Increased Reliability and Availability of a Geothermal Steam Turbine

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1 GRC Transactions, Vol. 38, 2014 Increased Reliability and Availability of a Geothermal Steam Turbine David Archambeault 1, Garth Larsen 2, and Mark Layton 3 1 TurboCare, Inc., Fitchburg, MA 2 PacifiCorp, Milford, UT 3 TurboCare, Inc., Fitchburg, MA Keywords Geothermal, casing, availability, reliability, life, extension, upgrade, redesign, efficiency, improvement, design, welding, optimizing, modernization, Blundell, resource, performance, steam, TurboCare, PacifiCorp, leak, erosion, corrosion, inlet, joint, inlay ABSTRACT PacifiCorp is one of the lowest cost electricity producers in the United States, providing energy to over 1.5 million customers in the West. PacifiCorp, owned by MidAmerican Energy Holdings Company is headquartered in Portland, OR, and has three divisions of its business; PacifiCorp Energy, Pacific Power, and Rocky Mountain Power. PacifiCorp Energy operates the power generation, commercial and energy trading as well as mining functions of PacifiCorp. PacifiCorp Energy was created after the purchase of PacifiCorp by MidAmerican Energy Holdings. There are 68 generating plants with installed capacity of 9,140 MW, 70% of which is thermal; the remaining is hydroelectric, wind, and other sources. The Blundell geothermal power plant is the first geothermal Unit Agreement approved by the U.S. Dept. of the Interior and is the only geothermal power generation facility owned by PacifiCorp Energy. Located approximately 15 miles north-east of Milford, Utah, the facility sits on the Roosevelt Hot Springs geothermal resource that spans more than 30,000 acres. Production depths are between 1,200 and 7,320 feet with reservoir temperatures between 464 F and 514 F. The 38 MW gross geothermal facility consists of two generating units. The first is a traditional single flash plant (unit 1), producing 26 MW gross (23 MW net) and was commissioned in The second (Unit 2), was an addition of a 12 MW gross (10 MW net) binary bottoming cycle commissioned in The binary unit employs a heat recovery process to extract more energy from the hot geothermal brine left over from the flash cycle. Over years of operation, the steam turbine casing for Unit 1 developed several internal and external leakage paths. Circa 1994, an attempted repair was made to the casing by adding stainless steel inlay on the horizontal joint of the casing. This repair was met with marginal success and the unit continued developing leaks in the horizontal joint to the point the casing had to be seal welded each time the unit was opened. In 2001, TurboCare, Inc. installed a new steam path which included a rotor, blades, and diaphragms made of 12% Chromium (12Cr) material. During this time, the inlet pressure was 115 psia, however the unit s output with the updated steam path provided the unit a re-rate of 26.1 MW output, an 11 percent increase due to the upgraded steampath. In 2009 TurboCare, Inc. was contacted to analyze the existing casing to determine if the pressure could be increased from 115 psia to 138 psia. The analysis of the high pressure area showed that there would be two leak paths at the current pressure in the horizontal joint. The locations of the two leaks was just forward of the vertical joint and the other was in the high pressure area of the N1 (axially forward most) packing casing seals. Therefore, the leaks would only get worse with an increase of pressure. Based on this analysis, TurboCare, Inc. recommended to PacifiCorp that they redesign a new casing for a maximum design pressure capability of 150 psia, for future increase in inlet steam pressure. In 2011, PacifiCorp Energy, in conjunction with steam turbine design and manufacturer, TurboCare Inc., undertook a major development and redesign to replace the steam turbine casing of Unit 1. This casing redesign and replacement was to address the significantly deteriorated geothermal steam turbine casing that had been in operation for 27 years. Over the course of its life, the OEM casing had developed several leakage paths allowing geothermal steam to exit the casing and enter the enclosed turbine hall. The goal of the project was to eliminate the steam leakage paths and increase the inlet pressure capability of the steam turbine. This included a complete engineering design and analysis of both the original and new designs to ensure reliable, safe operation with maximum performance and availability. The redesign consisted of adding stainless steel inlay to critical steam areas. All internal steam seal faces received stainless steel welded inlay during the fabrication process, ensuring any potential motive steam would not deteriorate the casing. This included the 721

2 axial steam seal faces of the diaphragm fits, the packing fits and the horizontal joint in both the high and low pressure sections of the casing. The design also provided opportunity to enlarge the steam turbine inlet from 20 inches to 24 inches, reducing the steam inlet velocity from over 200 feet per second to around 150 feet per second. During the design phase, the vertical joint as well as the unused steam extraction boxes were eliminated and the horizontal joint bolting design and configuration were improved. Lastly the supply of a newly designed real time monitoring steam turbine drain system was included. This drain system ensures maximum enthalpy drop per stage and provides feedback of potential blockages in any of the individual drain lines. This drain system also incorporated optimized external orifice plates and a by-pass loop which provide proper water removal, are serviceable while the unit is on-line and enhance the ability to water wash the unit while running. Project implementation was completed in April, 2013 with a back to back output measurement improvement consistent with engineering analysis, showing improved overall turbine performance. The results demonstrate the enhancement of the casing and drain system met all requirements. This has resulted in the optimization of the steam turbine output and provides real time monitoring of the steam turbine drain system, extending the life of rotating blades by ensuring water backup into the turbine does not occur. Introduction Blundell is a geothermal steam turbine facility located in Milford, Utah. The OEM turbine casing was a 23.5 MW GE designed unit installed in 1984 for Utah Power and Light. The turbine casing used a standard GE Lynn exhaust hood, which included two extraction boxes. These extraction boxes were unused and not required by Blundell. The OEM designed casing had a high pressure adaptor section bolted to the exhaust hood using a vertical 4-way joint. This vertical joint (VJ) was forward of the stage 1 diaphragm, therefore, the 4-way joint was exposed to 115 psia inlet steam. Approximately ten years after installation, the OEM repaired the horizontal joint (HJ) after leaks developed in the casing adjacent to the vertical joint. The OEM inlayed the entire steam path HJ with stainless steel on the high pressure section and the low pressure cone section. Additionally, it appears that at this time the OEM oversized some of the bolting in the HP adaptor section of the casing. In 2001, TurboCare, Inc. installed a new steam path which included a rotor, blades and diaphragms made of 12Cr. During the re-rate the inlet pressure remained the same, 115 psia, however, the units flow and efficiency were increased 11%. The unit was re-rated for 26.1 MW output. TurboCare, Inc. was asked to redesign the casing to accomplish three main objectives. First remove the VJ from the design and improve the HJ design to ensure no steam leaks. Secondly, the maximum design pressure of casing was to be 150 psia. Finally, the casing needed to accommodate a main steam inlet pipe size change from 20 inches to 24 inches. Preliminary Joint Analysis In November of 2009, the OEM design of Blundell casing s bolted joints were reviewed by TurboCare, Inc., to determine the acceptability of increasing the turbine s inlet pressure from 115 psia to 138 psia. The bolted joints of interest were the horizontal joint (HJ) of the inlet section, the vertical joint (VJ) between the HP inlet section and the LP/exhaust hood, and the four-way joint produced by these two flanges. The vertical joint was upstream of the first stage diaphragm and is exposed to the full inlet pressure. The bolted joints of the casing were reviewed using a Finite Element Analysis (FEA) approach with various pressures loading and contact elements on joints. The FEA model used solid geometry created from reversal data, site photos, and drawings. Half of the inlet casing was modeled and symmetry used at the vertical centerline of the unit. The model included a representation of the vertical joint flange and did not include the casing downstream of the flange. This flange was modeled to include the effect of the four-way joint and was assumed to be fixed in the axial direction. The original pressure run results showed that at 115 psia, there was a predicated leak at the HP packing ring near the corner of the casing wall. The gap was estimated to be across two bolts near the corner of the packing ring. The four-way and horizontal joints showed that these areas were starting to open up with minimal contact pressure. However, the FEA model did not show it as a complete leak path across the HJ. The increased pressure of 138 psia run results showed the leak near the HP packing rings would increase to over four bolts. The horizontal and four-way joints opened up to the bolt holes of the flange. See Figure 1. Figure 1. FEA Results from Preliminary Analysis - Gaps. The conclusion of the preliminary joint analysis was that given the current bolt sizes and geometry of the flanges, that the unit operating pressure should not be increased beyond its current setting of 115 psia. The analysis showed two areas of potential leaks on the horizontal joint. Both of the areas predicated by the FEA model are the exact areas that the OEM casing leaked. Observations Made on Site A TurboCare, Inc. reversal team traveled to Milford, Utah to scan and measure the casing in July The reversal team utilizes high accuracy white light image scanning to create a 3-dimensional model of the casing. During the reversal, the following observations were made at the site by the team. First, the current casing was leaking on the right side HP horizontal joint adjacent to 722

3 Archambeault, et al. the vertical joint, see Figure 2. The thermal insulation was soaked, damaged and partially removed from the upper half casing and a bucket was in place to catch the water. All the bolt heads and nuts in this area were severely rusted and corroded. Blundell had to seal weld the horizontal and vertical joints, after assembly, to reduce the leakage of the casing in this area. Additionally, the HJ was leaking on the turbine s left side, adjacent to the high pressure packing, matching the analysis, see Figure 3. This leak did not appear to be as severe as the leak adjacent to the VJ. However, the HP packing area also showed signs of corrosion. The OEM joint configuration offset the bolting from the steam path, adjacent to the unused extraction boxes. The upstream bolting was offset approximately 11.5 from the steam path and the downstream bolts had a 15.5 off set as illustrated in Figure 4. Additionally, the OEM vertical joint location is placed before the stage 1 diaphragm, which exposed the joint to inlet pressure and temperatures. The horizontal joint also showed signs of opening up during operation. The two areas of the joint that showed signs of discoloration were the LP bearing bracket cone, see Figure 5, adjacent to the LP packing holders and the forward wall of the exhaust hood. The discoloration of a joint is usually an indication that the area of the joint is not completely closed during operation and may be prying open. While the discolorations of the joint do not necessarily indicate a leak path for the steam, it would be an area of interest and would need additional review. The joint of the LP cone also showed excessive amounts of inlay that extended the entire length of the cone. Figure 2. Steam Leakage at HP horizontal Joint. Figure 3. Steam Leakage Adjacent to HP Packing. Figure 5. Steam Leakage at LP Cone. Finally, it was observed that the overall thickness of the horizontal flanges for the high pressure, low pressure and exhaust sections of the casing were inconsistent with TurboCare, Inc. design practices. For example, the upper half HP section s horizontal joint flange was 1.5 thick and is exposed to 115 psia steam. This was significantly thinner than the LP section s flange of 3.5, which is exposed to steam below 40 psia. Details of Redesigned Casing The process began with a full reversal of the existing casing geometry utilizing high-precision laser and white light scanners. Having designed, upgraded, manufactured, and installed the steam path, N1, and N2 packing back in 2001, this knowledge provided TurboCare, Inc. a solid understanding of the interface Figure 4. Horizontal Joint Geometry on OEM Casing. 723

4 for the internal components of the turbine. The reversal of the interface between the foundation and front standard were of the utmost importance. During the reversal it was determined that the first priority of the new casing design would be to eliminate the vertical joint. The next area of concern was to use TurboCare s design criteria to update the horizontal joint flange thicknesses around the entire machine to provide stiffness required to keep the horizontal joint sealed at the higher design pressure of 150 psia. The OEM casing had both high pressure and low pressure extraction boxes that were not being used. This created large distances between bolts and joint contacts. In the redesign of the new casing these extraction boxes would be eliminated. The final issue determined at the reversal was that the OEM casing inlet was a diameter of 20 while the plant piping was at a diameter of 24. It was decided that if possible the casing redesign would increase the inlet to a diameter of 24 to eliminate this flow restriction just before steam enters the turbine. The reversal data was used to begin modeling the casing for analysis purposes. The known internal components were used in this model to accept the current TurboCare, Inc. steam path and seals that were installed in Using ASME pressure vessel code equations and empirical corrosion rate data the casings minimum wall thicknesses were determined and added to the model. External interfaces from the reversal data were modeled to ensure the fit of the casing in the existing foundation and to the front standard. Flanges were thickened per TurboCare, Inc. design data height/ width ratio. This was a significant difference from the OEM casing. At this point the casing model needed only a bolt pattern before the first run analysis. For the first run, a similar bolt layout to the OEM design was used. The model was constrained, temperatures and pressures were applied and runs completed on the Finite Element Analysis (FEA) model. The FEA model showed that all of the casing stresses were below yield of the material. There were some peak stresses that were reduced by adding gussets where the extraction boxes would have provided stiffness. In the exhaust area of the casing the structural members made of pipe, referred to as struts, were included in the design to add stiffness and reinforcement to the LP bearing and external walls. To reduce some deflections in the casing some struts were added and one pipe was changed to bar stock. Once the casing was reacting within allowable stresses and design criteria, the focus changed to the horizontal joint contact analysis. With the larger flange ratio and the OEM bolt pattern there were still going to be leaks in the joint. First the sizes of the bolts were increased to provide more clamping area and an increase in bolt preloads. This change helped, but did not solve the joint leak completely. Utilizing covered nuts on the high pressure section, the bolt pitch was tightened to fit more hardware which increased clamping force and sealed the joint. The low pressure exhaust bolting was also increased in size with tighter pitching to fit more hardware. These changes in the analysis model solved the joint leak issue at 150 psia. 724 The design process used by TurboCare, Inc. during the redesign of this unit started with the reversal of the parts and components at the Blundell site in July, The preliminary analysis followed, which used design data, TurboCare design criteria, spread sheets and hand calculations to estimate the required new configuration of the casing. A detailed FEA model was then created and used to complete the thermal, mechanical, and horizontal joint gap analyses. After a detailed review of the results, the design model was modified and improvements were made to the design as necessary. Finally, the model was updated and re-analyzed using the final configuration of the casing. All final results were reviewed to confirm that the components met all TurboCare design rules. The assumptions used for the redesigned casing were that the maximum design pressure and temperature were 150 psia and 364 F, respectively. The casing was fabricated using carbon steel. The fabrication was made in two parts, the upper and lower halves, and there were no vertical joint on the assembly. The horizontal joint and axial steam faces of the diaphragms and packing holders had a 309L stainless steel inlay, see Figures 6 and 7. The main steam inlet pipe diameter was increased to 24 from the 20 OEM size. Finally, all material allowables were set to match proven design criteria as a percentage of yield strength. Figure L Stainless Steel Inlay areas. Figure L Stainless Steel Inlay. While redesigning a horizontal joint flange, two areas were reviewed carefully. First, the flange s height/width ratio needed to be as close to 0.8 as possible. Secondly, the bolt s position within the joint also needed to be the distance from the inside of the flange to the bolt centerline is less than or equal to the distance to the outer edge of the flange. These two geometric values ensured the flange was stiff enough and that the preloads required for clamping the joint tight could be generated. The OEM flange geometry had a height/width ratio, referred to as the H/T ratio, of This was significantly below the redesigned flange s H/T ratio design requirement. The redesign casing had flanges thicknesses which

5 Table 1. Casing Flange Thickness. Location OEM New HP Upper HP Lower Extraction UH Extraction LH Exhaust UH Exhaust LH 2 2 LP Cone UH LP Cone LH Figure 8. Extraction Box and Vertical Joint Removal. were thicker than the OEM casing. Most notably, the HP section had the upper and lower half flanges increased to 4.5 from 1.5 and 2.0, respectively. Table 1 summarizes the flange thicknesses for the OEM and redesigned casings. The new bolting design assumed a 45,000 psi preload on all bolts. Additionally, the new design had the following modifications; all the bolt sizes were increased from the OEM design. The OEM bolts adjacent to the extraction boxes were moved closer to the steam path. The extraction boxes, which were unused by Blundell, were removed from the casing as shown in Figures 8, 9 and 10. The total number of bolts on the horizontal joint was increased by spacing the bolts closer together. The high pressure section bolts were changed to a covered nut design to maximize the preload and minimize the bolt pitching. See Figures 11 and 12 for the details of the OEM and redesigned bolt patterns, respectively. All the bolts A = 1.00 hex B = 1.25 hex with washer C = 2.25 Double Nut D = 1.00 hex (pitch = 3.75 ) on exhaust E = 1.00 hex (internal & extraction boxes) D flange VJ VJ location E C A B Figure 9. OEM Casing Lower Half. A Figure 11. OEM Bolting and Pattern. Bolts loaded to 45 ksi Preload D flange E C A = 2.00 Covered Nut B = 1.75 Covered Nut C = 1.50 Covered Nut D = 1.25 hex (pitch = 2.85 ) E = 1.25 hex (internal HP) B C B A B C Figure 10. TurboCare, Inc. Lower Half Casing. Figure 12. Redesigned Bolting and Pattern. 725

6 were assumed to be installed hand-tight prior to the application of the required torques for a 45,000 psi preload. The maximum shear stress on the casing threads were 87% of allowable shear and occur in the 1.5 covered nuts on the forward wall of the exhaust. The casing s thread shear stresses were all acceptable. The minimum wall thickness required for the HP section of the casing was calculated using ASME Pressure vessel code equation using the following factors for consideration: t is the minimum thickness (inches), P is the delta pressure (psi), S is the allowable stress (psi), r is radius of casing (inches) and E is the weld efficiency. Weld efficiency is a function of the type of weld and its geometry. Minimum wall thicknesses for geothermal turbines must also consider the corrosion rate of the casing material when exposed to the geothermal steam. Therefore, the minimum wall thickness required for the HP section also includes the correction for the 25 year corrosion rate. Corrosion rates were based on test data from various geothermal sites. The worst-case corrosion rate was used when estimating the wall thickness loss over 25 years on the Blundell unit. The casing s final wall thickness was set above the required minimum wall thickness. The casing has a 309L stainless steel inlay on the horizontal joint and the axial steam faces where a pressure drop occurs. The inlay is on the axial steam face of all diaphragms and the six packing holders, the welding occurs during the fabrication phase. The required minimum depth of the inlay is set to ensure proper dissolution into the base metal. The expected minimum contact area of the horizontal joint inlay is also set, as is the minimum contact area on the axial steam faces. The exhaust casing had structural members, referred to as struts, included in the design to add stiffness and reinforcement to the LP bearing and external walls. The struts were used to transfer loads and minimize the casing s deflections. All of the OEM struts between the walls were 2.5 diameter pipe. The OEM strut between the LP bearing and the exhaust corner was a 4.0 pipe. The redesigned casing reviewed the stresses of the individual struts and re-sized and/or repositioned them as needed. The four struts in the upper half hood remained in the same position for the new casing and were made using schedule 80 pipe. The struts in the lower half exhaust casing were modified as follows: one pipe changed to 3 OD bar, one strut location changed, one strut s length increased and one strut was added. FEA Model and Results An ANSYS FEA solid model of the new casing design was created and analyzed, see Figure 13. The objective of the analysis was to review the new casing configuration without the vertical joint and two extraction boxes. The analysis was used to confirm the assumptions made by the hand calculations and determine the new temperature profile of the unit. Additionally, the FEA model was used to review the horizontal joint gaps, the contact pressures and the casing stresses. The FEA 3D model included the upper and lower half casings and the LP bearing cap cover. The parts were modeled with a symmetry plane at the rotor centerline. The model was supported vertically at the LP support pads of the exhaust casing lower half. The model was constrained in the axial direction at the exhaust casing centerline keys. The bolt preloads were applied to the model as a combination of pressures and forces. Pressures were applied at the nuts locations and forces used on the lower half casing threads. Both thermal and mechanical runs were completed on the final FEA model. All of the calculated casing gaps and stresses were from the combined mechanical run, which included both the maximum operating pressures and temperatures. See Table 2 for the boundary conditions used during these analyses. The temperature profile, calculated during the thermal run of the model, assumed that the HP packing, inlet pipe and HP section of the casing up to the forward wall of the exhaust was insulated. The FEA model showed that all of the casing stresses were below the yield strength of the material, see Figure 14. The casing met TurboCare s design criteria for allowable stresses. The horizontal joint gap analysis showed that there were several potential Table 2. Finite Element Analysis Boundary Conditions Location Pressure (psia) Temperature ( F) Figure 13. ANSYS FEA Model Inlet Stage Stage Stage Stage Stage Stage Exhaust LP Seal LP Vent Figure 14. Von Mises Equivalent Stress. 726

7 areas for the joint to open. All of these areas did not indicate a leak path to the outside of the casing joint, i.e. atmosphere. Diaphragm ledges had been inlayed with 309L SS to reduce any erosion damage caused by any potential motive steam. The FEA predicts there is no leakage path at the HP packing area or the LP cone area as seen in the OEM casing, see Figure 15. Figure 15. Horizontal Joint Closure Overview. The exhaust casing struts stresses were analyzed and reviewed during the combined run. The peak stress were found to be below 93% of allowable in strut #1, which was the 3.0 OD solid bar adjacent to the HP cone at the horizontal joint. Steam Turbine Stage Drain System The OEM drain system is shown in Figure 16. The drain piping is all 1 inch diameter pipe, and the effective orifice to the system is internal to the casing. This means if a blockage were to occur, it would require a shutdown of the unit and removal of the rotor in order to gain access to the blocked orifice. There is no feedback to an operator of the functionality of this system, so knowing whether or not a blockage is present and causing additional erosion of the rotating blades is nearly impossible to determine. The current practice is to simply touch the pipe and if it s cold it s likely not draining. Since all stage drains are the same diameter, this means that for some stages there is too large an opening and additional steam is flowing directly to the condenser which results in a loss of performance. For other stages, the diameter is simply too small and likely prevents all the water effectively removed from the diaphragm moisture separators to exit the casing. The drain system provided, see Figure 17 with this unit ensures the maximum amount of water is removed from each stage beginning with the individual diaphragm moisture separators. This begins with engineering calculations and is calibrated through real-time feedback of the monitoring panel. The centrifuged water is collected and removed in between each stage through oversize piping in the casing to properly sized external orifice plates which meter the flow. Isolation and by-pass valves are provided for online maintenance and water washing. The optimally designed and calibrated orifice plates increase the turbine efficiency by optimizing the re-heat effect, increasing the enthalpy drop across each stage. Immediate feedback is provided via a float switch arrangement (see Figure 18) connected to a display panel (see Figure 19) which indicates each stage is functioning properly. A single block and bleed arrangement is provided ensuring safe serviceability while on-line. The new drain system provides on-line in-situ serviceability, allowing the operating utility to clean and/or replace steam turbine orifice plates while the turbine remains in operation, thus reducing generator down-time, maintenance costs and lost revenue. Unlike traditional drain systems where the orifice is integral to the casing, these orifice plates are external and can be optimized during turbine operation in conjunction with an indication of the operational status of the orifice plate. The external piping configuration permits removal of the orifice plate and replacement with a plate having a different diameter opening. This is important for revenue generation since too large an orifice diameter would lead to excessive steam escaping decreasing turbine performance, and too small a diameter could allow water to back-up into the turbine where the blades rotate, causing excessive damage to the blades and in turn increased maintenance costs. The by-pass line in the each stage may be utilized during water wash to prevent the risk of the orifice plate being plugged Figure 16. OEM Stage Drain System. Figure 17. Stage Drain System Photo - As Built. 727

8 by contaminants. The discharge of the contaminants through the by-pass loop minimizes risk of orifice plate plugging. Indication of orifice plate fouling is incorporated using the float switch water level alarm connected to an indicator panel located locally at the turbine drain area. This indication can also be fed back to the control room for additional monitoring. This orifice plate indication panel can also be used to fine tune the diameter of the orifice plate during operation for optimum water removal and turbine efficiency. When the turbine is in operation, the plant operations personnel can start with a small diameter orifice plate which would indicate a back-up of water in the float sensor. Then, gradually increase the size of the orifice plate diameter until the alarm indication is clear, ensuring the proper sizing as compared with engineering calculations. This drain system design is currently patent pending. Local Indication Control Room Indication By others Float Switch Bleed Orifice Figure 18. Stage Drain System Arrangement. Normally Closed Drain Header Figure 19. Feedback Indication Panel. Conclusion The TurboCare, Inc. redesigned casing met or exceeded all of the objectives for the project. The turbine has run in base-load operation since it s installation in April, 2013 and is expected to provide long term reliable service for the Blundell power generation utility. The turbine has met performance expectations, and is expected to have maximum availability. The turbine s maximum design inlet pressure had been increased to 150 psia and included the larger 24 inlet pipe. The vertical joint and extraction boxes have been removed from the design. The horizontal joint had been redesigned to ensure no joint leakage and includes stainless steel inlay in the event motive geothermal steam is present. The drain system operates as expected and is expected to minimize down-time and provide for ease of service and reduced maintenance costs. Completion of this project ensures improved reliability as well as life extension for the power generation facility. When considering asset management - modernization by optimizing key turbine parameters can provide extended life and maximized revenue generation. 728