Gas Turbine Cleanliness and efficiency

[ma2mw]

I’m working on a new method for cleaning gas turbine compressors when off-line. All of my work so far has been with large stationary gas turbines used in power plants. But unlike gas turbines used for aircraft, stationary installations only use filtered air.

I need to find out if on large aircraft gas turbine engines, when being disassembled, if there is a noticeable difference in blade (airfoil) cleanliness comparing the low pressure side (convex) and the high pressure side (concave) of the same blades. In other words, does one side of the blades consistently look dirtier than the other side? Of particular interest are the first 8 stages, especially the stationary airfoils.

My research has shown that the low pressure sides of the first 8 stages (especially the stationary airfoils) have the greatest impact on compressor efficiency.

This new method for cleaning the compressor is especially effective on these low pressure sides, but may be unnecessary if existing aircraft methods (off-line washing and rain) are cleaning both sides effectively already.

Does anybody have any opened up compressors for large turbines around to check this out for me? I would be glad to share my extensive research on this subject if it applies to aircraft.

 

[astarguy]

while i do not have a direct answer to your question i feel that the following might be of some use to you. my company does not wash out jet engines on a regular basis. on the rare time that we do it is when the egt is climbing (trending) it helps cool the engine. To that end it is more for the combustors and turbine than the compressor. for turbo shaft engines that run a set rpm for days weeks months at a time i would supect that it might be more cost effictive to wash them. on question that i have is you say that they are getting washed off line. does that mean shut down? when we wash ours we run it at high speed and pump the soap thru a wand. how are you doing your's? hope this helps a little. astarguy

 

[ma2mw]

I clipped the following paragraph out of GE Power Systems GER-3620J 'Heavy-Duty Gas Turbine Operating and Maintenance Considerations'

Available at:

http://www.gepower.com/prod_serv/products/tech_docs/en/downloads/ger3620j.pdf

"Generally, axial flow compressor deterioration is
the major cause of loss in gas turbine output and
efficiency. Recoverable losses, attributable to compressor
blade fouling, typically account for 70 to
85 of the performance losses seen. As Figure 30
illustrates, compressor fouling to the extent that
airflow is reduced by 5%, will reduce output by
13% and increase heat rate by 5.5%. Fortunately,
much can be done through proper operation
and maintenance procedures to minimize fouling
type losses. On-line compressor wash systems
are available that are used to maintain compressor
efficiency by washing the compressor while at
load, before significant fouling has occurred. Offline
systems are used to clean heavily fouled compressors."

Typically in power plants, on-line washes are performed everyday (on continuously running turbines). This involves utilizing a built-in set of spray nozzles that inject heated distilled water and/or detergent into the compressor while at about 80% load. The washing continues for about 10 to 15 minutes. This recovers much of the deterioration seen on the on-line performance monitoring system that has accumulated over the previous 24 hours.

Since the on-line washes only recover about 95% of the degradation, this degradation builds up and eventually it becomes economically feasible to shut-off fuel to the turbine, cool it at cranking speed, then spray hot water and detergent into the compressor (while still at cranking speed), then shut down completely and let it soak for about 20 minutes. After soaking it is put back on its cranking motor and rinsed with distilled water. This recovers about 95% of the accumulated degradation since the last off-line wash. Typical intervals for off-line washing of a continuously operated turbine may be about 2000 hours (of course much more often in dirty environments).

Even with off-line washes deterioration of compressor performance increases to where it becomes necessary to remove the rotor and use power washing and good old elbow greased to restore performance.

Since gas turbine efficiency is directly proportional to profit (every dollar saved in fuel is a dollar more in profit), this is watch very closely in stationary turbine settings.

 

[yates]

Having considerable experience with heavy-duty industrial gas turbines, I can confirm that compressor washing is an essential factor in improving heat rate, and therefore fuel consumption. You mainly talk about water and detergent, but natural media (Carboblast, ground walnut shells, rice husks etc. have proved very effective in defouling compressor blades. I have no experience of aircraft gas turbines, which of course run without air filters, but I would imagine they stay much cleaner with the amount of rain, sand etc. ingested.
As astarguy says, the cleaning effect is also useful on fuel nozzles, which can experience carbon build-up when running on heavy distillate or residual fuels. An asymetrical build-up of carbon on the injectors can create an asymetrical flame, which tends to overheat one side of the combustion chamber.