Steam Turbine Analysis

A fertilizer manufacturer with a double controlled-extraction steam turbine requested assistance with a condenser vacuum leak. The vacuum leak was contributing to a loss of system performance. “Hogging jets” were required to be in service to maintain proper vacuum in the surface condenser. During the Field Engineering site visit, it was discovered that LP extraction was operating at less than design pressure. In addition, several leak locations were detected around the exhaust casing of the turbine, and later sealed with industrial sealant.

As a result of analyzing the turbine performance, it was determined that the turbine horsepower was being sacrificed at the expense of lower extraction pressures. By correcting this operating condition, the customer was able to recover over 1.3 MW of power, as well as reducing the wet steam conditions in the latter stages of the turbine.

The nominal extraction pressure of the steam turbine was 50 psig, based on the original design. However, Operations had gradually lowered the extraction pressure to satisfy improvements in production for downstream off-takers. The purpose of the engineering study was to determine the magnitude of the system impact by operating the LP extraction at a pressure that is less than design.

The analysis was done using a combination of hand calculations and computer modeling, using industry standard software (IPSEpro®) and generally accepted approximations where instrumented equipment data was not available. Physical turbine geometry was approximated using the most recent stationary steam path information. Using a lower extraction pressure, the stage pressures were recalculated. Based on the new turbine performance analysis, the available energy in the steam was consumed before the steam exited the turbine. This results in windage losses and heating for any turbine stages that are not doing work, as well as excess moisture in the remaining stages. The high moisture content of the steam places the turbine blades at greater risk for erosion and blade failure, as well as the potential for catastrophic failure. Larger blades that develop corrosion cracks and fail have a higher likelihood of destabilizing the rotor, creating severe vibration issues. If the imbalance is large enough, it can create a catastrophic event for the turbine and rotating equipment.

After modifying plant operations to maintain design extraction pressure, the turbine output increased by more than 1.3 MW. This represents a significant cost savings for the customer by reducing the Industrial kilowatt power consumption of the plant.