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49 MATERIALS PERFORMANCE: VOL. 58, NO. 6 JUNE 2019 average absolute temperature (°K). A com- pressibility factor Z was assumed as 0.9. = Superficia l g as velocity , V Operating flow rate Area g (4) Since the flow rate provided was under standard conditions, the combined gas equation is used to calculate the flow rate under the operating conditions, which upon rearranging the terms with compress- ibility factor gives us Equation (5): = Operating flow rate P * V * Z * T T * P 1 1 2 2 2 (5) where subscript 1 represents the quantities under standard conditions and 2 represents the quantities under operating conditions. With all parameters known, the critical angle is calculated using Equation (2). The previous equations 2 were used to calculate the critical angle for a gas flow rate of 237 kNm 3 /h. Table 1 shows the parameters cal- culated using the previous equations. Flow modeling calculations were performed to determine the critical angle for a range of flow rates and gas velocities, which is pre- sented in the next section, Results of Flow Modeling. Results of Flow Modeling Flow modeling results are used to iden- tify the locations at which water can accu- mulate in the pipeline. Determining the critical angles of inclination for a range of velocities and f low rates illustrate the results of f low modeling. Figures 3 and 4 show the plots of critical angle for velocity and flow rates. It can be seen from Figures 3 and 4 that as the gas velocity increases, the critical angle also increases. At low gas velocities and large critical angle, the water accumulates in the pipe. At higher gas velocities, water is carried further down- stream. Gravity that causes the liquid to f low upstream and the shear that carries the liquid downstream define the critical angle for water accumulation. The critical angle of the 0.91-m line for the current oper- ating conditions of 4.54 MPa (644 psi) at 20 °C is 0.6 degrees, as mentioned in Table 1. FIGURE 5 Dig site locations. Water first accumulates at a location where the pipeline inclination angle is greater than the critical angle. Figure 5 shows the locations where the pipeline inclination angle is greater than the critical angle of 0.6 degrees for the first 762 m (2,500 ft) of the pipeline. Comparing the actual pipeline inclinations with the critical angle helps us identify the locations of dig sites for direct examination. As shown in Figure 5, three locations were selected for direct examination. These three locations were selected because of large inclination angles. If no water accumulation/corrosion is found at these locations, it can be assumed that corrosion would be unlikely in the downstream segment of the pipeline. The pipeline was excavated at dig sites 1, 2, and 3 for detailed examination. Moderate corro- sion was found at dig site 1 using guided TABLE 1. FLOW MODELING RESULTS FOR FLOW RATE OF 212 MMSCFD 2017 DG-ICDA—36-in HCA, Louisiana, USA NACE SP0206 Dry Gas DG-ICDA Critical Angle Calculation for 212 MMSCFD Gas Flow Rate Parameters Value Gas density at standard temperature and pressure 0.692 kg/m³ 0.0432 lb/ft 3 Gas density at operating pressure 30.9 kg/m³ 1.932 lb/ft 3 Liquid density—water 998.3 kg/m³ 62.32 lb/ft 3 Acceleration of gravity 9.807 m/s² 32.174 ft/s 2 Nominal pipe outer diameter 0.914 m 36.000 in Nominal pipe wall thickness 11 mm 0.438 in Nominal pipe inner diameter 0.892 m 35.124 in Gas operating temperature 20 °C 68 °F Gas operating pressure (gauge) 4.54 MPa 644 psig Gas compressibility (Z) 0.9 Unitless Standard gas flow rate 237 kNm 3 /h 212.0 MMSCFD Flow rate at operating temperature and pressure 0.1297 MCMD 4,579 MCFD Superficial gas velocity 2.40 m/s 7.88 ft/s Critical angle 0.6 Degrees Flow Modeling to Predict Internal Corrosion in Gas Transmission Pipelines

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