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Gas Turbine Compressor Washing System

Innovative on line wash nozzle for large gas turbines

Would you like to watch Turbotect on line wash nozzle in operation? You are welcome to visit our Video Clips section.

As gas turbines have become bigger in output, with higher air mass flow and larger dimensions in air inlet conduits, experience with on line compressor washing has shown the need to improve the penetration and distribution of droplets into and across the air inlet stream to achieve uniform wetting of compressor vane surfaces. Turbotect has developed an advanced air-assisted nozzle system which is in operation in a number of large gas turbines up to 280 MW in power output.

More than ten years ago Turbotect introduced its on line compressor washing system and embarked on a comprehensive series of field trials at a district heating plant in the Netherlands. Since that time, gas turbines have become bigger and bigger, and the need for regular on line compressor washing is maintained in the competitive merchant plant environment.

The original Turbotect PSA Mk 1 on line nozzle has proved to be very successful on a wide range of gas turbine models. It creates a uniform distribution of very small water droplets under low, 4 bar, operating pressure, and enables good wetting and cleaning of first row vanes.

However, as gas turbine output has grown, mass flow has increased along with machine diameters and overall dimensions of the air inlet system. As a result it has been found that the droplets cannot penetrate across the air inlet stream so easily, and are instead deflected in their mid-course trajectories. This reduces the efficiency of wetting the blades, and reduces the effectiveness of on line cleaning.

The challenge, then, was to develop a nozzle which would allow droplets in the same small size range to penetrate much further into and across the air stream, and thereby provide more effective wetting of the blade surfaces.

Initial tests in gas turbines with variations in nozzle positioning and tip geometry were explored, and involved existing and various other nozzle designs. Separate studies were also made on a wind tunnel at a Swiss University. Results of these tests led to the introduction of the Turbotect PSA Mk 3 nozzle.

The PSA Mk 3 nozzle (patented) is also a low-pressure system operating at 4 bar, with a droplet size in the range 50 to 250 microns. This flush mounted nozzle design is an air-assisted type, with a flat-profile water spray that is shielded and sandwiched between two high velocity flat-profile air sprays. These dual air sprays protect the water spray, and punch it through the boundary layer and into the main air stream. Water droplets are protected from premature deflection, and a longer penetration trajectory is thereby enabled. In practice this means that for a larger gas turbine with larger inlet geometries, the Turbotect Mk 3 on line nozzle can deliver a more controlled and stable water spray pattern into the air inlet stream, which will improve wetting and cleaning performance.

The PSA Mk 3 nozzle is designed with the same fitting assembly as the PSA Mk 1 unit, so that easy retrofits can be performed. Also a fewer number of nozzles are required for a same gas turbine air inlet, so the overall water mass flow remains at the same low level as with Turbotect's Mk 1 nozzle, which is still an extremely low volume system compared with some others on the market without air assistance.

The risk of compressor blade erosion is greatly reduced due to very low total mass flow, small droplet size range, and optimised positioning of nozzles upstream and downstream of the bell mouth.

Fig. 1: The PSA Mk 1 nozzle (patented) has an adjustable spherical nozzle which projects into the intake stream.

Fig. 2: The PSA Mk 3 nozzle (patented) has low-pressure air jets at either side of the water nozzle and is designed for larger gas turbines above 120/140 MW output.

On line compressor washing

Fig. 3: The PSA Mk 3 nozzle filmed during actual on line compressor washing at base load. Photo shows how the air jets shield the spray, and that there is no early deflection or dribbling at the nozzle outlet.

Fig. 4: Development program using test rig with air velocities comparable to an operating gas turbine inlet. Plexiglas wind tunnel enables undisturbed observation of nozzle sprays.

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