1、 Standard Practice Internal Cathodic Protection (CP) Systems in Oil-Treating Vessels This NACE International standard represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone, whether he or s
2、he has adopted the standard or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not in conformance with this standard. Nothing contained in this NACE International standard is to be construed as granting any right, by implication or otherwise, to manufactur
3、e, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This standard represents minimum requirements and should in no way be interpreted as a restriction on the use
4、of better procedures or materials. Neither is this standard intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness of this standard in specific instances. NACE International assumes no responsibility for the interpretation or use of this standar
5、d by other parties and accepts responsibility for only those official NACE International interpretations issued by NACE International in accordance with its governing procedures and policies which preclude the issuance of interpretations by individual volunteers. Users of this NACE International sta
6、ndard are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determining their applicability in relation to this standard prior to its use. This NACE International standard may not necessarily address all potential health and safety problems or envi
7、ronmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this standard. Users of this NACE International standard are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with
8、appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this standard. CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may be revised or withdrawn at any time in accordance wit
9、h NACE technical committee procedures. NACE International requires that action be taken to reaffirm, revise, or withdraw this standard no later than five years from the date of initial publication and subsequently from the date of each reaffirmation or revision. The user is cautioned to obtain the l
10、atest edition. Purchasers of NACE International standards may receive current information on all standards and other NACE International publications by contacting the NACE International FirstService Department, 1440 South Creek Dr., Houston, Texas 77084-4906 (telephone +1 281/228-6200). Reaffirmed 2
11、007-03-15 Reaffirmed 2001-09-20 Revised March 1995 Approved October 1975 NACE International 1440 South Creek Drive Houston, TX 77084-4906 +1 281/228-6200 ISBN 1-57590-135-8 2007, NACE International NACE SP0575-2007 (formerly RP0575-2001) Item No. 21015 SP0575-2007 NACE International i _ Foreword Thi
12、s standard practice is a general guide for the application of effective cathodic protection to all oil-treating vessels. This standard covers design criteria, the selection and installation of applicable systems, and the operation, monitoring, and maintenance of installed systems. There are many des
13、ign variations in existing oil-treating vessels, with new designs being introduced continually. The preparation of a standard practice for the cathodic protection of each individual vessel design is not practical. Therefore, this standard is not specific with respect to one or more vessel designs. I
14、t is intended for use by corrosion engineers involved in oil and gas production, especially those concerned with surface facilities. Nothing contained in this standard is intended to conflict with applicable codes, including OSHA(1)regulations. This standard was originally prepared in 1975 by Task G
15、roup (TG) T-1E-6, a component of Unit Committee T-1E on Cathodic Protection of Oilfield Equipment, and revised in 1995 by TG T-1E-11. It was reaffirmed in 2001 and in 2007 by Specific Technology Group (STG) 35 on Pipelines, Tanks, and Well Casings. This STG is composed of corrosion consultants, corr
16、osion engineers from oil and gas producing companies, representatives from manufacturers, and others concerned with internal corrosion control in oil-treating vessels. In NACE standards, the terms shall, must, should, and may are used in accordance with the definitions of these terms in the NACE Pub
17、lications Style Manual, 4th ed., Paragraph 7.4.1.9. Shall and must are used to state mandatory requirements. The term should is used to state something considered good and is recommended but is not mandatory. The term may is used to state something considered optional. _ (1)Occupational Safety and H
18、ealth Administration (OSHA), 200 Constitution Ave. NW, Washington, DC 20210. _ SP0575-2007 ii NACE International _ NACE International Standard Practice Internal Cathodic Protection (CP) Systems in Oil-Treating Vessels Contents 1. General 1 2. Definitions 1 3. Determination of Need for CP. 2 4. Desig
19、n and Selection of CP System . 2 5. Anode Installation4 6. Reference Electrode Entrance 5 7. Criteria for Protection 5 8. Monitoring, Records, and Maintenance 5 9. Safety 6 References 6 _ SP0575-2007 NACE International 1 _ Section 1: General 1.1 This standard presents recommended practices for the c
20、athodic protection (CP) of internal surfaces of oil-treating vessels, heat exchangers, or the water side of process vessels. 1.2 The provisions of this standard should be applied under the direction of a corrosion engineer. The term “corrosion engineer,” as used in this standard, refers to a person
21、who, by reason of knowledge of the physical sciences and the principles of engineering and mathematics, acquired by professional education and related practical experience, is qualified to engage in the practice of corrosion control in oil-treating vessels. 1.3 Effective performance of the CP system
22、 requires operation within the limits of the design, monitoring of the system, and maintenance to replace damaged and consumed parts. 1.4 CP is not effective when applied to steel surfaces in the oil or gas phase because of the absence of an electrolyte. Coatings or chemical inhibitors should be use
23、d to control corrosion on the steel surfaces in the oil and gas phase. _ Section 2: Definitions Anode: The electrode of an electrochemical cell at which oxidation occurs. Electrons flow away from the anode in the external circuit. Corrosion usually occurs and metal ions enter the solution at the ano
24、de. Cathode: The electrode of an electrochemical cell at which reduction is the principal reaction. Electrons flow toward the cathode in the external circuit. Cathodic Protection: A technique to reduce the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. Co
25、ating: A liquid, liquefiable, or mastic composition that, after application to a surface, is converted into a solid protective, decorative, or functional adherent film. Corrosion: The deterioration of a material, usually a metal, that results from a reaction with its environment. Corrosion Engineer:
26、 A person, who by reason of knowledge of the physical sciences and the principles of engineering and mathematics, acquired by professional education and related practical experience, is qualified to engage in the practice of corrosion control. Current Density: The current to or from a unit area of a
27、n electrode surface. Driving Potential: Difference in potential between the anode and the steel structure. Electrode: A conductor used to establish contact with an electrolyte and through which current is transferred to or from an electrolyte. Electrolyte: A chemical substance containing ions that m
28、igrate in an electric field. For the purpose of this standard, electrolyte refers to the water, including the chemicals contained therein, adjacent to and in contact with a submerged metal surface. Galvanic Anode: A metal that provides sacrificial protection to another metal that is more noble when
29、electrically coupled in an electrolyte. This type of anode is the electron source in one type of cathodic protection. Holiday: A discontinuity in a protective coating that exposes unprotected surface to the environment. Impressed Current: An electric current supplied by a device employing a power so
30、urce that is external to the electrode system. (An example is direct current for cathodic protection.) Passivation: A reduction of the anodic reaction rate of an electrode involved in corrosion. Polarization: The change from the open-circuit potential as a result of current across the electrode/elec
31、trolyte interface. Reference Electrode: An electrode whose open-circuit potential is constant under similar conditions of measurement, which is used for measuring the relative potentials of other electrodes. Salt Bridge: A salt solution used with a reference electrode to bridge a gap in an electrica
32、l circuit to obtain potential data with a reference electrode. Steel-to-Water Potential: The potential difference between a steel vessel surface and a reference electrode immersed in the water with which the steel vessel surface is in contact (sometimes referred to as cathodic solution potential). S
33、P0575-2007 2 NACE International _ Section 3: Determination of Need for CP 3.1 General 3.1.1 Experience reveals that corrosion and metal loss is to be expected in any oil-treating vessel in which any portion of the internal steel surface is exposed to oilfield brines because of their aggressive corro
34、sive nature. The need for CP is contingent on the severity of existing or anticipated corrosion and the extent to which it affects equipment operation. Consistent with the latter, CP should be installed when it will accomplish one or more of the following: remove or minimize unsafe conditions caused
35、 by failure, provide economical control over equipment failures and losses, and remove or minimize the possibility of vessel content loss because of leaks or vessel collapse. 3.1.2 Internal coatings may be used in conjunction with CP to protect oil-treating vessels. Internal coating reduces the surf
36、ace area of steel to be protected. 3.2 Corrosion Rates 3.2.1 The corrosiveness of a fluid stream is a function of the following: 3.2.1.1 Corrosion usually becomes more severe as the conductivity (dissolved solids content) of water (electrolyte) increases, but low-conductivity water can be corrosive.
37、 3.2.1.2 Corrosion in produced oilfield brines usually increases as the partial pressure of acid forming components, such as carbon dioxide (CO2) or hydrogen sulfide (H2S), increases. 3.2.1.3 Corrosion is accelerated by even trace amounts of oxygen. 3.2.1.4 Corrosion usually increases with increasin
38、g temperature unless scaling is increased. 3.2.1.5 Corrosion usually increases with increasing flow velocity and turbulence. _ Section 4: Design and Selection of CP System 4.1 Basic Design Criteria 4.1.1 The design of a vessels CP system depends on the internal configuration of the vessel. Selection
39、 of the appropriate system depends on factors such as initial cost, maintenance, type and condition of coatings (if any), power availability and cost, and system life. 4.1.2 Vertical cylindrical vessels containing no baffles, compartments, firetubes, etc., are usually protected with anodes or string
40、s of anodes suspended from the deck (roof) of the vessel. This design method offers two advantages over other designs: (1) better current distribution because anodes are parallel to the vessel walls, and (2) deteriorated or depleted anodes can be replaced without lowering the water level or draining
41、 the vessel. 4.1.3 Compartmented vessels or those containing baffles, firetubes, spreaders, etc., should have at least one anode installed in each compartment exposed to the corrosive fluid. If a vessel is not designed to accommodate a CP system that will provide effective protection throughout, it
42、may be necessary to make modifications or redesign the vessel interior to provide access to areas needing CP by installing fittings in the vessel for inserting anodes through the walls. An example of such modification is positioning of a spreader in a heater treater below the firetubes so there is s
43、ufficient clearance to install an anode between the firetube and spreader. 4.1.4 Correct location and position of anodes in vessels are essential for proper current distribution. CP anodes should be placed such that their protective current can be distributed to all surfaces exposed to the corrosive
44、 electrolyte. Each anode should be located as near to the center of the compartment or center of the electrolyte as practical. 4.1.5 Factors determining the number, weight, and shape of anodes required for CP of vessels are: (a) area of bare water-immersed steel to be protected; (b) current density
45、required; (c) anticipated current output of the anodes; (d) vessel configuration; and (e) desired life of the CP system. 4.1.6 Current density requirements can range from 54 to 430 mA/m2(5 to 40 mA/ft2) of bare water-immersed steel. In the absence of specific current density data, 108 mA/m2(10 mA/ft
46、2) is commonly used for design. However, vessels handling water containing SP0575-2007 NACE International 3 depolarizers, such as H2S and oxygen, or operating at high temperatures or high flow rates, could require higher current density to maintain protective potentials. Internal coating of vessels
47、decreases the area of bare steel in contact with water and reduces the amount of current required for corrosion protection. 4.2 System Selection 4.2.1 CP can be provided by impressed current systems or galvanic anode systems. Typical performance data for commonly used impressed current and galvanic
48、anodes are shown in Table 1. Prior to the application of either impressed current or galvanic anodes, it must be ensured that treated electrolytes are chemically compatible with the anode. Table 1Typical Performance Data for Commonly Used Impressed Current and Galvanic Anodes Type of Anode A-h/kg(A)A-h/l
