We are operating Gas turbine Frame VI machine model PG6541B. We have been operating machine on gas from long time. occasionally it is required to be run on liquid fuel during gas crisis. Repeat failure of atomizing air cooler has been observed. more than half of the tubes found leaking. Tube failure was seen as a pin hole puncture.

We had changed original MOC of tube to SS304 anticipating that failure will not occur, but again all SS tubes found chocked. dechoking could not be done even with hydrojetting machine. Tubes seemed to be cracked.

Cooling water is used in circulation at 5 Kg/Cm2 (g) and Inlet temperature 30 Deg C.

Can u please guide us.

 

Pramod,

The most common problem with atomizing air pre-coolers is the incorrect adjustment of the temperature regulating valve.

The temperature regulating valve is to be set to maintain approximately 135 deg C (275 deg F) upstream of the Main Atomizing Air Compressor inlet (suction). This temperature setpoint is required for two reasons; first, to limit the temperature of the entering the Main Atomizing Air Compressor. If the air entering the Main Atomizing Air Compressor is greater than approximately 150 deg C (300 deg F) the internal clearances of the compressor can decrease and cause catastrophic failure of the Main Atomizing Air Compressor.

The second reason this temperature setpoint is important is to prevent condensation of air as it's being cooled by the atomizing air pre-cooler. Condensate in the cooler can damage the tubes, <b>AND</b> if a slug of condensate is passed along through the piping to the suction of the Main Atomizing Air Compressor the compressor can suffer damage and even fail catastrophically.

From the information provided, it would seem there is or has been moisture condensation in the pre-cooler, AND there is some contaminant entrained in the air which, when mixed with water is forming a corrosive which is attacking the tubing of the cooler bundle.

If you examine the P&ID for the Atomizing Air System you should see there is supposed to be a very small orifice in the shell drain to act as a continuous blowdown to allow any condensate which might collect in the cooler to be blown out by compressor discharge pressure. Many sites mistakenly remove this orifice incorrectly believing it is a "leak"; it is a very important part of protecting against the collection/pooling of any condensate or moisture in the cooler.

There should also be a cooler shell drain, with a manual valve, which can be periodically opened (slowly) to monitor for the presence of any condensate/moisture in the cooler.

Another common problem with collecting moisture in the cooler shell is improper valve positioning during off-line compressor water washing. During an off-line compressor water wash some detergent and/or rinse water is going to make it's way to the cooler shell, but if drains are open, and checked prior to closing, the detergent/water should be drained away prior to re-starting the turbine.

Lastly, because the atomizing air temperature regulating valve is a slow-acting valve, and it's trying to maintain 275 deg F when the turbine is shut down (such as during a water wash or maintenance outage) it has effectively shut off the flow of cooling water through the atomizing air pre-cooler. When the turbine is started, the compressor discharge air temperature is going to increase very quickly, but the temperature regulating valve is much slower to react and open and start allowing cooling water to flow through the cooler. Consequently, it's VERY COMMON for 'Atomizing Air Temperature High' Process Alarm to be annunciated for a few minutes while the valve is slowly opening. MANY people mistakenly run out and manually crank the valve open--and then leave it open, which causes the air temperature to be much lower than it should be, causing condensation.

BE SURE TO READ THE TEMPERATURE REGULATING VALVE MANUFACTURER'S INSTRUCTIONS for how to adjust the valve before making any adjustment--HINT: it's NOT adjusted with the handle on the side of the valve actuator. When making adjustments to the valve setpoint, do so when the turbine is running at rated speed, and when the load is fairly stable and hot changing. It takes patience when making the adjustment because, again, the valve is very slow to react. So, make a small adjustment (half a turn of the adjustor, for example) then wait about 10 minutes and observe the atomizing air temperature at the Main Atomizing Air Compressor inlet, and make small adjustment, waiting several minutes between checks to make any further adjustments.

Once adjusted, it should never need to be adjusted again. There may be an occasional 'Atomizing Air Temperature High' alarm on start-up of the turbine from a cold (ambient) condition, but BE PATIENT. If the alarm doesn't clear in a few minutes, then check to make sure the temperature is dropping, or if the valve has become stuck.

And, another problem with the temperature regulating valve is that the capillary tube quite often gets damaged because of careless and ignorant people. This means the valve has to be manually adjusted to obtain the proper temperature during various loads, and most sites don't do this. They don't replace the capillary, and they don't make the necessary adjustments, and this is one common cause of moisture condensing in the atomizing air pre-cooler. And of damage to tubes.

Hope this helps!

It would be most helpful to others who read these threads if you would tell us about how your temperature regulating valve is operating or being operated. Is it properly adjusted?

Is the capillary tube intact?

What is the temperature of the atomizing air entering the Main Atomizing Air Compressor inlet before you make any adjustment(s) to the temperature regulating valve?

Did you find the continuous blowdown orifice installed and clear, allowing moisture/condensate to drain from the cooler shell?

Did you find moisture in the cooler shell when you opened the shell drain valve?

If your site performs off-line compressor water washes, are cooler isolation valves closed and cooler shell drain valves opened as part of the process of preparation for the washing, then returned to normal operating position(s)?