Our State of the Art Frame-5 GE Gas Turbine tripped on high explosivity on sensing 10% of LEL of value on 2oo3 security installed at the outlet of the compartment exhaust duct. The compartment is a forced draft one with one fan in operation. Prior to tripping there was a spike of explosivity that came 4 hrs back and touched alarm value of 6% of LEL but came down in a minute or so to the normal value (set at 3 % of LEL as a 0 value). One spike also came 4 to 5 hours before the second last spike and normalized. Unfortunately we don't have a trend available and all the data is based on observations by operator. One thing that has been noted that plant start-up was in progress and there is a CO2 gas vent nearby that contains 1 to 2% H2. We are suspecting that some of the gas entered into the filter house and channeled into the Gas Turbine compartment via forced draft fan and caused the 3 sensors of explosivity to read above 10% LEL and tripped the turbine. Although as per own knowledge the explosivity meters are based on LEL of Methane gas, we are suspecting that they are also sensitive to H2. Has anybody experienced such spikes of explosivity which caused all the three sensors to reach trip value? It is interesting to note that sensors became normal after gas turbine tripping and did not read high values after gas turbine was started again and given upto 12 MW load. However this time the process plant (NH3 and Urea Units) was running normal with no start-up in progress. This ruled out the possibility that there was any leakage inside the compartment. We are now suspecting some outside source or malfunction of sensors. Even the chances of malfunction of sensors may be remote as all three sensors reached alarm values ten second before tripping before gas turbine tripped on reading two sensors at trip values. Please comment.

 

Lots of good information, but you didn't tell us what kind of hazardous gas monitors are installed or what fuel is being burned. We also don't know how long your 'State of the Art' turbine has been running without haz gas issues, or if you have experienced similar alarm "spikes" previously and/or frequently.

I have never been a fan of placing haz gas sensors in duct work or discharges. From everything I've learned dirt and oil--even oil from clean human hands--can contaminate the sensors. And, I've yet to see a turbine installation that has clean, filtered, and dry air flowing into or out of any compartment. And putting them in air paths doesn't seem to make the situation any better.

This means that the sensors, or rather the sintered metal covers used on most sensors, must be periodically cleaned--and that the humans doing the cleaning and re-installation of the assemblies should be using clean latex/nitrile gloves. Further, air ducts need to be cleaned periodically, as well.

<b>From the information provided, </b> it would seem like there was some kind of "leak" from some area, but there's just not enough information. As for what other gases might have an impact on the sensors installed at your site, we don't know what kind of sensors are being used, and you do, and you (should) have the instruction manuals for them.

One can always ask the sensor/monitor manufacturer for assistance understanding odd situations like this; most are willing to helpin order to improve their product reliability. And, it could be that an improper sensor was specified for the atmospheres at your site (they might work well for the majority of sites, but may not be correct for your site).

It has been my frequent experience when having odd issues like this that it's often very helpful to get the manufacturer involved--they don't want a misapplication of their product(s) to have an adverse affect on their brand name/standing. I am painfully aware, though, that in some parts of the world people are loathe to contact vendors because they fear bad information. But, in cases like this, the packager or their vendor may have incorrectly chosen a component, and again, any good manufacturer will usually readily work with someone having problems, and most want to keep their equipment being specified for GE-design heavy duty gas turbines.

I have seen improper shield grounding cause problems months or years after original commissioning when new wire and cable for high-current or switching application is (improperly) installed in cable trays with the improperly shielded wires (that usually takes a few days to track down and understand after asking a lot of questions and crawling around elevated or underground cable trays).

Hope this helps!

 

The explosivity meters measure %age of LEL based on Methane and can read explosivities based on Ethane and Hydrogen as well (The fuel in our gas turbine is 88% Methane). The explosivity meters are catalytic type and have shown the upper spikes for the very first time during last 2 years of operation. They had been showing error many times in past by going "bad" in lower direction (negative value) but almost always on only one transmitter reading at a time, not like this case where all three responded in one direction and that is 10% of LEL. Does anyone have knowledge about the possibility of explosivity sensor going "bad" in higher reading due to any reason like dust, short circuiting etc???

One thing that need to mention is that due to frequent errors on negative side our instrument team had calibrated the meter reading on positive side by setting 3% LEL as zero value. It means if it reaches 10% LEL, then the actual value will be 7% LEL. Unfortunately we could not change the trip set point to 13% LEL due to software limitation in the GE's provided design.

After knowing the above facts and those mentioned in my initial message, can anybody share their experience and comments to help in pinpointing the most probable cause?

What we are doing at the moment is to rise the trip setpoint to 20% LEL by suppressing the existing range. Can anybody share the trip setpoint given to the same security in their gas turbine? What I have read is that upto 25% LEL is usually permissible because technically speaking explosion can occur only at or above 100% LEL.

Also we are installing an additional sensor at the discharge of the forced draft fan for monitoring any explosivity in the surrounding air to warn the operator before it enters the compartment.

Regards,
Waqas Habib

 

Three sensors detecting a higher concentration of combustible gas simultaneously is not probable.

It has to be some kind of explosive gas detection by these sensors.

Please check leakage or ingress source.