Life Management of Above Ground Atmospheric Storage Tanks

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  • Life Management Of Above Ground Atmospheric Storage Tanks

    http://www.ndt.net/article/apcndt01/papers/935/935.htm[10/03/2015 7:37:04 AM]

    Home Table of Contents Industrial Plant & Structures

    Life Management Of Above Ground AtmosphericStorage Tanks

    C.J. Moss, RR Griffiths, A Bishop, M Dinon Contact

    Abstract

    The integrity of tanks needs to be well managed because they can contain a large inventory ofhazardous materials and because of the high costs such as cleaning and waste disposal prior toinspection and maintenance. The WTIA Petro Chem - Refinery " Save Money and Re-engineer withTechnology" (SMART) Group has collectively identified tanks as an area where collaborative workwould be beneficial.

    The damage mechanisms associated with tanks can be complex and varied. Mechanisms includeunderfloor corrosion (where cathodic protection and drainage issues are important), internal corrosion(where the contents of the tank, the presence of species such as sulphate reducing bacteria andtemperature control the corrosion rates) and non-corrosion related mechanisms such as differentialsettlement.

    When risk is defined as the product of likelihood and consequence, it is apparent that tanks deservehigh profile in a risk directed inspection program. It is maintained in the paper that it is possible todevelop inspection scopes directed on the basis of risk. Such an approach permits the use ofresources to be optimised while the overall costs of maintenance are minimised. Inspection andturnaround costs may be minimised and the risk of business and safety impacts reduced to anacceptable level whilst meeting statutory occupational health, safety and environmental requirements.

    This paper reviews Australian requirements pertaining to the scope and interval of tank inspection andidentifies gaps in requirements. Inspection needs are presented and techniques such as acousticemission and floor scanning are discussed. Case studies of tank asset management are presented.

    Background

    Tanks have been around since the beginning of hydrocarbon production. However, relative to pressureequipment, limited information is available for tank integrity management. Tanks vary considerably insize, from small Australian Standards (AS) 1692 class 4 or 5 tanks, where the size is typically 50,000litres, to American Petroleum Institute (API) 620 and 650 tanks where the size may be tens of millionsof litres. In the ten years of life assessment and life extension conferences in Australia, to the authors'knowledge, few papers have been presented on tank issues. Perhaps the perception that tanks aresimple, ambient pressure equipment leads to them receiving less attention in the technical literature.Additionally, the generally high reliability and perception of tanks as infrastructure rather than planthas meant that tank maintenance approaches have tended to be reactive. Whatever the case, reviewof tank design and operating experience shows that tank issues can be complex and responses toleaks have been costly and anything but simple.

    The failure of a tank can have several undesirable effects such as endangering personnel, affecting theenvironment and interrupting the operator's business. In a 1988 API worldwide survey, tank rupturesaccounted for 5 % of the 132 releases that occurred worldwide between 1970 and 1988 butaccounted for almost 19 % of the released material. An example of a failure with dramatic results wasin January 1988 in Pennsylvania, where 500,000 gallons of fuel flowed from an above ground tankinto the Monogahela River, the major source of water for many local towns. The cost of clean up,damage to the environment and adverse publicity associated with this and other releases spawned

  • Life Management Of Above Ground Atmospheric Storage Tanks

    http://www.ndt.net/article/apcndt01/papers/935/935.htm[10/03/2015 7:37:04 AM]

    present tank regulations and the development of API 653.

    Whilst pressure integrity management is well-developed in standards such as AS3788, tank integrityrequirements in Australia are evolving. Whether published standards for tank integrity are available ornot, it is apparent that well planned preventive, rather than reactive, measures should be taken intank maintenance and reliability. It is interesting to note that in the USA, tank regulations and rulesgenerally focus on mitigative rather than preventive aspects; for example leaks and spills aremitigated by secondary containment rather than prevented by design and inspection. The importanceof inspection and condition monitoring in avoiding failures, maintaining safety and optimisingavailability is unquestionable. However, in a competitive business environment, down time forinspection requires considerable justification.

    Facilities with tanks often present additional risks beyond site risks such as potential injury to sitepersonnel, damage to equipment and lost business. Tanks are often located in areas of environmentalvalue or, because of the encroachment of suburbia, are close to the community. Furthermore,incidents may create unfavourable publicity through media coverage. Consideration of the cost oflitigation and fines from accidental releases alone can warrant setting up an inspection program.Companies therefore require a consistent approach for assessing tank integrity and maintainingcompliance with industry standards and regulatory, that is, community requirements. Such anapproach must

    show that tanks are not leaking and will not leak before next inspectionreduce the potential for releasesmaintain tanks in safe operating condition, andmake repairs and determine if and when replacement is necessary.

    This paper presents a of information resources, regulatory requirements and describes case studies oftank asset management from three companies. Gaps and opportunities are presented to promotedialogue and raise awareness of needs.

    "SMART" Petrochemical group activities

    Three SMART groups representing the pipeline, power generation and petrochemical industries wereformed shortly after the launch of the Ozweld Technology Support Centres Network Project in late1998. The Petrochemical group has 12 member companies from refineries, gas plants and chemicalplants. The SMART Group has been successful in identifying technological needs by creating adiscussion forum at company level and a cooperative spirit of working together, despite participantsbeing associated with different companies. A fundamental aim of each SMART group is the creation ofa close network of key participants from that particular industry and to identify " expert technologytools" needed to help improve industry viability. In line with this theme, the SMART Petrochemicalgroup identified a need to review the most effective NDT techniques for tanks to improve tank assetmanagement.

    Regulatory Requirements and Knowledge Bases

    In Australia, tank in-service inspections are generally identified in petroleum regulations, occupationalhealth and safety regulations or dangerous goods regulations. Details vary from state to state butmost make reference to AS1642, AS1940 and AS3788, which address design, construction, operationsand maintenance in varying levels of detail. Beyond Australia, there are several design codes thatprovide good assurance on fitness for service: in particular, the standards and recommended practicesproduced by the American Petroleum Institute (API) are recognised as world class. Tank selection hashistorically been a complex process of optimising an array of requirements such as design, capacityand cost. Other factors include corrosion prevention systems and environmental regulations. Inplanning to design and construct new tankage, there are ample standards geared to provideagreement on design and fabrication between the supplier and purchaser. Such standards ensure thatthe tank will not fail when put into service and were not intended to deal with long term maintenance

  • Life Management Of Above Ground Atmospheric Storage Tanks

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    and inspection. There are a number of API standards and recommended practices which provideguidelines on design, fabrication, operation, cleaning, inspection and repair of tanks and which can beused to develop tank integrity programs and procedures. Selected information is contained inAppendix 1. The most important guide on in-service integrity is API 653.

    Summary of Useful Tank Inspection and Repair Standards and GuidesAS1692-1989

    Tanks for flammable and combustible liquids

    Details design and construction requirements for tanks, but makes no reference topost-construction inspection issues.

    AS1940-1993

    The storage and handling of combustible liquids

    States that a procedure for the inspection of tank vents and fittings shall beestablished to ensure that pressure/vacuum and emergency tank passages are clearand any relief valves are operating correctly. Such inspections shall be carried out atperiods not exceeding 12 months, or as necessary depending on the application.

    A procedure for the maintenance of tanks, including testing, inspection and monitoring.Clause 9.8.14 and Table 9.1 detail record keeping, repairs, limited filling heights,testing and inspection frequencies for category 6 tanks. Operational inspections shallbe carried out monthly, shell, bottom and roof integrity related inspections shall becarried out at a maximum interval of 10 years. States that the minimum allowable floorthickness is 4mm.

    AS3788 -1996Appendix T(Normative)

    In-service Inspection of tanks

    Deals with API 620 tanks, calls up AS1940 inspection interval category 6, ie. 10 yearlyinternal inspection required. Also references API 653, AIP CP 16 (10 yearly internalinspection intervals).

    AS3873 -1995

    Pressure Equipment - Operation and Maintenance

    Specifies requirements and owner and contractor responsibilities and gives guidance onoperation, maintenance, and operational surveillance and risk assessment of pressureequipment. Reference to this standard is provides for good working practice for tanks.

    API 575-1995

    Inspection of atmospheric and low-pressure storage tanks

    Details reasons for inspection and methods of inspection, methods of repair, recordkeeping and reporting.

    Provides check sheets for in service and out of service inspection.API RP651-1997

    Cathodic Protection of Aboveground Storage Tanks

    Details damage mechanisms and CP requirementsAPI 652-1997

    Lining of aboveground petroleum storage tank bottoms

    Details lining selection, cleaning and lining installation procedures.

    Makes no reference to post-construction inspection issues.API 653-1995

    Tank inspection, repair, alteration, and reconstruction

    Details minimum requirements for maintaining integrity of storage tanks, inspectionfrequency and methods of inspection, methods of repair, alteration, record keeping andreporting.

    Provides check sheets for in service and out of service, internal and external inspectionand API 620 and 650 code compliance.

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    Recommends monthly operational external visual, 5 yearly external inspection byauthorised inspector, 10 yearly internal inspection with possibility of 20 yearly bottominspection where corrosion rate has been measured.

    API 2000-1994

    Venting atmospheric and low pressure storage tanks

    Fire protectionAPI 2015-1994

    Safe entry and cleaning of petroleum storage tanks

    Details safety precautions and standards of cleaning for inspection.API 2021 Fighting fires in and around flammable and combustable liquid

    atmospheric petroleum storage tanksAPI 2207-1998

    Preparing tank bottoms for hot work

    Details safety precautions for hot work on tank bottoms.API 2200 Improving owner and contractor safety performanceAPI 12R1-1997

    Recommended practice for setting, maintenance, inspection, operation, and repair oftanks in production services

    Provides useful guidelines on tank corrosion mechanisms. Provides check sheets for inservice and out of service, internal and external inspection.

    API 327-1994

    Aboveground storage tank standards: a tutorial

    Summarises contents of all API standards and provides worked examples fordetermination of corrosion rate and inspection interval.

    API 334-1996

    A guide to leak detection for above ground storage tanks

    Summarises trials and validation on key 4 techniques.NACE RP0193-1993 External Cathodic Protection of On-Grade Metallic Storage Tank Bottoms

    Tank Asset Management and API 653

    The primary applicable Australian standards for in-service inspection of tanks are AS1940 and AS3788,which specify requirements for regular operational surveillance and a maximum internal inspectioninterval of ten years. API 653 is an important additional document that addresses suitability for serviceand repair and alteration requirements for large, atmospheric pressure above ground, steel storagetanks. API 653 cannot provide a cook book of answers to all issues and therefore should be regardedas outlining a program of minimum requirements for maintaining tank integrity. It outlines bestavailable, cost-effective technology to ensure that leaks or catastrophic failure do not occur.

    API 653 departs from most inspection specifications in that it requires an engineering analysis of theinspection data. Thickness measurements are evaluated to ensure that the tank is structurally sound,within allowable stresses for the required design conditions and will not leak before the nextinspection. Confirming that a tank will not leak goes beyond ensuring that it will not failcatastrophically, since even a small leak is unacceptable. API 653 emphasises the need for engineeringexperience when evaluating a tank's suitability for service. It requires that evaluation be conducted byorganisations that maintain or have access to engineering and inspection personnel who aretechnically trained and experienced in tank issues.

    API 653 recognises that fabrication and inspection records for older storage tanks may be incompleteand the original degree of inspection and construction material may be unknown. However, it stillprovides a structural integrity evaluation of such tanks by using conservative assumptions. In thesecases, shell thickness calculations use a low weld joint efficiency of 0.7 and assume the use of arelatively low-strength material. This reinforces the benefits of maintaining proper tank design,

  • Life Management Of Above Ground Atmospheric Storage Tanks

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    fabrication and inspection records. The concepts of three-stage life assessment, developed forpressure equipment and outlined in AS3788 Appendix U may be used. If records are inadequate or ifdamage mechanisms are not understood, the next stage of assessment is used. The information fromeach stage feeds into the next. Successive stages are more comprehensive and costly: therefore eachstage is performed only as required...