This Tip of the Month illustrates the impact of pipe insulation for natural gases being transported by pipeline. As the natural gases move along the pipe its pressure and temperature change due to the Joule-Thompson effect, frictional loss, elevation change, acceleration, and heat transfer to or from surroundings. Due to pressure and temperature change, liquid and solid (hydrate) may also form in the line which in turn affects the pressure profile.
In order to demonstrate the impact of pipe insulation, let’s consider the natural gas shown in Table 1. The gas enters a pipeline with an inside diameter of 18 inches (45.7 cm) at rate of 19800 lbmole/hr (8989 kgmole/h). The pipeline length and elevation profile are shown in Figure 1. The ambient temperature was assumed to be 60 ˚F (15.6 ˚C).
The calculation algorithms for computer simulation are discussed in the Gas Conditioning & Processing, Vol 3, Computer Applications and Production/Processing Facilities. In this work, the pipeline was simulated by JMC GCAP Vol. 3 software for two different cases. In the first case, it was assumed the pipe is well insulated and zero overall heat transfer coefficient was assumed, where as in the second case a typical value of 0.25 Btu/hr-ft2-˚F (1.42 W/m2-˚C) for the overall heat transfer coefficient was applied in the simulation. Figures 2 A, B, and C present the pressure, temperature, and liquid formation profile along the pipeline for both cases. Figure 2A indicates that there is less pressure drop for the case of fully insulated pipe (U=0) due to less heat exchange between pipe and surroundings resulting in the higher temperature and consequently less liquid formation. For the case of U=0.25 Btu/hr-ft2-˚F (1.42 W/m2-˚C), Figure 2B indicates that along the first 63 miles (100 km) the heat flows from pipe to the surroundings where as in the remaining portion of the line the heat flow direction is from surroundings to the pipe. The lower pipe temperature in the second portion of line is due to the Joule-Thompson expansion effect. However, for the fully insulated pipe (U=0), the pipe temperature remains above the ambient temperature; therefore, as shown in Figure 2C, liquid is formed only in the last portion of pipe. Finally, the pressure-temperature profiles for both cases are superimposed on the dew point portion of the gas phase envelope to show the crossing of pipeline profile with the dew point curve.
The work reported here clearly shows the impact of pipe insulation in practical design of gas pipelines. Improper use of overall heat transfer coefficient can lead to completely erroneous conclusions about the presence or absence of liquid, even to indicating a line will be handling dry gas when in reality the line will be in two phase gas – liquid flow.
Proper use of the overall heat transfer coefficient combined with calculations carried out in proper sequence will lead to more accurate and reliable forecasts of gas pipeline behavior. The overall heat transfer between the line and its surroundings has an impact on liquid formation in the line and, consequently, on the line pressure profile.
By: Dr. Mahmood Moshfeghian
References:
Maddox, R. N. and L. L. Lilly, Gas Conditioning and Processing, Vol. 3 (2nd Edition), Campbell Petroleum Series, Norman, Oklahoma, 1990
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