**1. Introduction**

The beauty of numerical simulations is its ability to reveal the detailed phenomena or nature of complicated practical engineering problems, which are difficult and sometimes impossible from experimental studies. Based on the obtained numerical results, adequate solutions can easily be identified.

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Five keepers are inserted inside the vessel to maintain the proper radial positions of these elements. The nominal diameter of the inlet and exit pipes is 80 mm. The computational domain covers the whole flow field of the heater from the inlet to the exit, including heat elements and five keepers. It is important to mention that the whole natural gas heat exchanger assembly

Failure Analysis of a High-Pressure Natural Gas Heat Exchanger and its Modified Design

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The mesh for one section of the heat exchanger vessel is shown in **Figure 3(a)**, and the mesh at a cross section cutting through heating elements is illustrated in **Figure 3(b)**. In these two plots, the meshed areas are where the natural gas flows, and the unmeshed hollow regions or circles are where the heating elements are located. **Figure 3(c)** is the mesh cutting through one keeper. As shown in **Figure 3(c)**, there are hundreds of small holes (11.5 mm in diameter) on the perforated plate of the keeper, 276 holes are considered blocked by the heating elements, and the rest meshed are flow passages. A narrow annular flow channel surrounding the keeper is used to keep the heating elements away from the vessel inner surfaces. Adjacent to the annular flow channel, parts of full circles are cut out by a flat bar of the keeper. The

The inlet boundary conditions for the numerical simulations are listed in **Table 1**.

The heavy-duty vessel was wrapped with insulation material, and so it was reasonable to assume adiabatic boundary condition for its external wall boundaries. A heat flux of 4.48 KW/m2 was specified at the heating-element surfaces starting from the middle cross section of the inlet pipe to the end of the heater elements. An increase in natural gas temperature by ~200 K, from the room temperature, was expected. In the investigation, the natural gas was consid-

Steady, turbulent, thermal flows were considered in the present work, and a commercial software package, Fluent, was used for all simulations. The governing Favre-averaged conservation equations of mass, momentum, and enthalpy are not reproduced here, but can be readily

For closure of these partial differential equations, the realizable k-ε turbulence model was applied to model turbulent momentum transfer. A benchmark study on turbulence models indicated that this model was superior to other four popular two-equation models and could provide similar results as those from the Reynolds stress model, a second-momentum closure [3]. The Reynolds analogy [4] was used to account for turbulent enthalpy transfer, and for this type of pipe flows, the turbulence Prandtl number of 0.7 was used [5, 6]. The gravity of 9.8 m/s2 was assigned in the direction consistent with the heat exchanger mounting orientation.

For the thermal properties of methane, polynomials derived from the NIST JANAF tables [7] were used to calculate the specific heat as a function of temperature. Data from NIST [8] were used to obtain polynomials to determine the molecular viscosity and thermal conductivity of

was mounted horizontally.

**2.2. Boundary conditions**

ered as pure methane.

**2.3. Numerical methods**

methane as functions of temperature.

found in [1, 2].

mesh size is ~4.0 million in the number of cells.

**Figure 1.** The natural gas heat exchanger: (a) the damaged heat exchanger; and (b) a close view of the cracks on the top surface of the vessel.

This chapter gives an excellent example for this type of approaches. To identify the reasons for the rupture of a heavy-duty, high-pressure natural gas heat exchanger, as shown in **Figure 1**, the flow field of the heat exchanger was numerically examined. Based on the findings, a new configuration was suggested and the corresponding flow field was studied. The installed modified heat exchanger has been trouble-free used to date. In the following sections, the computational domain, mesh, numerical methods, flow features of the two designs, and the cause of the heat exchanger damage are presented and discussed.
