Parametric Study of On-Line Compressor Washing Systems Using CFD Analysis

Abstract

Compressor fouling is a significant operational problem faced by gas turbine operators. It negatively affects the overall gas turbine performance in terms of power output and heat rate. In order to recover gas turbine performance, operators use compressor washing. When the gas turbine cannot be stopped, on-line compressor washing is used. This technology utilizes water spray injection in front of the compressor inlet at full rotational speed. To provide good cleaning, water spray nozzles should be properly positioned and directed in order to allow uniform wetting of the compressor blades. At the same time, water droplets should not be collected on the air duct walls upstream of the compressor. Additionally, the water droplets should be big enough to be able to clean the compressor and, at the same time, small enough to avoid erosion of the blades. In order to provide gas turbine operators with guidelines for the proper selection and use of on-line washing systems, a numerical study has been carried out, based on typical washing systems installed on large industrial gas turbines. The study includes air flow analysis inside the air duct upstream of the compressor and calculations of the motion of water droplets injected into the air flow at different locations. The influence of the following parameters has been studied: droplet size, both injection direction and velocity, spray cone angle and location of the spray nozzles. “Fluent” CFD software was used. Effectiveness of the washing systems was evaluated by: a) amount of droplets entering the compressor (as mass percent of total injected water), b) uniformity of droplet mass distribution along the compressor first blade length and c) droplet accumulation on the surrounding surfaces (this being a negative phenomenon). The results showed that the effectiveness of the washing systems depends highly on the spray nozzle location (a total of six locations have been analyzed) and in each location — on droplet size and initial velocity. Special diagrams have been developed to illustrate the required optimal combinations of droplet size and initial velocity in each location of the spray nozzles. The general conclusion was that the proper design of any on-line washing system should be based on detailed CFD analysis of the motion of the injected droplets. The results could be helpful for gas turbine operators as well as for designers of the on-line washing systems.