OPERATING EXPERIENCE WITH IMPROVED STEAM TURBINE PACKING RINGS by Steve Williams. and Ronald Brandon

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OPERATNG EXPERENCE WTH MPROVED STEAM TURBNE PACKNG RNGS by Steve Williams Central Diagnostic Team Coordinator Performance Projects Department Potomac Electric Power Company and Ronald Brandon Senior

He is a Registered Professional Engineer in the State of Maryland and a member of ASME. He has been employed with Potomac Electric Power Company for 16years. For the past seven years, Mr.

1 OPERATNG EXPERENCE WTH MPROVED STEAM TURBNE PACKNG RNGS by Steve Williams Central Diagnostic Team Coordinator Performance Projects Department Potomac Electric Power Company and Ronald Brandon Senior Engineer Power Technologies, ncorporated Schenectady, New York,,, Steve Williams gradated from George Washington University in 1977, with a B.S. degree in Mechanical Engineering. He is a Registered Professional Engineer in the State of Maryland and a member of ASME. He has been employed with Potomac Electric Power Company for 16years. For the past seven years, Mr. Williams has been in charge of the Central Diagnostic Team which is responsible for performance testing and diagnostics of steam trbines and axiliary eqipment. He also spent several years working at Chalk Point and Benning Generating Stations as a Plant Performance Engineer. Mr. Williams has been actively involved in the EPR project to evalat e Power Plant Performance nstrmentation Systems at PEPCo, s Morgantown Generating Station. Ronald E. Brandon is a Senior Engineer with Power Technologies, ncorporated in Schenectady, New York. Mr. Brandon gradated with a B.S. degree in Mechanical Engineering from MT. He was formerly with General Electric. Since joining PT, he has participated in a wide variety of thermal plant optimiation projects. He has also refined his long standing specially regarding performance problems of steam trbines. This work entails diagnosis of internal steam path problems from test reslts; carefl inspection of disassembled trbines; analyses identifying root cases of poor efficiency; economically sond recommendations for repair; and sggestions for improved operating practices to prevent recrrence of the dedced losses. Mr. Brandon is an instrctor in PT corses dealing with power plant performance and steam trbine optimiation techniqes. n the corse of his professional work, Mr. Brandon has been awarded eight patents. He is a Registered Professional Engineer in the State of New York, and a Senior Member of ASME. He has a nmber of professional pblications. ABSTRACT The losses in trbine performance de to excessive leakages in the labyrinth seals can reslt in significant heat rate degradation to the trbine cycle. The modified packing arrangement descrbe helps eiminate the rbs that occr dring startps, which S the major case of excessive clearance. This packing was first installed at two tility generating stations. The performance of these nits were monitored to. evalate the packing and these reslts are discssed in detail. NTRODUCTON Potomac Electric Power Company (PEPCO) has historically had problems maintaining proper clearances in their steam trbine seals. This has reslted in significant degradation to trbine efficiency and nit performance. The packing modification installed on the Dickerson Unit and later on the Chalk Point Unit has reslted in better control of steam seal leakage. This has reslted in significant improvement in nit performance which are described in detail herein. TURBNE PACKNG A trbine is designed to convert the potential energy in steam into kinetic energy to prodce sefl work. n order to obtain this kinetic energy, high differential pressres are present throghot the trbine to accelerate the steam throgh the nmeros trbine noles. The steam will attempt to take any path it can find into the lower pressre chambers of the trbine. Unfortnately, leakage paths arond the trbine noles reslt in a loss of kinetic energy, and often reslt in an even greater loss where it distrbs the flow path pon reentry into the lower pressre section. The manfactrer attempts to control the leakage very closely, in order to maintain good trbine performance. The ba ic method sed by the trbine manfactrer in sealing the rotatmg shaft from the stationary parts of a. trbine ses springbacked labyrinth packings. These packings restrict the amont of steam leakage flow for a given pressre drop. The teeth of these packings are arranged in a high/low manner simil r to that shown in Figre 1. This design cases the steam passmg throgh the packing to enconter the back of the next tooth reslting in eddies, which destroys the kinetic energy of the steam. The steam mst then accelerate pass the next tooth and this process contines throgh all the packing teeth which considerably restricts the amont of flow passed for a given clearance. 77

2 78 PROCEEDNGS OF THE SEVENTEENTH TURBO MACHNERY SYMPOSUM V 1 B pstream specific volme a fnction of the nmber of teeth and the downstream to pstream pressre ratio Effective area = clearance X diameter X pi (2 ) After the trbine is designed and bilt it is normally recognied that the only remaining controllable parameter is the shaft clearance. Holding all parameters constant, except clearance and combining Eqations (1) and (2 ) redces Martin's formla to: Leakage flow = constant X clearance (3) Figre 1. Thrbine PackingLabyrinth Seal. The packing ring holds the teeth in proper position relative to the shaft and sally has for or more teeth per ring. The clearance of the packing is determined pon assembly, by appropriate machining of the packing ring sholder or the tooth length. This clearance is maintained by the spring clip which holds the packing ring sholder against the ring holder. Design clearance is approximately.1 in per inch of shaft diameter. This is normally.3 in for a typical trbine design. t is also important to maintain the sharp edge on the tooth to minimie the leakage effect. The packing ring (Figre 2) consist of several segments arond the circmference which allows the packings to move independently, while the spring clips maintain a tight sholder fit. Using Eqation (3), it becomes clear that if the clearances are dobled then so is the leakage flow. The problem is made worse if the sharp edge packing tooth is blnted, reslting in even larger flows de to increasing the flow coefficient (normally reflected by a change in k). PROBLEM The ability to maintain tight clearances at Dickerson has always been a problem as it is in most other trbines. Dring startp, the trbine is ssceptible to vibration as the rotor is broght throgh its critical speeds. The end bearing vibration may not be excessive, bt the center span is sbject to large defection de to the long lengths of nspported shaft. Dickerson nits have no shaft prewarming capabilities, which adds to the shaft bowing problem. The inner shells, diaphragm and packing boxes which hold the stationary packing rings are sbjected to large temperatre differentials dring startp. This condition will reslt in the normally rond packing ring taking on an egg shape. This condition can lead to distorted clearances which may reslt in a packing rb. The manfactrer designs the packing segments and spring clips to move if a shaft rb occrs. This design is intended to save the trbine shaft from excessive ctting, bt will not prevent damage to the relatively soft packing teeth. The reslting condition of shaft rbbing conseqentially prodces heat, possibly bowing the shaft, increasing the intensity of the rbbing, casing increasing clearance and also casing the packing teeth to become somewhat embrittled which makes them ssceptible to breakage and even larger clearances. The bowed rotor condition rbs the labyrinth packings and also the tip seals designed to prevent leakage arond the rotating bckets. This tip seal leakage loss often exceeds the losses fond in the labyrinth packing. f packing clearances cold be opened at startp and closed dring normal load operation most severe packing rbs cold be avoided. Figre 2. Packing Ring. The flow of steam that passes the packing is calclated by Martin's formla Eqation (1). where flow flow constant effective area pstream pressre (1) MODFED DESGN The modification to the Dickerson Unit was designed to accomplish the objective of increased clearance at startp and tight clearance at fll load. The original packing design was field modified to accomplish this objective dring the Fall 1986 overhal. The original rings of packing are shown in Figre 2 with the spring clips inserted. The major change reqired is to remove the spring clips, which force the packing ring toward the shaft and install coil springs which force the packing rings away from the shaft (Figre 3). The springs sie and depth of the holes drilled in the packing rings are predetermined based on the expected pressres arond the packing.

OPERATNG EXPERENCE WTH MPROVED STEAM TURBNE PACKNG RNGS 79 ficients was part of the preliminary research work partially sponsored by the Department of Energy.

3 OPERATNG EXPERENCE WTH MPROVED STEAM TURBNE PACKNG RNGS 79 ficients was part of the preliminary research work partially sponsored by the Department of Energy. Also, the pressre drop across the packing teeth from P1 to P2 is somewhat nonlinear and frther complicates the analysis. The packings are held open ntil the predetermined nit flow (five to twenty percent) is obtained. The open clearance of approximately.15 in is determined by the back of the tooth ring fit with the hosing. This is sfficient to avoid normal startp problems and at the predetermined flow the packings close to normal rnning clearances of.25 in. The pressre forces to close the packings were felt to be marginal on nmeros rings. To resolve this problem, the design reqired removal of several of the last short teeth, and this reslted in less force holding the packing open (Figre 6). Fewer packing teeth reslted in increased leakage flow, bt are far offset by the improved clearances being maintained. Figre 3. Modifwd Packing. The basic theory sed in modifying the packing is that the pressre forces behind the packing increases with the nits' flowrate and can be sed to overcome the spring and friction forces acting on the individal packing segments. By proper design of the coil springs, the pressre forces acting on the packing can be tilied to case the packing to move from a large clearance to a small clearance at a predetermined flow condition that is known to be beyond the condition where severe rbbing is likely to occr. This method then determines the nit load at which the packing rings close and the tight clearances are established. The static pressre forces acting on the cross section of packing are shown in Figre 4. By sing a vector diagram similar to that in Figre 5, the forces can be calclated for a given load and the springs sied to balance the pward and downward forces. This oversimplifies the problem, since the friction forces act against the closing force. The determination of the friction coef PRESS. Pl Pl OPENNG FORCE WTH LAST TOOTH Pl OPENNG FORCE W/ LAST TOOTH Figre 6. Effect of Tooth Removal. PRESS. P2 P2 Figre 4. Pressre Forces. CLOSNG FORCES >OPENNG FORCES Figre 5. Vector Diagram. PRELMNARY TEST The new packing design was developed and patented by Ronald Brandon, of Power Technologies, ncorporated. The preliminary work was sponsored by the Department of Energy and New York State Energy Research, which inclded test work on a prototype packing assembly, followed by installation in a boiler feed pmp trbine at Niagara Mohawk. Preliminary test work inclded cycling stdies for reliability and development of the friction coefficients. After the preliminary investigation was completed the decision was made by the tility company to install the packing in the Dickerson Unit dring the Falll986 overhal. PERFORM ANCE FACTORS A variety of benefits were expected to reslt from the effects of the improved packing. These inclded: Decreased stage packing leakage. This creates a direct improvement on the high pressre (HP) or intermediate pressre (P) trbine section efficiency. Decreased tip (or shrod) leakage. This benefit reslts from the avoidance of bowed rotors (normally cased by packing rbs)

4 8 PROCEEDNGS OF THE SEVENTEENTH TURBO MACHNERY SYMPOSUM that reslt in damaged tip seals. This saving can be expected to even exceed the benefits of the improved diaphragm packing clearance. Decreased (nmber two packing) N2 leakage flow. Flow leaking throgh this critical six iings of packing which separates the first high pressre stage from the first reheat stage (Figre 7), will bypass all bt the first HP stages, casing direct kilowatt (kw) losses. n addition, the leakage flow will redce the P section bowl enthalpy with a reslting redction of reheat (RH) section available energy. This latter loss can often be abot 5 percent as big as the HP loss. ncreased first stage shell pressre. t is common to find the trbine first stage shell pressre lower than design by from five to fifteen percent. This has the effect of taking energy off the relatively efficient later HP stages and ptting more energy on the first stage (the least efficient stage). The added energy drops the already low first stage efficiency even lower. t was expected that the improved packing wold improve first stage shell pressre to be closer to design vales. Decreased excess flow capacity. Excessive leakage has a secondary effect of increasing the trbine flow capacity. While this may have some side benefits, it also cases some bad HP efficiency effects across the load range by reqiring increased throttling of control valves for any normal level of flow. This cases significantly poorer HP section efficiency. Note that where the excess flow capacity is of vale, it may still be available by way of increased initial pressre five percent overpressre being commonly acceptable on most trbines. NSTALLATON The modified packing was installed in the N2 packing and in all the HPP diaphragm packings (Figre 7). This inclded seven diaphragm iings in the HP, five diaphragm rings in the P, and six N2 rings. The rings were being replaced as a part of the Dickerson Unit 3 overhal; therefore, the cost did not inclde the rings which were standard packing. Additional cost inclded the prchase of the springs, machining the spring holes, lapping the seal joint and ctting slots to assre the back of the packing was pressried. The field work involved ctting the teeth and sholders to obtain the desired closed clearances. This is a normal company overhal procedre sed with stationary packing. XOVER ONLNE ANALYSS The tility company decided to condct online performance monitoiing to evalate the new packing design. Steam pressres and temperatres were being monitored arond the HP and P trbines and the data was stored on a desktop compter. The nit was continosly monitored throgh the end of 1987, and incldes several startps which exercised the packing opened and closed. The nit was also tested nder controlled conditions sing precision instrmentation to verify the reslts obtained with the online monitor. PERFORMANCE RESULTS MONTHLY FUEL HEAT RATES The l2 month rnning nit heat rate averages for the three Dickerson nits is shown in Figre 8. Unit 3, with the improved packing, shows a consistent improvement, which is expected to level off at abot 2 Bt/kWh. Most of this difference is credited to the improved packing, bt other improvements made dring the inspection peiiod also contribted to the improvement. LJ 1 LJ <C 1!: 1 <C LJ 1 :r: 99 C J LJ LJ ::::> > LL.. 93 MONTH Figre Monthly Fel Heat Rate. HP SHAFTLP SHAFT MEGAWATTS The Dickerson nits are cross compond. Therefore, a comparison of HP shaft to low pressre (LP) shaft otpt ratio can be made after correcting for standard conditions. The following reslts have been obtained. CONTROL VALVES Date Design Jly 85 (preoverhal) December 86 (postoverhal) Jne87 November87 Megawatt Ratio This ratio is a good indicator that the HPP performance is being maintained. t is significantly better than the preoverhal ratio of.97. For a given throttle flow rate, the HPP KW otpt is p by abot 5 kw. Figre 7. Trbine Cross Section. HGH PRESSURE TURBNE EFFCENCY A plot of HP trbine efficiency, (taken at nmber for control valve crack point) over the twelve months test period since the

5 OPERATNG EXPERENCE WTH MPROVED STEAM TURBNE PACKNG RNGS 81 overhal is presented in Figre 9. The 81.5 percent efficiency calclated shortly after startp is 1. 5 percentage points higher than the best efficiency ever obtained on any Dickerson Unit in the last seven years. and nonsteadv state conditions. As a reslt, a control led test was condcted sing calibrated resistance temperatre detectors (RTDs). Apparent P trbine efficiencies were calclated and are shown in Figre J.. 83.J.. w w :... ct:j :: ::::> ::: <. DATA TAKEN AT #4 VALVE PONT. <.:::> r. co r. N r. r. r. r. ("")... co N <::t N '1 co (V') <. <.... C'..1 N N N N (V') L() r. '1 N ELAPSED TME N DAYS SNCE THE OVERHAUL Figre 9. High Pressre Trbine Efficienc Trend {TME SNCE OVERHAUL),, v85 TEST ;::, ( 4 YR.)...._ '" (5 MONTHS) TEST '. \ \ ' /....../ Unit 3 HP trbine has degraded 2% percentage points since the overhal. The most likelv case of the degradation is de to weld head damage, deposit; and possibly some erosion. Based on previos overhal inspections, not all of the degradation can be assmed to be de to the above mentioned items, so some degradation mst be de to packing and excess seal leakage. How mch is indeterminable. t shold be noted that the Brandon design is f<jr packings only, therefore, spill strips are still vlnerable to shaft rbbing, althogh to a lesser degree. A plot of the Jne 1987 enthalpy drop test and a trbine test rn in 1985 is depicted in Figre 1. As stated earlier, the trbine efficiency has dropped since the startp in October, bt it is still above the trbine perf(>rmance level. The design efficiency crve is a valve best point type crve and, theref(jre, it does not show the valve loops as the test data shows. NTERMEDATE PRESSURE TURBNE EFFCENCY Trend data was fond to be misleading in evalating the P trbine performance. This is a reslt of poor instrmentation 88 (MODFED PACKNG) Figre 11. Apparent P Ttwbine Efficiency. Apparent P efl:iciency is calclated fi om the hot reheat to the crossover state points and incldes the afl'eet of the relatively cold N 2 packing leakage on the trbine efficiency. Three P efficiency crves are compared in Figre 11. The post overhal 1981 test shows the level obtained after five months of operation, while the 1985 data was obtained for years later. The rise in the efficiency dring this period indicates the amont of N2 packing leakage had significantly increased. The 1987 data obtained after the packing modification was installed, indicates the leakage is significantly lower than the 1981 reslts. This is a good indicator that the N 2 packing clearances are tighter than the 1981 overhal clearances..j..!j... w co 73 ::: ::::> 1 69 :c 65 4 // // "/.... / TE., "" "",.,.. / /.::_ ;;:, 1 ".. 85 TEST CALCULATED MAN STEAM FLOWKLB/HR Figre 1. HP Efficiency Test..,. t FRST STAGE HP SHELL PRESSURE First stage shell pressre will normally remain constant for a given throttle flowrate. f the pressre changes, it indicates a change in flow passing area, either the second stage diaphragm, diaphragm packing or the N2 packing. This pressre has been trended since the packing modification and shows no sign of degradation in the 12 months of operation, frther it is eqal to manfactrer design vale which strongly sggests design vale bypass leakage. Previos testing showed the first stage pressre was 6.5 percent low. N2 PACKNG RATO MONTOR A somewhat niqe approach to monitoring the N2 packing determines the ratio of clearances before the blowdown divided the clearances after the blowdown. This reqires pressres and temperatre measrements in the blowdown line.

6 82 PROCEEDNGS OF THE SEVENTEENTH TURBO MACHNERY SYMPOSUM A plot of this data is presented in Figre 12 and step changes are shown at low load that cold be a reslt of the packing opening. A more thorogh investigation revealed that the clearance ratio change is correlated to the temperatre in the blowdown pipe which drops off significantly at low loads. (Figre 13) This phenomenon is a reslt of temperatre differences between the first stage shell and the P bowl which reslts in inner shell distortion. The first stage temperatre changes significantly at low loads. The effect has been seen on nits with packing boxes and with integral inner shell packing holders similar to Dickerson. This problem is not niqe to the modified packing design.. (/) (/) 15 LJ. 12 1;; 1 :Q N2 PKG CLR. RATO 1 /'... 1.,... LJ.... ' ELAPSED TME Figre 12. Packing Clearance Ratio. LL. L&J a::: HR ffi 12 ffi (/) ::>... ::> < 3 78 c m N \... \ \ SLOWDOWN\, /..., TEMP. 2 4 ELAPSED TME,. Figre 13. Temperatre/Clearance Ratio. 6 HR. L:5... c.!:! if 1.8 L&J <.6 L&J c.!:!.2 if L&J 3, / <, L&J / LL.... 2,., :E: c.!:! L:5 (/) 1 if START UP TME HR. Figre 14. Startp Packing Clearance. packing closres is somewhat interpretative bt the online clearance ratio does indicate that all packings are closed. CHALK PONT UNT 1 The packing modification was installed in Chalk Point Unit 1 dring the major overhal dring the Fall The nit is a spercritical, doble reheat, tandem compond trbine. The primary trbine was overhaled and the modified packings were installed in the N2 packing and the HP trbine and second reheat primary P trbine diaphragms. The improvement in pre and post overhal test of the HP trbine is shown in Figre 15. The gain in efficiency is approximately 7. 7 percentage points and reslts in an improved heat rate of 11 Bt/kWh ST STAGE PRESSURE Figre 15. Chalk Point HP Trbine Efficiency. 26 STARTUP MONTOR The N2 packing ratio was monitored dring startp to observe the pressre changes that wold indicate the packing is closing. The expected reslts shold be six niqe steps as areslt of each of the six N 2 packing rings closing. The data in Figre 14 were taken dring a start p of Dickerson Unit No. 3. The changes in packing ratio are indicative that the packing is closing as the nit is loaded. The nmber of step changes reslting from The primary second P trbine efficiency im provement is shown in Figre 16. The improvement in actal efficiency of 5.5 percentage points reslts in a 53 Bt/kWh heat rate improvement. The difference in actal and apparent efficiency of percent prior to the overhal and 3.5 percent after the overhal shows a dramatic decrease in the N2 packing flow. The estimated packing flows at fll load are 16, lblhr prior to the overhal and 48, lblhr after the overhal. This redction in packing flow reslts in a 12 Bt/kWh improvement in heat rate.

7 OPERATNG EXPERENCE WTH MPROVED STEAM TURBNE PACKNG RNGS 83 LL LL LJ ' PREOVERHAUL APPARENT _. t.... co c::: POST OVERHAUL APPAR, ENTft J( D ::;) PREOVERHAUL ACTUAL EGN.. POST OVERHAUL ACTUAL ;;r ST STAGE PRESSURE Figre 16. Primary Second P Trbine Efficiency. CONCLUSON The reslts show the new packings have improved the performance of Dickerson Unit 3 and Chalk Point Unit 1 significantly. The modification cost was minor compared to the expected gains in performance realied on these two nits. f the packings contine to perform over its overhal cycle as it has to date, then the pay back in fel savings will be significant. ACKNOWLEDGEMENTS The testing, data analysis and presentation of the reslts for Dickerson was developed by Mr. Greg Staggers, a Project Engineer with Potomac Electric Power Company. The testing, data analysis and reslts for Chalk Point were the work Dave Schnetler, a Test Engineer with Potomac Electric Power Company. The overall improvement in performance at Chalk Point Unit 1 is 283 Bt/kWh. Based on the overhal inspection of repairs, approximately 24 Bt/kWh is de to the redction of tip seal and packing leakage losses. The remainder is the reslt of steam path repairs.

8 84 PROCEEDNGS OF THE SEVENTEENTH TURBO MACHNERY SYMPOSUM

ANALYSIS OF STEAM TURBINES FOR FEED WATER PUMPS ON LNG SHIPS

14. 15. Jne 018 ANALYSIS OF STEAM TURBINES FOR FEED WATER PUMPS ON LNG SHIPS ABSTRACT Martina Živković, B.Sc. Antna Barca 9D, Zadar, Croatia martina.latinovic@gmail.com Josip Orović, D.Sc. Igor Poljak,