We have recently upgraded our contro system from speedtronic Mark-II to Mark-VIe control system. The turbine is Frame-III and LP turbine and HP turbine are not physically linked.

Now we are facing problem that during startup as LP turbine failed to cut-in at max of 50% of NHP. Unit tripped on LP shaft locked trip alarm. This setting can be changed and increased up to 52-54%, but i would like know know the hazards involve and effect on turbine specially HP rotor.

Second i would like to know why LP turbine is unable to cut-in at desired state? For last 1 year for every start-up it rotate under 48% speed of NHP.

 

Owais,

How long has this problem been occurring? Since commissioning of the new contro system? Some time after the new contro system was commissioned? You stated, "... For last 1 year for every start-up
it rotate under 48% speed of NHP." Was this before, or after, the contro system upgrade.?.?.? It makes a great deal of difference to the responses you will receive. The responses I've provided below presume the LP shaft broke away from zero speed slightly below 50% with the older Speedtronic turbine control system, and that since the darned Mark VIe was installed it's not breaking away until slightly after 50% speed.

So, you upgraded your contro system--but what, if any, upgrades did you do to the turbine and auxiliaries? Lots of older Frame 3s did not have variable second-stage nozzles, and also did not have a hydraulic pump for fuel- and nozzle actuation.

Does the Frame 3 have a variable second stage nozzle? If so, is it possible that the calibration of the LVDTs is different from before the contro system upgrade, or that if variable second stage nozzle control was added during the contro system upgrade is it possible that the LVDT calibration is not accurate or correct?

Was there any work done on the LP turbine shaft/bearings during the contro system upgrade outage? Perhaps, new bearings or a new LP rotor was installed? How about the driven device (I presume the LP turbine drives a compressor of some sort)--was anything done to the driven device's shaft and/or bearings during the contro system upgrade outage?

Have you physically rotated the LP shaft somehow to be sure it's not binding or has excessive friction? Are you certain of the oil flow to the LP and driven device bearings during starting?

Was the compressor control (presuming this is a compressor drive unit) incorporated into the Mark VIe? If so, are you certain the compressor valve sequencing logic is working correctly?

What does the contro system provider say about the issue? Are they concerned about the later-than-expected LP shaft breakaway?

You see, something else has likely changed with respect to mechanical components (such as the second stage nozzles, or bearings, etc.)--or, it could be that there was so much fuel going into the unit with the older contro system that the LP shaft was breaking away and accelerating sooner. This would be likely indicated by a slower acceleration of the HP shaft since that darned Mark VIe was installed--because EVERYTHING ALWAYS WORKED PERFECTLY before the Mark VIe was installed. (A former colleague--and I--heard that a LOT from many (most) Customers, and subsequently it was almost always learned that there was little if any change in unit operation (it was just a perception that it was operating differently), or that it had never really worked properly before but was operating more correctly since the darned Mark VIe was installed.)

Many times new bearings, along with a bearing/casing alignment, often results in slightly different starting characteristics until everything gets "run in" for a few months.

There are lots of questions above to be answered. We're not there, and you didn't provide very much information in your original post--so, if you want more help, you're going to need to provide more information, which means even if you don't think the questions are relevant since we're not there to see what is happening and what's been done or not done please provide answers to all the questions.

As far as the "...hazards involved..." with changing the LP acceleration/breakaway logic, you should discuss this with the contro system provider and/or the turbine OEM. It doesn't seem like such a small change would make much of a difference (and I'm guessing you've already forced the breakaway logic to see when the shaft actually does breakaway.?.?.? to know how much the value needs to be increase by), but it may be masking another problem. And, you may be tempted to keep increasing the value over time as the root cause goes unresolved--which may definitely result in damage, most likely not to the HP shaft but to the LP turbine section (nozzles; buckets).

Again, please be more precise about when the problem started occurring, and provide the answers to all the questions so that we can provide the best answers going forward.

 

Has anything changed that may be affecting the load on the LP compressor? I wouldn't see any problem in increasing the setting to above 50% to see where the LP will break away.