Background

During operation, several steam strainer baskets reportedly failed at a power plant comprised of Gas Turbine Combined Cycle Units (CCGT). Each of the units consisted of two Gas Turbines with steam cooled combustion transition components. Each of the units had two gas turbines (GTs) exhausting into individual Heat Recovery Steam Generators (HRSGs) that generated steam. Some of the steam was supplied to the GTs from the HRSGs to cool the transitions. As a precaution to avoid getting debris into the GT cooling channels, a steam strainer with a fine mesh basket was designed to be inserted in the steam line between the HRSG and the GTs.

Steam strainers are passive components (no moving parts), which consist of a body installed into the steam piping by welding or flanges, and contain an internal mesh basket (strainer), that removes large and small particles of debris from the high quality steam before it is supplied to the GTs.

The units were taken off line for the removal, inspection, and possible cleaning of the mesh baskets for the steam Stop Valves and Reheat Valves. A bore scope was used to inspect other strainers in the plant without having to remove the components. It was at this time that the steam strainers appeared to have been damaged during plant operation.

Services Provided

Rimkus was retained to determine the root cause of the strainer basket failures. Due to the complexity of the evaluation of the basket failures, four separate tasks needed to be completed:

• An evaluation of the design and operation of the steam system that the baskets were installed in.
• A Computational Fluid Dynamics analysis of the pipe layout and the steam strainer which is used to evaluate the steam flows in the pipe and the strainer.
• A Finite Element Analysis (FEA) which is used to determine the flow-induced load on the baskets.
• A metallurgical evaluation which would evaluate the material and the failure modes.