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Controlling liquid leaks from tanks
Liquid loss from a storage tank is generally caused by localized material failure in the form of localized corrosion. Tank bottom leaks can be a result of improper foundation design or operating a tank outside the recommended design pressure or temperature boundaries. Product liquid leakage remains a significant environmental concern. Any tank used to contain a hydrocarbon product can be prone to develop leaks sometime during the service life. Tank design options that reduce the risk of a leak can be considered, or in the event of a leak, any product that escapes is contained and detected in a realistic time frame.
Design options are generic with respect to the type of storage tank. Similar details are used on fixed-roof and floating-roof tanks alike. Options considered by most tank owners include internal and external corrosion protection and bottom cathodic protection systems. Secondary containment and detection systems are also considered an essential part of a tank installation.
A primary method used to protect metal surfaces against surface corrosion is to apply a suitable coating. Exterior surfaces generally require protection only from the elements, although in some chemical production plants, chemical vapors can be prevalent in the atmosphere and might impact selection of the coating material. Applying a suitable primer and topcoat per manufacturer’s recommendations normally provides adequate protection of the external tank surfaces at onshore locations. More elaborate multicoat epoxy-based paint systems are used at offshore locations.
Internal surfaces can be more problematic. Water and other corrosive products naturally collect on the bottom. In many cases, only the bottom and 18 to 24 in. of the shell are coated.
Various types of coatings are used depending on the service requirements stipulated in the coating specification. Some of the more common coatings that remain in use in petroleum storage are coal tar, various two-part epoxy paints, and conventional fiberglass coatings. Internal flexible liners may be used for the most severe product applications.
For tanks in petroleum service, internal cathodic protection in conjunction with coatings has not gained widespread use. Under certain conditions, it can be effective in protecting against corrosion at holidays in the coating. More detailed information on internal cathodic protection is available in the National Association of Corrosion Engineers (NACE) RP05-75 and RP03-88.
Corrosion of the steel tank bottom may be reduced or eliminated with proper application of cathodic protection. Systems may be used in new tank construction or may be added to an existing structure when the original bottom is replaced.
With cathodic protection systems, the entire bottom surface acts as the cathode of an electrochemical cell. Two methods currently used to protect the underbottom surfaces against corrosion are the impressed current system or the galvanic/sacrificial anode system. Each is described in some detail in the second edition of the API RP651, Cathodic Protection of Aboveground Petroleum Storage Tanks.
The operation of any cathodic protection system can be affected by the tank foundation design, the use of secondary containment liners, and general site conditions. The system designer should complete a thorough review of all tank details.
A typical galvanic system, shown in Fig. 1, uses a metal more active than the structure to be protected to supply the current required to limit or stop corrosion. The more active metal is called the anode, commonly referred to as the galvanic anode or a sacrificial anode. A galvanic corrosion cell develops, and the active metal anode corrodes (is sacrificed) while the metal bottom (cathode) is protected. Metals commonly used as the anodes are magnesium and zinc in either cast or ribbon form.
Impressed current systems use an external power source through a rectifier to provide direct current (DC) to the anode and then on to the tank bottom, as shown in Fig. 2.
Secondary containment or leak detection
Appendix I, "Under-Tank Leak Detection and Sub-grade Protection" in the API Standard 650, provides acceptable construction details that may be used to detect and contain leakage from above ground storage tanks. It is noted that the API supports a general position that owners consider the installation of release prevention barriers (RPB) under new tanks during initial construction.
Acceptable RPB includes second steel bottoms, impermeable clay materials, or synthetic materials such as high density polyethylene (HDPE) materials. The API Standard 650, Appendix I provides several different construction details for consideration; however, the tank owner must determine whether the undertank area is to include leak detection. If required, the owner must then select the method or methods to be used.
Whenever a new bottom is going to be added to an existing tank, the owner should consider adding some type of RPB. In many cases, a new bottom is only added after the original bottom has corroded through and product has leaked through to the foundation. This is covered in the second edition of API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction.
Examples of typical RPB systems that are used are shown in Figs. 3 and 4. Each of these uses a synthetic liner fabricated from sheets of HDPE material (60–90 mil thick) welded together to form a continuous barrier. Fig. 3 shows a detail of a system used primarily for new tank construction on earthen birm or a concrete ring wall when leak detection is required.
The example in Fig. 4 is specifically used when a new bottom is to be added to an existing tank using the slotted shell method of construction. The Endolock™ system also relies on a synthetic HDPE liner as the barrier. This system also includes leak detection and can be used either with granular fill or concrete between the old and new bottom. The systems shown in Figs. 3 and 4 are patented by CB&I. Other potential systems are available, as well as systems that use adhesives or nails to secure the liner at the ringwall or inner tank shell surfaces.
Cathodic protection systems may be incorporated into each design. In most cases, an impressed current system is installed with the ribbon anodes installed between the flexible liner and the new bottom.
Evaporative Loss Measurement. 1997. Manual of Petroleum Measurement Standards, Chap. 19, Sec. 2-E. Washington, DC: API.
API RP12R1, Setting, Maintenance, Inspection, Operation, and Repair of Tanks in Production Service, fifth edition. 1997. Washington, DC: API.
API RP575, Inspection of Atmospheric and Low-Pressure Storage Tanks, first edition. 1995. Washington, DC: API.
API RP651, Cathodic Protection of Aboveground Petroleum Storage Tanks, second edition. 1997. Washington, DC: API.
API RP652, Lining of Aboveground Petroleum Storage Tank Bottoms, first edition. 1991. Washington, DC: API.
API RP2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents, fifth edition. 1991. Washington, DC: API.
API Spec. 12B, Bolted Tanks for Storage of Production Liquids, fourteenth edition. 1995. Washington, DC: API.
API Spec. 12D, Field-Welded Tanks for Storage of Production Liquids, tenth edition. 1994. Washington, DC: API.
API Spec. 12F, Shop-Welded Tanks for Storage of Production Liquids, eleventh edition. 1994. Washington, DC: API.
API Standard 650, Welded Steel Tanks for Oil Storage, tenth edition. 1998. Washington, DC: API.
API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction, second edition. 1995. Washington, DC: API.
API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks (Nonrefrigerated and Refrigerated), fifth edition. 1998. Washington, DC: API.
API Standard 620, Design and Construction of Large, Low-Pressure Storage Tanks, tenth edition. 2002. Washington, DC: API.
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