API RP 1632:2010 pdf download

API RP 1632:2010 pdf download.Cathodic Protection of Underground Petroleum Storage Tanks and Piping Systems.
3.1 Description
3.1.1 Corrosion protection with sacrificial anodes may be used in preference to an impressed-current system where current requirements are low. Sacrificial anodes are usually constructed of zinc or magnesium metal, packaged in a low- resistivity backfill. Zinc anodes are best utilized in low- resistivity soils where their lower CutTeilt output will result in longer anode life. Magnesium anodes, because of their higher driving potential (— I .6 volts compared to —1. 1 volts for zinc), are frequently used in higher resistivity soils or soils with high concentrations of carbonates, bicarbonates, or phosphates.
3.1.2 Low driving voltages and low-current outputs (usually less than 0.10 amperes per anode) generally limit sacrificial anodes to cathodic protection of well-coated structures. Sacrificial anode corrosion protection systems are particularly suited to new installations involving coated tanks and/or distribution piping.
3.2 Factory-Installed Anodes
3.2.1 Underground storage tanks are available from tank manufacturers with sacrificial anode cathodic protection systems already attached. Current requirements are minimized by factory application of long-lasting corrosion resistant coatings. Sacrificial anodes are attached at the plant, with anode weight and size determined by the tank surface area. Nonconductive bushings and flanges are provided with each tank to isolate the tank from any extra current demand resulting from electrical contact between the tank and associated metal piping. These bushings and flanges must he used to protect the tank and maintain the suppliers warranty. These tanks may also be obtained with factory- installed test leads which facilitate field monitoring of anode current output and tank-to-soil potential.
Note: Steel product piping isolaied from the tank by nonconductive bushings and flanges will require its own cathodic proed ion sys(em.
3.2.2 Factory-installed cathodic protection systems for new tanks are designed to satisfy requirements for most soil situations. The purchaser should compare conditions present at the proposed installation sites with the conditions under which long-term corrosion protection is provided by the factory-installed system.
3.2.3 Tanks with factory-installed cathodic protection systems must be handled carefully during transportation and installation to protect against coating damage or rupture of anode packages. Anode wires, test leads, tank coating, and tank isolation bushings should be inspected for obvious damage prior to final installation. As with any cathodic protection system, a regular postinstallation monitoring program is necessary to determine that corrosion protection is being maintained.
3.3 Field-Installed Anodes
3.3.1 Field-installed, sacrificial-anode cathodic protection systems may also be used to protect new or existing well- coated tanks and piping. A coal tar coating routinely applied to steel petroleum storage tanks should not be considered an adequate coating for cathodic protection purposes. Many of the corrosion resistant coatings used on cathodically protected petroleum pipelines (see NACE RP-O1-69) may also he used as coatings fbr buried petroleum-storage tanks and dispensing piping. Effective coatings reduce current requirements of these structures to levels that can be supplied over a long term by sacrificial anodes.
3.3.2 Current requirements for new or existing buried structures may be calculated if the soil resistivity, total surface area to be protected, and coating quality are known. For existing structures, it is also possible to quantitatively determine current requirements by electrically connecting buried structures to a variable DC power source and measuring structure-to-soil potentials versus a copper-copper sulfate electrode. The observed current output needed to achieve a desired cathodic protection potential may then be used for designing a cathodic protection system for the structures.
3.4 Anode Types and Placement
3.4.1 Three sacrificial anode materials are commonly used for soil installations:
a. High-potential magnesium alloy.
b. Standard magnesium alloy.
c. Zinc.
3.4.2 Magnesium and zinc anodes prepackaged in cheniical backfill are readily available in a number of size and weight configurations to meet various current output and anode-life design requirements. The use of a chemical backfill with anodes is necessary for reliable operation in soil environments. Chemical backfill promotes anode efficiency, lengthens anode life, and keeps the anode environment moist.