Materials Performance

JUN 2019

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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48 JUNE 2019 W W W.MATERIALSPERFORMANCE.COM CM CORROSION MANAGEMENT intervals for the pipeline. If any corrosion is foun d , cal cul ation s to d et ermin e th e remaining strength/fitness for service and corrosion rate may need to be performed. Fluid Flow Modeling Basics Flow modeling is used to predict loca- tions where water can accumulate when it enters the pipeline. Water accumulates at sags and low points on the pipeline. If the water holdup locations predicted using f low modeling have not corroded, then other locations in the pipeline that are less likely to accumulate water can be consid- ered corrosion free. For dry gas transmission pipelines, liq- uid holdup strongly depends on the gas velocity and the pipe angle of inclination. For a given gas velocity, the higher the incli- nation angle, the higher the probability for water to accumulate. Conversely, for a given inclination angle, the lower the gas velocity, the higher the probability is for water to accumulate. At the water holdup location, there are additional pressure losses due to shearing encountered at the gas/liquid interface as well as along the pipe wall. Also, the highly compressible gas expands as the pressure decreases along the flow path. Gravity and shear at the gas/liquid interface are the two important forces that influence the critical angle for water accu- mulation. 4 Figure 2 shows the influence of gravity and shear on the liquid holdup. Note that liquid holdup is more prominent in uphill inclined sections where gravity drives the liquid upstream and the shear drives the liquid downstream. In the downhill section of the pipe, the gas f low and gravity both move the liquid downstream. In the hori- zontal section of the pipeline, if the gas is moving, no water can accumulate. There- fore, gravity that causes the liquid to flow upstream, and the shear that carries the liq- uid downstream, define the critical angle for water accumulation. Flow Modeling Calculations As previously described, the critical angle for liquid accumulation is a function of the velocity of the gas. Since there are no injection points or valves to control the flow in the section of the pipeline under assess- ment, it is fair to assume that the f low is under steady state. It is also assumed that the gas temperature along the pipeline is constant (isothermal approximation). The equation that relates critical angle to the velocity of the gas is: u = arcsin 0.675 r g r l –r g × V g 2 g * ID ⎛ ⎝ ⎜ ⎜ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎟ ⎟ ⎟ 1.091 (2) where ρ g = density of the gas at the operat- ing conditions, g/cm 3 ; ρ l = density of liquid, g/cm 3 ; ID = internal diameter, m; and V g = superficial gas velocity, m/s. The gas density at the operating condi- tions is calculated using the ideal gas law using compressibility factor Z: = r P * MW Z * R * T g (3) where MW is the molecular weight, R is the ideal universal gas constant, and T is the FIGURE 3 Critical angle for a range of gas velocities. FIGURE 4 Critical angle for a range of gas flow rates.

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