SPECIAL ISSUE: OUTAGE HANDBOOK TURBINE BYPASS SYSTEMS How to verify proper operation of bypass, spray-water valves S team-turbine bypass sys tems are used in combinedcycle plants during startup, shutdown, and load rejection. The most common arrangement, shown in Fig 1, allows plant operators to route high-pressure (HP) superheated steam through a pressure control valve/desuperheater station to the cold-reheat line, thereby bypassing the steam turbine’s HP cylinder. This typically is referred to as the “HP bypass.” In addition to the HP bypass, an IP/LP-turbine bypass allows operators to condition and dump steam from the hot-reheat line directly to the condenser via another pressure control valve/desuperheater station. This generally is called the “hotreheat bypass,” or more simply, the “HRH bypass.” The service life of a bypass system can be shortened dramatically by poor design and installation practices and by ignoring control issues that cause severe thermal stresses conducive to metal fatigue. Cracking attributed to thermal fatigue has occurred in valve trim, bodies, desuperheaters, and downstream piping, and is most acute in plants cycling daily (Fig 2). All plants with turbine bypass systems should establish preventive maintenance and inspection programs to determine if cracks are HP bypass to cold reheat pressurecontrol valve/desuperheater (details in Fig 3) Spray-water control valve HP IP LP Block valve Cold-reheat line Pressure control valve/desuperheater Hot-reheat or IP/LP bypass to condenser details in Fig 3) HP superheater HRSG Reheater HP evaporator Condenser Sparger Hot-reheat line Deaerator Economizer Feedwater heater Feedwater pump Spray-water valve Condensate pump 1. Steam-turbine bypass arrangement is typical for a combined-cycle plant. HP bypass routes high-pressure superheated steam to the cold-reheat line; IP/ LP bypass reduces the pressure and temperature of hot-reheat steam OD 2. Life of bypass system components can be shortened dramatically by poor design and installation practices and by failure to confirm periodically the proper operation of critical valves during startup, shutdown, and normal operation. Photo A shows crack originating in the heat-affected zone between the bypass valve and its desuperheating section; crack in B is in the heat-affected zone of another weld in the bypass system; cracking of valve trim caused by thermal shock is in C; and a valve-body crack caused by quenching of hot metal is evident in D COMBINED CYCLE JOURNAL, Third Quarter 2008 ID A B C D 27 TURBINE BYPASS SYSTEMS SPECIAL ISSUE: OUTAGE HOT-REHEAT BYPASS VALVE/DESUPERHEATER HANDBOOK Planning forensics The case history that follows chronicles work undertaken by personnel at InterGen’s La Rosita Power Project, Mexicali, Mexico (sidebar), and CCI ~10 ft engineers to determine the root cause of cracks discovered in the HRH and TC1 TC3 TC7 HP bypass systems for one of the plant’s three gas turbine/heat-recovTC4 TC6 ery steam generator trains (so-called Unit C). Details were provided by La Rosita’s J Andres Felix, associate TC5 TC8 TC2 Cross section 1 Cross section 2 Cross section 3 plant engineer, and CCI’s Dan Watson, development engineer. Many plant-operations experts recommend that the type of diagnostic study conducted at La Rosita be done during the commissioning of every plant and repeated about TC9 ~10 ft six months before each hot-gas-path HP-TURBINE BYPASS VALVE/DESUPERHEATER or major inspection—depending on service duty—to ensure needed parts 3. Bypass systems should be checked for proper operation about six months are available for the outage. before each hot-gas-path or major outage (depending on duty cycle) to allow Felix said La Rosita contacted sufficient time for ordering replacement parts that might be required. It’s relaCCI well in advance of an upcoming tively simple to install the necessary thermocouples and collect data, tasks that major inspection/overhaul and that could be done by the plant staff with proper training the vendor recommended the study present or likely to develop. Normal lowing must be inspected, reviewed, profiled here. Reasoning was simple: measures of plant cyclic life—such and evaluated periodically to obtain Knowing the root cause of the crackas number of starts, duration of a true assessment of condition: ing, the plant could implement corstarts, number of trips, etc—are not n Temperature gradients in the rective during the outage. La Rosadequate for gauging the condition bypass system. ita runs base-load in summer and of bypass systems. Rather, the fol- n Adjacent pipes and butt welds. cycles (usually daily) during most n Steam valves, spray valves, desuother months. It had operated about perheaters, and dump devices. 40,000 equivalent hours by the time n Desuperheater control this study was conducted La Rosita thumbnail logic, and startup and in March 2008. shutdown data from the Review of information InterGen’s La Rosita Power Project, plant historian. gathered during annual a 1065-MW, 501FD2-powered, 3 x Perhaps the last thing inspections and sent to 1 combined-cycle facility, designed a roving operator wants to CCI for the development and built by Bechtel Power Corp, see on rounds is steam (or of a meaningful proposal Frederick, Md, began commercial water) leaking out from the revealed the following peroperation in August 2003. insulation at a bypass statinent details about Unit C: Approximately 500 MW of the tion. No way to tell at first n HRH bypass. A crack plant’s output is purchased by sight if a catastrophic event was discovered a few inches Mexico’s Comision Federal de ElecWatson is about to occur. downstream of the first tricidad (CFE) under a 25-yr BOO Most plants have proweld after the spray nozzles (build/own/operate) power purchase grams in place for periodic (Fig 3). The crack extended agreement. Remaining capacity is inspection of bypass syscompletely around the P22 available to meet energy needs in tems, as outlined by Steve (2¼ chrome) pipe, but only the border region. Natural gas is Freitas of CCI-Control Comextended through the pipe supplied via a 126-mi dedicated, ponents Inc, Rancho Santa wall at the 6 o’clock posicross-border pipeline, owned by Margarita, Calif, in the 2008 tion, as evidenced by a Sempra Energy/PG&E Corp, that Outage Handbook (access water drip. Bulging of the runs from Ehrenberg, Ariz, to the www.combinedcyclejourpipe at that position was plant. nal.com/archives.html, click noted by engineers. Availability since startup is 3Q/2007, click “Key elements n HP bypass. Cracks invisFelix greater than 95%. La Rosita, which of successful PM programs ible to the naked eye were ranks among the cleanest electric for turbine bypass systems” on cover. found on the (1) pressure-control generating plants in Mexico, is parBut relatively few combined-cycle valve’s body drain pipe after the ticularly proud of its environmental facilities run sophisticated diagfirst elbow, (2) first weld downrecord. NOx emissions are virtually nostics on their bypass stations to stream of the spray nozzles, (3) on eliminated by the latest SCR emisensure they are operating as designthe weld connecting the bypass sions control technology. Water use ers intended and to verify that the pipe to the cold-reheat header (P22 is minimized by a treatment plant designers’ intentions are correct for to P91 joint). Note that P91 is 9% that processes municipal wastehow plants must operate today (for chrome/1% molybdenum. water into makeup for the coolingexample, cycling rather than baseIt took about a day for CCI engiwater system. load as designed). neers to instrument both bypass sta28 COMBINED CYCLE JOURNAL, Third Quarter 2008 A Leader in Electrically and Hydraulically Actuated Combustion Turbine Fuel Controls. IGV Severe Service Actuator Y&F fuel control valves, isolation valves, inlet guide vane actuators, and other accessories control an install base of over 10,000 turbines. Our Global Sales and Service Network delivers engineered fuel control solutions for industrial gas turbines worldwide. ELECTRIC Liquid Bypass Valve Naphtha Dump Valve IGCC Valves EMA Gas Control 1”, 2”, 3” Combined Valve EMA Controller HYDRAULIC Natural Gas Valve Liquid Fuel Splitter Isolation Valve Whether it’s liquid, natural gas, Naphtha, or IGCC Syngas, we can meet your needs. PID with Dual LVDT Conditioner www.yf.com • Tel. 315.457.3110 • [email protected] Data recorded by CCI engineers on the HRH bypass during one startup are shown in Fig 4; information captured for the second startup essentially was the same. At numerous points during the startup—those identified by the number “1”—TCs 2, 7, and 8 registered sharp temperature drops. Watson said that when pipe temperatures drop so quickly, the cause almost certainly is water contacting the pipe’s internal surface. Next, look at points marked “2.” They show that when the bypass valve closes, the spray-water valve still is at approximately 25% open and proceeds to close completely over the next 10-15 seconds. The drop in temperature recorded by TC7 around 6 a.m. confirms that water is flowing but steam is not. For this startup, Watson said, the magnitude of the temperature gradient was relatively small. However, he added, during full bypass operation on a steam-turbine shutdown, the gradient would be much larger. 30 900 100 Hot-reheat bypass to condenser (CCI data) TC1 800 90 80 Turbine-bypass valve stroke 700 70 TC2 600 60 3 Spray-water valve stroke 1 500 400 40 300 30 TC7 200 2 20 100 0 50 10 4:27:53 5:17:53 Time 0 6:07:53 4. Sharp temperature drops recorded by CCI engineers almost certainly indicates that water is contacting the pipe’s internal surface. One cause was control logic that allowed the spray-water valve to remain open for a few seconds after the bypass valve had closed 600 (1112) 100 Hot-reheat bypass to condenser (DCS data) 500 (932) 2 Turbine-bypass valve stroke 80 70 400 (752) 60 300 (572) Spray-water valve stroke 200 (392) Spray-water flow 100 (212) Steam temperature recorded by permanent plant thermocouple downstream of turbine bypass valve near cross section 3 in Fig 3 5:17:53 6:07:53 Time 0 (32) 90 4:27:53 50 40 1 30 20 10 Spray-water flow, tonnes/hr; Valve stroke, % open HRH bypass 1000 Steam temperature, F tions for diagnostic evaluation. Fig 3 shows this includes installation of eight Type K thermocouples (TCs) on both pressure control valves, plus another on the body of the HP bypass valve. Transducers also were installed on both the HRH and HP bypass and spray-water-control valves to accurately measure valve position. The diagram shows where the TCs were located to detect temperature patterns indicative of abnormal or improper valve and/or desuperheater behavior. The transducers help identify errors between the valve demand signals transmitted by the plant control system (DCS) and the actual positions of the valves. Data were recorded continuously over two-day periods by CCI engineers on both the HRH and HP bypass systems. The plan was to capture data from two startups and two shutdowns of the gas and steam turbines. While this goal was achieved for Unit C, the steam turbine was in continuous operation during the test period. The analysis that follows focuses on the startups. During shutdowns, the HRH and HP bypasses did not operate, or opened only a small amount for a very brief period. However, the primary goal, to identify the root cause of cracking, was achieved. Finally, engineers noticed some unusual behavior of the HP bypass system during normal plant operation, as well as during startup, and that is covered as well. HANDBOOK Valve stroke, % open SPECIAL ISSUE: OUTAGE Steam temperature, C (F) TURBINE BYPASS SYSTEMS 0 5. Data on HRH bypass operation accessed from La Rosita’s DCS confirmed CCI’s findings and helped identify the need for new valve trim as well as controllogic modifications Another point to note is that approximately a half hour after the completion of startup, TC2 experiences a very sharp drop (follow arrows from Point 3). It records a temperature about 775 deg F lower than the temperature indicated by TC1. A significant temperature differential remains over the next 24 hours—although it slowly decreases. What this suggested to Watson was that water probably was leaking by the “closed” spray-water valve. HRH bypass, DCS data. Data on HRH bypass operation captured from the DCS, at the same time CCI instruments were monitoring that component, are presented in Fig 5. Note how closely the DCS data for bypass and spray-water valve strokes match the CCI curves in the previous figure. Also, the temperature measured by the DCS control thermocouple downstream of the bypass valve (near cross section 3) exhibits the same sharp temperature drop as CCI’s nearby TC7. InterGen’s Felix and CCI’s Watson and Joe Polidan, and others in their respective organizations, carefully reviewed design data and compared them to the information compiled from the plant DCS and CCI field instruments. Engineers concluded COMBINED CYCLE JOURNAL, Third Quarter 2008 SPECIAL ISSUE: OUTAGE 1000 900 Steam temperature, F 800 HANDBOOK 100 TC2 HP-turbine bypass to cold-reheat line (CCI data) 90 TC6 Turbine-bypass valve stroke 700 80 TC4 70 600 60 Spray-water valve stroke 500 50 TC5 400 40 TC7 300 30 2 200 20 100 0 Valve stroke, % open TURBINE BYPASS SYSTEMS 10 1 4:53:14 0 7:06:34 5:59:54 Time 6. Analysis of the HP bypass showed the spray-water valve remained open after the bypass valve closed, as it did in the HRH bypass system Steam temperature, C (F) 500 (932) 400 (752) 100 Turbine-bypass valve stroke HP-turbine bypass to cold-reheat line (DCS data) 90 80 Turbine-bypass valve, steam 70 inlet pressure 3 Spray-water valve, water inlet pressure 300 (572) Spray-water valve stroke 200 (392) Spray-water flow 1 0 (32) 50 40 30 20 2 100 (212) 60 10 4:53:14 Time 5:59:54 7:06:34 0 Pressure, bar (abs); Spray-water flow, tonnes/hr; Valve stroke, % open 600 (1112) 7. DCS data confirmed the behavior of bypass and spray-water valves characterized by the CCI test. Engineers learned that both the bypass and spraywater valves were leaking and required new trim that approximately 20% more spray water was being injected into the steam flow than was indicated by designers’ calculations. Startup data from the technical specifications indicated a fully open bypass valve handling a steam flow of 219 tonnes (T, a metric ton or 2205 lb)/hr and a matching spray-water requirement of 72.4 T/hr. Test data captured during the carefully monitored start last spring showed the bypass valve was never more than 80% open. All other operating conditions either matched, or were very close to, those calculated by designers. When the bypass valve is at 80% stroke, spray-water requirement should be 57.9 T/hr, or 80% of the 32 72.4-T/hr design value. DCS data show that the actual flow rate was about 69 T/hr—nearly 20% more than it should have been. The difference could be caused by instrumentation error or an error in control. Engineers concluded that the HRH bypass system had the following four major problems, presented in the order of their severity: 1. Spray-water valve was leaking, allowing water to flow into the bypass piping. Evidence: The sharp temperature drop recorded by TC2 (Fig 4) about a half hour after startup was completed. The continual flow of water cooled the bottom of the pipe to near spray-water temperature while the top of the pipe at the same cross section (Fig 3) was 775 deg F hotter. A temperature gradient of this magnitude creates enormous stresses on the pipe and may be the primary reason that the pipe cracked. Recommendation. Replace the plug, seat, and trim in the spray-water valve to eliminate the leak. This work was done. 2. Poor synchronization of the bypass and spray-water valves during closing. As mentioned earlier, the spray-water valve closed about 15 seconds after the bypass valve closed. Never allow water to enter the bypass system when there is no steam flow. This is one of the primary causes of pipe quenching. During the La Rosita startups analyzed by plant and CCI engineers, the thermal gradients measured on the pipe wall were relatively small (not shown on the diagrams to reduce clutter). But the effects on the bypass-valve outlet diffuser were almost certainly more severe. Also, bypass operation during a steam-turbine shutdown would likely cause even more severe temperature quenching of the outlet piping and the valve’s outlet diffuser. Such sharp reductions in temperature create additional piping stress and are conducive to cracking, especially during cyclic operation. Recommendations. (A) Change control logic to ensure that the spraywater valve opens after—and closes before—the bypass valve, so there never is spray-water flow without steam flow. (B) Modify control logic to inject the proper amount of spray water during startups. (C) Review DCS data for other operating cases to verify accuracy of spray-water flow. Felix said the service firm La Rosita uses for its control-systems work was assigned the task of troubleshooting spray-water valve control logic. One issue identified was with the enthalpy-calculational routine and the equation governing that was corrected. Also, the interlock that prevents spray water from entering the bypass system when steam flow ceases was adjusted simply by tweaking the 4-20-mA signal setting that controls the opening and closing of the spray valve. Tests conducted by the plant following these adjustments confirmed proper valve operation because no delta T was observed. 3. Too much spray water was injected for the given startup steam flow. This was corrected with changes to the control logic described in the previous item. 4. Sharp temperature drops during startup are evident from the TC2 data shown in Fig 4. 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Other recommendations made by CCI included the following: (1) Inspect the inside of the pipe and the valve’s outlet diffuser for damage that my not be visible from the outside. (2) Service and inspect all spray nozzles. (3) Calibrate spraywater flow meters. Plant completed most of the work suggested and 900 Steam temperature, F 800 TC2 scheduled the remainder for future outages. HP bypass Data recorded by CCI engineers on the HP bypass during one startup are shown in Fig 6; information captured for the second startup was similar. Lower two arrows originating at Point 1 in the diagram show HP-turbine bypass to cold-reheat line, “normal” plant operation (CCI data) 100 TC2 90 TC4 TC4 80 700 70 600 60 500 400 300 200 100 0 50 TC5 1 Turbine-bypass valve stroke 40 Turbine-bypass valve stroke 30 2 20 3 10 Spray-water valve stroke Day 1, 12:06:34 Day 1, 21:49:54 Time Valve stroke, % open 1000 888-881-7118 Day 2, 7:33:14 0 8. Sharp temperature spikes recorded for the HP bypass during normal operation confirmed that water was entering the steam system via the spray-water line 34 www.comrent.com that when the bypass valve closes, the spray-water valve is still open approximately 8% and proceeds to close over the next two minutes. The negative temperature gradient observed on the curves for TCs 2, 4, 5, and 6 at about 6:30 a.m. confirmed spray water was flowing when steam was not. As noted in the section on the HRH bypass, such a large gradient causes an enormous amount of pipe stress. It could eventually lead to cracking if the root cause is not corrected—especially considering the unit’s daily-start/ shutdown regimen. Point 2 reveals that TC7 was at saturation temperature for the duration of the startup. This suggests that the pipe was either in constant contact with water or the steam at that location actually was at saturation temperature. Either way, too much spray water was being injected. DCS data. Information on HP bypass operation captured from the DCS, at the same time CCI instruments were monitoring that component, are presented in Fig 7. Note that DCS data on the stroke of bypass and spray-water valves exhibit the same behavior as that identified by CCI in Fig 6. Comparison of design and operating data for the HP bypass, as was done for the HRH bypass, revealed COMBINED CYCLE JOURNAL, Third Quarter 2008 Produce More Power Protect Your Blades Reduce Heat Rate Reduce Water Washes PHONE: 323.832.8316 that approximately 82% more spray water was being injected during startup than specified. Specifically, a wide-open bypass valve handles 166 T/hr of main steam and requires 18.8 T/hr of spray water. During the startup evaluated, the bypass valve reached a maximum stroke of 97% and should have been supplied 18.2 T/hr of spray water. But actual flow was 33.2 T/hr. As mentioned earlier, this could be caused by instrument or control error. The spray-water inlet pressure during the startup observed was 55 bar (abs). According to the design specifications, the inlet pressure should have been 42.2 bar (abs) for all operating conditions. Thus the spray-valve inlet pressure was about 30% higher than specified. Conclusions. Engineers concluded that the HP bypass system had the following three major problems, presented in the order of their severity: 1. Poor synchronization of the bypass and spray-water valves during closing. During the startups evaluated, injection of spray water after steam flow had ceased resulted in the rapid reduction in pipe wall temperature as measured by TCs 2, 4, 5, and 6 in Fig 6. The 500-deg-F temperature drop experienced qualifies as “thermal shock” and one of the main reasons for the cracks at the desuperheater outlet. www.GasTurbineInletAirFilters.com 2. Bypass and spray-water valves were leaking. 3. Too much spray water was injected for the given startup steam flow. Recommendations a n d a c t i o n taken by the plant to correct the root causes of the HP bypass problems experienced were the same as those for the HRH bypass described earlier, with one exception: Plug, seat, and trim were replaced for the bypass valve, in addition to the spray-water valve. Normal plant operation HP bypass data also were recorded between the two startups analyzed—that is, during normal plant operation. However, the TC curves in Fig 8 were not what engineers considered “normal,” exhibiting sharp temperature spikes while the unit was in service (region between the startup peaks for the first and second days at the left- and righthand sides of the chart). During this time there was no movement of the bypass or spray-water valves. From the detailed information collected during the first startup (Figs 6 and 7), engineers knew that the HP bypass valve was leaking and constantly passing steam to the downstream piping. This would cause high- COMBINED CYCLE JOURNAL, Third Quarter 2008 er downstream temperatures, ones that should be relatively constant. However, curves TC2 and TC4 reveal sharp drops in the downstream temperature, as the arrows from Point 1 highlight. Only injection of water could make this occur. Engineers concluded that either of the following was happening: The spray-water valve was slightly open and constantly passing water, or it was closed and leaking. More evidence is provided by curve TC5, which remained at or very near saturation temperature and exhibited only small temperature variations (Point 2). Data from TC3 (not shown to minimize graphics “clutter”), collected at the top of the same pipe cross section as TC5, was constantly 300 to 400 deg F higher than the TC5 readings, proving water was flowing along the bottom of the pipe. Finally, position-feedback data from the spray-water valve indicated that it might have been from 2% to 4% open during operation (follow arrows from Point 3). But based on DCS data, engineers determined that the zero point for the spray-water valve’s position-feedback measurement may have been skewed by movement of the feedback transducer. Conclusion: The spray-water valve actually was shut off during normal operation, reinforcing the belief that it was leaking. ccj 35
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