Masanori ADACHI*
Hidenobu OKAMOTO**
Takashi OKAZAKI***
Takashi HORI****
*
Marketing Department, Marketing Division
**
Oversea Business Development Department, Development Division, Building Service & Industrial Company
***
Planning and Administration Department, Strategy and Administration Division, Building Service & Industrial Company
****
Elliott Ebara Turbomachinery Corporation
By comprehensively utilizing our expertise in development, design, and maintenance, we are able to address fluid-related issues that may be difficult for users to handle, and consultations are provided to users according to the situation. This paper introduces a case of consulting on cavitation erosion issues that occurred during the partial flow operation of a pump. The internal flow of the pump was evaluated by flow analysis, concluding that the backflow vortex cavitation that occurred at the impeller inlet was the cause. To address this issue, the inlet diameter of the impeller was adjusted to suppress the backflow at the impeller inlet, creating an improved impeller. The delivered improved impeller did not experience cavitation erosion even after one year of use, and the issue was resolved.
Keywords: Consultation, Retrofit, Computational fluid dynamics, 3D scan, Cavitation erosion, Backflow vortex cavitation, Suppression of inlet backflow
EBARA CORPORATION's pumps are used in various applications such as public works, industrial plants, and building construction. By comprehensively utilizing our know-how in development, we can address issues caused by fluids that are difficult for users to deal with. By comprehensively utilizing our expertise in development, design, and maintenance, we are able to address fluid-related issues that may be difficult for users to handle. From the investigation stage, our maintenance instructors and engineers are dispatched to the site to provide consultations to users.
This paper introduces a consultation on a cavitation issue that occurred during the partial flow rate operation of a pump used for gas absorption liquid. The overview includes interviews on operational conditions, cause estimation, 3D scanning, flow passage modeling, confirmation of the estimated causes by fluid analysis, retrofit parts manufacturing to solve the problem, and verification during periodic inspections one year later.
Many users in industrial plants recognize that pumps will operate without issues if they are used between the minimum and maximum flow rates specified by the manufacturers and various standards. However, in practice, if the pump is operated at a point far from the Best Efficiency Point (BEP), the energy that has not been converted into hydraulic energy can easily lead to failures.
The Barringer-Nelson curve1) shows the pump operating points and Mean Time Between Failures (MTBF) as shown in Fig. 1. It is shown that MTBF becomes very low, especially at the points where cavitation occurs on the low and high flow sides.
Fig. 1 Impact on reliability when pump operating range deviates from BEP
The cause of the shortened MTBF on the high flow side from the BEP is the increase in the Net Positive Suction Head Required (NPSHreq) due to the rise in suction speed, resulting in a shortage of Net Positive Suction Head Available (NPSHav). This leads to the occurrence of cavitation and the destruction of parts due to cavitation erosion.
When operating on the low flow side of the BEP, the pump must be operated at the manufacturer-specified minimum flow or higher to prevent damage from heating due to circulation inside the pump. However, it is not generally known that, even above the minimum flow rate, if cavitation occurs on the impeller suction port at the operating point far from the BEP, it can cause damage to parts due to a synergistic effect with the backflow vortex generated at the suction port (caused by fluid that has not been sucked in leaving the suction port).
Our service lineup includes parts supply, parts repair, dispatch of field service engineers for customer inspections, consultation, and retrofit of parts as solutions. The retrofit service introduced in this paper can be applied not only to our pumps but also to other company pumps.
The background of this development is that we had been working on accurate 3D scanning, a method to predict the shape of parts that cannot be scanned, drafting of production drawings from the scan results, training of engineers, and application to both domestic and overseas projects with the aim of applying the service to shape inspection of castings, which are considered to affect pump performance.
In two BB3 pumps (horizontal split casing multistage pumps) handling CO2 absorption liquid, operated at partial flow rates, severe erosion penetrating the back side of the 1st stage impeller blades was consistently observed during annual periodic inspections (Fig. 2). As a result, annual disassembly inspections were conducted and expensive impeller replacement costs were incurred. The results of an investigation of the operating conditions at that time are shown in Fig. 3. The operating range in Fig. 3 is considered to correspond to the short service life of the impeller in Fig. 1.
Fig. 2 Pump structure and photo of damaged 1st stage impeller
Fig. 3 Relationship between pump’s BEP and operating range
To estimate the cause of the erosion, Computational Fluid Dynamics (CFD) was first conducted to understand the flow conditions inside the pump. Since the erosion occurred on the 1st stage impeller, the downstream flow from the 2nd stage onwards was considered to not affect the upstream flow. Therefore, the CFD range was limited to the 1st stage only. 3D scanning of the impeller and casing was performed to reflect the shape of the actual equipment, creating a 3D model of the pump interior (Fig. 4). In addition, as the 1st stage is double-suction (symmetry type), the analysis range was limited to one side only to reduce the analysis load. Ansys CFX Ver. 18.2 was used as the CFD analysis software. CFD was conducted with the boundary conditions set to match the actual operation: rotor speed of 2 950 min-1, suction pressure of 0.1 MPa, and flow rate of 324 m³/h, which corresponds to 60% of the pump’s best efficiency flow rate.
Fig. 4 Analytical shape reflecting the actual equipment shape by 3D scanning
An example of the visualization of the internal flow of the pump based on the CFD results is shown in Fig. 5. Fig. 5 (a) shows the positive and negative axial velocities at the impeller inlet cross-section. It is also shown that the flow is flowing out of the impeller into the suction flow passage side (left side of Fig. 5 (a)) in the range of positive flow velocity (blue). Fig. 5 (b) shows the streamlines starting from the inside of the impeller. The extension of streamlines from the impeller inlet cross-section into the suction flow passage indicates the range of inlet backflow. Fig. 5 (a) shows that the axial velocity is predominantly positive at the outer periphery of the impeller inlet cross-section, indicating the occurrence of significant inlet backflow. The large number of streamlines in Fig. 5 (b) further revealed that significantly strong inlet backflow occurs in the original impeller. In addition, the possibility of cavitation occurrence was also evaluated by visualizing the isosurface of the saturation vapor pressure of water at 90℃ shown in Fig. 5 (c). The isosurface was visualized near the suction surface side of the impeller leading edge, suggesting the occurrence of cavitation in this vicinity. Fig. 5 shows the simultaneous occurrence of inlet backflow and cavitation in the original impeller.
Fig. 5 Visualization of the internal flow of the original impeller: inlet backflow and cavitation occurrence
On the other hand, a lot of research has been conducted on cavitation in turbomachinery, and a document summarizing cavitation damage has been published2). Table 1 shows an extract of the damage types corresponding to the damaged location (on the pressure surface side of the impeller) and the internal flow (inlet backflow and cavitation) of this case, derived from a table classifying the location of cavitation damage occurrence and the type of cavitation occurring in the pump, which is summarized in the aforementioned document.
Table 1 Location of cavitation damage occurrence and type of cavitation corresponding to this case
According to Table 1, the mechanism of the erosion issue is the occurrence of backflow vortex cavitation due to the combination of two conditions: occurrences of the cavitation in the low static pressure area and inlet backflow resulting from low flow rate operation. This leads to cavitation being caught in the backflow vortex, causing significant cavitation collapse on the pressure surface of the adjacent blade where the other end of the backflow vortex reaches: exposing to the impact pressure generated at that moment.
To address the erosion issue, backflow vortex cavitation must be suppressed. However, the occurrence of cavitation is highly dependent on the installation conditions of the pump, making it difficult to suppress in the plant under conditions where the installation conditions cannot be changed significantly. Therefore, suppressing another factor, the occurrence of inlet backflow was considered. To suppress inlet backflow, correcting the shape of the impeller near the suction port to suit low flow rates was considered. Two methods were considered: reducing the diameter of the impeller inlet (suction diameter) to decrease the suction cross-sectional area or correcting the blade angle near the leading edge to match the low flow rate. The former method, which is easier to correct, was applied this time. Impellers with gradually reduced suction bores were designed, and each impeller was replaced with the original one to conduct CFD in order to confirm the inlet backflow suppression status, selecting the improved impeller with the appropriate suction bore.
The flow visualization results for the CFD results of the improved impeller with a reduced suction cross-sectional area of up to 60% are shown in Fig. 6 in comparison with Fig. 5. From Fig. 6 (a) and (b), it can be seen that the improved impeller has reduced the positive range of axial velocity components and the number and range of streamlines protruding upstream, indicating that the inlet backflow has been significantly suppressed. The location and size of the isosurface of the saturation vapor pressure in Fig. 6 (c) are not much different from those in Fig. 5, suggesting no significant difference in the occurrence of cavitation. However, since the occurrence of the inlet backflow was suppressed, it was considered that the occurrence of the backflow vortex cavitation, presumed to be the cause of cavitation erosion, has also been suppressed.
Fig. 6 Visualization of the internal flow of the improved impeller: Inlet backflow has disappeared
The modification of the impeller may change the hydraulic performance characteristics of the impeller, leading to changes in pump specifications and potentially disrupting plant operations. It was assumed that correcting the shape near the inlet would hardly affect the hydraulic performance, given that the impeller's performance is primarily influenced by the shape of the outlet side. However, to verify this assumption, a comparison was conducted by calculating the hydraulic performance of both the original and the improved impellers by CFD within their operating range at the plant, ranging from partial flow rate to near the best efficiency point. As a result, no difference was observed between the original and improved impellers in terms of impeller hydraulic performance.
Based on the above results, a retrofit of the parts was proposed and an improved impeller was manufactured and installed into the pump. During the test run after impeller replacement, it was confirmed that there was no change in the hydraulic performance of the pump. A photo of the impeller condition checked during a periodic inspection one year later is shown in Fig. 7. No cavitation erosion was observed on the pressure surface of the impeller, confirming the effectiveness of the improved impeller.
Fig. 7 Internal state of the improved impeller after one year of operation
This paper introduced a case of solving the vortex backflow due to partial flow operation and erosion due to cavitation by retrofitting parts with the utilization of our expertise in fluids and CFD technology.
Our consulting services can be applied not only to cavitation problems but also to vibration and corrosion problems. Not only domestic users but also overseas users have adopted our consulting services to solve problems that were difficult for them to handle.
1）Barringer, Paul. : API Pump Curve Practices from Variability About BEP, Private Weibull Analysis Course, Ref 2, p621
2）Turbomachinery Society of Japan. (2003). Ponpu no Kyabitēshon Sonshō no Yosoku to Hyōka (Guideline for Prediction and Evaluation of Cavitation Erosion in Pumps). TSJ G 001:2003, Tokyo: Japan Industrial Publishing *This citation is available only in Japanese.
This paper is reprinted from "Petrotech Vol. 45, no. 12 (2022)."