Myths about online water washing

A paper by Klaus Brun and Terence Grimley of SWRI, Rainer Kurz of Solar Turbines, and William Folles at ASME/IGTI, 2014 dealt with ‘Experimental evaluation of the effectiveness of online water washing in gas turbine compressors.’ Below are excerpts from the paper.

"An investigation of the effectiveness of online combustion turbine axial compressor washing using various purity grade waters and commercial washing detergents was performed. For this project, blade surface fouling dirt was obtained from gas turbine axial compressor blades installed at various field sites.

The dirt was analyzed to determine the composition and consistency of typical blade surface fouling materials. A representative dirt formula and blade coating procedure was developed so that comparative tests could be performed using various cleaning fluids. Dirt coated blades were installed in a wind tunnel capable of simulating compressor operating conditions. A spray nozzle upstream of the blade test section was used for washing blades with five different test liquids to determine the effectiveness or advantages of any liquid.

Once this testing was completed, a similar test setup was then utilized to inject a mixture of formulated fouling dirt and the various online cleaning liquids upstream of the blade into the wind tunnel to assess redeposit characteristics. The effect of high-purity water versus regular water on fouling dirt was also studied in separate residue experiments. Results indicate that spraying cleaning fluid into a flowing air stream is a viable means of cleaning a compressor blade. Each of the fluids was able to clean the test blade at both low and high air velocities, and at different blade incident angles.

Within the parameters/ fluids tested in this study, the results indicate that:

1.    Spraying cleaning fluid into a flowing air stream is a viable means of cleaning a compressor blade. Each of the fluids was able to clean the test blade at both low and high air velocities and at different blade incident angles. However, for all tested cleaning cases, there was always an area of the blade where some fouling deposits remained.

2.    The blade cleaning is primarily a mechanical (droplet impact) function and does not depend on the fluid used for cleaning. Test results show that most of the cleaning occurs shortly after the cleaning fluid is introduced into the flow stream. The type of fluid used did not have a significant impact on the cleaning effectiveness.

3.    Dirt removed from the blades will redeposit in downstream stages as the cleaning fluid is evaporated. Redeposit occurred in flow recirculation zones during the cleaning tests, and heated flow tests demonstrated dirt deposit in the presence of a cleaning fluid. The type of fluid used for cleaning has no effect on the redeposit characteristics of the dirt.

4.    Blade erosion was not found to be a significant issue for the short durations that online water-washing is performed. However, uncontrolled water-washing (or overspray) for extended periods of time does result in measureable leading and trailing edge blade erosions.

5.    The results suggest that it may be beneficial to the cleaning process to slow the compressor speed or vary the cleaning fluids spray rate while the online wash is being performed. The results show that most of the cleaning occurs shortly after the cleaning fluid is introduced into the flow stream.

Field tests with online water-washing should include tests with varied cleaning times. The issue of redeposit should be addressed during field testing; if possible, inspection of downstream compressor stages should be used to assess the transport and redeposit of dirt from early to later compressor stages.

The objectives and results of this study can be summarized with the previously posed four questions and answers:

1.    Does online cleaning work? Yes, wind tunnel blade test results indicated up to 95 percent removal of blade fouling is possible.

2.    Is there any difference between any of the online cleaning liquids? No, there was no clear evidence that any of the liquids or detergent mixes improved the overall blade washing efficiency.

3.    Will dirt be removed during online cleaning redeposit once the cleaning liquid has been evaporated? Yes, redeposit tests showed that a significant fraction of the dirt will redeposit on downstream blades. The actual quantity of the redeposit depends strongly on the local flow field and the type of particles that are being carried in the free stream.

4.    Do any liquids reduce the redeposit of dirt in the online washing process? No, testing showed that redeposit occurred with all liquids tested, and there was no clear evidence that any mixtures or detergents reduced particle redeposit."