Performance Boost with Wet Compression (WetC)

The different locations in a GT application where water can be used for performance improvements are:

· evaporative cooling with water in the air filter house,

· fogging makeup water in the air intake

· wet compression with makeup water in the air intake

· makeup water injection (PAG operation) into the combustor

The following focuses on the particularly effective wet compression upgrade. After Siemens and Westinghouse became one company, they were able to develop wet compression in an R&D program for the V94.2 frames for both the 50-Hz and 60-Hz fleet based on applications and operating experience with Westinghouse W501 engines.

In wet compression, atomized water is injected through a nozzle rack into the compressor air intake. Part of the injected water evaporates in the air intake; the remaining water enters the compressor in liquid form (droplets of approx. 20μm diameter at 90% probability). This achieves an inter-cooling effect. The injected water evaporates in the compressor stages. The energy required for evaporation is taken from the compressed air mass flow, which is thus continuously cooled. This cooling, coupled with the mass flow increase of the working fluid drawn in, results in a significant performance gain in both output and efficiency. In the baseline wet compression the performance gain is independent from ambient conditions.

During the development of this upgrade, the design criteria assembled were analyzed and met, and validation tasks for first-time application were defined. For example, we needed to ensure that water injection is homogeneous to prevent casing deformation due to non-uniform temperature fields. The spray pattern in the intake duct was therefore specified in advance on the basis of 3-CFD analyses and temperature field of the casings measured during the validation run.

The amount of water is controlled by a mass flow control loop comprising the injection pump, a variable-frequency drive (VFD) and a controller. In order to maintain a desired mass flow the controller activates the VFD to set the appropriate speed at the pump motor. The pump directly feeds the desired amount of water into the feeding line. Thus an additional return line is no longer necessary. The entire equipment is arranged on the high-pressure wet-compression skid.

As for thermodynamic performance a typical increase in the efficiency of the overall gas turbine in an open GT cycle is up to 5%, with an output increase of up to 20%.

Before wet compression implementation, the GT was operated with power augmentation (PAG), i.e. water injection in the combustion chamber, achieving an output increase at a NOx limit of 9 ppm and a turbine inlet temperature of 1040°C.

When using just the wet compression system without simultaneous power augmentation, the result was an efficiency increase of 6%, compared to an efficiency loss of 5% in the case of water injection into the combustion chamber.

The measured output increase for a relative ambient humidity of approximately 90%

was around 13% at 17.5°C ambient temperature. In contrast to wet compression, inlet cooling systems such as evaporative cooling or fogging coolers yield only a negligible increase or no increase whatsoever in output or efficiency under ambient conditions with high humidity or cold temperatures.

As for combustion performance at a turbine inlet temperature of 1040°C, NOx

emissions were reduced from 9 ppm to 6 ppm, making it legitimate to conclude that a level of 9 ppm will not be exceeded at a turbine inlet temperature of 1060°C. Any potential corrosion occurring on the compressor blades and vanes can be limited by coating the compressor parts.

The first compressor stages must be monitored for erosion and corrosion during inspections performed at the standard intervals. Vibration measurements of blade row

#1 of the first unit will be repeated after a sufficient number of wet compression operating hours and compared to the initial testing in order to evaluate erosion effects on the vibration behavior of the compressor rotor blading. This will be done at the fleet leader in wet compression operation.

In addition an R&D program was started this year to provide new corrosion-resistant compressor blade materials and protective measures against erosion in the near future.

Experience gathered at a specific Siemens Westinghouse GT W501D5A after some 25,000 operating hours with wet compression demonstrated that GT maintenance follows the standard inspection intervals. As is also the case for the implementation of power augmentation, the water factor must be included in the calculation of equivalent operating hours.

Benefits

Wet Compression can be an effective system for recovering power loss experienced at high ambient temperature. The mutual occurrence of peak load electricity demand and high ambient temperature make Wet Compression more beneficial and valuable.

Benefits can include:

· Power increase of up to 15% and potentially more depending on the frame and operational requirements *

· Up to 3% gas turbine heat rate improvement *

· Higher exhaust energy for increased steam production

· Greater operational flexibility.

These benefits can lead to the ability to produce more power in peaking and base load operation.

Wet Compression is largely independent of the ambient relative humidity. While somewhat higher performance improvement can be available in a very hot dry climate, Wet Compression can also be very effective at times of high humidity.

* Actual results may vary

Scope of supply

Besides a diligent original equipment manufacturer assessment of the gas turbine and the involved power plant components, the scope of this modernization includes:

· Compressor inlet Wet Compression water distribution system with nozzles

· Inlet duct treatment

· Wet Compression pump skid

· Piping between pump skid and distribution system

· Compressor coating (where required by gas turbine frame and version

· Compressor upgrade (depending on gas turbine frame and version)

· Modification of existing control logic Gas turbine customization (e.g. axial trust compensation and compressor drains where applicable).

Installation and commissioning of Wet Compression can require an outage from one to three weeks depending on plant and gas turbine configuration.