Design and Modification of Large Pump Turbine Stay Vanes

Design and Modification of Large Pump Turbine Stay Vanes

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American Hydro (AH) was contracted by a customer to supply two new 330 MW pump turbines in the United States. AH’s scope of supply also included new wicket gates, stay vane modifications, new motor-generator rotors, along with both new and rehabilitated stationary components. During the hydraulic engineering development phase of the contract, AH analyzed dozens of stay vane iterations, finalizing a design that significantly lengthened the vanes in the direction of flow. The newly designed stay vane profile better aligned the vanes to the hydraulic flow, improving the turbine efficiency by over 2%, and improving the stability of the pump.

By design, each end of the OEM stay vanes included extensions, to be welded to the vanes and material was also removed in order to better streamline the overall profile. After installation, the internal cavity of the stay vane was pressure grouted to support compressive loads during operation. A general concept sketch of the installation is shown below in Figure 1.

Figure 1: General concept sketch for stay vane extension installation.

AH modeled a complete 3D OEM spiral case and stay ring, then modified the geometry to reflect the new hydraulic design. This 3D model was imported into ANSYS Mechanical along with a point cloud from the CFD analysis to accurately load the spiral case.

Figure 2: Applied load diagram.

Figure 3: Stay vane and spiral case loading.

The analysis was initially completed using a relatively coarse mesh that was refined sufficiently to represent component stiffnesses without the refinement required to resolve converged stresses. This process identifies high stress areas so that sub-models could then be used in these specific regions to examine local geometry while limiting the computing power and time requirements to solve. Sub-models included “gaps” in the weld root areas where component geometry prevented welds from being performed as complete joint penetration welds (CJP).

Figure 4: Typical results for spiral case inlet sub-model.

Results from Max Power, Pump Prime, Load Rejection and Shutdown load cases were then used in conjunction with the ASME Boiler & Pressure Vessel Code (BPVC) to ensure all designs and inspections of weldments were completed with strict adherence to the code. Stress levels in the OEM and modified designs dictated that the welds be volumetrically inspected in order to limit the Fatigue Strength Reduction Factor within the BPVC calculations.

The new weldments were not practical CJP welds as no backer or back gouge was readily accessible – these welds would have to be limited to near full dimension, partial joint penetration welds (PJP).  Due to both the complexity of the geometry and volumetrically inspecting a PJP weld, AH worked closely with a company to develop a specialized custom UT procedure. This company supplied an NDE procedure for advanced UT examination using phased array techniques. The company used detailed geometry models with their software to determine the appropriate equipment and process required. The vendor’s inspectors, along with AH inspectors and engineers, travelled to site to perform the UT examination to inspect fusion lines, root depths, and weld quality for inclusions or other flaws to determine if the welds met the design requirements and quality requirements within the BPVC.

Figure 5: Example from phase array UT examination procedure.

Field fitment is of critical importance as poor fitment of the new extensions can have a significant impact on both cost and field schedule. In order to ensure accurate information, AH recommended performing a comprehensive 3D scan of the OEM stay ring geometry to map out the stay ring decks and stay vanes. This information is used to create a 3D shell of the existing geometry which is used to design and manufacture custom extensions for each stay vane. Each vane is serialized for its specific location within the unit. Custom bevels and fillet welds were designed in the 3D cad models to calculate the necessary quantity of weld volume to be deposited in the field which on this project approached 6,000 lbs. per unit. Custom templates were manufactured to be used in conjunction with vent and other measurements to allow each stay vane extension to be placed in accordance with IEC and contractual requirements for the hydraulic flow passage.

While stay vane modifications are not performed on all projects, AH has used this large pump turbine to improve and standardize the method in which we collect information, design, implement and install stay vane modifications.