As due to high exhaust temperature seismic vibration sensor in frame 6B gas turbine is failing frequently in our plant, we have also asked to other neighbour industry for same they are also facing an issue can anyone suggest solution for it.?

Installed sensor's temperature range : 375 degc.

 

Well if you say that high exhaust temp cause this issue
Then you should review why you got high temperature

 

Well if you say that high exhaust temp cause this issue
Then you should review why you got high temperature

High temprature comes due to combustion of fuel, and the installed matrix 5485c sensor is also hight temperature vibration sensor its rating is 375 degc, but still it fails so we are looking for other alternative or solution in existing design.?

 

You should look at air circulation around the #2 bearing--which is inside the load tunnel. Either there's insufficient cooling air flowing in the load tunnel, or there is an exhaust leak in the load tunnel. It may actually be a combination of the two, less than normal cooling air flow and an exhaust leak.

You can mount a T/C on or near the area where the vibration pick-ups are mounted, and run a cable outside the Load Compartment and use a multi-meter with T/C input capability and monitor the temperature in the area to see if it's higher than the rating of the sensor.

375 deg C (above 700 deg F) is hot, and we don't know why you are so certain that it's high temperature causing the failures.

Getting good air flow into and out of the load tunnel is challenging. We don't know how old (or young) the unit is at your site or what the condition of the Load Compartment Vent Fan and motor is. Some Load Compartment cooling arrangements have dampers which can be improperly set up and reduce air flow, and some Load Compartment Vent Fans have dampers to prevent leakage when a fire is detected (to keep fire extinguishing agent in the compartment).

MOST GE-design Frame 6B heavy duty gas turbines have two (redundant) vibration pick-ups on the #2 bearing. If the unit at your site has two sensors is the same sensor always failing? That would be odd, because the two sensors are mounted very close to each other (mm). So it would be hard to understand why the same sensor is always failing--if there are two sensors....

 

You should look at air circulation around the #2 bearing--which is inside the load tunnel. Either there's insufficient cooling air flowing in the load tunnel, or there is an exhaust leak in the load tunnel. It may actually be a combination of the two, less than normal cooling air flow and an exhaust leak.

You can mount a T/C on or near the area where the vibration pick-ups are mounted, and run a cable outside the Load Compartment and use a multi-meter with T/C input capability and monitor the temperature in the area to see if it's higher than the rating of the sensor.

375 deg C (above 700 deg F) is hot, and we don't know why you are so certain that it's high temperature causing the failures.

Getting good air flow into and out of the load tunnel is challenging. We don't know how old (or young) the unit is at your site or what the condition of the Load Compartment Vent Fan and motor is. Some Load Compartment cooling arrangements have dampers which can be improperly set up and reduce air flow, and some Load Compartment Vent Fans have dampers to prevent leakage when a fire is detected (to keep fire extinguishing agent in the compartment).

MOST GE-design Frame 6B heavy duty gas turbines have two (redundant) vibration pick-ups on the #2 bearing. If the unit at your site has two sensors is the same sensor always failing? That would be odd, because the two sensors are mounted very close to each other (mm). So it would be hard to understand why the same sensor is always failing--if there are two sensors....

We have load tunnel's 2 number of venting fan arrangement with damper mechanism, our unit normally runs on part load around 22 MW, i dont doubt on the cooling air flow as venting fans are working fine, but we have doubt of exhaust frame flue gas leakages we have told same to maintenance team but they replied that complete 100% sealing of exhaust Compartment is not possible and as sealing ages its prone to leak some flue gases and temperature around bearing #2 will rise.
Our unit is relatively young commissioned in 2016, and timely maintenance also done, we also have installation of 2 sensors(bb4 and bb5) on bearing #2 but in our GT-04 BB5 sensor failed 2nd time in less than a year, and BB4 also failed one time, in GT-01, BB4 failed once and BB5 failed once.
We are planning for following 2 solutions please suggest me if we should go ahead or not?
1) we will run a air tube inside load tunnel from external source (plant air) as we have doubt of improper cooling over there.
2) also, we are planning to install integral type(in which extension cable is integrated with sensor) of sensor instead of current extension cable and sensor separate version.
Matrix : 5485c , sensitivity : 150

 

Your maintenance team is correct; there is, effectively, no way to seal the exhaust area around the #2 bearing and even around the Load Compartment edge of the Load Tunnel (where the flex seals are located) from any and all leakage. There is always going to be some leakage--BUT the ventilation system is designed to handle normal leakage, which should be pretty minimal.

There is insulation around the inside of the Load Tunnel and I have seen several machines where the insulation has blown out and that causes the normal leakage rate to become excessive, as well as adding to radiant heat in the area. Also, the flex seals on the Load Compartment edge of the Load Tunnel have insulation behind them and that can deteriorate or be blown out and lead to excessive leakage into the Load Compartment.

If you want to counter the maintenance team, take some temperature measurements as suggested in the area of the #2 Bearing. It's pretty simple--a pretty accurate T/C can be made by stripping some insulation from the two conductors of any thermcouple extension cable, twisting them together with a couple of pairs of pliers, and then placing the twisted conductors on an anvil or vise and using a ball peen hammer to smash them together--a single firm strike should do the trick! You can test the home-made T/C by connecting it to a multimeter with T/C capability or a T/C monitor (most instrument shops have one or both of these) and reading ambient temperature, and then placing the T/C on hot areas and cold areas. It should be pretty accurate; but, you don't need perfect accuracy, just a means of getting a good approximation of the temperature in the area you're trying to measure. Have someone record temperatures say once per hour for a couple of days, or once every two hours for a couple of days.

Oh, I can hear it now, "The machine is running and we can't access the #2 bearing area!" So, get everything ready in advance and if there's a forced outage or an opportunity to jump in the Load Tunnel--you are ready to go without needing too much time for preparation.

You can also ask the maintenance team to go inside the exhaust compartment and look for the presence of insulation where it should be around the Load Tunnel and the aft side of the exhaust area. A quick way to check for any leaks and missing insulation is to have a strong lamp/light in the Load Compartment pointed at the aft wall of the exhaust compartment and the #2 Bearing. If there is missing insulation and/or the flex seals have shifted it will be possible to see light coming through the cracks where there shouldn't be any visible light.

The maintenance team ALWAYS assumes the mechanical equipment is perfectly assembled and in excellent condition and that the problem is ALWAYS the turbine control system (or in this case the vibration sensors in the #2 Bearing area). But, often, when forced to check the equipment they can be seen to be scrambling to fix unexpected problems--MECHANICAL problems--and the problem often goes away without any acknowledgement from the maintenance team that there had been a problem or that the problem was minor and couldn't have caused the symptoms being troubleshot.

The maintenance team is part of the plant team--and as such, they need to step up and do their part and check things when asked. Have them show you the drawings from the Maintenance Manuals where the seals and insulation would be present in the Load Tunnel and the aft wall of the exhaust compartment and they will quickly realize, "Oh, there could be a problem there...." Operations and Supervision and Management should be holding all the teams (Operations, Instrumentation & Controls, and Maintenance) accountable and asking them to participate in the troubleshooting. Again, the simple act of having them show other members of the plant team what the seals look like in the exhaust compartment and Load Tunnel and Load Compartment can very often be enlightening to the maintenance team--and to everyone participating in the troubleshooting (especially the Management team...).

I think you don't really understand how the air is circulated from the Load Compartment Ventilation fans into and through the Load Tunnel, especially when the unit is running. AND, most people don't understand how high the temperatures can be in the #2 Bearing area in the Load Tunnel when the unit shuts down and the Load Compartment Ventilation fan(s) shut down. It gets VERY hot when there's little or no air circulation in the Load Compartment/Load Tunnel. VERY HOT. Unless there have been some MAJOR changes to air flow from the Load Compartment Ventilation fans on GE-design Frame 6B heavy duty gas turbines (and THAT'S always a possibility when GE Belfort is involved...!) there really isn't, historically, any air directed into the Load Tunnel when the unit is running. It's pretty much the rotation of the shaft that helps "rotate" the air in the Load Tunnel and temperature differentials that help to move air into and around and out of the Load Tunnel. I have even seen some 6B machines that have expanded metal coverings over the entrance to the Load Tunnel--and that actually slightly inhibits air flow into and out of the Load Tunnel. AND, if the flex seals of the aft wall of the exhaust area are leaking into the Load Compartment then that hot air is being circulated in the Load Compartment before it's forced out of the Load Compartment.

But, if you want to get more help from the maintenance team and more buy-in from Operations, Supervision and Management you need to have data--actionable data. And the best way to get that is by measuring temperatures in the #2 Bearing area (and don't forget to also measure the temperatures as the machine coasts down to zero speed, and while it's on Cooldown--especially after the Load Compartment Ventilation fans shut down). Actionable data is hard to argue against; conjecture and ideas and estimations are easy to argue against. Get some actionable data, analyze it and then make your presentation to the other members of the plant team and you will seem some action.

OR, you may find that there is an inherent design flaw with the air circulation in the #2 Bearing area and Load Tunnel. In that case, as the owner/operator of the machine you can work on solution(s) with other members of the plant team to reduce temperatures in the area. You can even ask GE to participate in the resolution IF YOU HAVE ACTIONABLE DATA TO SHOW THEM THE TEMPERATURES IN THE AREA ARE NOT AS LOW AS THEY THINK THEY ARE!

Good luck; plan your work and work your plan. A failure to plan is a plan to fail--especially when you are troubleshooting something like this. Getting the data isn't hard; it's getting the opportunity to install the T/Cs to measure the temperature. Install several, and have the leads easily available outside the Load Compartment so that readings can be taken and recorded on a regular basis--when the machine is running, and when it's being shut down and while it's on Cooldown. You don't need fancy T/Cs with stainless steel sheaths; you just need a few hose clamps to route the leads and position the tips of the home-made T/Cs in a more or less stationary position. If you see large changes when recording the data over time when the the unit is operating more or less stably, then have someone check the T/C tips to make sure they haven't moved or aren't laying against hot metal. (I would even suggest having one or two home-made T/Cs secured to each of the vibration sensors to get the temperatures of them! and not just the area around them.)

Make a plan. Follow the plan. Analyze the data. Present the information to the other members of the Plant Team. And take appropriate action.

(Personally, I don't think changing the type of cabling is going to make that much of a difference. You can always ask the supplier of the sensors for help with the problem--but you gotta have actionable data to show them. Thoughts and estimates and feelings are NO substitute for actionable data.)

Let us know how this goes!

 

SOME GE-design heavy duty gas turbines (non F-class machines with hot end drive) have multiple T/Cs installed in the Load Tunnel area to continuously measure temperature in the area. Your machine may or may not have them installed, but if they are you need to look at the proper P&ID to see if they are installed on your machine, and if they are, you can monitor and trend them very easily.