We have a GE Frame 9E (PG 9171E) gas turbine, operation controlled by speedtronic MARK VI controller running on Permeate Low-BTU Gas.

Recently, we observed a transient spike in Exhaust Spread-1(132°C) during normal operation. Upon investigation it was found that one of the 24 exhaust thermocouples, TTXD_12 recorded a temperature of 383°C, while adjacent thermocouples TTXD-11 and TTXD-13 measured 518°C and 515°C, respectively.
At the time of the spike, the gas turbine was loaded at 69 MW. The corresponding values for Spread 2 and Spread 3 were 10.069 C and 9.488 C, respectively, and the allowable spread was 117.588 C.
As per the trend-graph it was seen that the thermocouple reading increased smoothly from 11:58 PM, peaked at 12:03 PM and returned to normal at 12:05 PM. After which all the parameters went back to normal. No anomaly has been seen in the behavior of the thermocouple or spreads since then.
I want to know:
- Assuming the culprit was the thermocouple, what could cause it to do so?
- If it was an actual reading because of an actual irregular flow of exhaust gases, what could cause so?

 

Irregular flow distribution, seems to be the most likely in such a case

 

A loose connection in the thermocouple wiring, at the junction box, or in the Speedtronic MARK VI control system can cause spurious spikes.

 

@somiamehar,

WHAT ALARMS (Process AND Diagnostic) were active and annunciated when the problem occurred?

What kind of combustion system does the machine have—conventional or DLN-I?

What trend data were you looking at—Live Data (from the processor the T/C is connected to) or Trip History data

How many years has the problem T/C been in use on the machine?

Was there any maintenance activity prior to the problem? If so, were the T/C terminations disturbed?

Have you checked all the terminations in all the junction boxes between the exhaust compartment and the Mark* VI—including the cables between the T/C terminal boards and the Mark* VI T/C input card, making sure BOTH ends of the cable are clean and securely seated and locked into the receptacles?

Irregular flow distributions are not something that happens in a GE-design heavy duty gas turbine; there’s just too much air moving through the machine. If there’s a flow distribution problem it’s usually the result of a fuel flow problem and the number of exhaust T/Cs and the number of combustors make it pretty difficult for just one single T/C to experience such a decrease (or even increase) for the time span you described.

While it could be a Mark VI problem, it’s more likely a wiring problem or a T/C thermocouple cable (the one that runs between the terminal board and the input card in the Mark* VI panel). Unless there are Diagnostic Alarms that are related to the input you haven’t provided.

I’m not familiar with the specific gas you mentioned but unless it has some unusual properties/characteristics it’s probably not the fuel. For a T/C to actually see a low temperature such as you described it would mean an awfully unusual flow restriction in the fuel nozzle(s), and I suspect the machine has conventional combustors, probably with a single nozzle per combustor (but it may also have multiple nozzles per combustor with an unusual fuel. A sudden, and shortlived, fuel restriction might be the cause but we don’t have enough information to say anything with any degree of certainty.

My experience says it’s a failing exhaust T/C or a wiring problem or a cable issue in the Mark* VI, but not a software or T/C input card problem (unless there were Diagnostic Alarms that weren’t reported or were ignored).

It would really be great if you would write back with the answers to ALL the questions and provide a progress report and/or the solution as many people read and follow these threads and could benefit from your experience in resolving this problem.

In the scheme of things exhaust T/Cs are pretty inexpensive and relatively easy to replace (as long as proper connection methods for T/C wiring are observed).

 

Thank you @WTF? for the detailed response. As for your questions:

WHAT ALARMS (Process AND Diagnostic) were active and annunciated when the problem occurred? The process alarms included the following for obvious reasons:

• Exhaust Thermocouple Trouble Alarm L30SPTA
• Combustion Trouble Alarm L30SPA

However no Diagnostic alarms were annunciated.

What kind of combustion system does the machine have—conventional or DLN-I? Conventional

What trend data were you looking at—Live Data (from the processor the T/C is connected to) or Trip History data. It was historical trend data

How many years has the problem T/C been in use on the machine? The thermocouple was installed two years ago on the machine

Was there any maintenance activity prior to the problem? If so, were the T/C terminations disturbed? No there was no maintenance activity carried out recently

Have you checked all the terminations in all the junction boxes between the exhaust compartment and the Mark* VI—including the cables between the T/C terminal boards and the Mark* VI T/C input card, making sure BOTH ends of the cable are clean and securely seated and locked into the receptacles? Yes, the terminations were verified from all the junctions. However, no issues/abnormalities were found.

To provide an update on the progress. The thermocouple went bad, the root cause for its failure (thermocouple/cable/connection) is now to be checked.

 

Thank you for the update!

This machine--if the Mark* VI was provided with the equipment when originally installed--may, or may not, have the exhaust T/Cs with the long, stainless-steel sheaths. Most every site I've ever visited with this type of thermocouple just leave the stainless-steel sheaths unsecured and they end up rubbing against UniStrut brackets or walls and eventually, sooner or later, a hole develops in the sheath and leads to an open circuit--which GE designed the Mark* turbine control systems to interpret as a very low reading. Usually, when they are beginning to fail they will exhibit the kind of behavior you are describing (intermittently going low).

The other type of T/C which may be used on the machine has flexible leads which can be unscrewed from the T/C itself, and is MUCH easier to install or replace--but still, some site personnel feel the need to loosen the clamps holding the flexible lead when replacing the T/C--and then not replacing the flexible lead in the clamp when finished replacing the T/C. (By the way, these types of T/Cs are often referred to as "Frankenstein" T/Cs because the two threaded bolts on opposite sides of the exterior part of the T/C look like the bolts on Frankenstein's neck....)

There seem to be several vendors of this type of T/C these days (the Frankenstein T/Cs) and as with everything the quality can vary greatly. One does have to remember that temperatures in the gas turbine exhaust can get VERY high--even under normal circumstances. Machine which "regularly" experience trips from loaded operation cause high thermal stresses on components in the combustor, turbine section and exhaust when fuel is suddenly shut off. Also, improperly installed T/Cs can also cause issues with premature failures (T/Cs inserted too far in the radiation shields, touching metal, or even "jammed" into the end of the radiation shield during installation (dropping the T/Cs can also bend/damage the T/C tips). Again, in the whole scheme of the cost of parts for the machine T/Cs are inexpensive (I know, the sourcing manager and the Plant Manager and the plant owner(s) are ALWAYS looking for ways to save a few pennies and cut costs) and purchasing low-cost exhaust T/Cs which fail sooner than expected, often resulting in a machine trip and unreliable operation is like ignoring a piece of paper money on the ground to stop and bend over to pick up a small denomination coin. It's senseless. Reliability should be the big driver for parts purchases, especially something as easily replaceable and as "inexpensive" as an exhaust T/C.

But bean counters seem to drive most decisions these days and it's tough to quantify reliability and availability to a bean counter. (I worked at a cogen plant at a refinery where the employees could earn bonuses for high reliability and availability and efficiency. Those people were amazing at how they worked together to convince the Plant Manager and the Sourcing Manager that cheap parts were costing the plant owner money (and the plant employees lots of bonus money). They had spreadsheets and quantifiable data which they kept updated and used whenever they felt poor purchasing (sourcing) decisions were being made. Those people made LOTS of bonus money with their attention to detail and data--and as a result the plant was extremely reliable and profitable. They even had a say in hiring decisions because they wanted good qualified people to join their team and help keep the bonuses large and plentiful. Needless to say, the refinery the cogen plant sold heat and steam to was also extremely profitable as they were also able to send unusable refinery gases to the cogen plant to burn. Those guys had every tool and meter and knew how to use them. It was a pleasure to work at that plant for me, especially because it was well organized, had a good parts warehouse, and was kept very clean and tidy. And I know it was VERY profitable for the cogen plant employees as well--many of them worked there for 20 or 30 years. The wages were good, but they weren't great--it was the bonuses they earned that made it extremely worthwhile for them. AND, the plant rarely experienced forced outages, but when it did, they had the tools and the parts onsite and got it quickly up and running again. And, the cogen plant employees were a very large part of the reason.)