There are some negative velocities in the lower region of the plot. Some of the flow is actually exiting the impeller in this plane, indicating that a certain amount of backflow is present. Similar plots were created for the 50% BEP and 120% BEP flow conditions. The flow variations were much less severe at the lower flow but increased at the higher flows.
There are also radial and circumferential variations of the static pressure in the same plane. One particular point of interest is the localized pressure zone found near the splitter. This location corresponds to a localized area of pitting (cavitation damage) found on the top half of the casing, thus providing some credibility to the accuracy of the analysis prior to any experimental verification.
Further upstream of the impeller, one side (right) of the suction passages' velocity distribution includes the flow traveling from the suction nozzle while the other (left) side of the distribution includes only flow that passes around the casing wearing ring and enters the impeller from the opposite side because of the location of the cut plane (Fig. 4).
The inner wall of the suction nozzle has a deep valley that lies just upstream of the wearing ring. This profile is created by the wall of the discharge volute that wraps around the impeller and passes through the suction passage. The valley is formed by the blending of the impeller housing and the discharge volute. Some of the flow entering the pump tends to follow the contour of this valley but then must "climb back out" on its way over the wearing ring. This redirection of flow causes some vortexing in the valley region.
For the portion of flow that follows this contour and continues over the wearing ring, there is yet another vortex within the impeller. The inside profile of the wearing ring creates a sharp discontinuity in the flow boundary. As a result, the fluid momentum carries the high-velocity fluid over the discontinuity but viscous forces cause a small portion of it to slow down and turn back into a somewhat stagnant region. Thus, the vortex is formed.
Thus far, the CFD analysis of the pump casing had revealed that the suction inlet design is less than optimal. The most significant conclusion at this stage of the project was that cavitation was likely to occur irrespective of the impeller design.
The casing wearing ring was chosen to be altered to smooth the flow because it is a replaceable part in this double-suction pump design.
The radius on the inside of the ring was increased significantly to avoid separation of flow as the fluid accelerates into the impeller eye. In addition, large tabs or ears were added on a portion of the ring circumference facing the suction nozzle. The desired effect was to create a bridge over the valley formed by the curvature of the discharge volute (compare Fig. 6 with Fig. 7).
The CFD model was updated to reflect the new wearing ring geometry. Then the analysis was rerun using the same boundary conditions as before.
The circumferential and radial variations in velocities normal to the plane of the impeller are much less prominent than they were with the original wearing ring. There is no longer a region of negative velocities, thus no flow exits the eye. Overall, the flow is much more uniform in velocity. As in the velocity plot, the variations in a revised static pressure distribution plot still exist but are much less severe.
Improvements can also be seen in the inlet velocity distribution (Compare Fig. 4 with Fig. 5, above). The extension on the new ring appears to serve its intended purpose. The entrance flow moves smoothly along the outer surface of the ring and accelerates much more smoothly into the impeller. The vortex patterns are no longer visible in the valley created by the discharge volute nor are they visible inside of the impeller. The magnitude of velocity is more nearly equal from one side of the impeller to the other as compared to the case with the original wearing ring.
The acceleration into the eye is less than with the original rings as a direct result of providing a large radius turn into the eye. These effects can also be seen in the static pressure plot (not shown), where the gradients are less severe than the originals. The isolated low-pressure region has collapsed into nothing more than a gradient near the inner surface of the wearing ring.
All of these analysis results point toward a better pump design for a quieter pump. Recirculation losses upstream of the impeller were virtually eliminated. Recirculation in the impeller eye has disappeared. The magnitude and direction of flow entering the impeller are more uniform in the circumferential and radial directions. The final CFD analysis has predicted a greatly improved inlet flow to the impeller and one would expect better pump performance.
A series of tests was conducted at the pump manufacturer's R&D facility using a spare pump identical to the other four in service at the Dow plant with a new bronze casing wearing ring to prove that the analysis was correct.
Efforts were also made to verify the analysis results by means of measurements within the suction flow field. A static pressure probe and pitot (total-static) probe were used to record pressures and velocities at specific locations within a plane that lay just outside of the wearing ring, based on the output of the CFD analysis.
The measured data were plotted along with the theoretical predictions and an excellent correlation was observed.
The primary cause of noise in the 30 x 30-38 horizontally split case double-suction pump was flow separation occurring in the suction chamber. Classic techniques would not have led to a solution.
CFD was used successfully to identify the problem area within the suction passage (volute suction nozzle), rule out impeller recirculation as a problem and predict an improvement with a new ring.