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BB5 Vibration Probes Problems
Changing location of gas turbine bearing probes away from load gear, since they frequently are not working correctly

hello..this is Himasnhu Khoriwal from Indian Oil working as Electrical Engineer.

We are having 3 gas turbines which are single shaft and of 30 MW rated capacity under standard conditions. now the problem is that BB5 and BB4 vibration probes are becoming out of order frequently in all the GTs.

the reason behind this according to me may be that since these vibration probes are located in the tunnel between load gear and turbine which has higher temperature as compared to the other probes areas. As measured through temperature gun, temp of tunnel was found to be nearly 250degc in all the three GTs.

Kindly suggest whether any relocation (like outside the tunnel or outside the compartment) for the same probes can be done, so that they are not exposed to such high temperature.

2 out of 2 members thought this post was helpful...

Himasnhu Khoriwal,

It's simply not feasible to monitor #2 bearing vibration from outside the load tunnel.

Your theory about heat is likely correct. There are vibration sensors, and then there are high-temperature vibration sensors. Of course, the vibration sensors cost less than the high-temperature vibration sensors, and it's all too common for sourcing and purchasing personnel to choose the lesser-cost component because they both have virtually identical characteristics, so, why shouldn't the lesser-cost component work just fine?

They just don't understand the application. Yes, you could definitely use the (low temperature) vibration sensors on the #1 bearing, or on the aft generator bearing and save a little money over using the high temperature vibration sensors in those locations. But, the load tunnel and the load compartment are high temperature locations and require high temperature components and high temperature wiring.

And, that's also another frequent cause of failure of devices in high temperature locations such as the load tunnel--the use of wire with the wrong temperature rating, or installed improperly. If the wire insulation melts and allows the conductors to come in contact with each other then strange signals can arise, sometimes intermittently (which is the worst scenario). And, a lot of times, after maintenance outages, the wiring doesn't get reconnected properly, either. And, while this is extremely difficult to believe, mechanics and laborers working on turbines don't always treat the electrical components and wiring and conduit with the most respect (like I said, it's hard to believe--but it does happen; a LOT).

If the vibration sensors ARE the high-temperature version, then it's possible that there is too little air flow in the load tunnel and the temperatures are too high (which can occur). Or, that there is a problem with the wiring. The vibration sensors usually use stainless steel-sheathed cables with special connectors on them, and which have to be routed through conduit and terminated in a junction box. The cables are not like normal, two-conductor cables, there is a shield and and working with the stainless steel sheath isn't always the most pleasant task. And, like the vibration sensors there are cables and high-temperature cables. They look pretty much alike, and have identical characteristics in all other regards--except the temperature rating.

While rare, there have been cases where the hold-down bolts of the sensors came loose. And, the sensors are usually mounted on a plate that is bolted to the bearing cap, so those bolts can also loosen if some kind of thread-locking liquid/gel wasn't applied. Some of the socket-head cap screws used to mount the sensors to the plate have small holes drilled through them through which stainless steel lock-wire can be threaded to prevent loosening. And, the bolts on the mounting plate usually have lock-tabs.

The velocity ("seismic") vibration sensors used on GE-design heavy duty gas turbines have a moving magnet inside a coil (or a moving coil inside a magnet--I can never remember) and when the bearing cap moves because the shaft is vibrating the moving element generates a small voltage which is connected to the Mark VI which is scaled and displayed as vibration. So, you could mount the sensors outside the load tunnel, but they wouldn't be able to measure the vibration.

Hope this helps!

"Feedback is the most important contribution!"(c) here at control.com. If you haven't used the 'Search' feature yet, there are more than 10 years of GE-design heavy duty gas turbine control-related threads, many of which have feedback provided by the original poster indicating whether or not the information provided was helpful--or not. But, it's that feedback that makes the threads here at control.com so much better than a lot of other technical World Wide Web forums, because one can see if the information was helpful, or not. So, if you can take a few minutes to write a post asking for information or help, we think you can take a few minutes to let us know if the information or help was useful, or not. A LOT of people read these threads, not just now, but years later. That's another one of the great things about this forum--that it's searchable and old posts are accessible. So, we like feedback. We want feedback (good and bad). We crave feedback. And the longer you are a reader/contributor here the more you will begin crave feedback.

I guarantee it.

thanks for your informative response. Whether it was useful or not, is going to take little time.

thanks a lot.
respect for your knowledge.

0 out of 1 members thought this post was helpful...

I think it is not like that himangshu...

I am working with frame 5 and frame 3 gas turbine continuously operating since more than 6 yrs. If temp would have been an issue, then i could have faced the same problem. I never faced such an issue. You can talk to bggts....mr majumdar.

Also speak to your inst maint engineer. ensure proper termination of probes. where is your jb location? It should be outside hot & vibrating area. Ask a root cause analysis from your inst hod. I think you problem will be solved.

How many times you faced vib probe failure? You experienced it or you just heard from your seniors?