as insuring anyone against liability for infringement of letters patent. The verbal forms used to express the provisions in this document are as follows. Shall: As used in a standard, “shall” denotes a minimum requirement in order to conform to the standard. Should: As used in a standard, “should” denotes a recommendation or that which is advised but not required in order to conform to the standard. May: As used in a standard, “may” denotes a course of action permissible within the limits of a standard. Can: As used in a standard, “can” denotes a statement of possibility or capability. This document was produced under API standardization procedures that ensure appropriate notification and participation in the developmental process and is designated as an API standard. Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should be directed in writing to the Director of Standards, American Petroleum Institute, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001. Requests for permission to reproduce or translate all or any part of the material published herein should also be addressed to the director. Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. A one-time extension of up to two years may be added to this review cycle. Status of the publication can be ascertained from the API Standards Department, telephone (202) 682-8000. A catalog of API publications and materials is published annually by API, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001. Suggested revisions are invited and should be submitted to the Standards Department, API, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001, [email protected]. iii https://t.me/MyAPI https://t.me/MyInspectors https://t.me/MyAPI https://t.me/MyInspectors Contents Page 1 Scope............................................................................... 1 2 Normative References................................................................. 1 2.1 Codes, Standards, and Related Publications............................................... 1 2.2 Other References..................................................................... 1 3 Terms and Definitions................................................................. 4 4 Types of Storage Tanks................................................................ 9 4.1 General.............................................................................. 9 4.2 Atmospheric Storage Tanks........................................................... 10 4.3 Low-pressure Storage Tanks........................................................... 23 5 Reasons for Inspection and Causes of Deterioration....................................... 28 5.1 Reasons for Inspection............................................................... 28 5.2 Deterioration of Tanks................................................................ 29 5.3 Deterioration of Other than Flat Bottom and Non-steel Tanks................................ 31 5.4 Leaks, Cracks, and Mechanical Deterioration............................................. 32 5.5 Deterioration and Failure of Auxiliary Equipment.......................................... 35 6 Inspection Plans..................................................................... 36 6.1 General............................................................................. 36 6.2 Inspections Planning and Reports...................................................... 37 6.3 RBI Plans........................................................................... 39 7 Interval/Frequency and Extent of Inspection.............................................. 40 7.1 Interval of Inspection................................................................. 40 7.2 Condition-based Inspection Scheduling.................................................. 41 7.3 Inspection Scheduling Based on Minimum Acceptable Thickness............................ 41 7.4 Similar Service Methodology for Establishing Tank Corrosion Rates.......................... 43 7.5 Fitness-For-Service Evaluation......................................................... 45 8 Inspections......................................................................... 45 8.1 Preparation for Inspections............................................................ 45 8.2 External Inspection of an In-service Tank................................................ 48 8.3 External Inspection of Out-of-service Tanks.............................................. 62 8.4 Internal Inspection................................................................... 66 8.5 Hydrostatic and Pneumatic Testing of Tanks............................................. 79 8.6 Inspection Checklists................................................................. 80 9 Leak Testing and Hydraulic Integrity of the Bottom........................................ 80 9.1 General............................................................................. 80 9.2 Leak Integrity Methods Available During Out-of-service Periods............................. 82 9.3 Leak Detection Methods Available During In-service Periods................................ 86 10 Integrity of Repairs and Alterations..................................................... 90 10.1 General............................................................................. 90 10.2 Repairs............................................................................. 90 10.3 Special Repair Methods............................................................... 93 11 Records............................................................................ 95 11.1 General............................................................................. 95 11.2 Records and Reports................................................................. 95 11.3 Form and Organization................................................................ 97 v https://t.me/MyAPI https://t.me/MyInspectors Contents Page Annex A (informative) Selected NDE Methods.................................................... 98 Annex B (informative) Similar Service Evaluation Tables.......................................... 103 Bibliography............................................................................... 106 Figures 1 Cone Roof Tank...................................................................... 11 2 Umbrella Roof Tank.................................................................. 12 3 Self-supporting Aluminum (Geodesic) Dome Roof Tank.................................... 12 4 Self-supporting Dome Roof Tank....................................................... 13 5 Externally Stiffened Pan-type Floating-roof Tank.......................................... 13 6 Annular-pontoon Floating-roof Tank..................................................... 14 7 Double-deck Floating-roof Tank........................................................ 14 8 Cross-section Sketches of Floating-roof Tanks Showing the Most Important Features........... 15 9 Floating-roof Shoe Seal............................................................... 16 10 Floating-roof Log Seal................................................................ 16 11 Floating Roof Using Counterweights to Maintain Seal...................................... 17 12 Floating Roof Using Resilient Tube-type Seal............................................. 17 13 Typical Arrangement for Metallic Float Internal Floating-roof Seals........................... 18 14 Typical Internal Floating-roof Components............................................... 19 15 Cable-supported Internal Floating-roof Tank.............................................. 19 16 Plain Breather Roof Tanks............................................................. 21 17 Tank with Vapor Dome Roof........................................................... 22 18 Balloon Roof Tank................................................................... 22 19 Cutaway View of Vapor Dome Roof...................................................... 23 20 Plain Hemispheroids.................................................................. 24 21 Noded Hemispheroid................................................................. 25 22 Drawing of Hemispheroid.............................................................. 25 23 Plain Spheroid....................................................................... 26 24 Plain Hemispheroid with Knuckle Radius................................................. 26 25 Noded Spheroid..................................................................... 27 26 Drawing of Noded Spheroid............................................................ 27 27 Foundation Seal..................................................................... 30 28 Cracks in Tank Shell Plate............................................................. 33 29 Extensive Destruction from Instantaneous Failure......................................... 34 30 Cracks in Bottom Plate Welds Near the Shell-to-bottom Joint................................ 34 31 Cracks in Tank at Riveted Lap Joint to Tank Shell......................................... 35 32 Failure of Concrete Ringwall........................................................... 49 33 Anchor Bolt......................................................................... 50 34 Corrosion of Anchor Bolts............................................................. 50 35 Corrosion Under Insulation............................................................ 51 36 Close-up of Corrosion Under Insulation.................................................. 52 37 Corrosion (External) at Grade.......................................................... 53 38 Caustic Stress Corrosion Cracks....................................................... 55 39 Small Hydrogen Blisters on Shell Interior................................................ 56 40 Large Hydrogen Blisters on Shell Interior................................................ 56 41 Tank Failure Caused by Inadequate/Obstructed Vacuum Venting............................. 58 42 Roof Overpressure................................................................... 59 43 Example of Severe Corrosion of Tank Roof............................................... 64 vi https://t.me/MyAPI https://t.me/MyInspectors Contents Page 44 Collapse of Pan-type Roof from Excessive Weight of Water While the Roof Was Resting on Its Supports.............................................. 65 45 Pontoon Floating-roof Failure.......................................................... 65 46a Rolling Scaffold Used for Inspection and Repairs Inside of Tank............................. 67 46b Tank Buggy Used for Inspection and Repairs on Exterior of Tank............................ 67 47 Remote Control Automated Crawler..................................................... 68 48 Example of Vapor-Liquid Line Corrosion................................................. 70 49 Corrosion Behind Floating-roof Seal.................................................... 70 50 Localized Corrosion-erosion at Riveted Seam in a Tank Bottom.............................. 73 51 Example of Extensive Corrosion of a Tank Bottom......................................... 73 52 Shell-to-bottom Weld Corrosion........................................................ 74 53 External View of Erosion-corrosion Completely Penetrating a Tank Shell...................... 75 54 Internal Corrosion on Rafters and Roof Plates............................................ 77 55 Failure of Roof Supports.............................................................. 77 56 Fin-tube Type of Heaters Commonly Used in Storage Tanks................................. 78 57 Example of Corrosion of Steam Heating Coil.............................................. 78 58 Hydraulic Integrity Test Procedures..................................................... 81 59 Vacuum Box Used to Test for Leaks..................................................... 83 60 Vacuum Box Arrangement for Detection of Leaks in Vacuum Seals........................... 84 61 Helium Tester....................................................................... 85 62 Method of Repairing Tank Bottoms...................................................... 92 63 Temporary “Soft Patch” Over Leak in Tank Roof.......................................... 94 64 Mastic Roof Coating.................................................................. 94 65 Tank Jacked Up for Repairing Pad...................................................... 95 A.1 Automatic Ultrasonic Testing......................................................... 100 A.2 MFL Scanner....................................................................... 100 A.3 Ultrasonic Examination.............................................................. 101 A.4 Guided Wave Ultrasonic Testing....................................................... 101 A.5 Robotic Inspection Tool.............................................................. 102 Tables 1 Tools for Tank Inspection............................................................. 46 2 Useful Supplemental Tools............................................................ 47 B.1 Selected Considerations for Performing Similar Service Assessments....................... 103 B.2 Similar Service Example for Product-side Corrosion...................................... 105 https://t.me/MyAPI https://t.me/MyInspectors Inspection Practices for Atmospheric and Low-pressure Storage Tanks 1 Scope This document provides useful information and recommended practices for the maintenance and inspection of atmospheric and low-pressure storage tanks. While these maintenance and inspection guidelines may apply to other types of tanks, these practices are intended primarily for existing tanks that were constructed to one of the following four standards: API Std 12A, API Spec 12C, API Std 620, or API Std 650. This document addresses the following: a) descriptions and illustrations of the various types of storage tanks; b) new tank construction standards; c) maintenance practices; d) reasons for inspection; e) causes of deterioration; f) frequency of inspection; g) methods of inspection; h) inspection of repairs; i) preparation of records and reports; j) safe and efficient operation; k) leak prevention methods. This recommended practice is intended to supplement API Std 653, which provides minimum requirements for maintaining the integrity of storage tanks after they have been placed in service. 2 Normative References 2.1 Codes, Standards, and Related Publications The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction 2.2 Other References The following codes and standards are cited in the text of this recommended practice or included in the knowledge base to develop this document. Familiarity with these documents is suggested as they provide additional information pertaining to the inspection and repair of aboveground storage tanks. API Standard 12A, Specification for Oil Storage Tanks with Riveted Shells (withdrawn) API Specification 12B, Specification for Bolted Tanks for Storage of Production Liquids https://t.me/MyAPI https://t.me/MyInspectors 2 API RECOMMENDED PRACTICE 575 API Standard 12C, Specification for Welded Oil Storage Tanks (withdrawn) API Specification 12D, Specification for Field-welded Tanks for Storage of Production Liquids API Standard 12E, Specification for Wooden Production Tanks (withdrawn) API Specification 12F, Specification for Shop-welded Tanks for Storage of Production Liquids API Specification 12P, Specification for Fiberglass Reinforced Plastic Tanks API Standard 12R1, Installation, Operation, Maintenance, Inspection, and Repair of Tanks in Production Service API Publication 306, An Engineering Assessment of Volumetric Methods of Leak Detection in Aboveground Storage Tanks API Publication 307, An Engineering Assessment of Acoustic Methods of Leak Detection in Aboveground Storage Tanks API Publication 315, Assessment of Tankfield Dike Lining Materials and Methods API Publication 322, An Engineering Evaluation of Acoustic Methods of Leak Detection in Aboveground Storage Tanks API Publication 323, An Engineering Evaluation of Volumetric Methods of Leak Detection in Aboveground Storage Tanks API Publication 325, An Evaluation of a Methodology for the Detection of Leaks in Aboveground Storage Tanks API Publication 334, A Guide to Leak Detection for Aboveground Storage Tanks API Publication 340, Liquid Release Prevention and Detection Measures for Aboveground Storage Facilities API Publication 341, A Survey of Diked-area Liner Use at Aboveground Storage Tank Facilities API 510, Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration API 570, Piping Inspection Code: In-service Inspection, Rating, Repair, and Alteration of Piping Systems API Recommended Practice 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry API Recommended Practice 572, Inspection Practices for Pressure Vessels API Recommended Practice 576, Inspection of Pressure-relieving Devices API Recommended Practice 579-1/ASME FFS-1, Fitness-For-Service API Recommended Practice 580, Risk-Based Inspection API Recommended Practice 581, Risk-Based Inspection Methodology API Recommended Practice 583, Corrosion Under Insulation and Fireproofing API Standard 598, Valve Inspection and Testing API Standard 620, Design and Construction of Large, Welded, Low-pressure Storage Tanks https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 3 API Standard 625, Tank Systems for Refrigerated Liquefied Gas Storage API Standard 650, Welded Tanks for Oil Storage API Recommended Practice 651, Cathodic Protection of Aboveground Petroleum Storage Tanks API Recommended Practice 652, Linings of Aboveground Petroleum Storage Tank Bottoms API Bulletin 939-E, Identification, Repair, and Mitigation of Cracking of Steel Equipment in Fuel Ethanol Service API Standard 2000, Venting Atmospheric and Low-pressure Storage Tanks API Recommended Practice 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents API Standard 2015, Requirements for Safe Entry and Cleaning of Petroleum Storage Tanks API Standard 2610, Design, Construction, Operation, Maintenance, and Inspection of Terminal and Tank Facilities ACI 376 1, Code Requirements for Design and Construction of Concrete Structures for the Containment of Refrigerated Liquefied Gases and Commentary AISC 2, Steel Construction Manual ASME Boiler and Pressure Vessel Code (BPVC) 3, Section V: Nondestructive Examination ASME Boiler and Pressure Vessel Code (BPVC), Section VIII: Rules for the Construction of Pressure Vessels ASME Boiler and Pressure Vessel Code (BPVC), Section IX: Welding, Brazing, and Fusing Qualifications ASNT CP-189 4, Standard for Qualification and Certification of Nondestructive Testing Personnel ASNT SNT-TC-1A, Personnel Qualification and Certification in Nondestructive Testing ASTM D3359 5, Standard Test Methods for Rating Adhesion by Tape Test EEMUA 159 6, Users’ Guide to the Inspection, Maintenance and Repair of Aboveground Vertical Cylindrical Steel Storage Tanks—Volume 1 NFPA 30 7, Flammable and Combustible Liquids Code OSHA 8, 29 CFR Part 1910.23, Ladders OSHA, 29 CFR Part 1910.106, Flammable Liquids 1 American Concrete Institute, 38800 Country Club Dr., Farmington Hills, MI 48331, www.aci-int.org. 2 American Institute of Steel Construction, 130 East Randolph, Suite 2000, Chicago, IL 60601, www.aisc.org. 3 ASME International, 2 Park Avenue, New York, NY 10016-5990, www.asme.org. 4 American Society for Nondestructive Testing, 1711 Arlingate Lane, P.O. Box 28518, Columbus, OH 43228, www.asnt.org. 5 ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428, www.astm.org. 6 Engineering Equipment and Materials Users Association, 16 Black Friars Lane, Second Floor, London EC4V 6EB, UK, www.eemua.org. 7 National Fire Protection Association, 1 Batterymarch Park, Quincy, MA 02169-7471, www.nfpa.org. 8 U.S. Department of Labor, Occupational Safety and Health Administration, 200 Constitution Avenue NW, Washington, DC 20210, www.osha.gov. https://t.me/MyAPI https://t.me/MyInspectors 4 API RECOMMENDED PRACTICE 575 OSHA, 29 CFR Part 1910.146, Permit-required Confined Spaces STI SP001 9, Standard for Inspection of Aboveground Storage Tanks UL 142 10, Standard for Safety Steel Aboveground Tanks for Flammable and Combustible Liquids Steel 3 Terms and Definitions For the purposes of this document, the following definitions apply. 3.1 alteration Any work on a tank that changes the physical dimensions or configuration, in accordance with API Std 653 guidelines. 3.2 applicable standard The original standard of construction, such as API standards or specifications or Underwriter Laboratories (UL) standards, unless the original standard of construction has been superseded or withdrawn from publication; in this event, applicable standard means the current edition of the appropriate standard. See API Std 653, Annex A for background on editions of API welded storage tank standards. 3.3 atmospheric pressure When referring to vertical tanks, the term “atmospheric pressure” usually means tanks designed to API Std 650, although API Std 620 uses the term atmospheric pressure to describe tanks designed to withstand an internal pressure not exceeding the weight of the roof plates. API Std 650 also provides for rules to design tanks for “higher internal pressure” up to 2.5 psi (18 kPa). API Std 653 uses the generic meaning for atmospheric pressure to describe tanks designed to withstand an internal pressure up to, but not exceeding, 2.5 psi (18 kPa) gauge. 3.4 authorized inspection agency Any one of the following that can provide inspection services of authorized tank inspectors: a) the inspection organization of the jurisdiction in which the aboveground storage tank is located; b) the inspection organization of an insurance company that is licensed or registered to write and does write aboveground storage tank insurance; c) the inspection organization of an owner or operator of one or more aboveground storage tank(s) who maintains an inspection organization for activities relating only to their equipment and not for aboveground storage tanks intended for sale or resale; or d) an independent organization or individual that is under contract to and under the direction of an owner or operator and that is recognized by the jurisdiction in which the aboveground storage tank is operated. The owner or operator’s inspection program should provide the controls necessary when contract inspectors are used. 9 Steel Tank Institute/Steel Plate Fabricators Association (STI/SPFA), 944 Donata Court, Lake Zurich, IL 60047, www.steeltank.com 10 Underwriters Laboratories, 333 Pfingsten Road, Northbrook, IL 60062, www.ul.com. https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 5 3.5 authorized inspector An employee of an authorized inspection agency, who is qualified and certified to perform inspections under API Std 653, Annex D. Whenever the term “inspector” is used in API RP 575, it refers to an authorized aboveground storage tank inspector. 3.6 bottom sediment and water BS&W Bottom sediment and water (BS&W) that may settle out or exist in the non-agitated portion of the tank. 3.7 change in service A change from previous operating conditions involving different properties of the stored product such as specific gravity, corrosivity, or different service conditions of temperature or pressure. 3.8 condition monitoring location CML Designated areas on tanks where periodic examinations are conducted in order to directly assess the condition of the tank. Condition monitoring locations (CMLs) may contain one or more examination points and utilize multiple inspection techniques that are based on the predicted damage mechanism to give the highest probability of detection. CMLs can be a single small area on a tank (e.g. a 2-in. diameter spot or a plane through a section of a nozzle) where recording points exist in all four quadrants of the plane. NOTE CMLs include, but are not limited to, thickness measurement locations (TMLs). 3.9 corrosion allowance Additional material thickness available to allow for metal loss during the service life of the tank component. 3.10 corrosion rate The rate of metal loss due to erosion, corrosion, a chemical reaction(s) with the environment, either internal or external. Corrosion rates depend on how and where the measurements are taken. Guidance on determining corrosion rates in tanks is provided in API Std 653. 3.11 corrosion specialist A person, acceptable to the owner/operator, who has knowledge and experience in corrosion damage mechanisms, metallurgy, materials selection, and corrosion monitoring techniques. 3.12 corrosion under insulation CUI Refers to all forms of corrosion under the insulation (CUI) including stress corrosion cracking (SCC). 3.13 damage mechanism Any type of deterioration encountered in the refining and chemical process industry that can result in flaws or defects that can affect the integrity of tanks (e.g. corrosion, cracking, erosion, dents, and other mechanical, physical, or chemical impacts). See API RP 571 for a comprehensive list and description of damage mechanisms. https://t.me/MyAPI https://t.me/MyInspectors 6 API RECOMMENDED PRACTICE 575 3.14 defect An imperfection whose type or size exceeds the applicable acceptance criteria and is therefore rejectable. 3.15 documentation Records containing descriptions of specific tank design, personnel training, inspection plans, inspection results, nondestructive examination (NDE), repair, alteration, operating heights or capacities, Fitness-For-Service (FFS) assessments, procedures for undertaking these activities, or any other information pertinent to maintaining the integrity and reliability of the tank. 3.16 examinations Quality control (QC) functions performed by examiners in accordance with approved NDE procedures. 3.17 examiner A person who assists the API authorized tank inspector by performing specific NDE on the tank and interprets the results for compliance with the applicable acceptance criteria, whereas the authorized inspector evaluates the results of those examinations in accordance with API Std 653 and this recommended practice. 3.18 external inspection A visual inspection performed from the outside to find conditions that could impact the tank’s ability to maintain integrity or conditions that compromise the integrity of the supporting structures (e.g. ladders, platforms, supports). The external inspection may be done either while the tank is operating or while the tank is out-of-service. 3.19 Fitness-For-Service evaluation FFS evaluation A methodology where flaws and other deterioration or damage or operating conditions contained within a tank are assessed in order to determine the integrity of the tank for continued service. 3.20 general corrosion Corrosion that is distributed more or less uniformly over the surface of the metal, as opposed to localized corrosion. 3.21 heat-affected zone The portion of the base metal whose mechanical properties or microstructure have been altered by the heat of welding or thermal cutting. 3.22 hydrostatic pressure The pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity. Hydrostatic pressure increases in proportion to the depth measured from the surface because of the increasing weight of the fluid exerting downward force from above. 3.23 indications A response or evidence resulting from the application of NDE that may be nonrelevant or could be flaws or defects upon further analysis. https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 7 3.24 in service Designates a tank that has been placed in operation and has not been emptied and cleaned. 3.25 in-service inspection All inspection activities associated with a tank once it has been placed in service but before it is emptied and cleaned. 3.26 inspection The external, internal, or in-service evaluation (or any combination of the three) of the condition of a tank conducted in accordance with API Std 653. 3.27 inspection plan A strategy defining how and when a tank or its devices will be inspected, repaired, and/or maintained. 3.28 inspector (See definition for authorized inspector.) 3.29 internal inspection An inspection performed from the inside of a tank using visual and NDE techniques. 3.30 jurisdiction A legally constituted governmental administration that may adopt rules relating to tanks. 3.31 localized corrosion Corrosion that is largely confined to a limited or isolated area of the metal surface of a tank. 3.32 magnetic flux leakage MFL An NDE method utilizing electromagnetic or permanent magnets to detect corrosion and pitting on ferromagnetic steels. 3.33 minimum acceptable thickness The lowest thickness at which a tank component should operate, as determined by the parameters in the applicable tank design standard (such as API Std 650, API Std 653, etc.), the FFS principles in API 579-1/ASME FFS-1, or other appropriate engineering analysis. 3.34 out of service Designates that a tank is clean and empty of product. A tank during original construction, removed from service and cleaned for repair, or a retired tank. 3.35 out-of-service inspection All inspection activities that are conducted after the tank has been cleaned and declared to be vapor free. https://t.me/MyAPI https://t.me/MyInspectors 8 API RECOMMENDED PRACTICE 575 3.36 owner/operator The legal entity having control of and/or responsibility for the operation and maintenance of an existing storage tank. 3.37 procedures A document that specifies or describes how an activity is to be performed. It may include methods to be employed, equipment or materials to be used, qualifications of personnel involved, and sequence of work. 3.38 product-side The interior surface of a tank bottom, usually used when describing corrosion. Another term with the same meaning is “top-side.” 3.39 reconstruction The work necessary to re-assemble a tank that has been dismantled and relocated to a new site. 3.40 reconstruction organization The organization having assigned responsibility by the owner/operator to design and/or reconstruct a tank. 3.41 release prevention barrier RPB Steel bottoms, synthetic materials, clay liners, and all other barriers or combination of barriers placed in the bottom of or under an aboveground storage tank, which have the following functions: a) preventing the escape of contaminated material, and b) containing or channeling released material for leak detection. 3.42 repair Any work necessary to maintain or restore a tank to a condition suitable for safe and leak-free operation, in accordance with API Std 653. 3.43 risk-based inspection RBI A risk assessment and management process that considers both the probability of failure and the consequence of failure due to a defined damage mechanism (ref. API RP 571) and that is focused on inspection planning to prevent loss of containment of storage tanks due to material deterioration. NOTE 1 Refer to API RP 571 and API RP 580 for further details on damage mechanisms and risk-based inspections (RBIs), respectively. NOTE 2 These risks are managed primarily through inspection in order to influence the probability of failure but can also be managed through various other methods to control the probability and consequence of failure. 3.44 soil-side The exterior surface of the tank bottom, usually used when describing corrosion. Other terms with the same meaning are “under-side” or “bottom-side.” https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 9 3.45 sounding An inspection technique that relies on impacting the surface with a tapping rod and listening for hollow sounds that indicate delamination or loss of continuity. 3.46 storage tank engineer An engineer acceptable to the owner/operator who is knowledgeable and experienced in the engineering disciplines associated with evaluating mechanical and material characteristics affecting the integrity and reliability of tank components and systems. The tank engineer, by consulting with appropriate specialists, should be regarded as a composite of all entities necessary to properly address technical requirements and engineering evaluations. 3.47 tank The term tank refers to aboveground storage tanks, typically either a fixed or floating-roof design, or both. 3.48 temporary repairs Repairs made to storage tanks to restore sufficient integrity to continue safe operation until permanent repairs are conducted. 3.49 top-side See definition for product-side. 3.50 water bottom See definition for BS&W. 4 Types of Storage Tanks 4.1 General Tank Design and Materials Storage tanks are used to store fluids such as crude oil, intermediate and refined products, gas, chemicals, waste products, water, and water/product mixtures. Important factors such as the volatility of the stored fluid and the desired storage pressure and temperature result in tanks being built of various types, sizes, and materials of construction. In this document, only atmospheric and low-pressure storage tanks are considered. Guidelines for inspection of pressure vessels operating at pressures greater than 15 psi (103 kPa) gauge are covered in API RP 572. There are two types of tanks commonly found in industry: fixed roof and floating roofs. Fixed roof tanks can be constructed out of carbon steel, alloy steel, aluminum, or other metals. In addition to these materials, tanks can be constructed out of nonmetallic materials such as concrete, reinforced thermoset plastics, and wood (API Std 12E). Floating-roof tanks are commonly made out of carbon steel but can also be made of other alloys. Atmospheric tanks are generally welded but can also be riveted (API Std 12A) or bolted (API Spec 12B). Storage Tanks with Linings and/or Cathodic Protection Where internal corrosion is experienced or expected, tanks can be lined with a variety of corrosion-resistant materials such as coatings of epoxy or vinyl, fiberglass, poured or sprayed concrete, alloy steel, aluminum, rubber, lead, synthetics such as high-density polyethylene (HDPE) or other synthetic rubber, and glass. https://t.me/MyAPI https://t.me/MyInspectors 10 API RECOMMENDED PRACTICE 575 See API RP 652 for provisions for the application of some types of tank bottom linings to both existing and new storage tanks. Cathodic protection systems are often provided for control of external bottom corrosion and, combined with internal linings, may also be used to protect tank bottoms internally. See API RP 651 for design, maintenance, and monitoring recommendations for such systems. Storage Tanks with Passive Leak Detection Systems Leak detection systems are installed for early awareness of a potential leak. API Std 650, Annex I provides design guidelines for leak detection and subgrade protection. Reference also API Publication 306, API Publication 307, API Publication 315, API Publication 322, API Publication 323, API Publication 325, API Publication 334, API Publication 340, and API Publication 341 for additional information on leak detection systems for storage tanks and dike containment areas. Storage Tanks with Auxiliary Equipment Most storage tanks are provided with some of the following auxiliary equipment such as liquid-level gauges, high-and low-level alarms and other overfill prevention systems, pressure-relieving devices, vacuum venting devices, emergency vents, gauging hatches, roof drain systems, flame arrestors, fire protection systems, and mixing devices. Stairways, ladders, platforms, handrails, piping connections and valves, manholes, electric grounding connections (as required), and cathodic protection systems are considered examples of storage tank auxiliary equipment. Insulation may also be present to maintain product temperature. Insulation can vary from externally jacketed panel systems to sprayed-on foam systems to loose-fill systems in double-wall tank construction. Inspection and failure of auxiliary equipment are covered in 5.5. 4.2 Atmospheric Storage Tanks Construction, Materials, and Design Standards Atmospheric storage tanks are designed to operate with internal gas and vapor spaces at pressures close to atmospheric pressure. Such tanks are usually constructed of carbon steel, alloy steel, aluminum, or other metals, depending on service. Additionally, some tanks are constructed of nonmetallic materials such as reinforced concrete, reinforced thermoset plastics, and wood. Some wooden tanks constructed to API Std 12E are still in service. Atmospheric storage tanks are generally welded. Some riveted tanks constructed to API Std 12A and some bolted tanks constructed to API Spec 12B can also be found still in service. Information for the construction of atmospheric storage tanks is given in API Std 12A (withdrawn), API Spec 12B, API Spec 12C (the predecessor to API Std 650 and now withdrawn), API Spec 12D, API Std 12E (withdrawn), API Spec 12F, API Std 650, API Std 620, and API Std 2000. API Std 625 covers the selection, design and construction of tank systems for refrigerated liquefied gas storage on land. API Std 653 provides information pertaining to requirements for inspection, repair, and reconstruction of aboveground storage tanks. Use of Atmospheric Storage Tanks Atmospheric storage tanks in the petroleum industry are normally used for fluids having a true vapor pressure that is less than atmospheric pressure. Vapor pressure is the pressure on the surface of a confined liquid caused by the vapors of that liquid. Vapor pressure increases with increasing temperature. Crude oil, heavy oils, gas oils, furnace oils, naphtha, gasoline, and nonvolatile chemicals are usually stored in atmospheric storage tanks. Many of these tanks are protected by pressure/vacuum vents that limit the pressure difference between the tank vapor space and the outside atmosphere to a few ounces per square inch. https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 11 Non-petroleum industry uses of atmospheric tanks include storage of a variety of chemicals and other substances operated in closed-loop systems not vented to atmosphere and with pressure control and relief devices as required. These tanks may be designed and operated as low-pressure storage tanks per API Std 620. See 4.3 for additional information on tanks operated at low pressure. Additional uses for atmospheric storage tanks can include the storage of liquid (both hydrocarbon and non-hydrocarbon), storage in horizontal vessels, and the storage of process liquids or granular solids in skirt-supported or column-supported tanks with elevated cone bottoms (non-flat bottom), and the storage of process water/liquids in open-top tanks. Types of Atmospheric Storage Tank Roofs 4.2.3.1 Cone Roof Tanks The most common type of atmospheric storage tank is the fixed cone roof tank (see Figure 1). Fixed cone roof tanks may typically be up to 300 ft (91.5 m) in diameter and 64 ft (19.5 m) in height, although larger-diameter tanks have been built. These roofs are normally supported by internal structural rafters, girders, and columns but can be fully self-supporting in smaller diameters [typically 60 ft (18.3 m) diameter or less]. Self-supporting aluminum dome roofs (e.g. geodesic dome roofs) may be applied to most diameter tanks without the need for internal supporting columns. 4.2.3.2 Umbrella Roof Tanks The umbrella roof tank (shown in Figure 2) and the self-supporting aluminum dome roof tank (shown in Figure 3) are variations of the fixed roof tank. The umbrella roof has radially arched segmental plates with integral framing support members (usually without internal support columns). The aluminum dome utilizes the geometric properties of the geodesic design and tubing members covered by aluminum sheeting for strength. In the steel dome roof tank shown in Figure 4, the roof plates are usually formed with curved segments joined to be self-supporting. Figure 1—Cone Roof Tank https://t.me/MyAPI https://t.me/MyInspectors 12 API RECOMMENDED PRACTICE 575 Figure 2—Umbrella Roof Tank 4.2.3.3 External Floating-roof Tanks General The floating-roof tank is another common type of atmospheric storage tank. The floating-roof tank is designed to minimize filling and breathing losses by eliminating or minimizing the vapor space above the stored liquid. The shell and bottom of this type of tank are similar to those of the fixed roof tanks, but in this case, the roof is designed to float on the surface of the stored liquid. Older styles of external floating roofs include single steel deck designs without annular pontoons as shown in Figure 5. This external floating roof type is no longer permitted under API Std 650, Annex C. Such roofs have no reserve buoyancy and are susceptible to sinking in service. Figure 3—Self-supporting Aluminum (Geodesic) Dome Roof Tank https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 13 Figure 4—Self-supporting Dome Roof Tank Figure 5—Externally Stiffened Pan-type Floating-roof Tank Variations Annular-pontoon and double-deck roofs are shown in Figure 6 and Figure 7, respectively. Some floating-roof tanks have fixed aluminum geodesic dome roofs installed on top of the tank shell to reduce product vapor loss or to eliminate the need to drain rainwater from the roof. These are considered internal floating roofs for purposes of emissions control, confined space entry, etc. https://t.me/MyAPI https://t.me/MyInspectors 14 API RECOMMENDED PRACTICE 575 Figure 6—Annular-pontoon Floating-roof Tank Figure 7—Double-deck Floating-roof Tank Features Cross-sectional sketches showing important features of floating roofs are shown in Figure 8. Floating-roof sealing systems are used to seal the space between the tank wall and the floating roof, typically with a mechanical seal. This type of seal consists of a shoe that is a plate pressed against the tank wall by springs (or by counterweights in older designs) or other tensioning system, with a flexible vapor membrane attached between the shoe and the floating-roof outer rim. Typical examples of this type of floating-roof seal are shown in Figure 9, Figure 10, and Figure 11. One alternative seal detail occasionally still found in existing tanks is the tube seal shown in Figure 12. These tubes are filled with solid foam, liquid, or air. Figure 13 illustrates various pontoon roofs and seal details. https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 15 Continuous fabric Roof support brace Gauge hatch Roof support Truss rod Center truss ring Turnbuckle Roof support Truss post Manhole Truss rod Screen Deck Sealing Sump Liquid ring Gusset plate Roof Truss structural member surface Liquid level Tank shell Pipe drain Liquid PAN FLOATING ROOF Continuous fabric Roof supports Automatic bleeder vent Gauge hatch Roof supports Pontoon Bleeder vent Emergency manhole drain Siphon Deck manhole Pontoon Deck drain Pipe Pan Liquid Sealing Tank shell ring Liquid Roof Liquid surface level PONTOON FLOATING ROOF Continuous fabric Automatic bleeder vent Open drain Gauge hatch Manhole Roof support Manhole Manhole Deck Roof support Emergency drain Sealing ring Liquid Roof Liquid surface level Liquid Tank shell DOUBLE DECK FLOATING ROOF Figure 8—Cross-section Sketches of Floating-roof Tanks Showing the Most Important Features https://t.me/MyAPI https://t.me/MyInspectors Figure 9—Floating-roof Shoe Seal Figure 10—Floating-roof Log Seal https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 17 Figure 11—Floating Roof Using Counterweights to Maintain Seal Figure 12—Floating Roof Using Resilient Tube-type Seal https://t.me/MyAPI https://t.me/MyInspectors 18 API RECOMMENDED PRACTICE 575 Figure 13—Typical Arrangement for Metallic Float Internal Floating-roof Seals 4.2.3.4 Internal Floating Roof Tanks Another type of tank has both a fixed roof and an internal floating roof. The fixed roof is usually a supported cone or dome (of steel or aluminum). The internal floating roof can be constructed of steel, aluminum, or other material, as shown in Figure 14. Such tanks are usually built to alleviate weather-related concerns about the flotation of an external floating roof, to reduce vapor emissions, or to prevent product contamination. An existing fixed roof tank often can be modified by the installation of an internal floating roof. Cone roofs with an internal floating roof supported by cables suspended from the fixed cone roof are another design that is being used (see Figure 15). https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 19 Figure 14—Typical Internal Floating-roof Components Figure 15—Cable-supported Internal Floating-roof Tank https://t.me/MyAPI https://t.me/MyInspectors 20 API RECOMMENDED PRACTICE 575 API Std 650, Annex H classifies internal floating roofs into the following types. a) Metallic pan internal floating roofs have a peripheral rim above the liquid for buoyancy. These roofs are in full contact with the liquid surface and are typically constructed of steel. See Figure 5. b) Metallic open-top bulk-headed internal floating roofs have peripheral open-top bulk-headed compartments for buoyancy. These roofs are in full contact with the liquid surface and are typically constructed of steel. c) Metallic pontoon internal floating roofs have peripheral closed-top bulk-headed compartments for buoyancy. These roofs are in full contact with the liquid surface and are typically constructed of steel. d) Metallic double-deck internal floating roofs have continuous closed top and bottom decks that contain bulk-headed compartments for buoyancy. These roofs are in full contact with the liquid surface and are typically constructed of steel. e) Metallic internal floating roofs on floats have their deck above the liquid, supported by closed pontoon compartments for buoyancy. These roof decks are not in full contact with the liquid surface and are typically constructed of aluminum alloys or stainless steel. f) Metallic sandwich-panel/composite internal floating roofs have metallic composite material panel modules for buoyancy compartments. Panel modules may include a honeycomb or closed cell foam core; however, cell walls within the panel module are not considered “compartments” for purposes of inspection and design buoyancy requirements. These roofs are in full contact with the liquid surface and are typically constructed of aluminum alloys or purchaser-approved composite materials. g) Hybrid internal floating roofs shall, upon agreement between the purchaser and the manufacturer, be a design combination of roof types having bulk-headed compartments with closed-top perimeter pontoon and open-top center compartments for buoyancy. These roofs are in full contact with the liquid surface and are typically constructed of steel. h) Other roof materials or designs if specified and described in detail by the purchaser. 4.2.3.5 Other Roof Designs General Other less commonly used atmospheric storage tank roof details include the lifter-type roof and the breather-type roof. Lifter-type Roof Tanks Lifter-type roofs prevent vapor losses from the tank by means of liquid or dry seals. Liquid-seal lifter roofs have a skirt on the roof edge that fits into a trough filled with liquid. Dry-seal lifter roofs have a flexible membrane connected to the tank wall and a skirt on the roof edge. In these two lifter-type roofs, the roof is free to move up and down within limits as the tank is filled and emptied or when a change in temperature causes vaporization of the stored product. These types of lifter-roof tanks are less commonly found in service today than in the past. Breather-type Roof Tanks In the breather-type roof, a number of methods are used to provide expansion space for vapors without using a loose external roof. The plain breather-type tank (shown in Figure 16) has a flat roof that is essentially a flexible steel membrane that is able to move up and down within rather narrow limits. The balloon-type roof (shown in Figure 18) is a modification of the plain breather-type roof that is capable of a greater change of volume. A tank with a vapor-dome roof (shown in Figure 17 and Figure 19) uses an added fixed dome with a flexible membrane attached to the walls that is free to move up and down. This type of vapor roof may be designed to provide for any desired change in volume. Vapor recovery systems may use this type of tank. https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 21 Figure 16—Plain Breather Roof Tanks https://t.me/MyAPI https://t.me/MyInspectors 22 API RECOMMENDED PRACTICE 575 Figure 17—Tank with Vapor Dome Roof Figure 18—Balloon Roof Tank https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 23 Figure 19—Cutaway View of Vapor Dome Roof Vapor Recovery Systems Vapor recovery systems can be provided on several types of tanks such as a fixed cone roof like Figure 1, umbrella roofs like Figure 2, and vapor dome roofs like Figure 17. Adjustment in relief valve settings will be required to accommodate the operating parameters for the vapor recovery system. 4.3 Low-pressure Storage Tanks Construction, Materials, and Design Standards Low-pressure storage tanks are those designed to operate with pressures in their gas or vapor spaces exceeding the 2.5 psi (18 kPa) gauge permissible in API Std 650 but not exceeding the 15 psi (103 kPa) gauge maximum limitation of API Std 620. These tanks are generally constructed of carbon or alloy steel and are usually welded, although riveted tanks in low-pressure service are still found. Rules for the design and construction of large, welded, low-pressure storage tanks are included in API Std 620. Venting requirements are covered in API Std 2000. API Std 625 addresses tank systems designed for storing refrigerated liquefied gas. A tank system consists of one or more containers together with various accessories, appurtenances, and insulation. The metal storage containers themselves are addressed by API Std 620, Annex R for steel containers for refrigerated products from 40 °F (4 °C) to −60 °F (−51 °C) and by API Std 620, Annex Q for steel containers for −60 °F (−51 °C) to −325 °F (−198 °C). ACI 376 addresses concrete containers for 40 °F (4 °C) to −270 °F (−168 °C). Use of Low-pressure Storage Tanks Low-pressure storage tanks are used for the storage of the more volatile fluids having a true vapor pressure exceeding the pressure limits of API Std 650 but not more than 15 psi (103 kPa) gauge. Light crude oil, gasoline blending stock, light naphtha, pentane, volatile chemicals, liquefied petroleum gas (LPG), liquefied natural gas (LNG), liquid oxygen, and liquid nitrogen are examples of liquids that should be stored in low-pressure storage tanks. API Std 620, Annexes R and Q, and ACI 376 provide single-wall and double-wall construction details. https://t.me/MyAPI https://t.me/MyInspectors 24 API RECOMMENDED PRACTICE 575 Types of Low-pressure Storage Tanks 4.3.3.1 Cylindrical Tanks Tanks that have cylindrical shells and cone or dome roofs are typically used for pressures less than about 5 psi (34.5 kPa) gauge. Tank bottoms may be flat or have a shape similar to the roof. Hold-down anchorage of the shell is generally required. For pressures above about 5 psi gauge (34.5 kPa) gauge, hemispheroid, spheroid, and noded spheroid tank types are commonly used. Tanks for this application are now typically constructed as spheres. Such tanks are designed to withstand the vapor pressure that may be developed within a tank having no devices or means to change or relieve the internal volume. As with atmospheric storage tanks, these tanks are provided with relief valves to prevent pressures from rising above design values. 4.3.3.2 Plain and Noded Spherical Tanks Tanks with a plain spherical roof and tanks with a noded spherical roof are shown in Figure 20 and Figure 21, respectively, and cross-sectional view is shown in Figure 22. Figure 23 shows a plain spheroid, while Figure 24 shows a spherical roof with a knuckle radius or smooth transition at the intersection of the shell and top head. Figure 20—Plain Hemispheroids https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 25 Figure 21—Noded Hemispheroid Figure 22—Drawing of Hemispheroid https://t.me/MyAPI https://t.me/MyInspectors 26 API RECOMMENDED PRACTICE 575 Figure 23—Plain Spheroid Figure 24—Plain Hemispheroid with Knuckle Radius https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 27 4.3.3.3 Spheroids The spheroid uses elements of different radii, resulting in a somewhat flattened shape as shown in Figure 20. The noded spheroid, shown in Figure 25, is used for larger volumes, and internal ties and supports help to distribute the shell stresses. Figure 26 shows a cross-section of a noded spheroid. Noded spheroids are no longer constructed; they have been replaced either by spheres or by vertical cylindrical storage tanks as referenced in API Std 620. Figure 25—Noded Spheroid Figure 26—Drawing of Noded Spheroid Tank Systems for Refrigerated, Liquefied Gas Storage API Std 625, Section 5 defines and describes various storage concepts for refrigerated liquefied gas tank systems. These include single, double, and full containment concepts. Some of these concepts are briefly described as follows. https://t.me/MyAPI https://t.me/MyInspectors 28 API RECOMMENDED PRACTICE 575 4.3.4.1 Single Containment—This system incorporates a liquid-tight container and a vapor-tight container. There are several variants to the single containment concept such as the following. 1) Single wall—Single low-temperature steel or concrete tank containing the cold liquid with warm vapor-containing roof, suspended deck with insulation, and external wall insulation. (See API Std 625, Figure 5.1.) 2) Single wall—Single low-temperature steel or concrete tank containing the cold liquid with low-temperature vapor-containing roof, external roof insulation, and external wall insulation. (See API Std 625, Figure 5.2.) 3) Double wall—Single low-temperature steel or concrete tank containing the cold liquid with a warm vapor-containing roof, suspended deck with insulation, annular space insulation, and warm vapor-containing outer tank (concrete or steel). (See API Std 625, Figure 5.3.) 4) Double wall—Single low-temperature steel or concrete tank containing the cold liquid with low-temperature vapor-containing roof, external roof insulation, annular space insulation, and warm vapor-containing outer tank (concrete or steel). (See API Std 625, Figure 5.4.) 4.3.4.2 Double Containment—An inner tank (low-temperature steel or concrete) containing the cold liquid surrounded by a secondary containment tank of steel or concrete that holds any leaked liquid but not any leaked vapor. There are several variants to the double containment concept such as the following. — Low-temperature steel or concrete primary liquid container, secondary low-temperature steel or concrete secondary liquid container, suspended deck with insulation, warm vapor-containing roof, and insulation on primary liquid container shell. (See API Std 625, Figure 5.5.) 4.3.4.3 Full Containment—An inner tank (low-temperature steel or concrete) containing the cold liquid surrounded by a secondary containment tank of steel or concrete that holds any leaked liquid and provides for a controlled release of vapor. There are several variants to the full containment concept as follows. 1) Low-temperature steel or concrete primary liquid container, secondary low-temperature steel or concrete secondary liquid container, suspended deck with insulation, warm vapor-containing roof, and annular space insulation between the liquid containers. (See API Std 625, Figure 5.7.) 2) Concrete primary liquid container, concrete secondary containment, and concrete roof. (See API Std 625, Figure 5.10.) 5 Reasons for Inspection and Causes of Deterioration 5.1 Reasons for Inspection General The basic reasons for inspection are to determine the physical condition of the tank and to determine the type, rate, and causes of damage mechanisms and associated deterioration. This information should be carefully documented during each inspection (see Section 10 for a list of example documentation). The information and data gained from an inspection contribute to the planning of future inspections, repairs, and replacement and yield a history that should form the basis of good quality permanent inspection records. Process Safety and Environmental Protection One of the primary reasons to conduct periodic scheduled inspections is to identify deficiencies that could result in a process safety incident, such as loss of containment, which could lead to fire, toxic exposure, or other environmental hazard. These deficiencies should be addressed as soon as practical through evaluation, further inspection, or repair. https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 29 In-service NDE In-service inspections performed while the equipment is in operation using nondestructive techniques, including techniques such as robotic magnetic flux leakage (MFL) or acoustic emission examination, may reveal important information without requiring entry into the tank. With such data and information, FFS or RBI evaluations can be performed, which can aid in maximizing the period of operation without taking the tank out of service. In addition, repair and replacement requirements can be planned and estimated in advance of taking the tank out of service to more effectively utilize downtime. These efforts can therefore contribute to overall plant availability by minimizing required downtime. Regulatory Requirements 5.1.4.1 General In general, regulatory bodies require the compliance with an industry standard or code or adherence to Recognized and Generally Accepted Good Engineering Practice (RAGAGEP) when performing any inspection and repair activities. Many regulatory groups (including OSHA and PHMSA in the United States) require that operating companies follow internal procedures in addition to applicable codes and standards. 5.1.4.2 Regulatory API Std 653 was developed to provide an industry standard for the inspection of aboveground storage tanks. It has been adopted by a number of regulatory and jurisdictional authorities. Internal procedures regarding inspection of tanks should encompass the requirements outlined in API Std 653 to help ensure compliance with many of the regulatory and jurisdictional authorities. Owner/operators should be familiar with the regulatory requirements, including which standards and editions are recognized by their regulatory agencies, as the latest edition of the standard is not always the one referenced. 5.1.4.3 Emissions Control In the United States, the Environmental Protection Agency (EPA) has issued a Spill Prevention, Control, and Countermeasure (SPCC) rule that covers most petroleum storage facilities. These regulations allow tank owners and operators to use industry standards and practices to implement and ensure an effective storage tank integrity program. Currently, the most widely recognized tank inspection standards are API Std 653, API Std 12R1, STI SP001, and EEMUA 159. Regulatory requirements for emission sources (such as floating-roof seals and tank vents) should be considered when establishing the inspection plans for tanks, as some environmental regulations require shorter intervals than those stipulated by API Std 653. In some cases, more frequent inspections or additional inspections of some emission sources are required. 5.1.4.4 Mechanical Integrity and Reliability Inspections are an important part of avoiding failures, maintaining safety, and optimizing availability. Therefore, it is prudent to take a proactive approach toward storage tank inspection and maintenance to ensure continued integrity and reliability of the assets. 5.2 Deterioration of Tanks General Corrosion is the prime cause for the deterioration of steel storage tanks and accessories. Locating and measuring the extent of corrosion is a major reason for storage tank inspection. If left unchecked, tank deterioration can progressively lead to failure, which may have adverse effects such as endangering personnel or the public, environmental and property damage, and business interruptions or damage to reputation. https://t.me/MyAPI https://t.me/MyInspectors 30 API RECOMMENDED PRACTICE 575 External Corrosion 5.2.2.1 Atmospheric Corrosion Atmospheric corrosion can occur on all metallic tank components exposed to the atmosphere. The type of tank, construction details, and environmental conditions can all affect the location, extent, and severity of external corrosion. For example, a sulfurous, acidic, or marine atmosphere can damage protective coatings and increase the rate of corrosion. External surfaces of the tank and auxiliary equipment will corrode more rapidly if they are not protected with coatings where surfaces are in contact with moisture or the ground. Extended contact with water is likely to cause localized corrosion. Such susceptible areas should be protected with coatings designed to withstand long-term immersion, or the owner/operator should consider alternative mitigation strategies. Inspections should target areas where tank construction details cause water or sediment to accumulate. For further information on atmospheric corrosion, refer to API RP 571. 5.2.2.2 Tank Bottoms External corrosion of tank bottoms can be significant. The foundation material used for forming a pad that is directly in contact with the steel bottom plates can contain materials that promote corrosion. For example, cinder is known to contain sulfur compounds that become very corrosive in the presence of moisture. The presence of clay, wood, gravel, or crushed stone as contaminants in a sand pad can cause pitting corrosion at each point of contact. Faulty pad preparation or poor drainage can allow water to remain in contact with the tank bottom. If a tank previously leaked corrosive fluid through its bottom, accumulation of the fluid underneath the tank can cause external corrosion of the bottom plates. For tanks that are supported above grade, an improperly sealed ringwall, as shown in Figure 27, can allow moisture to accumulate between the tank and the support, thereby accelerating corrosion. The failing sealant should be repaired or removed. Asphalt-impregnated fiberboard is not a recommended sealant for tanks sitting on concrete ringwall foundations, as the fiberboard deteriorates and gaps are known to develop over time. The lower tank shell can be exposed to accelerated external corrosion near the grade line where soil movement has raised the grade level to cover the lower portion of the shell. Containment areas should be drained as soon as possible after water accumulates to minimize the possibility of bottom or lower shell corrosion. For further information on soil corrosion, refer to API RP 571. Figure 27—Foundation Seal 5.2.2.3 Corrosion Under Insulation (CUI) External corrosion also occurs when insulation absorbs ground or surface water by wicking action, or when damaged or improperly sealed openings around nozzles and attachments allow water behind insulation. For further information on CUI, refer to API RP 571 and API RP 583. In cases where the wall of the tank is partially below grade, the inspector should pay special attention at the soil interface for accelerated corrosion. https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 31 5.2.2.4 Riveted Tanks Riveted tanks are becoming increasingly rare in refinery and petrochemical plant operations, as they are difficult to design, construct, and maintain. However, some riveted tanks are still in service and are included in inspection plans. API Std 650 does not provide guidance for the construction of riveted tanks. Therefore, other design and construction standards should be referenced. Riveted tanks have many niches where concentration cell corrosion can occur (see 8.2.9.7). Leaks at the seams of riveted tanks can cause failure of external coatings, allowing external corrosion to develop. Internal Corrosion/Deterioration 5.2.3.1 General The occurrence of internal corrosion in a storage tank depends on the contents of the tank and its materials of construction. API RP 571 is a primary resource document for damage mechanisms and should be consulted when developing the inspection plan to ensure that proper inspection and examination techniques are applied. Annex A provides information on the more common NDE methods. 5.2.3.2 Product/Vapor/BS&W Corrosion Crude oil and petroleum product tanks are usually constructed of carbon steel. Internal corrosion of these tanks in the vapor space (i.e. above the liquid level) can be caused by hydrogen sulfide vapor, water vapor, oxygen, or any combination of these. In the areas in contact with the stored liquid, corrosion is commonly caused by acid salts, hydrogen sulfide or other sulfur compounds, dew point corrosion, or contaminated water that settles out and mixes with solids on the bottom of the tank, typically referred to as BS&W. Issues of SCC can be of particular concern when the product is known to be corrosive to welds and other heat-affected zones. Ethanol, diethanolamine (DEA), and caustic products are just a few of the products that can contribute to this condition when in contact with bare metal. 5.2.3.3 Linings In some cases, it is necessary to use linings (see API RP 652) that are more resistant to the stored fluid than are the materials of construction. In some particularly corrosive services, it may be necessary to construct the tanks of a more resistant material such as aluminum, stainless steels, or other alloys. These materials can experience deterioration from less common mechanisms such as caustic or chloride SCC, acid erosion, flow erosion, electrolytic reactions, and cyclic fatigue, among others. 5.3 Deterioration of Other than Flat Bottom and Non-steel Tanks General Tanks can be constructed of materials other than carbon steel. Aluminum, stainless steels and other alloys, wood, and concrete tanks can occasionally be found in refineries, chemical plants, and terminals. Wooden Tanks Tanks constructed of wood can rot unless they are adequately protected. They also can deteriorate from infestation by insects such as termites. Unless kept continually moist, these tanks can shrink and can leak when refilled. If steel bands are present, the bands can be subject to atmospheric corrosion and can loosen under repeated cycling. https://t.me/MyAPI https://t.me/MyInspectors 32 API RECOMMENDED PRACTICE 575 Concrete Tanks Concrete tanks can be attacked by the tank contents, crack because of ground settling or temperature changes, or spall due to atmospheric conditions, resulting in exposure of the steel reinforcement to further atmospheric corrosion. Non-steel Metal Tanks Tanks constructed of materials such as aluminum, stainless steels, and other alloys are usually used for special purposes, such as food processing (for product purity), or because of product corrosion concerns. They are subject to some of the same mechanical damage mechanisms as carbon steel tanks. In addition, external SCC of stainless steel tanks should be a concern if chlorides in insulation get wet and attack the stainless steel. Aluminum can be affected by impurities such as acids or mercury compounds in process streams or wastewater. Alternative Tank Construction Tanks can also be constructed of details that are not vertical, cylindrical, or flat bottomed. These tanks are usually classified as horizontal (axis of tank is in horizontal plane), skirt-supported, or column-supported with cone bottoms (with a vertical major axis). These latter tanks can be classified as bins or silos and very often hold granular or non-petroleum liquids or solids such as grain, cement, process liquids, carbon black, coker fines, and similar materials. Bins and silos, especially in granular product service, can suffer mechanical damage in operation including shell deformation and fatigue (from agitators or vibrators). Due to moisture entrapment, horizontal tanks on saddle supports can experience external corrosion at the saddle-to-tank interface. These areas are often inaccessible and difficult to inspect with normal inspection methods. Variations It is not possible to present all of the different or specific details that can be present in wooden tanks, concrete tanks, or steel bins and silos in this document. Careful examination and assessment should be planned based on prior inspection or similar service issues, type of construction details, and materials of construction. Structural integrity assessment shall require use of a qualified engineer familiar with the type of tank design and operation in question. 5.4 Leaks, Cracks, and Mechanical Deterioration General Storage tanks should be inspected for leaks (current or imminent) or defects to minimize or prevent loss, hazard to personnel, pollution of air, ground water, and waterways, and damage to other equipment. Brittle Fracture Brittle fracture and sudden loss of the contents of a tank can result in injuries to personnel and extensive damage to equipment in surrounding areas. Pollution of streams or waterways can result when such a tank failure occurs near a waterway or is connected to one by a sewer or other flow channel. Figure 29 illustrates the complete loss of a tank from brittle fracture. Proper design, fabrication, operation, and maintenance will minimize the probability of brittle fracture. A detailed discussion of brittle fracture can be found in API RP 571. API Std 653, Section 5.3 provides a procedure to assess the risk of tank failure due to brittle fracture and guidance for lowering the risk of brittle fracture. https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 33 Leaks Leaks are primarily the result of corrosion but can occur at improperly welded or riveted joints, at pipe thread or gasket connections or cover plates, or at crack-like flaws (including arc strikes on plates) in welds or in plate material. Three-plate laps in lap-welded tank bottoms are particularly prone to defects that can lead to leaks. Cracks Crack-like flaws can result from a number of causes including deficiencies in design, fabrication, and maintenance. The most likely points for crack-like flaws to occur are at the bottom-to-shell details, around nozzle connections, at manholes, around rivet holes or around rivet heads, at welded brackets or supports, and at welded seams. The lower shell-to-sketch plate or shell-to-bottom weld is especially critical because in relatively large or relatively hot tanks there is a higher likelihood for this detail to develop a crack-like flaw due to high stresses. Potential for this occurrence can be minimized by the use of thicker, butt-welded annular bottom plates, which are required by API Std 650 for higher design stress tanks and for larger elevated temperature tanks (see API Std 650, Sections 5.5 and M.4.1, respectively). Photographs of typical crack-like flaws in tanks are shown in Figure 28, Figure 30, and Figure 31. Other cracking mechanisms are possible, including SCC. All cracking mechanisms are discussed further in API RP 571. Figure 28—Cracks in Tank Shell Plate https://t.me/MyAPI https://t.me/MyInspectors 34 API RECOMMENDED PRACTICE 575 Figure 29—Extensive Destruction from Instantaneous Failure Figure 30—Cracks in Bottom Plate Welds Near the Shell-to-bottom Joint https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 35 Figure 31—Cracks in Tank at Riveted Lap Joint to Tank Shell Mechanical Deterioration Many other types of mechanical deterioration can develop over the service life of a storage tank. If such deterioration is discovered early through inspection, continued deterioration can be minimized and potential failures and leaks can be prevented. Early detection of deterioration and conditions that cause deterioration permit cost-effective maintenance and repair to be done on a scheduled basis, minimizing the risk of failure. Examples of conditions that cause other types of mechanical deterioration can include severe service conditions like frequent fill/withdrawal cycling or elevated temperature (see API Std 650, Annex M) affecting the integrity of the shell-to-bottom weld, bulging, peaking, denting, overpressure, etc. Settlement Settlement of a tank due to soil movement under the tank or tank foundation can also cause mechanical deterioration. Uniform settlement of the entire tank would not necessarily cause structural damage or be considered a serious issue. Large or uneven amounts of settlement can cause nozzles with attached piping to become overstressed and possibly deformed or cracked or interfere with the normal operation of a floating roof. Significant amounts of uneven settlement should be cause for concern and for further investigation. Edge settlement in tanks with cone-down bottoms can trap BS&W, resulting in bottom and lower shell corrosion in this area. Soil and water can also be retained against the external shell when such settlement is present. 5.5 Deterioration and Failure of Auxiliary Equipment Fixed Roof Equipment Pressure/vacuum vents and flame arrestors can fail to operate for the following reasons: a) the presence of fouling material or debris; b) corrosion between moving parts and guides or seats; c) deposit of foreign substances by birds or insects; https://t.me/MyAPI https://t.me/MyInspectors 36 API RECOMMENDED PRACTICE 575 d) formation of ice; e) accumulation of grit-blasting material; f) the covering of the vent opening with plastic or the plugging of vent openings with paint during coating operations that is not subsequently removed; g) tampering by unauthorized personnel; h) improper setting of the pressure relief actuation set point or vacuum relief actuation set point. Examination of tank venting devices should be included in a periodic inspection to ensure that their proper operation and protection are maintained. API RP 576 provides information regarding inspection of pressure-relieving devices and, specifically, weight-loaded pressure/vacuum relief devices in Section 4.3.2. Internal Equipment Gauge float leakage can be caused by corrosion or cracking. Inoperative pulleys, bent or broken float tapes, or plugged guides can cause float-type gauging devices to become inoperative. Equipment for draining water from floating roofs can be rendered inoperable by plugging or by mechanical damage caused by debris, ice, or rotation of the floating roof. Drain piping, mechanical joints, and hoses can develop leaks that will allow the tank contents either to leak from the roof drain system or allow water to flow into the tank. For single-deck floating roofs, leakage of the tank contents onto the floating roof can submerge or sink the roof. Inoperative drains with installed plugs (or with closed valves) can cause enough rain water to accumulate on the roof to sink a pontoon-type floating roof. Shell Attachments Deterioration of auxiliary equipment—such as ladders, stairways, platforms, wind girders, and shell stiffeners—can occur from corrosion, wind, and other external forces. Mechanical equipment such as mixers, swing line pontoons, piping and swing joints, diffusers, jet nozzles and other flow direction details, baffles, rakes, and agitators can suffer from deterioration due to corrosion, wear from flow erosion, and mechanical defects. Miscellaneous API Std 653, Annex C includes inspection checklists for many types of deterioration of storage tank auxiliary equipment and other appurtenances. The tank inspector should be thoroughly familiar with these checklists. 6 Inspection Plans 6.1 General Developing Inspection Plans An inspection plan is often developed and implemented for tanks within the scope of API Std 653 through the collaborative work of the inspector, engineer, corrosion specialist, and operations personnel. An inspection plan should contain the inspection tasks, scope of inspection, and schedule required to monitor damage mechanisms and assure the mechanical integrity of the tank. Knowledge of the capabilities and limitations of NDE techniques allows the proper choice of examination technique(s) to identify particular damage mechanism in specific locations. The plan should typically: a) define the type(s) of inspection needed, e.g. external, internal, thickness measurements, NDE, associated piping, etc.; https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 37 b) identify the regulatory intervals for seal or other component inspection needs, any applicable jurisdictional requirement, and the date for each inspection type; c) describe the inspection and NDE techniques, extent, and locations; d) describe any surface cleaning requirements needed for inspection and examinations; e) describe the requirements of any needed pressure or tightness test, e.g. type of test, test pressure, and duration; and f) describe any required repairs. Incorporating Operations Information in Inspection Plans Inspection plans should consider information such as operating temperature ranges, process fluid corrosive contaminant levels, material of construction, tank design and configuration, service changes since the last inspection, and inspection/maintenance history. Ongoing communication with operating personnel when process changes or upsets occur that could affect damage mechanisms and rates are critical to keeping an inspection plan updated. Other common issues in an inspection plan include: — describing the types of damage mechanisms anticipated or experienced in the tank; — defining the location of the damage; — defining any special access requirements. Inspection Records/Software Inspection plans for tanks can be maintained in spreadsheets, hard copy files, and proprietary inspection software databases. Proprietary software, typically used by inspection groups, can assist in inspection data analysis and recordkeeping. 6.2 Inspections Planning and Reports General Prior to conducting internal or external inspection, the inspector should thoroughly review any available inspection records to become familiar with previous problems and recommendations noted. Tank Specific Plans Specific plans should address the following details for the tank to be inspected: a) locations for inspection; b) access requirements (scaffolding or other supports); c) access limitations (road condition or width, infringements, etc.); d) insulation removal; e) considerations for roof inspection and access; f) vents and vacuum breakers; g) foundation issues; https://t.me/MyAPI https://t.me/MyInspectors 38 API RECOMMENDED PRACTICE 575 h) seals and floating-roof components; i) non-pressure-containing components required for tank operation; j) cathodic protection; k) linings and coatings; l) removal of water or debris. After the inspection plan has been implemented, each inspection report should address a number of factors, including: a) corrosion rate calculations, b) next inspection interval recommendations, c) API Std 653 requirements, and d) inspection results, providing a description of the types of damage mechanisms in the tank and their exact locations. Inspection Methodologies API Std 653 provides criteria for condition-based inspection and scheduling of tanks utilizing internal and external visual inspection results and data from various NDE techniques (also refer to 6.2). API Std 653 also recognizes the use of alternative inspection methodologies. For example, robotic inspection is one possible approach to perform an assessment of the tank bottom and other internal components without personnel entry. Unmanned aerial vehicles (UAVs) or drones can be utilized for better access to rafters, thereby eliminating the need for man lifts or scaffold installation. Leak Detection There are several different leak detection technologies or approaches, such as: a) volumetric/mass leak detection methods; b) acoustic emissions leak detection methods; c) soil-vapor monitoring leak detection methods; d) product dyes or treatments for detecting the existence of leaks: e) inventory control leak detection methods; f) monitoring the interstitial space in a double bottom tank. Many of the technologies used in leak detection are identified in API Publication 334. These control measures should be inspected or tested periodically, as appropriate for the particular system and the risk involved. For example, external cathodic protection systems should be tested for performance as recommended by API RP 651. Appurtenances API Std 2610 provides additional guidance regarding the inspection of tank appurtenances, accessories, and the surrounding area. https://t.me/MyAPI https://t.me/MyInspectors INSPECTION PRACTICES FOR ATMOSPHERIC AND LOW-PRESSURE STORAGE TANKS 39 6.3 Risk-based Inspection (RBI) Plans General Inspection plans based upon an assessment of the risk associated with a tank failure (by determining the likelihood of failure and the consequence of failure) is referred to as risk-based inspection (RBI). RBI can be used to determine inspection intervals and the type and extent of future inspection/examinations. API RP 580 provides minimum requirements that must be considered to carry out a systematic evaluation of both the likelihood of failure and consequence of failure for establishing RBI plans. API Std 653 outlines the requirements and limitations for performing an RBI assessment for a storage tank. In addition, regulatory requirements in the applicable jurisdiction should be considered to determine acceptability of using RBI for inspection plann

Inspection Practices for Atmospheric and Low-pressure Storage Tanks PDF (API RP 575, 2020)

Document Details

Tags

Related

Summary

Full Transcript

Upgrade to continue