1、 Item No. 24156 NACE International Publication 10A392 (2006 Edition) This Technical Committee Report has been prepared By NACE International Specific Technology Group 35* on Pipelines, Tanks, and Well Casings Effectiveness of Cathodic Protection on Thermally Insulated Underground Metallic Structures
2、 September 2006, NACE International This NACE International technical committee report 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 from manufacturing, marketing, purchasing, or u
3、sing products, processes, or procedures not included in this report. Nothing contained in this NACE report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemni
4、fying or protecting anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictabl
5、e circumstances may negate the usefulness of this report in specific instances. NACE assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents an
6、d for determining their applicability in relation to this report prior to its use. This NACE report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this rep
7、ort. Users of this NACE report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use o
8、f this report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACE r
9、eports may receive current information on all NACE International publications by contacting the NACE FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 281/228-6200). Foreword The present trend in establishing an effective level of external metallic surface corr
10、osion control is the application of a barrier coating or adhesive on the metallic surface prior to the application of a thermal insulating material. Experience has shown that there is generally a limited beneficial effect from the application of cathodic protection (CP) to a bare or ineffectively co
11、ated metallic surface under thermal insulation. This NACE technical committee report was prepared as an information guide for external corrosion control of thermally insulated underground metallic surfaces and considerations of the effectiveness of CP. This report is intended for those dealing with
12、thermally insulated structures or pipelines. Although pipelines are the primary focus of this report, the principles discussed would be applicable when a thermal insulating material has been applied on or in the immediate proximity of an underground metallic surface. This report was originally prepa
13、red in 1992 by NACE Task Group (TG) T-10A-19, a component of Unit Committee T-10A on Cathodic Protection and was reaffirmed with editorial changes in 2006 by Specific Technology Group (STG) 35 on Pipelines, Tanks, and Well Casings. It is published by NACE under the auspices of STG 35. _ *Chair Paul
14、R. Nichols, Shell Global Solutions, Houston, Texas. NACE International 2 BACKGROUND On most thermally insulated oil and gas transmission pipelines installed prior to 1980 to 1981, a shop mold-formed thermal insulation was placed directly over the bare steel pipe, with an outer jacket applied to mois
15、ture-proof the system. At the field joint, preformed insulation half shells were applied over the joint area to fit between the ends of the shop-applied insulation. After the insulation was fitted, a heat shrink sleeve or a tape wrap was applied over the insulation. When the integrity of the outer m
16、oisture barrier was compromised, the space, gap, or void between the edges of the preformed half shells and the shop-applied insulation allowed oxygenated water to diffuse to the bare steel beneath. Damage to the outer moisture barrier has also occurred remote from the joint, allowing oxygenated gro
17、und water ingress. Thermally insulated pipelines have experienced relatively aggressive corrosion, with some failures occurring within three years of service, although acceptable industry standards of CP had been applied and maintained shortly after line construction. The most predominant failures h
18、ave been those occurring at joints; however, moisture has migrated along the pipeline steel surface to create electrochemical corrosion cells remote from the field joint, culminating in extensive replacements of substantial lengths of line. An article titled “Corrosion of Underground Insulated Pipel
19、ines1supports this committees conclusions that sufficient CP current from an external source may not reach the insulated metallic surface in sufficient quantity to establish adequate corrosion control. BASIC CORROSION MECHANISM External failure of thermal insulated metallic surfaces has been primari
20、ly attributed to electrochemical corrosion cells generated from oxygenated ground waters, although some have found and concluded that failures are due to microbiologically influenced corrosion (MIC). When conventional CP is applied to a thermally insulated pipeline where an annular void exists, prot
21、ection along the length of the void often does not occur. In a paper titled “Cathodic Protection Levels Under Disbonded Coatings,2presented at CORROSION/82, the authors submitted experimental data that suggested a distance limitation of effective corrosion control by the use of externally applied CP
22、. The amount of bare metallic surface under the thermal insulation (which can be equated to a severe condition of disbonded coating) would be the major factor limiting the area effectively protected by the externally applied CP. Figures 1a, 1b, and 1c detail various metallic surface conditions and a
23、nnular spaces where oxygenated water has migrated to a location remote or shielded from the external environment. Figure 1a shows a joint on which the joint wrap or sleeve, for some reason such as line movement, has become disbonded from the exterior coating and allows water to ingress to the pipe s
24、urface. The oxygenated water then migrates through the annulus, and active corrosion cells could be established if the foam or another barrier is not bonded to the pipe surface. Sufficient CP current generated externally cannot reach the metallic surface because of the shielding effect of the therma
25、l insulation and the natural phenomenon of electrochemical reactions that result in active corrosion cells. FIGURE 1a: Typical Joint with Damaged Wrap or Sleeve Shop-applied foam insulation Joint wrap/sleeve Foam half shell Pipeline External coating Potential water ingress NACE technical committee r
26、eports are intended to convey technical information or state-of-the-art knowledge regarding corrosion. In many cases, they discuss specific applications of corrosion mitigation technology, whether considered successful or not. Statements used to convey this information are factual and are provided t
27、o the reader as input and guidance for consideration when applying this technology in the future. However, these statements are not intended to be recommendations for general application of this technology, and must not be construed as such. NACE International 3 Figure 1b details a close-up of a met
28、allic surface on which a discontinuity (holiday) in the exterior coating and a void in the thermal insulation has been created. The externally applied CP current provides protection to the surface area at the void and for a limited distance beyond the void. Active corrosion cells result beyond the e
29、ffective coverage of CP in which the insulating material affects or significantly inhibits current flow through the thermal material. It has been observed that when the rigid foam insulation adheres to the metallic surface, it forms an effective barrier and corrosion usually does not occur. Figure 1
30、c is similar to 1b except a discontinuity also exists in a coating that was applied directly to the pipe surface. Dye test experiments have indicated that impressed currents, such as those applied by CP systems, affect the water migration pattern once water has broken through the insulation. These i
31、mpressed currents in fact cause the water to migrate further from the point of entry than it may otherwise have done. The conditions described are not necessarily the worst- or best-case situations. When the metallic surface temperature has been maintained so that moisture is not allowed surface con
32、tact, the presence of active corrosion cells by oxygenated water is usually eliminated; however, elevated temperatures cannot be relied on as a corrosion control method. FIGURE 1b: Insulation Material Not Continuously Bonded to Metal NACE International 4 FIGURE 1c: Pipe with Discontinuity in the Bar
33、rier Coating on the Pipe Surface TYPES OF THERMAL INSULATION The most common thermal insulation utilized by the pipeline industry has been a polyurethane foam. Table 1 reviews some of the properties of materials that have been utilized for thermal insulation and Table 1a reviews the water permeabili
34、ty of various types of thermal insulation. TABLE 1Properties of Thermal Insulation Materials Type Typical Use Application Method Feasible Operating Temperature Heat Transfer Coeff. ”K W/m2-K Compression Strength kPa (psi) Rigid Polyurethane Pipelines Shop Molding or Spray to 93C (200F) 0.12 207-414
35、(30-60) Isocyanurate Pipelines Shop Molding or Spray to 150C (302F) 0.18 193 (28) (VERTICAL) 138 (20) (PARALLEL) Polystyrene Tank Bottoms Board Stock Laid in Sheet Form Cryogenic to 74C (165F) 0.26 at 4.4C (40F) 0.25 at -6.7C (20F) 241 (35) (VERTICAL) 138 (20) (PARALLEL) Fiberglass Pipe Half Shells
36、to 316C (600F) 0.23 N/A Cellular Glass Pipe/Structures Board Stock/Half Shells -268 to 538C (-450 to 1,000F) 0.33 at 10C (50F) 689 (100)3Calcium Silicate Geothermal Pipe High Temperature Hot Water Lines Half Shells to 593C (1,100F) 0.4 1379 (200)(A)NACE International 5 TABLE 1aWater Permeability of
37、Various Types of Thermal Insulation Type ASTM Methods Typical Value Rigid Polyurethane D284240.7 g/cm (0.05 lb/ft) Isocyanurate D284240.7 g/cm (0.05 lb/ft) Polystyrene C272450.3% by volume Fiberglass N/A Less than 1% by volume Cellular Glass C24060.2% by volume Calcium Silicate Calcium silicate has
38、a very high moisture absorption rate and may not be suitable for use on underground pipelines Cellular glass and calcium silicate insulations have been used underground in “pipe-within-a-pipe” systems. In these systems, the thermal insulation is placed as half or quarter shells in the annular space,
39、 and metallic spacers provide concentricity. Protective coatings supplemented with CP are provided for the outer casing. This is necessary to ensure a dry environment within the annulus. APPLICATION PROCESSES Early methods of insulating steel line pipe intended for buried service copied the technolo
40、gy used for above-grade refinery piping. Preformed halves of polyurethane insulation were placed over the pipe and held in place with ties or bands of material and subsequently covered with an outer wrap, such as polyethylene tape or polyethylene heat shrinkable sleeves of the type commonly utilized
41、 in pipeline applications. From this technique evolved a process of placing steel line pipe, one joint at a time, in a “dunk” tank or mold into which hot polyurethane was injected and allowed to form and set around the pipe. Mold tolerances of different dimensions are available to provide the desire
42、d thickness of insulation. Subsequently, a patented process was developed for applying polyurethane insulation to a steel pipe that is rotating and travelling past a fixed point through a fixed nozzle or jet. This method represents a recent development for the application of insulation to steel pipe
43、s intended for buried service, permitting greater control of such variables as compressive strength and application temperature. Variables in the polyurethane formulation, method of application, and physical conditions at the time of application determine the properties of the as-applied product. Fr
44、om a physical property point of view, end users are concerned with the integrity of the bond between the insulation and the steel pipe, the compressive strength of the insulation material, and maximum temperature at which the system can be operated without altering or damaging the properties of the
45、insulation or its associated coating(s). Traditionally, pipeline coatings have determined the temperature at which a pipeline can be operated. In order to realize maximum benefit and heat transfer efficiency from an insulated pipeline system, coating products that maximize this feature are selected.
46、 In circumstances in which a coating or corrosion barrier is being applied to a bare steel pipe prior to application of insulation, a coating or barrier is selected to withstand the application temperature of the insulation. In other words, its effectiveness as a corrosion barrier remains intact aft
47、er the insulation has been applied. The principal change in the construction of thermally insulated line from what was practiced prior to 1980 to 1981 is the addition of a corrosion barrier coating on the steel pipe prior to applying the insulation materials. The typical shop preparation and applica
48、tion process listed below refers to the use of rigid polyurethane insulation that requires both an external vapor/moisture barrier to maintain the thermal integrity of the insulation system and a barrier at the steel surface to control corrosion: 1. Incoming pipe is inspected to ensure it is free of
49、 grease, oil, etc., that would impede proper coating application. 2. Pipe is preheated to specified temperature. 3. Pipe is blast cleaned to the specified finish. 4. Corrosion coating is applied to specification with proper cutback. 5. The polyurethane insulation is applied to specified properties and thickness. 6. The external moisture barrier jacket is applied according to specifications. 7. The specified quality control tests are conducted to confirm conformance to specifications. NACE International 6 8. Half shells of the required si
