Numerical investigation of erosion of tube sheet and tubes of a shell and tube heat exchanger
Introduction
Shell and tube heat exchangers are widely used for cooling and heat recovery in oil and gas production and other chemical processes. Erosion is one of major problems of the heat exchangers, when either fluid flowing through tubes or fluid passing through the shells contains solid particles, for instance, seawater is utilized for cooling, and runs at a velocity that likely causes the metal wear from the tube surfaces at its operating temperature (Kuźnicka, 2009, Lai and Bremhorst, 1979). The impingement of solid particles most often occurs on the inside surface of the tubes and near the tube entrances for shell and tube heat exchangers and also along the U bend for U-tube heat exchangers. Severe metal loss in some local areas of a heat exchanger not only requires frequent maintenance but also results in the failure of the heat exchanger, leading to very costly planned and unplanned maintenance, production loss and potentially environmental disasters.
Erosion process has been investigated extensively since decades with respect to the mechanism (Research and Markets, 2011) and the relationship between the amount of erosion and the variables dominating the erosion, including the materials of targets (Laguna-Camacho et al., 2013), fluid properties and velocity (Kesana et al., 2013a, Kesana et al., 2013b), properties and velocity of the solid particles (Kesana et al., 2013a, Kesana et al., 2013b), and temperature (Naz et al., 2015). Despite these fundamental studies and recent advances in computational fluid dynamics the erosion process has yet to be fully predicted with reasonable accuracy even for fairly dilute suspension of solid particles. Most prediction studies mainly concern the erosions on simple geometries, such as straight and sudden contraction tubes (Badr et al., 2005, Duwig et al., 2008, Habib et al., 2007, Habib et al., 2008), elbows (Chen et al., 2004, Mazumder et al., 2008, Njobuenwu and Fairweather, 2012, Safaei et al., 2014; H. Zhang et al., 2012), tees (Chen et al., 2004), U bends, orifices (Nemitallah et al., 2014), pipe and wall cavities (Lin et al., 2014, Wong and Solnordal, 2012, Wong et al., 2013a, Wong et al., 2013b) and flat plates (Wong et al., 2012). Predicting the erosion process in shell and tube heat exchanger is more challenging because of the complexity of flow field in the regions likely to be worn, i.e., the inlet head and the downstream regions from the tube entrances. A single fluid stream expands into a large area and then divides into multiple smaller streams; turbulence leads to a very high velocity in some local regions. The flow field much depends on the geometry of the heat exchanger and the properties and velocity of the fluid.
The tube entrance of shell and tube heat exchanger is the most critical region with respect to erosion failure. The rate of erosion greatly relates to the flow characteristics in this region. It has been found that a cross flow pattern near some tube inlets considerably contributed to the erosion of the tubes (Bremhorst and Lai, 1979). Low velocity caused accumulation of deposits, reduction of tube diameter and sometimes complete blockage of tubes (Ranjbar, 2010). The effects of flow velocity and sand particle size on the rate of erosion in a shell and tube heat exchanger were investigated numerically (Habib et al., 2006, Habib et al., 2005), where k-ε turbulence model was used for fluid and the Lagrangian approach was employed for tracking particles. The influence of particle motion on the fluid flow field was neglected. Half of the heat exchanger having 38 tubes were considered based on the assumption of symmetrical flow. However, both experimental and numerical studies of a sudden symmetric expansion flow (Bremhorst and Lai, 1979, Duwig et al., 2008, Sugawara et al., 2005, Ternik, 2009, Velasco et al., 2008) have shown that the flow field is asymmetry, which is attributed to the competing effects of shear thinning and inertia on the size of the corner vortex (Mishra and Jayaraman, 2002). The asymmetric flow in the head of shell and tube heat exchanger is similar in nature to asymmetries noticed in plane expansion flow. A full geometry was used in the modelling of flow field in the symmetrical head of a shell and tube heat exchanger (Bremhorst and Brennan, 2011). In the modelling, the Reynolds-Averaged Navier–Stokes (RANS) equations for continuity and momentum equations and the SST-k-ω turbulence model were used. The prediction of erosion/corrosion of the tube inlets was based on the characteristics of flow field as solid particles were not included in the modelling. Accurate prediction of erosion rate depends on the determination of the particle impact velocity, impingement angle and the frequency of impacts on concerned surfaces. These particle variables can be derived from their trajectories. The Lagrangian approach has been demonstrated in modelling particle motion in various geometries for dilute systems (Badr et al., 2006, Parsi et al., 2014, Wong et al., 2013a, Wong et al., 2013b).
Although erosion of tube sheet, tubes and tube ends is a common problem that influences the performance of shell and tube heat exchangers, there is no research published in the literature that deals with the effect of various parameters on the erosion of these targets. The present work aims to study the erosion at the entrance region and tubes of the heat exchanger and the effect of particle behaviors and fluid flow on the rate of erosion. A physical model is proposed based on initial computational fluid dynamics simulation of fluid flow in a shell and tube heat exchanger, which provides the flow characteristics in the heat exchanger and the dependence of results on the momentum and turbulence models adopted. The simulation is performed for different feed fluid rates and particle sizes in a range of 0.1–1000 μm.
Section snippets
Physical model
The simulations were performed for the inlet head inside and the flow development regions in tubes close to the tube sheet of a typical shell and tube heat exchanger with two-pass construction. Fig. 1 shows the geometry of the physical model. Fluid enters from the side entry into the head and then is distributed into 230 tubes at the tube sheet. The liquid through each tube first experiences a 400 mm pipe flow and then a 200 mm porous flow. In another word, first 400 mm from the tube sheet of each
Results
Erosion rates are calculated for tube sheet, the part of copper tube protruded outside the tube sheet and the surface of the copper tubes, since the practical evidence shows that these are the critical sections with respect to erosion. To demonstrate the dependence of erosion on flow pattern and solid particle behaviors close to the likely erosion areas, the impact angle, velocity and concentration of particles at upstream distance of 5 mm to the tube sheet, at the tube entrances and at tube
Conclusions
The erosion of a two-pass shell and tube heat exchanger was simulated by modelling the flow and particle motion with a physical model, where each individual tube was presented by a short tube plus a porous plug and a porous plate was used to adjust the backpressure in the rear head. This approach revealed more flow aspects affecting the erosion of the head and tubes of the heat exchanger, compared to the treating not including the modelling of flow in the rear head and the second pass. The
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No. 51274082).
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