Figure 2. Seal faces and flow channel shown with barrier fluid analysis domain.

In this study, ways were sought to improve the cooling function of barrier fluids. One effective means of enhancing the removal of heat from seals is to increase the axial circulation of fluid to warmer regions of the seal where sliding friction occurs, namely the sealing interfaces (Figure 2). It has been found that simple, reliable, and cost-effective designs can provide significant increases in forced convection cooling of mechanical seal, thereby improving their reliability and extending their anticipated life of service.

Method of Design Analysis

Although seal designs must ultimately be validated by extensive testing in the laboratory and in real world (field) applications, computer simulation is becoming an increasingly valuable tool for predicting design performance and guiding the prototype development process. For example, the thermomechaical behavior of seal faces can often be determined with considerable accuracy using specialized and general purpose numerical models. Two such models have found extensive use in our efforts to design new generations of seals. The first of these is a customized finite element code used to study the thermoelastic behavior of mechanical seal faces. This model computes heat generation at the sliding interface and the deformations produced by hydrostatic loads and thermal expansion. The model relies, however, on a number of inputs, one of which being the coefficient(s) of heat transfer representing the convective exchange of thermal energy between the seal faces and their surrounding fluid media.

In the analysis process, the coefficients are first estimated using empirical relationships developed from available experimental data. Using these estimates for the convective heat transfer, the seal face model then computes the frictional heating and distributions of temperature within the faces. The next step is to apply the computed surface temperatures of the faces as input boundary conditions to a commercial CFD code which then solves the governing equations of fluid motion and associated heat transfer. From the CFD analysis, the convection coefficients are then calculated for the specific problem of interest. These coefficients are then compared with those originally assumed, and, if necessary, the entire process is repeated until satisfactory agreement (within 10%) is achieved between the values used by the two models. After obtaining what is believed to be a reliable solution, the results of the analyses are selectively validated against actual physical measurements made in the laboratory.

Scope of Investigation

For the present study, a dual seal configuration was examined for a 48-mm diameter centrifugal pump shaft (Figure 1). The operating conditions were assumed to be 687 kPa, 66 C for the process (sealed) fluid, and 1,031 kPa, 38 C (inlet temperature) for the barrier fluid. Of particular concern were the effects of seal design and operating conditions on the efficiency of heat removal by barrier fluid circulation. Among the variables studied were radial clearance between stationary and rotating components, effects of tapered surfaces, effects of shaft rotational speed, effects of barrier fluid throughflow, and effects of fluid property variations.

The investigation centered around steady-state fluid flow analyses made using the finite volume CFD model FLUENT. Two turbulence model options available within the code were utilized in conducting the flow simulations - a two-equation model which assumes directional uniformity of the turbulence field (sufficient for most cases of interest), and a more sophisticated Reynolds Stress model suitable for strongly nonisotropic flows.

Examination and improved understanding of the CFD results were provided by Data Explorer. Use of this versatile and powerful visualization tool enabled the computed data to be studied in unique and enlightening ways.

Sample Results

Shown in Figure 3 is a typical example of a 2D (axisymmetric) finite element mesh used to analyze the thermoelastic behavior of seal faces. The non-rotating silicon-carbide face is shown in red, while the softer carbon-graphite face which rotates is shown in blue. The axially tapered inner radius of the stationary face (discussed later) is also apparent. The

Figure 3. Meshed representation of seal faces for thermoelastic analysis.

thermal results obtained from the analysis of these faces is shown in Figure 4. This result represents the inboard set of faces, i.e., those nearest the pump impeller (Figure 1), for the case of water as both process and barrier fluid, a shaft speed of 377 rad/sec, and a barrier fluid throughflow of 1.4 l/m. Not surprisingly, the warmest temperatures are predicted to occur near the sliding interface where frictional heat is generated. Up to 80% of this heat is conducted away from the interface through the silicon-carbide material , due, largely, to its much greater thermal conductivity compared with that of the carbon-graphite.