Control Differential Expansion of Turbine

Is it possible to control Differential Expansion of Turbine? What steps to be taken if one finds it alarmingly high?

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Tahoor Alam KhanS KhanSr. Engineer-Operation at ADHUNIK POWER & NATURAL RESOURCES LTDTop Contributor Unlike Comment (21) Follow Reply Privately 10 days ago Comments You, Jan Giedroyc and 2 others like this 21 comments Jump to most recent comment Joseph F Byrd, Jr Home health and personal care aide at Unpaid

here is an interesting discusion of similair inquiry http://www.linkedin.com/groups/Can-anyone-explain-reason-High3866056.S.5822624396421906436?trk=groups_search_item_list-0-b-ttl&goback=%2Egna_3866056 and another http://www.linkedin.com/groupItem?view=&gid=2920902&type=member&item=180029392&trk=groups_search_i tem_list-0-b-ttl&goback=%2Egna_2920902 Udhayakumar Venkataraman Executive Director Operations, Gurgaon, NCR Delhi Hi Thakoor Reducing steam flow DE ( Rotor minus casing expansion) may be reduced, but the root cause like any variation of steam temperatures, mismatch , deposits in the initial stages etc may be checked. Measurement probe aspects too may be checked. K Arun Kumar Vice President - Power at BMM ISPAT LTD. Hi, the source for expansion is from the inlet steam and its temperature. Rotor expands faster than the casing. Shutoff / trip the inlet steam from further heating during abnormal expansions. After getting diff expansion under control, re-roll the turbine with minimum superheat temperature and or sliding pressure operation (ensure your inlet steam temperature to be higher than the casing temperature). Few adopt a practice of rolling it more than once to achieve desired casing temperature. You can also review your cold startup ramping procedure with more soaking  period to avoid such abnormal expansions. We had an experience of proven cold startup procedure causing such abnormal expansion and tripping on high vibrations. Later found a restriction in the movement of casing during expansion had caused such abnormality. Jim Hartline Project Mgr at MD&A Install shell warming device. Engineer at CCM/Thermal

we at Thermo International design and build turbine warming systems, the control cabinet, and heaters to be attached to the inner side of the turbine shell insulation. General manager (power) at Indo engineering project corporation, Nagpur

@Khan, can you give some past history of your turbine, I mean is it newly installed machine or old which gives occasional problem of diff. expansion ? This piece of information is required to ascertain if there is operational mistake of fast heating to save time. In case of new machine, it is always advisable to follow OEM guidelines in case of cold start up and every step is important as it imply metallurgical issues initially. Al Peters Supervisor, Maintenance at NiSource Tahoor: Differential expansion can be a misnomer. If front standard is supposed to move during start up, but doesn't move at an adequate rate to avoid the perception of differential expansion, there are a couple of common caused. 1. Firing at an improper rate (too hard or not hard enough) can modify when the front standard moves (as opposed to when it is supposed to move). 2. Lubricating the front standard slides: lack of lubrication or using the incorrect lubricant can inhibit movement. Verify in your turbine manufacturer's manual the full description of the grease. I know of one plant that used a lithium-based EP-1, while the manual clearly called for a black or dark grey grease with a high molybdenum content. After they purged their plugged grease channels, they started using Dow Corning 41 grease. The issue has subsided or it has been resolved. Prakash Surana Consultant (Power Plants) @ Tahoor Alam, Please check all the sliding supports of your turbine casing. I think the expansion of the casing is getting restricted at one or more places this is resulting in high differential expansion. The LP casing bolts should be loose so as to allow free expansion. Brent Homes Principle KR Incorporated I will admit that I have not read a single comment above.....when I saw this post I had to respond..... Thermal expansion is an absolutely NOT controllable...Period..... It is a result of temperature change and thermal expansion and is an absolute! Thermal expansion is an uncontrollable force and must be accepted and dealt with. Nuf siad Satyam Gupta Power Plant Engineer at Thermax India Ltd. I think wheel chamber pressure also should be checked , if it is higher than standards then there is probable cause of scaling on the HP blades which might be due to fluctuating drum level. Can we know how much diff expansion is occuring in your m/c. Fredy LeJeune Shift Supervisor at NAES Corporation Follow your OEM Starting And Loading Instructions!!!! Rajgopalan sv SKV Energy Services Pvt Ltd Dear Sir, Surge tank drains to be kept crack open. Also Please note that the extraction line drains are properly drained out. Provide the orifices if any in the extraction line drains, Clean the drain lines completely and keep the valves full open with orifice in line when the extraction is in service. Hope this should resolve your problem.

S.V.Rajagopalan Director SKV Energy Services Private Limited Mobile: 9999985716 Jan Giedroyc Senior I&C Engineer at Alstom Power The title question is extremely interesting, but very broad too. Especially, if turbine layout is not known. Imagine please big turbine with three cylinders: HP, IP and LP. The thrust bearing located between HP and IP. Very typical configuration. Turbine has three differential expansion measurements: - at front pedestal for HP DE - at IP/LP pedestal for IP DE - at LP rear (LP-Gen) for LP DE. So we have 6 cases to analyse, shaft short and shaft long for each DE measurement. Some of above answers are for one and only one case. Example: for HP DE shaft long, there is possibility, that front pedestal not moves smoothly on fundation (steps on absolute elongation trend diagram). But...there are many other reasons for HP shaft long. Can we please try to give answers, with precise explanation, to which case they belongs? Example: For LP DE shaft short, the pressure in condenser can be too high, and improving the vacuum can improve LP DE. Tahoor Alam Khan Sr. Engineer-Operation at ADHUNIK POWER & NATURAL RESOURCES LTD Top Contributor I am very greatfull to all of you for adding valuable comments to this discussion. Here I would like to draft some details of my plant. The plant is BHEL make 270 MW PF boiler unit with 3 cylinder tandem compunded turbine. For last 3 months the unit was under overhauling. Before going for overhauling, the value of DE of LPT as shown in DCS was 20mm. After taking the unit in operation again, it was observed that the value climbed up to 27.3mm. My  biggest concern is upto how much value can the turbine be run ensuring its safety a s we haven't incorporated any  protection for high DE. If anybody is having exp with turbines supplied b y BHEL, kindly share views. Joseph F Byrd, Jr Home health and personal care aide at Unpaid IF you have applied all the above information about "controlling DE" and you are sure the steam conditions are the same now as prior to the outage, I would suggest you perform an extensive review of your outage documentation, especially wheel clearances, rotor positions during wheel check, rotor positions once coupled and thrust set and then finially and the most probalbe is the calibration of the DE. It could be posible the DE is calibrated correctly and durring assembly the rotor positions have change the 7mm after couplings were made You might want to ask someone that was responsibile for DE calibration if they have an external "witness" measurement that can verify the current indication (discussed in the threads I linked earlier) Andy Wearmouth Chief Engineer at Synergy So it sounds like the issue is only in the LP cylinder? Can we assume the HP and IP are operating in their historically normal positions? . Where is the thrust bearing - between HP and IP ? Given the anomaly started following overhaul I'd be suspiscious of the set up of the instrumentation in the first instance. By the way - are we

talking a DE of 2.0mm and 2.7mm ?? 20's sounds very large even for a very big LP - for a 270MW machine this sounds very odd given its relatively short length and fairly low temperatures. Tahoor Alam Khan Sr. Engineer-Operation at ADHUNIK POWER & NATURAL RESOURCES LTD Top Contributor Dear Andy Sir, there is no system by which we can measure HPT & IPT DE in our unit. The only value shown in DCS is for LPT. The thrust brg is located in 2nd pedestal. The value shown is 20mm and not 2 mm. I will crosscheck with operation manuals for its correctness. Andy Wearmouth Chief Engineer at Synergy Be very careful then Tahoor, the axial clearances inside the HP and IP cylinders will be much tighter than the LP. If the issue is the incorrect line off of the turbine at overhaul you could do (have done ?) real damage here. 7mm long is a huge dimension change so I would be amazed if it is genuine. Have you had any issues of vibration during run ups or shut downs ? Axial rubs can be difficult to spot as they usually do not produce rotor bowing. Tahoor Alam Khan Sr. Engineer-Operation at ADHUNIK POWER & NATURAL RESOURCES LTD Top Contributor Dear Andy sir, we have encountered high vibrations in brg no 7 (exciter rear brg) but later the values were found to  b faulty. But again we have observed high vibrations in brgs 2X , 2Y and 3X. Can this be due to high DE? Andy Wearmouth Chief Engineer at Synergy Quite possibly. The first place I would expect to see rubbing would be the steam glands if the turbine shaft is sitting aft. It it were shaft short it would be blade contact in the HP first. However I am assuming the steam flows through the HP and IP are in opposite directions to balance the thrusts - in which case with shaft aft you may have touched the IP axial seals and hence the vibration of brgs 2 and 3. It is very difficullt to comment more specifically without understanding the exact configuration of the machine. Jan Giedroyc Senior I&C Engineer at Alstom Power 27 mm seems typical value of ABSOLUTE elongation of turbine THAT size (it is standard value for LMZ 200MW machines or clones like 13K215, in configuration described in my previous mail), when AE sensor is mounted at front pedestal and turbine fix point (bolted to foundation) is at the middle of double-flow LP turbine. For above turbines, LP differential expansion measurement range is (minus 4 to plus 8) mm or (minus 3 to plus 9) mm. DE indication of 20 or 27 mm is simply immposible for standard instrumentation mounted on such turbines. For some of DCS configuration, not all TSI parameters are visualised. Can be separate system/monitor/data recorder installed for TSI only, sometimes under the name "Vibration monitoring". Between both systems there are hardwired (4...20 mA) connections or information data links. Especially when DCS was installed as an retrofit, some  paramereters, signal names and ranges can be not correct. It is very common case. Tahoor, I don't know any turbine that size without HP and IP DE measurements, if is younger than 40 years. Sometimes LP DE measurement can be missing (eg old Metropolitan Vickers machines), because big clearances within LP part and no sealing strips on the shaft at gland area (no any rubbing possible).

Look please at full configuration of I Jan Giedroyc Senior I&C Engineer at Alstom Power .. I&C system in your plant. Jan Giedroyc Senior I&C Engineer at Alstom Power Differental expansion, Differential elongation, Relative elongation, Relative expansion, means the same (for steam turbine). Look at other nice discussion: http://www.linkedin.com/groupItem?view=&gid=1513267&type=member&item=5838887919091204098&qid=508  b932e-68de-4d2f-864d-9b23c6d4006a&trk=groups_items_see_more-0-b-cmr 

steam turbine Relative expansion

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sajid saleemOperation Engineer at Orient operating company Pvt Ltd. Dear All, I want to know about relative Expansion of Steam Turbine. As our steam turbine relative expansion on base load in alarming zone. our Relative expansion tripping at -3.8mm and alarm limit at -3.2mm. so please apprise your comments for understanding this issue. Like (1) Comment (14) Follow Reply Privately 15 days ago Comments Muhammad Iftikhar likes this 14 comments Jump to most recent comment Joseph F Byrd, Jr Home health and personal care aide at Unpaid

it is unusually for relative (differential) expanision to go negative except during load reduction or hot restart. some machine history would help with making guesses. Is this the very first time the unit has been placed in service? Is this the first time at full load following an outage? if so, what were the past values at same load. what I would guess is that the polarity is incorrect and "-" should be "+" indication. Nicholas Schroeder Power generation engineer Top Contributor The case has to float, if it's stuck that could be a possible problem. sajid saleem Operation Engineer at Orient operating company Pvt Ltd. Dear Joseph F B , Actually this problem persisting since COD 2010. when we start steam turbine the relative expansion of ST going on alarming zone but when we achieved base load and increased low pressure steam to

turbine the relative expansion will be normal decreased up to -2.6mm. we take up this issue with SKODA power engineering they replied little bit modification required to resolve this issue. Joseph F Byrd, Jr Home health and personal care aide at Unpaid when I made my initial comment about the rarity of a steam turbine running rotor short, I failed to consided the various new turbine and plant configurations that have been devolped since I was comissioning. So maybe it is  possible for the turbine to have a casing recieving heat with its rot or being cooled. By being a new unit, I would think SKODA is aware of the design problem with thier design and hopefully can  provide some guidance. If you were to provide aditional infor mation here like a cross section of the unit and operational conditions, the replies would only be cursory. The possible amount of effort required might make it worth while for you to hire a specalist engineering company to review and recomend since SKODA is not repling David Ford Process Engineer at METKA S.A. Maybe you should clarify what you mean by negative diff expansion. We have very old Parson machines with Siemens instrumentation and a negative diff means the casing is e.g. expanding at a slower rate than the rotor or confusingly a negative diff menas the rotor is expanding faster than the casing. Chris Comi Steam Turbine Engineer Most ST are designed so that the relative expansion is well within normal bounds at base load so that it can easily accommodate transient conditions. To be in alarm after some time at baseload indicates, to me, a suspect measurement device calibration, or as Joe suggests, a significant design issue. A good starting point is to check the cold readings for near zero values (or expected cold values). OEM should also be able to provide expected values at  baseload conditions for comparison. Nicholas Schroeder Power generation engineer Top Contributor When was the turbine last apart & was the "K" value confirmed, the relative rotor, case positions? Was it listed in the overhaul report? As found/as left? Ryszard Nowicki FAE at GE Company Polska Sp. z o.o. Div. BENTLY NEVADA If DE goes behind of ALARMS –  it is danger for Your turbine. The situation can be due to: 1. There is something wrong w/ the turbine 2. The setup of transducers / measurements have not done correctly (during measurement commissioning) 3. A characteristic of the transducers used for DE measurement is out of specification. There are many of transducer configurations for DE monitoring. If You would like to discuss more about the measurement contact me at [email protected] Ray Beebe Speaker, Author, Trainer, Consulting Engineer I experienced this some years back when trialling air forced cooling. Diff Expan went near the limit so air cooling was stopped - yet diff kept going to past the limit before settling. Urgent call to the OEM. Response "limit revised". So there was some margin available. Abdul Nassar Technical Director @ SoftInWay Turbomachinery Solutions Pvt. Ltd.

Sajid, SoftInWay has been assisting many global OEM's on transient thermal analysis to analyse the performance of steam turbines under different conditions during start up, shut down and base load operations. If you need any design evaluation to analyse the issue, we can help on that, professionally. You can write to [email protected] velmurugan valkier MECHANICAL ENGINEER RESPECTED SIR ; I WOULD LIKE TO PRESENT MYSELF AS A CANDIDATE TO DISCUSSES HOW MY SKILL CAN BE VALUE TO POWER PLANT EMPLOYMENT OPPORTUNITY AS MECHANICAL ENGINEER IN YOUR ESTEEMED ORGANIZATION . I AM POST GRADUATE DIPLOMA IN THERMAL POWER PLANT ENGINEERING FROM THE NATIONAL POWER TRAINING INSTITUTE (NPTI)-SR , NEYVELI UNDER THE MINISTRY OF POWER , GOVT. OF INDIA IN THE YEAR (2012-2013) IN FIRST DIVISION ; AFTER COMPLETING MY B.TECH. MECHANICAL ENGINEERING. I BELIEVE THAT , I HAVE NECESSARY QUALIFICATION FOR THE POST MECHANICAL ENGINEER AND I AM ENCLOSING MY RESUME AND MARKSHEET FOR YOUR KIND REFERENCE . I WOULD BE GRATEFUL AND OBLIGED ; IF YOU GIVE ME AN OPPORTUNITY TO PROVE MY ABILITY. HOPE THAT YOU WOULD GIVE KIND RESPONSE . THANKING YOU

FROM R.VELMURUGAN S/O MR . S. RAJENDRAN  NO.36 , AKILANDAGANGA PURAM , AMMERI POST ,  NEYVELI-607802. CELL NO.- 91 9940343315 Jerry Clarke Operations Manager Commissioning at Forge Group Ltd On Start up Lower vacuum to -30 kpa and increase gland steam temp as high as possible this will prevent the HP diff and expansion problem. Andy Wearmouth Chief Engineer at Synergy Sajid,can we have a bit more information about the machine configuration please. Also, as David asked, so we are all on the same page, can you confirm that a negative differential expansion is describing a shaft "short" condition some manufacturers do not follow convention. Is the machine a two cylinder (HP & LP [maybe IP?]) - you mention admitting more steam to the LP - is this unit  part of a CCGT with differing steam sources ? I assume with these dimensions we are talking LP differential expansion. I'll make the bold assumption that as  Nicholas has asked, your instrumentation is all set up correctly. Clearly the OEM knows of the issue. If it is a two (or more) cylinder machine it may be as simple as resetting the cold offset position of the LP by a change in the patch plate thickness to get the running position of the LP in the correct spot. It may be necessary to compromise a little for start ups. Marius Caragea Chief dispatcher central shift at SC Termoelectrica SA Sucursala Electrocentrale Braila

I agree Jerry Clarke, gland steam sistem or vacuum should be the problem...

Can anyone explain the reason of High LPDE ,we are facing problem in our 135 MW machine ,which is having one HIP and one LP casing sendhil kumar135 MW ,operation manager at BALCO Can anyone explain the reason of High LPDE ,we are facing problem in our 135 MW machine ,which is having one HIP and one LP casing Comments Al Cauwenbergs likes this 46 comments Comments are closed Jump to most recent comment Joseph F Byrd, Jr Home health and personal care aide at Unpaid Top Contributor

even a few detail would be appreaciated! Throw us a bone! can you provide a cross sectional view of the unit. If I recall, this is one of multiple similiar units at your station, thus do you have an unit comparison of expansions, heater/stage temps and pressures. as for reasons; 1 calibration error, consider gain as well as zero 2 something too hot, posible ruptured heater bellows 3 something too cold, posible ruptured heater bellows 4 something stuck 5 design error, in that the actual growth should be expected Is mentioned in one of the prior postings, Is the LP expansion a true differitail or is it just rotor expansion? Bradley Piatt Manager Peaking Power at PPL Corporation Yep like where the thrust bearing is and the casing fixed point. Paolo Tolomei Service, power plants field engineer presso Ansaldo Energia Hi sendhil, further information are necessary to evaluate the root cause. I would begin checking the HP dxd if available, with LP gland steam temperature, all temperatures in compliance with design. Paolo sendhil kumar 135 MW ,operation manager at BALCO Joseph sir it is measuring only rotar expansion we have four similar units all are having similar problem sendhil kumar 135 MW ,operation manager at BALCO Bradley sir thrust bearing is at second pedestal hip casing is rested at second pedestal which is fixed one LP outer casing also fixed at second pedestal can expand towards generator Hugo de Koningh Principle Mechancial Engineer at Worleyparsons

Your information is a bit to short. I assume you have high vibrations on the LP DE? What does the vibration spectrum exist off (is this a high 1 x or 2 x)? Furthermore at what load do the vibrations appear? If dependable on the load it could be caused by the generator rotor. Are you talking about shaft or bearing vibrations? sendhil kumar 135 MW ,operation manager at BALCO dear sir, vibration levels are normal ,to reduce LPDE we are maintaining less HRH (re-heater) temp Joseph F Byrd, Jr Home health and personal care aide at Unpaid Top Contributor Hugo, I believe the concern is Low Pressure Diferrential Expansion (not vibration on Low Pressure Driven End) are you sure the LP casing is fixed at its out board bearing and free to expand towards the generator. All I have ever seen were fixed near centerlines and expand outward in all directions. If this is occuring on multiple units, then is it possible this could be the way the unit must grow and the book value of normal is in error. It doesn't take to much effort to look through the cutaway drawings and determine the DE that would become a  problem and estimate what should happen. can you share such drawings? Thomas Galambvari Sr. Commissioning Engineer at Alstom Power Unless these are four new machines that have never been operated under these conditions before, I think it's safe to rule out mechanical design error or freezing of the pedestal. If these are reheat machines, it could be possible that the HRH steam is operating abnormally high temp (or cold if the DE is short). Is the reheat steam passing a desuperheater station? There is also a chance that the gland steam system temperature is running too high or the sealing steam pressure is too low or a combination of both. Localized heating of the gland area can cause this problem as can gland steam  blowing on the probes. If these units are coming out of some type of outage, is it possible that calibration was performed on the measuring and that this was done in error? radulescu ioan-radu control room operator at Petrom-CC Power Plant (860MW/h) Brazi water circulate in condenser syst is problem....or new tubes in condenser..............))))))))))))))))))))))))))))))))))))))))))))))) Bradley Piatt Manager Peaking Power at PPL Corporation You are saying the rotor and casings are both fixed at the second pedestal. Then if the rotor is long seal steam is too hot or somehow the rotor is warmer. It could be the casing is stuck and can not expand. Usually LPs are fixed in the center but it is possible they can be fixed at one end. Another possibility is the reading is wrong. Do you have a reference reading on the generator normal position at load? Look to brush rigging or a seal wear mark. John Mitchell

Owner, Power Prof, LLC Top Contributor

How is the hood temperature? Do you have good vacuum? Jan Giedroyc Senior I&C Engineer at Alstom Power Can be a problem with proper mounting location of DE transducer (DE probe). If DE transducer is mounted directly on outer LP gland, everything should be OK. If is mounted inside LP-GEN pedestal, many things can go wrong. What is location of transducer, type of monitoring system, shape of shaft (collar, cone, two cones)? In case of measurement on single cone (ramp), is there compensating sensor foresseen ? In case of mounting inside LP-GEN  pedestal, how pedestal is connected with LP casing and with foundation? sendhil kumar 135 MW ,operation manager at BALCO Sir, Probe is mounted in LP-gen pedestal,type of monitoring collar,pedestal is connected with lp casing through a key Thomas Galambvari Sr. Commissioning Engineer at Alstom Power Is it possible for you to burn a set of wedges to verify the measurement? Wedges don't lie... Jan Giedroyc Senior I&C Engineer at Alstom Power Thomas, it is very good method (old BBC...), but usually LP shaft has flat surface in gland area. For BBC/ Amrein "Pendel geber" it was small collar on the shaft, say 5x7 mm, always enabling local measurement by wooden wedges. But it is very specific solution. Generally, confirmation of remote indication by local mechanical measurement is the only proof, that remote indications are OK. Most important: first measurement must be done at cold zero position, as a base. Sometimes elementary error can be done: adjusting of DE measurement "zero position" not in real "zero expansion" of shaft. "Zero DE" means: uniform cold status of turbine, no vacuum, no gland steam, no warm oil to bearings, shaft pushed to axial shift "zero position" within thrust bearing. Sendhil, after long stillstand of turbine and cold status, what are ALL indications of relative and absolute elongations? Are all ZERO mm , plus minus 1mm ? To find a reason of abnormal DE indication, you must be sure that measurement itself is OK (zero, range, plusminus direction). It is a task of TSI service man/producer. If OK , than parameters of turbine start-up/loading should  be verified with turbine OEM data. At t he end, more detailed analysis of sensor mounting place and connections  between LPcasing-pedestal with transducer. What system of TSI do you have (producer)?, Is TSI installed by turbine OEM, or as retrofit? Joseph F Byrd, Jr Home health and personal care aide at Unpaid Top Contributor "... burn a set of wedges..."- Thomas Galambvari I have never heard of that! and know I would not have the nerve to have done it without seeing some else do it first. BUT I really like the ideal and if possible would have included it for comsissioning routine. Could you provide a narritive just incase I every get a chance again. oh, would it still work with those new fragile looking prox probes? Al Cauwenbergs Senior Project Manager at Algonquin Power Systems

Jan, I find this discussion very interesting however I have not heard of the process "burning wedges" Could you explain the process please? Al Cauwenbergs Senior Project Manager, Algonquin Power Thomas Galambvari Sr. Commissioning Engineer at Alstom Power Glad to see I've stirred some interest here! Burning wedges is old school and as elementary as it gets. Funny that Jan noted that it was a BBC technique as that is the company I started my career with! The process involves making a half dozen wooden wedges that are cut very thin - basically looking like a hatchet or axe blade. A reference reading is taken with a cold machine, that hopefully has a motor turning gear, burning a wedge by forcing it into a recess or ridge on the rotor while holding it against a stationary part of the bearing  pedestal near the oil seal. The cold measurement is then taken from the wedge and the DE monitor zero is confirmed. A second or several more wedges can be burned once t he machine is up to speed and on load using the same method. This measurement is then compared to the cold reading and then to the DE monitor. The measurement of the wedge differences is a true mechanical measurement of DE and it doesn't lie. This would be an absolute method used to confirm the proximeter measurement. Wish I had a picture! Jan Giedroyc Senior I&C Engineer at Alstom Power Maybe Thomas remembers better, but OK, I can share old stagers tricks. Joseph, the only problem was with heat resistant gloves, not shorter than 25 inch :-). In old good times made from asbestos, now forbiden. It was (and probably is in particular plants) standard procedure to check and verify REAL relative elongation (diff. expansion) during first start-up, after overhauls and in case of problems as above. Al, imagine cuboid piece of hard, dry (very important) wood - we used oak. Square, for example 25x5x1,5 cm, exact machined. Each piece cutted diagonally to obtain 25x5 cm wedges, on one side 1,5 cm thick, other (working) side sharp. Now slide carefully (!!! 3000 rpm, 300 C on HP gland surface!!!) side of wedge on gland surface, touching sharp edge against small collar on shaft (it works for BBC/ABB design of shafts, usually there are also some usable steps on shaft diameter in other OEM's design). If everything goes ok, you can smell campfire and on the wedge there is sharp mark. By comparison of cold mark with mark taken during such test you have very exact DE value. From my  practice, error was around plus minus 0,2 mm onl y. Used wedges, labelled, were stored for future comparison (keep dry !). If DE probe is mounted far away from outer gland (eg eddy current probe inside near pedestal) it is good method to know the difference between reality and control room indications (there is ALWAYS additional error from place of mounting, additionally from probe behaviour). Jan Giedroyc Senior I&C Engineer at Alstom Power Thomas, Fantastic, we both have the same VERY GOOD school ! Joseph F Byrd, Jr Home health and personal care aide at Unpaid Top Contributor

OK, so the wedges are burnt on a reference external to the standards. I was invisioning buring the wedge in the gaps used to calibrate the DXD probes:) inside the standard! My external checks was a scribe lines on external shaft and visial with scale Svetoslav Katerinski Mechanical Engineer at Bright Engineering Ltd Thomas and Jan, this is a really good and useful trick if there is enough space between casing/bearing housing and on the rotor are some roughness which will be printed on the pieces of wood. And the pieces have to be applied on the shaft on the opposite bearing housing of the axial bearing, where should be mounted the probe. Also, the wood  pieces have to be kept pressed on one side in contact with one of the walls (ST casing or bearing housing casing). On that way, we will have a base. Did I understand it correctly as a principle? I guess, if RTG is to slow (impulse type), something like less than 2 rpm, I can take the first sample at the beginning of the acceleration of the rotor. Of course, there will be some thermal influence of the sealing steam on the casing and on the shaft too. What would you say? For some machine probably it will be essential but for some will not so much… Regards Svetoslav Katerinski Mechanical Engineer at Bright Engineering Ltd By the way, we have had a discussion named “Hi Everybody, Can someone throw the light on the effect of vacuum loading on differential expansion in a steam turbine. How does it affect the trip setting?” by Anil Saxena. I am still feeling that this topic is not completely clarified, therefor I will be very happy if someone of you takes part of it too. Thanks Jan Giedroyc Senior I&C Engineer at Alstom Power Svetoslav, u are corect except "zero condition". For ZERO, look at my comment above.  No oil, no turning gear, no gland steam. Position of marking collar at ZERO is taken by mechanical measurement (Johannson gauges, caliper) .Of course later, on barring gear, you can take second measurement : "DE at barring speed". Then on "barring AND vacuum" Never the same ZERO. Barring speed no mater, mark will be made, except lack of camping fire smell :-).  Never trip the turbine by DE, see related disscussion. There is simple influence of vacuum on LP casing: casing will be smaller, sucking gland body inside, means DE indication will grow in PLUS direction (shaft long). Later, with increasing speed, shaft will go MINUS (shorter,by centrifugal forces, esp. for reaction turbines). Example 250 MW reaction turbine: vacuum suction is around 2 mm at the gland, and shortening of shaft is similar, sometimes LP DE is very small at full load. Alert and danger settings of LP DE (NO TRIP VALUES) are given by turbine designers, taking into consideration  particular turbine design and much more parameters, using very sofisticated software. Res ults are precisely calulated for DE transducer mounting location. Sorry, it is not task of power plant people to decide about DE alarm values. For LP, it is not simple thermal calculation of DE, transducer location is always far from ideal and some compromise is a must. Around 45 years ago Philips Germany decided to make DE transducer (as part of his TSI) suitable to be mounted INSIDE LP casing, in critical points. Uses outer space technology and sofisticated design, is still in production (look at former EPRO, now EMERSON TSI). Maybe mounted in some power plants inside LP.... To make LP DE problem even more complicated, remember, that for HP-thrust bearing- IP-LP configuration, IP DE has direct influence on LP DE :-))). That's all, folks !

Joseph F Byrd, Jr Home health and personal care aide at Unpaid Top Contributor

Jan, I wanted to comment on a couple points you made. I agree that compromise in probe location can be a problem. the GE unit I recall was an HP/IP and DFLP, thrust at #1, but the DXD was at the LP/Gen coupling, thus I refered to it as a combination DXD and RXD. the alarm values were as if it was just for the HP/IP so during HOT restarts, the LP rotor expainsions were masking a real rotor short and consitantantly wiping packing teeth. the solution of moving the short alarm made an interesting operating paramater that I actually liked. when the unit was cold, they were in short alarm. Only after the unit at been completly prewarmed (heat soak on TG) would the alarm clear. You also mentioned a DE mounted in the LP being designed as if for outer space use! I know I have lost a few brain cells over the years, but I seem to recall setting a Lynn GE mounted in the LP and it was the same large 4 coil device that GE used in the standards. I guess there might have been some epoxy around the coils instead of just wrapped glass cloth. Thomas Galambvari Sr. Commissioning Engineer at Alstom Power On the vacuum problem, I have seen large hoods under high vacuum affect the reading towards rotor short. You can usually see this on a trend as soon as they start to pull. Again, you can compare with the wedge. The most important thing I can say on taking the external wedge measurement is that you act on a fixed part of the gland or bearing and reference something fixed on the rotor. The key here is to take the wedge and simultaneously read the DE monitor under ANY turbine condition. These need to match at any given time within reason or the measurement is wrong. Get your zero reference as best as the machine condition allows and consider the variables... this is what engineers do! Lol. Brainstorming... if there is no recess or ridge on the rotor... if you found the fixed point you wanted to use and you made a mark or put an indicator on the shaft (like a temporary keyphasor), you could take a wedge or something similar with a mechanical scale attached (special tools!!) and use a strobe to dial in the shaft at speed This won't be nearly as accurate as a burn but it could make a good rough measurement. You could also mark the alarm and trip  points on the wedge in advance for a quick eye on the situation. Just an idea... Jan Giedroyc Senior I&C Engineer at Alstom Power Joseph, You brain is in perfect condition and you are our Guru ! I'm not familiar with GE measurement solutions and turbines configuration, here in continental Europe only Philips designs DE sensor PR6418 working up to 350 C with 40mm range and 1,5 % error at 300C. That time it was idea to monitor axial clearances between moving and stationary blades inside LP blade carriers. I can imagine epoxy sealed probe up to 100-150 C, means can be mounted near or after last LP stage only. But idea of mounting sensor inside of LP blading was never popular and usually we must rely on external sensors. In your example, probably additional partial DE sensor will solve the problem, if I understand your configuration. Tom, we like possibility of simple checking of electronic devices functionality, but maybe we are old fashioned? We fly by wire and nothing happens... Joseph F Byrd, Jr Home health and personal care aide at Unpaid

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One last comment (maybe). I think the concept of calibration of a device, and including an external validation has  been a "trick of the craft' that was not full y documented and carried on with new generations of turbine supervisory instrumentation TDs Thomas Galambvari Sr. Commissioning Engineer at Alstom Power I would say you hit the nail right on the head... Kerim Kenan KİREMİT

2x615MW Operation Engineer / Eren Enerji Elektrik Üretim A.Ş. Cause of increase in LP Turbine differential expansion are listed below. 1. Low vacuum at high loads 2. High reheat steam temperature 3. High main steam flow 4. High gland steam temperature Svetoslav Katerinski Mechanical Engineer at Bright Engineering Ltd Thank you Jan. Thank you Thomas. Of course Zero position is defined at conditions as you said and it will be used as base pattern to compare later taken samples with. Generally, according to me, everyone monitoring system on DE has to be set exactly on that way on zero position to get real readings on the DE during changeable operation (normal or abnormal). Jan, I would like to ask you about several points from your posts which are not completely clear to me? Probably,  because of my English, I don‟t understand what you mean with your question to Sendhil „‟ Are all ZERO mm , plus minus 1mm ? “ If you speak about deviation, plus/minus 1mm is too much to my mind for such measurement. I guess, you mean +/- 0.1mm. And one more question, please. What does it mean “: vacuum suction is around 2 mm at the gland, and shortening of shaft is similar” ? Excuse me Jan, I could not find out the related discussion about “Never trip ST by DE” This statement makes me t o think really hard about it and I will not come to rest until I get it figured out completely. Why I don‟t have to trip the ST by DE? At the moment I don‟t have the answer on it. Only what I think about, which could be the answer is that immediately, when the SV is closed the rotor will be cooled down faster than the casing by the decreasing temperature in the condenser and this will increase the DE between rotor/casing. . But, in such case, the vacuum  breaker has to be opened. Here, have to mention, that I haven‟t had experience with measurement of differential thermal expansion. Such measurement was not implemented by the manufacture of the STs, which machines I have been doing commissioning on. That‟s way it is so important and interesting to me to clar ify out all aspects of the topic. Jan and Thomas, as I see you are very experienced in that field. So, I would be very thankful to you if you explain me a little bit more about it. Thanks Joseph F Byrd, Jr Home health and personal care aide at Unpaid Top Contributor Jan, Thomas, please excuse for for jumping back in given Svetoslav ask you directly. It seems as if DE questions has been asked multiple times in a couple of the groups for Steam Turbines. In several of the other threads, I have linked a write up I provided to a customer to explain the expansions and the unique way GE LSTG scaled the values. this may be of intrest so I'll post the link here.

http://www.slideshare.net/slideshow/embed_code/8284819 the two unit discriptions in above ARE NOT the original posters unit the question of why GE does not provide a DE trip is mentioned in the above. Please correct me if not stated fully,  but the phylosiphy was in the case of rotor l ong alarm, operator actions were the cause and it should be operator actions to correct. in general going long was do to rapid of loading/firing and thus should be held or reversed to allow long DE to normalize. Tripping a long rotor will make it longer by relaxing the centipedal forces that made it  bigger diameter and pulled in rotor lenght AND by the heat added to the rotor by spinning in bott led up casing  pressure (no cooling steam flow) Jan Giedroyc Senior I&C Engineer at Alstom Power Svetoslav, my English is not native, sorry. Will try to explain. Tom comment/corrections welcomed. Joseph, thank you for help, I know your Slideshare paper. ZERO: if you run and completely cool the turbine periodically (for example for overhaul), theoretically all absolute and relative elongation should be zero at the same ambient temperature. It never happens. If it is within plus minus 1mm, it is perfect. If you have enough long experience with turbine, probably you can sleep well also with +- 2mm deviation. Exactness: typical eddy-current sensor with 12 mm range and 5% class, if not specially calibrated can indicate with +- 0,6 mm error. Plus error from non-standard shaft material plus error from temperature influence on probe itself and on shaft material behaviour. Any human error (mounting position and holder design, zero, calibration,  programming) not included ! Don‟t overestimate absolute value, for TSI (esp. for vibr ation) analyse of trends is much more important. Shaft shortening by centrifugal forces: shaft at 3000rpm has bigger diameter and is shorter than at 0 rpm, say 2,3 mm. Suction by vacuum: try to breathe from empty plastic bottle. The walls will collapse. The same for LP turbine casing, sometimes collapses. For 6 sq. meters casing wall and one bar pressure difference, there is 60 Tons of force on small portion of casing. Abrams tank fully equipped. Such a force can shorten axially the casing (say 1.9mm), disturbing DE measurement. Taken both into consideration, at 3000rpm and full vacuum, there is only (2,3-1,9=0,4)mm additional error in minus direction.  Never trip by DE: look at Joseph answer, instead of trip, t urbine operator action is required, to bri ng turbine back to normal. For some abnormal turbine status, trip can be fatal. There is too much possible combinations to explain here, usually no problem for operators with long experience. Svetoslav Katerinski Mechanical Engineer at Bright Engineering Ltd First of all thank you Jan. I meant my English, not yours. My native language is far way of all West European languages. Jan, Unfortunately 80% of your explanation is not acceptable. If you afford me, I would give you an example with Bently Nevada 25mm transducer with linear range of 12.7mm (suitable to measure the range of DE) has deviation less than plus/minus 0.3 mm.  Next, excuse me but any shaft at Zero speed has “banana shape” and exactly by means of the centrifugal force it takes the normal straight line, which means exactly the opposite on your statement.  Next, 1 bar is 10 197 kgf/m2 (or 1.0197 kgf/cm2 or 0.010197 kg f/mm2) and it influences equally on every one square meter (or cm2 or mm2) of the casing surface. The values of the concentrated stresses in welded elements and angles are result of this load and they are known of the designer. So, I don‟t worry about that . Why you made a sum of 60 tons, even if you speak about surface of 6 m2, I don‟t know!!! But it is for sure is wrong. Excuse me again please, if I didn‟t catch something. For sure you are more experienced than me and I am very thankful to you for your shared knowledge.

With deep respect, Svetoslav Joseph F Byrd, Jr Home health and personal care aide at Unpaid Top Contributor "any shaft at Zero speed has “banana shape” and exactly by means of the centrifugal force it takes the normal straight line" the shape is cantenary. excluding any deformation resulting from vibration, the rotor's cantenary remains at speed. the shorting of a rotating shaft is the poisson's ratio as a result of centrifugal force with speed Jan Giedroyc Senior I&C Engineer at Alstom Power Svetoslav, more sense of humor, please. OK, I'm 80% wrong. Maybe I'm 90% wrong (easier calculation), 3 topics, means 30% per topics. Lets try to recalculate. Shortening of shaft: answered by Joseph. Minus 30%, 60% remains. Bently Nevada is better than average, it really has +-0,31 mm deviation from best straigh line. But it is not total error, and at full temperature range deviation is +- 0,92 mm. Taken from Bently Specifications and Ordering Information, Part Number 163236-01,Rev. C (04/07), page 3 from 28. My example (example !) was for 12 mm sensor (no producer given, hopfully) with total +-5% error. 12mm x 0,05=0,6 mm. Minus 30%, remains 30 %. The force acting from outside on every square meter of LP casing at full vacuum (about minus 1 bar) is ca 10 000 kgf or 10 tons of force. For 6 square meter, it is 6x10 = 60 tons of force.What is wrong ? OK, the force is wrong, but  by nature laws. The question was, what is a reason of vacuum influence on LP DE indication. It is the reason: deformation of LP outer casing by external air pressure. Minus 30%, remains 0%. From above calculation, I'm 0% wrong. But it is impossible. I must be wrong, OK, I'm always 10 % wrong :-). Don't excuse, agree ? Joseph F Byrd, Jr Home health and personal care aide at Unpaid Top Contributor history of TSI calibration--the 50's through 90's vintage of GE TSI came with a 0-10VDC anolog panel meter that was supose to be used for calibration. substituting a 4 1/2 digit multimeter just causes unnnescearry headaches and delay in calibration Thomas Galambvari Sr. Commissioning Engineer at Alstom Power And who calibrated the panel meter??? Lol. I think in the world of TSI, it's safe to say that a few mils, give or take, here or there, will not wreak havoc on a machine.

When a machine thrust trip is set at 32 mils and it thrusts 31 mils, there is still something wrong. When a vibration trip is set at 8 mils and the unit runs at 7.9 mils, there is still something wrong. You get my point... These measurements were never intended to be, nor are they, rocket science. External influences on the mounting and probes caused by temperature, relative vibration and other anomalies will always induce small errors and nonideal behavior. The probes themselves sometimes expand when they get too hot... I have (almost) never seen a 200 mv/mil  proximeter actually produce that output... Generally speaking, we hope to protect a machine from it's operators or itself. In this day and age where control rooms for 10 machines are operated by 1 or 2 guys, we only hope that when something goes terribly wrong, we close a stop valve. The rest will go down the way it's going to go down and that's why they need guys like us to come in and fix it. Jan Giedroyc Senior I&C Engineer at Alstom Power Tom You makes me happy, thank you ! "Protection of machine from it's operators.." It is so simple and so true. Will use in my presentations about protection systems :-) John Mitchell Owner, Power Prof, LLC Top Contributor Thank you, all. I've appreciated rereading this posting over the last hour-or-so. Well done and I'd like to summarize some of your conclusions. 1) Rotors get longer with heat and expand axially away from the thrust bearing. 2) Rotors get shorter with speed since they get larger from centrifugal forces. 3) The thrust bearing may move axially or not, depending on unit configuration. 4) Low pressure hoods do not expand substantially because of low temperatures. 5) A measure of differential expansion (DE) is good to limit axial rub damage potential. 6) Condenser vacuum pulls the ends of the hood in, based on the forces and stiffness. 7) If the DE probes are mounted outside the hood, vacuum deflection is not seen. 8) If the DE probes are mounted inside the hood, the RELATIVE GROWTH is better seen. 9) All instrumentation is subject to limits of range and accuracy. 10) Designers need to understand and incorporate laws of physics. 11) Field people need to do things correctly and ask when they don't understand. 12) We all can learn new things if we are willing to listen. Svetoslav and Jan, I appreciate your use of my native English and I'm sure that Joseph agrees. Thomas has not indicated his native language. Thomas Galambvari Sr. Commissioning Engineer at Alstom Power John, my native language is the same as yours. Born and raised in the USA. Svetoslav Katerinski Mechanical Engineer at Bright Engineering Ltd

Hi Jan, as I see you didn‟t notice, that my attention here is concentrated on the principle, because when the base is clear, then the conclusions are easier and almost 99% correct. Also what I wanted to pay attention here is, that very often are given directly values (as you make it), like they are always correct for everyone machine. I hope, all of you will agreed that this is very dangerous. Thanks to the Joseph, who remind me about Poisson for elasticity of the materials, could you please gave us an example of calculations that will confirm your result of shorter rot or with 2.3mm? And please, don‟t forget to mention what would able to be the material of such shaft. And the last about that point. I don‟t believe that you or someone else of the group has not heard about shaft catenary (probably it is known with different terminology by you) in stationary position. By the way such monitoring is also possible to be implemented in Bently Nevada. And  because I am not so familiar with this monitor ing system like you, I would not come to details which are very well given in the specifications (as you very well quoting as well) I just would like to ask you, if you have had monitoring on the shaft catenary, why this monitoring is active in range let say RTG speed to 600 rpm (depend on the machine shaft) ? And the last about measurements and their deviations. Do you operate in their whole temperature range? Or how many voltages is the deviation from the best straight line at the normal operation ambient temperature for the given example? Finally, I would like to ask you. If we have one pipe 1 meter long with differential pressure in/out of 1 bar and if we have the same pipe 1 km long in the same conditions, does it mean that the second pipe walls are 1000 times more loaded than the first one? One think I will agree with you, if you insist to be 90% wrong. OK. John Mitchell Owner, Power Prof, LLC Top Contributor Svetoslav, I'm not sure what you're asking about the "shaft catenary." All turbine and generator rotors have a sagging shape that looks a catenary. They have a mass distribution, specific material properties like elasticity, "moments of inertia," and so forth. Therefore, one can calculate the precise elevations at every location and the axial effects, too. I believe that the shaft sagging doesn't change substantially from turning gear speed to full speed. The oil film, the thermal growth and vacuum / pressure deflections of the stationary elements, etc., all impact the relative centerlines of the rotor and stationary components, radially. The Poisson effect and thermal expansion / pressure effects impact the relative axial positions of the rotor and stationary components. Instrumentation, like Bently Nevada probes, can observe the radial and axial positions and, therefore, changes like differential expansions. Regarding your question about "the second pipe walls [being] 1000 times more loaded than the first one," naturally they have the same load but 1000 times more deflection. Svetoslav Katerinski Mechanical Engineer at Bright Engineering Ltd You are right John, I just would add that it is not recommended rotor long stay in horizontal stationary condition. That‟s why the rotors are stored always in vertical position. About the question for the second pipe walls loading, it is a rhetoric question at Jan, whose calculations are “F or 6 square meter, it is 6x10 = 60 tons of force” (from pressure 1 bar!!!!!!) . John Mitchell Owner, Power Prof, LLC Top Contributor I agree that storing a rotor vertically has some marginal advantage. However, horizontal storage is common without significant issues. Rotor materials are unlikely to permanently and plastically yield due to sagging at ambient conditions. If you were anxious, you could temporarily block the rotor in mid-span. As a worst-case, one may need to rebalance at the end of the storage time.

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While this subject is maybe appropriate, it does not fit nicely into a discussion about "the reason of High LPDE" (Low Pressure Differential Expansion). 7 days ago

we have two lp turbine during trip the differential expansion of LP 2 reach to the alarm point i do not know why but during the normal operation the alarm d id not appeared Comments 22 comments Jump to most recent comment

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Ahmed Ahmed Mohamed Gebril Instrumentation and Control Senior Engineer

I don't know Either I'm sorry Bob Taylor Sr. Mechanical Engineering Technologist at American Electric Power If your LP rotor is welded disc construction similar to an Alstom LP rotor the centrifugal force will shorten the rotor as it is brought to speed and lengthen the rotor following a trip. If the turbine rotor is fixed at the front (non drive end) and DE is measured at the generator end, once the rotors reach operating temperature the expansion following a trip could put you into alarm. John Mitchell Owner, Power Prof, LLC I assume you went rotor long, not short, probably from higher hood temperatures and windage. The differential expansion is mostly a result of temperature of the rotating elements, but stationary temperatures are also involved. As Bob Taylor said, rotors also get shorter at speed and longer on shutdown (material comes from somewhere as the ID expands). David Barrow Utility Operations Technician at Teekay Corporation Hello gents. Thanks for the discussion. Bob, you mentioned welded-disc rotors specifically. Are they more prone to this centrifugal effect than other rotors? Atia Mohamed mechanical engineer at abo qir power plant dear mr john thank you for your replay but hood temperature and windage normal during the trip but i think there is diffrent between the desgin calculation and actual condition Atia Mohamed mechanical engineer at abo qir power plant Dear mr bob thanks alot but our rotor not welded type its forged type John Mitchell Owner, Power Prof, LLC A single piece rotor (monoblock or welded) will get longer and shorter - more than a built-up or stacked rotor. David Barrow Utility Operations Technician at Teekay Corporation

Thanks for the clarification, John. Figured it was the Poisson effect on a single-piece rotor. Mohamed Khalil FSE at Elliott Company Dear Atia; Would you please make it more clear. It'll help if you send a schematic for the two turbines showing how they're coupled, Thrust Bearings locations, the Fixed & Flex leg orintation relative to casing DE. My contact mail is [email protected] Atia Mohamed mechanical engineer at abo qir power plant Dear mr mohamed let me check because this drawing for MHI company if possible i will send Mohamed Khalil FSE at Elliott Company Dear Atia;  No need for a cross sectional dwng. A P&I or even a hand sketch will be good enough. Atia Mohamed mechanical engineer at abo qir power plant ok i will send to you Atia Mohamed mechanical engineer at abo qir power plant thrust bearing location in the HP pedestal beganing of hp turbine before bearing no 1 and fixed point is lp1 Mohamed Khalil FSE at Elliott Company Dear Atia; Your description is not clear. Better you provide either drawings or sketches. Joseph F Byrd, Jr Home health and personal care aide at Unpaid Sounds similair to GE's LSTG G8 configuration. where are the expansion detectors located? I would expect the DXD to be at "A" coupling and a RXD at "C" coupling. (and the sliding front standard to have a SXD)? Mohamed Khalil FSE at Elliott Company Dear Atia; What's the cooling rate recommended & your unit power capacity? Did you follow during shutting down the unit or was it an emergency trip? Amitabh Srivastava Adviser at Jindal Power Limited One has to analyse if this problem was there right from commissioning of this Unit or it has started after some incident / event e.g. Capital Overhauling / repair etc of Steam Turbine, or so. Any way following are to be checked: 1. Calibration of the DE pickup;

2. Correctness of mounting of DE Pickup. 3. Shifting of the Thrust Bearing which is the FIXED/ ANCHOR POINT for rotor system. This should result in change of DE values of all the rotors. For this installation / location of Thrust Bearing is to be checked & corrected. The operating engineer has to analysed the problem in totality. Ken Marien Co ordinator- Adaptive Skiing at Wachusett Mountain Ski Area could it be the LP 1 tripped first causing an overpressure situation in LP2? Chester Auble Field Manager at Wood Group What was the cause of the trip? Jan Giedroyc Senior I&C Engineer at Alstom Power Generally speaking, DE of LP part is a complex function, not as simple as for HP and IP parts. First of all, influence of IP DE (assuming thrust bearing is between HP-IP and LP is between IP and GEN). Second, shortening of rotor for reaction type blading (centrifugal force:-hollowed, as for Alstom, or not, doesn' matter). Third, influence of vacuum suction on outer casing-can shift sensor mounting, say, 2,5 mm ! No known TSI system can addapt to above conditions: in fact alarm for LP DE should be function of speed, vacuum and measured DE. Kerim Kenan KİREMİT

2x615MW Operation Engineer / Eren Enerji Elektrik Üretim A.Ş. ıt ıs too normal. because during trip there is a little load at the system. after trip lp turbines relax and then alarm comes. Amitabh Srivastava Adviser at Jindal Power Limited It appears that the differentioal expansion of LPT-1 & LPT-2 remains positive and DE of LPT-2 remains more than LPT-1. After tripping, the length of LP Rotor increases so the differential expansion values of LPT rapidly increase in the positive direction. This is reason why differential expansion of LPT-2 reaches the alarm value. To overcome this problem you may check the caliberation of LPT DE Pickup and ensure its proper mounting at correct location.