hello all,

can any one explains to me what the purpose of the false start drain thermocouple sensors (WWTD-1, WWTD-2) in VA17-1 pipe line Frame 9E? In the location, they are very close to each other (1 m between them). So why in the GT start they give different values? And when they trip the unit?

 

hello,

They are there to detect leakage of hot combustion gases when the unit is running to prevent igniting vapors in the gas turbine drains tank(s) which could result in an explosion, fire, death and dismemberment.

As for why they are reading differently, when did this problem start? After a maintenance outage? Were the T/Cs replaced or disturbed during the maintenance outage?

Is one T/C inside the turbine compartment and one outside? Or are both inside or outside the turbine compartment?

Is it possible that one of the T/Cs has hot air blowing on it from a leak in the turbine compartment?

Is it possible that one of the T/Cs is being cooled more than the other when the turbine is running?

Usually, the false start drain valves are air-operated by axial compressor discharge pressure, and they should close during acceleration sometime before reaching FSNL. I don't have a P&ID for a Frame 9E at this writing, but if I recall correctly VA17-1 is the three-way valve that controls purge air flow to the liquid fuel nozzles and has nothing to do with false starts.... And, the outlet of VA17-1 is called the Tell-Tale Leak-off because it indicates a failure of one or more of the Liquid Fuel Purge Check valves allowing liquid fuel to flow backwards through the purge piping and to the gas turbine drains tank. (Refer to the Liquid Fuel Purge P&ID or the Fuel Purge P&ID for your unit for details of the construction of VA17-1 and its purpose.)

I suppose if one of the purge air check valves was leaking when running on gas fuel that it would allow purge air to flow backwards through the purge piping to the gas turbine drains tank. But, that still has nothing to do with the false start drains.

As for when these T/Cs might trip the turbine, well, that would depend on the sequencing running in the turbine control panel of the turbine at your site. These T/Cs are rather "new" to control and protection, mostly added because no one ever bothers to check the Tell-Tale Leak-off or the False Start Drain Valves during turbine operation (because operators aren't required to make rounds and take readings and look for problems) and there has been at least one explosion and fire as a result of a leaking false start drain valve. That's usually when sensors like this and sequencing are added to the turbine control system--when something bad happens.

So, to answer your questions it would be necessary to know precisely where the T/Cs are located on the piping in question (both inside the turbine compartment; both outside the turbine compartment; one inside the turbine, one outside the turbine compartment); if the T/Cs were recently "disturbed" during a maintenance outage; if there might be a leak of hot air blowing directly on one of the T/Cs, or if cooling/ventilation air is somehow being directed more at one T/C than the other; and, what the sequencing/application code running in the Speedtronic turbine control panel at your site does with the inputs from the T/Cs. It would seem that if they are within 1m of each other they would read similarly, but that depends on a lot of factors. I've also seen lots of problems with the re-mounting of T/Cs on piping after maintenance outages.

Hope this helps!

 

Thanks CSA for your reply.

The thermocouples are both inside the turbine compartment and about 1m away, and this pipe drain to atmosphere by 3-way valve at its end. one goes to water side and other goes to oil side (sump tank). at water wash we turn the valve to the water side, and at normal state it's at oil side. The unit usually trips at starting before 2500 rpm. we getting the first alarm "false start drain tc diff high," and then the unit trip by alarm "loss of false start drain temperature sensors ns."

 

AhmedWaleed,

You're missing the main point of troublshooting problems like this:

When did this problem start?

After a maintenance outage, when the thermocouples might have been removed and re-installed, or might have been disturbed by the highly caring, careful and sensitive mechanics working in the turbine compartment?

If this problem just started, do you have any data from earlier operations to say what the differential was when the start-up was normal?

If there is a leak of one or more of the purge check valves, then during start-up the piping is going to heat up, and the piping will not heat up uniformly--the upstream piping is going to heat up faster than the downstream piping. So, this could be an indication of a real problem.

If the trip is caused by a "loss of signal" condition, that could indicate bad wiring/loose terminals between the T/C and the Speedtronic. Has anyone verified the wiring along it's entire length from the T/C to the Speedtronic T/C input terminals?

The usual construction is for the two outlets of the three-way valve to be flowing into open "cups" (funnels) to the two drains tanks. This way, flow and/or pressure can be observed when there is a leak. If there is no open pipe to allow visual/audible indication of a flow, then there should be flow indicators in sight glasses in the piping--small wheels which rotate behind glass to allow visual flow indication.

The point is: If this problem just recently started, what changed just prior to this problem which might be the cause of the problem? If the T/Cs were removed, and not properly re-installed, then that could be the cause of the problem. If there was a maintenance outage and highly caring, careful and sensitive mechanics working in the turbine compartment <i>accidentally</i> disturbed the T/Cs, then that could the cause of the problem.

If the purge check valves are leaking while starting/running on gas fuel, then that could be the cause. They do fail from time to time, and depending on the type of check valve some fail more often than others. If the unit doesn't run on liquid fuel very often, then the check valves can also fail more frequently.

But, someone needs to use the Liquid Fuel Purge or Fuel Purge P&ID and go out to the discharge of VA17-1 piping and check to see if there is flow during start-up--if the piping system was constructed so as to allow visual indication. And, someone is going to have to look at the T/Cs and the T/C wiring to determine if it was disturbed or not re-installed properly.

If you require more assistance with this problem, please list the temperature differential or the readings of the two T/Cs (which would be better) during start-up. And tell us what fuel the unit is being started on when this problem is occurring, and if the outlet of the VA17-1 piping allows for visual checking/indication of flow/pressure coming out of the pipe.

I presume someone is currently forcing the logic to block this trip to be able to start and run the turbine, and if so, what are the temperatures of the two T/Cs while running?

Again, please write back with the requested information to allow us to be of more help.

And, we would like very much to know when and how you resolve this problem!

 

Dears

We have the same issue (still not solved). Our design (7FA) uses only one thermocouple "WWTD1-1A" (Type K) located inside the turbine compartment. The thermocouple is reading ambient temperature when the unit is shutdown. At high loads (like 140 MW), sometimes it is reading high (350 F). There were some instances where the load decreased and brought back to 140 MW, but the thermocouple didn't read high again. We have 3 thermocouples for measuring the turbine compartment temperature. The didn't show any increase when false start drain thermocouple started reading high. The issue is old before I started working in the plant. Sorry I don't have enough history.

We thought the false start drain valve or the water wash manual valve were leaking by (passing). We checked downstream of these valve and found normal.

I am uploading a plot showing how the thermocouple is reacting.
Also I am putting the P&ID showing the false start drain thermocouple (WWTD1-1A)

https://drive.google.com/open?id=1dcLmyy1o7o1eiqvqQdF6J_S3LczMGQnv

https://drive.google.com/open?id=15dLz6squR_JgGpGJtN9jrAdB38iV9bWq

Thanks

 

Hi,

Looking to the trend, you missed put axes for different reading as well as highest, lowest temp. Could you please re-do it putting the axis on different places (on the pi process book), for the same period.

You can also check leakage by get close to gas turbine compartment in order to hear something. Take a look also on GMB GT FSD/WW drain tank for hot gases exhaust!

Before I forget, did you check with another new TC?.