Nozzle for a new generation

8 July 2004

The new PSA Mark 3 nozzle assembly from on-line turbo-wash specialist Turbotect is designed for the later generations of large gas turbine. The nozzle operates at a pressure of 4 bar and guarantees a droplet size in the 50 to 250 micron range.

It has become the common practice to clean compressor inlet guide vanes with an on- or off-line process. With an off-line procedure, the power recovered is greater, but that has to be offset against the loss of production. On-line washing is performed while producing energy. It does not recover all the power loss, but can usefully be combined with off-line washing performed at longer intervals.

However, bell mouth size, mass flow, and air velocity of newer, larger gas turbines have increased to an extent that has outgrown the first generation of water spray nozzles. In fact, the combination of these characteristics makes it more difficult for the water spray to penetrate the boundary layer, allowing the air stream to deflect the water droplets in their mid-course trajectories before they have reached the required penetration to uniformly wet the vane surface. An additional danger of poor performance is that excessive and often deflected water concentration will cause erosion in a defined blade area, while soot will accumulate in the dry area.

According to Turbotect, it is increased penetration, not increased water flow, that improves the washing performance.To increase the water spray penetration through the boundary layer and avoid premature deflection of the spray from the main stream at the compressor intake, the PSA Mark 3 nozzle is of the air-assisted type. The flat profile water spray 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.

Tests on a special test rig and also in the wind tunnel of a Swiss university showed that a Mark 3 nozzle placed horizontally at a height of 1.2 m wetted a surface at a distance between 0.8 and 2 m, but when air-assisted, the same spray wetted a surface up to 4.5 m away. The first field applications on large size gas turbines have confirmed these results.

Water consumption compared to the earlier Mark 1 version is alomst identical, because although water consumption is almost double the surface coverage is extended, reducing the number of Mark 3 nozzles required. Compared to other nozzles on the market total fluid consumption can be less by a factor of up to six.

Both nozzles are flush mounted on the bell mouth in separate ring manifolds spraying upstream and downstream and both feature the same nozzle holder so the new nozzle can readily replace existing Mark 1 units.

Mk 3 nozzle spray during on-line compressor washing Mk 3 nozzle spray during on-line compressor washing

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