DESIGN AND CONSTRUCTION OF THE HYBRID CP SYSTEM FOR THE MEXICO CITY AIRPORT JET FUEL PIPELINE UNDER THE 100MV POLARIZATION CRITERION.
L. M. Martinez de la Escalera* and J. Canto* Corrosion y Proteccion Ingeneria, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290. A. Rios Aeropuertos y Servicios Auxiliares Avenida 602 No. 161. México, Distrito Federal. Mexico. 62290. H. Carrillo Facultad de Ciencias, Universidad Nacional Autonoma de Mexico. Circuito Interior. Ciudad Universitaria. México, D.F. CP 04500. H. C. Albaya Sistemas de Protección Catódica S.A. Tronador 1126, piso 5° A. Buenos Aires. Capital Federal. Argentina. C1427CRX. J. A. Ascencio and L. Martínez-Gomez Instituto de Ciencias Físicas, Universidad Nacional Autonoma de Mexico. Avenida Universidad s/n, Colonia Chamilpa Cuernavaca, Morelos. 62210, Mexico. *Also at Facultad de Ciencias Quimicas e Ingenieria, Universidad Autonoma del Estado de Morelos. Avenida Universidad 1001, Colonia Chamilpa Cuernavaca, Morelos. 62210, Mexico.
ABSTRACT The Mexico City international airport underwent a major expansion/transformation in the last few years, which included the growth or renewal of some segments of the jet fuel network of pipelines and hydrants. A strategy to maximize the space utilization in the current location forced to
build the new terminalâ€™s apron using a tailored construction methodology based on concrete decks above the wet clay soils characteristic of the former Lake of Texcoco. Structures are prone to sink with time in this type of soil; therefore the decks were made of a low enough density to hold above the wet clay soil. The selected construction system for the apron was a sandwich of steel reinforced concrete layers, with a 2 meter layer of a porous volcanic material called Tezontle in the middle. A significant part of the new jet fuel pipeline was located within this sandwiched structured deck. The CP system was a hybrid design so that the pipeline transporting jet fuel into the hydrant pits of Terminal 2, within the sandwiched deck structure, could have a continuous anode configuration, and the rest of the jet fuel pipeline network would be protected by a remote deep three anode CP array. The complexity of the jet fuel pipeline network due to the combination of a very old poorly coated segment, the new FBE coated segments, as well as the many fittings associated to the hydrants, obliged a design targeting the compliance to the 100mV criterion of NACE RP 0169 â€“ 2002. Keywords: Cathodic protection, airport, hybrid CP system, continuous anode, deep anode, jet fuel pipelines. INTRODUCTION. The increasing needs of passengerâ€™s flow at the Mexico City International Airport, MCIA motivated a large scale investment for the construction of a full new terminal and the modernization and expansion of the former single terminal, both in new passenger gates and parcel company operations. Ultimately in the last few years the airport experienced a near 100% growth in jet fuel pipeline extension and hydrants. Presently the jet fuel supply is over 3.5 million liters per day. External corrosion control is a very important task for the proper integrity management of the jet fuel supply of the airport. The soil is very corrosive in this part of Mexico City. The airport is located in the former Lake of Texcoco. The high saline content of the clays of the once bottom and now soil of the lake is directly related to the very low resistivity of the soil and ultimately to the high corrosivity of the environment where the pipelines are buried. In this paper we report the experiences in designing and installing the cathodic protection system for the jet fuel pipelines motivated by the recent expansion of the MCIA. Control of the external corrosion of jet fuel pipelines in airports is usually a complicated task. Multiple requirements about security and logistic constraints are normal in operating airports, but due to the strategic location of the Mexican main port of entrance and departure the difficulty is increased. Several particularities made this CP project interesting and challenging. Two different generations of materials, pipeline steels, coating types and conditions as well as building techniques were involved in the final jet fuel configuration and had to be addressed for the solution design.1-2 Security and therefore technically no negotiable issues established a fixed framework for the CP design work. No power devices were to be installed inside the terminal areas, therefore all rectifiers could only be installed in the tank farm. Also in the terminal fields all connections should be housed in flame-proof boxes and only underground test stations were allowed in the terminals. We could convince the airport authority in charge of construction to install isolating joints where needed, mainly to divide the CP circuits in order to treat separately the old and the new jet fuel pipelines. However we could not convince constructors of not using what we thought was an excessive number of casings that in the middle or long term may not remain isolated from the jet fuel pipeline.
Due to the construction constraints described above, and the many fittings and groundings near the jet fuel hydrants, we targeted this CP system at least to provide external corrosion control under the 100mV criterion.3-5 SITE AND CONDITIONS The problem understanding requires determining the conditions for the construction, logistics, involved materials and conditions that derivate into bigger complications; so it is convenient to identify the site and installations distribution. MCIA jet fuel pipelines distribution is illustrated on figure 1, which corresponds to an air view of the MCIA region. The jet fuel pipeline consists of two networks. One was constructed about 30 years ago using API B2 type of steel and coal tar coating to fuel about 40 hydrants existing before the expansion project. An API X65 steel and FBE coated new pipeline was constructed to fuel the new Terminal 2, and a portion of the old pipeline going into Terminal 1.
NEW PIPELINE SEGMENT OF OLD PIPELINE TO BE DECOMMISIONED
NEW TO OLD PIPELINE JOINTS
C Figure 1. Scheme of MCIA with the two segments of the jet fuel pipeline network.
The scheme of figure 1 shows the run ways (marked with red arrows), besides the services area, the older terminal and the new one (marked as A, B and C respectively over the photograph). The jet fuel
pipelines system (with more than 20 km) is schematized using three line sets, which denote the localization of the new pipe (pink), old one (green) and the both pipes joining sections (blue) in order to understand the complexity in the common consideration of them for the CP system. The current demand for the older pipeline was measured 60A. The soil electrical resistivity is too low, just 300 to 600 Ohmcm the is mainly because this airport is on the lands that used to be the Lake of Texcoco, The terrain is of very low shear strength, particularly in the zone of the last section of the jet fuel pipeline (marked with the yellow rectangle), which is characterized by clays saturated with water and it produces the imminent risk of sinking. In this way, it was necessary to design complex and hybrid solutions in order to reduce the sinking risks and to apply a cathodic protection system for the corrosion control of the jet fuel pipelines. ANTI-SINKING PLATFORM The strategy to maximize the space utilization in the current location forced “Aeropuertos y Servicios Auxiliares (ASA)”, the Mexican airport authority, to build the new terminal’s taxi drives using a tailored construction methodology based on concrete decks above the wet clay soils characteristic of the former Lake of Texcoco. Structures are prone to sink with time in this type of soil; therefore the decks were made of a low enough density to hold above the wet clay soil. This must have a high rigidity in order to support the airplanes traffic that can be forwarded to the gates of the Terminal 2.
JET FUEL PIPELINE JET FUEL PIPELINE REINFORCED CONCRETE
SUPERIOR LAYER OF REINFORCED CONCRETE
TEZONTLE INFERIOR LAYER OF REINFORCED CONCRETE
Figure 2. Anti-sinking platform for Terminal 2 of MCIA. Schemes in a) longitudinal and b) transversal views; and c) a photographic registry of the building process.
The construction system that was selected for the taxi drives was a sandwich with three layers; two steel reinforced concrete layers and in between filled with Tezontle particulate a very porous and light volcanic material. Figures 2a, b and c illustrate the construction scheme. An asphalt cover on the upper concrete deck allows the stress distribution and pressure over a wide area, where airplanes circulate and park at the passenger’s zones. As mentioned before in the middle of these concrete layers, a zone of red gravel (around 2’’ to 4’’ diameter) with large porosity called Tezontle was placed. The density of Tezontle allows filling high volumes with a relatively low weight. The lower density of the whole
structure prevents it from sinking in the wet clays of the lake. The jet fuel pipeline was located within the Tezontle layer in a trench of sand. The three layer structure is not water insulated at laterals which contact the wet clay soils. Since humidity may thus reach the pipeline, the CP design included a solution for the pipeline in the middle of this structure. In figure 2c, the construction process is shown, denoting how the first stage was the collocation of the steel reinforced concrete, followed by the Tezontle refill and the sand bed for the pipeline. Above are to be the final slab of reinforced concrete with an asphalt surface. In the photography can be also observed the space between the concrete plate and the pipeline (to be filled with Tezontle and sand), which allows a mechanical protection for the pipe supports. CP CURRENT DEMAND As pointed out before around 30% of the new jet fuel pipeline and hydrants are built inside the described three layer structure, which may act as an isolating cavity to the electrical potential induction for a impressed-current CP system. The rest of the pipeline and the hydrants of Terminal 2 are buried in natural soil where a remote bed impressed current CP system is indicated. The corrosion control system must be designed considering two different conditions, and therefore of hybrid nature both because of the remote and distributed anode beds, and also sacrificial and impressed current anode beds. TABLE I. CURRENT DEMAND VALUES AND PARAMETERS TO CONSIDER. MEXICO CITY AIRPORT JET FUEL PIPE DIAMETER
TOTAL AREA OF PIPE
BURIED JET FUEL PIPE
JET FUEL PIPE IN SANDWICH
COPPER COATED GROUNDINGS
CURRENT DEMAND PRESENT
CURRENT DEMAND IN 30 YEARS
AMPERE/ SQ METER
NET METAL AREA TO UNIT CURRENT PROTECT DEMAND
The current demand calculations are resumed in table 1. There is shown that it is necessary to protect about 14,400 m2 of pipelines in natural ground. The segments that are inside the anti-sinking platform, which totalize 5,750 m2 of jet fuel pipeline, associated to connections and hydrants that sum 287 m2. From these amounts of pipe surfaces exposed to ground, and considering extreme conditions, where the pipelines in the natural ground (including old and new ones) would have a coating minimal efficiency of 80%, while the pipelines inside the deck would show a value of 95% and the hydrants 90% in the worst scenery (used for the extreme requirements of security). In this way, the bare metal considered amount is 2,873, 287 and 29 m2 respectively for the mentioned sections, which present a unit demand of 0.020 A/m2 (characteristic to steel). Also to the copper coated groundings that correspond to 18m2 with a unit
current demand of 0.060 A/m2. So the total current demand present is 64.9 A, and the total current demand in 30 years is 113.5 A. PROPOSED SOLUTION. Under the current demand considerations, the mechanism for administrate the required current was determined. Based on the complication of the sandwich deck, which acts as isolating cavity, the proposed solution involves a hybrid system. With an impressed-current CP system based on remote ground beds for the pipelines in the natural ground, and using a galvanic anode CP system for the pipeline that is inside the anti-sinking platform. A. IMPRESSED-CURRENT CP SYSTEM.The selection of the impressed-current CP system was based on the proposal of a set of three deep anodes, localized in the tank farm area, where it was possible to make the excavations, but also can be used for the control, maintenance and operation of the CP system with no major logistic complications. It was measured the maximum distance and because of the soil properties of low resistivity and high humidity, it was concluded this as the optimal solution.
RECTIFIER 0.60 m (+)
AT LEAST 3M
PIPE 35m 20m CATHODIC PROTECTION SYSTEM WITH THREE DEEP ANODES ARRAY
5 CM OF CEMENT AROUND PVC
PVC STRUCTURE WITH PERIODIC HOLES
ANODES BASED ON DESIGN
Figure 3. Scheme of a) three deep ground bed system and b) each deep anode configuration.
Figure 3a shows the configuration of the three deep anodes, it can be observed that the distance between the anodes of 20 m, while the established deep was 100m and the active zone (defined by the liquid carbon) is 35m. The system of three ground beds is regulated with independent controls of two rectifiers, one of 100A X 100V DC output and the other with 50A X 50V DC output. Each deep anode is considered a structure as shown in figure 3b, where the dimensions of the anode body are illustrated. The selected anodes were mixed metal oxides deposited over titanium tubes, while the active column is
immersed in low resistivity carbon coke. For each anode there is an independent wire attached to a shunt box, where they are all connected by a single wire to the rectifier. This system provides enough current to keep the jet fuel pipelines with enough electronegative potential to control the corrosion behavior. It is important to establish that the impressed current part of the CP system was targeted to provide corrosion protection mainly to the jet fuel pipeline segments buried in soil. The CP of the segments of the jet fuel pipeline in the two concrete slabs of the anti-sinking deck was designed by employing the reinforcement of a magnesium (Mg) continuous ribbon anode parallel to the pipeline due to the plausible blockage of the CP current coming from the impressed current configuration. B. GALVANIC ANODE CP SYSTEM. Considering that about a third part of the new jet fuel pipeline was located within this sandwiched structured deck. The freatic level in the former Lake of Texcoco reaches ground level most of the year, especially in the rainy season that lasts about 7 months. Water leakage into the sandwiched deck structure was considered unavoidable. In this way, the proposed solution for the protection of the jet fuel pipelines and hydrants inside the anti-sinking platform was the use of a magnesium sacrificial continuous ribbon anode following the same trajectory than the pipe at a distance sufficiently short enough to present the effects of a distributed galvanic anode system. Associated to this effect, the electrolyte must be considered, and then the zone between the pipeline and the magnesium anode is filled with sand, which has the adequate resistivity and it also allows the mechanical manipulation for the sacrificial ribbon along the trajectory. The placement of this anode and the sand electrolyte can be observed in figure 4, where the installation is registered.
JET FUEL PIPELINE
CONTINOUS MANGANESIUM ANODE
Figure 4. Localization of Mg sacrificial anode besides the pipeline.
In order to determine the specifications of the magnesium ribbon to be used, the amount of mass required for the sacrificial anode must be calculated. This analysis is resumed in table II, which corresponds to the worst scenario, where it is necessary 5.6 A to protect the jet fuel pipeline during 30
years in extreme conditions. It is derived from calculations that the minimal mass requirement of 1,344 kg of Mg. TABLE II. REQUIRED MASS FOR GALVANIC ANODE. Unit Mg anode cm 1.92 Wide m 5000 Length Kg/m 0.36 Linear density Kg 1804.05 Mass
Following the condition of keeping the pipeline protected in all the segment, the solution of the galvanic continuous anode involved the selection of Mg with characteristics listed in table III, and because the jet fuel pipeline length (sacrificial anode is parallel and will have similar length) is 5 km, the total amount of mass is more than 1800 Kg, covering the established requirements. TABLE III. SPECIFICATIONS OF THE Mg ANODE, CONSIDERED AS SACRIFICIAL ANODE. Mg Consumption rate, Cr, kg/A-yr 18 Current demand, A. 5.6 Life in service, years. 30 Mass og Magnesium, Kg. 1344
RESULTS. The effects of this designed CP hybrid system were evaluated with current interrupters in order to determine the on-off soil/pipeline potentials. The sites for the analyses were selected considering 8 points over the old pipeline and 8 more for the new one. These values are shown in table IV, which denoted for positions with values higher than the -0.850 V, considered in the criteria for steel structures. The lowest values are basically for the terminal 1, which has the poorest coating efficiency, mainly because the age of the pipeline. TABLE IV. EVALUATION OF PIPE/SOIL POLARIZATION POTENTIALS FOR POSITIONS IN BOTH TERMINALS. TERMINAL 1 TERMINAL 2 POSITION POTENTIAL POTENTIAL POSITION POTENTIAL POTENTIAL ON (V) OFF (V) ON (V) OFF (V) Tank farm -2,060 -1,160 1 -1,800 -1,140 14 -2,040 -1,140 2 -1,800 -1,130 15 -1,923 -1,120 3 -1,790 -1,110 16 -1,700 -1,100 4 -2,400 -1,090 17 -1,826 -1,040 5 -2,870 -1,130 18 -1,840 -1,120 6 -2,220 -1,136 19 -1,200 -0,890 7 -1,460 -1,120 20 -1,340 -0,920 8 -1,456 -1,110 21 -1,310 -0,904 9 -1,550 -1,120 22 -1,170 -0,900 10 -1,100 -1,000 23 -1,120 -0,904
11 12 13
-1,382 -1,259 -0,940
-1,100 -1,000 -0,880
24 25 26
-1,120 -1,170 -0,985
-0,870 -0,845 -0,860
Since we could not find any evidence of microbial induced corrosion, here it is important to notice that the ground conditions allow considering the -100mV criteria with no detriment on the security of the jet fuel pipelines. This would allow the CP system with less expenses and keeping the corrosion control for the whole set of structures. Here it is important to notice that the ground conditions allow considering the -100mV criteria with no detriment on the security of the jet fuel pipelines. This would allow the CP system with less expenses and keeping the corrosion control for the whole set of structures. CONCLUSIONS The CP system was a hybrid design so that the pipeline transporting jet fuel into the hydrant pits of Terminal 2, within the sandwiched deck structure, could have a continuous anode configuration, and the rest of the jet fuel pipeline network would be protected by a remote deep three anode CP array. The complexity of the jet fuel pipeline network due to the combination of a very old poorly coated segment, the new FBE coated segments, as well as the many fittings associated to the hydrants, obliged a design targeting the compliance to the 100mV criterion of NACE RP 0169 â€“ 2002. We report the experience in the design, construction and commissioning of this hybrid CP system. Full CP coverage of the jet fuel pipeline and hydrant branches was achieved. The tests during the commissioning provided valuable experiences, including the mapping of the CP current distribution in the complexity of the Mexico City airport employing a clamp ammeter. The selected configurations and systems for both; the jet fuel pipeline in the anti-sinking deck, where an galvanic continuous anode, and for the use of the impressed current CP system, based on the use of three ground beds allow establishing a CP along the different segments of the jet fuel pipelines at least for a period of 30 years. This was gotten with a hybrid system that can be used in future cases where equivalent conditions are found. Beyond our original expectations, most of the test stations read a polarization potential complying with the standard -850 mV criterion. Natural potentials ranged between -500 and 625 mV; at the point where the potential read was -845 mV, the natural potential measured value was -542mV. Then, we can conclude that the hybrid CP system was applied successfully, and that the -100 mV criteria can be applied directly to this jet fuel pipeline network. This project provided a sound cathodic protection hybrid system helping for the compliance of the current regulations for the corrosion control in this major airport in Mexico. ACKNOWLEDGMENTS We are grateful to Aeropuertos y Servicios Auxiliares for the valuable support while performing the field work. We specially thank Tom Lewis for his valuable advice in designing the deep anode configurations, as well as the field supervision of Joseph Tatum III. We also thank the technical support of Arturo Godoy, Hernan Rivera, Anselmo Gonzalez, Maura Casales, Miguel Lobato and Osvaldo Flores; as well as Evelia Sandoval in project management.
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