Steam Turbine Analysis

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 in service to maintain proper vacuum in the surface condenser. During site visit, it was discovered that LP extraction was operating at less than design pressure. Several leak locations were detected, 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 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 , nominally 50 psig, per the original design, circa 1966. The system impacts were quantified in terms of operational risk, financial risk, and production risk. The objective of this study is not to develop a detailed performance model. The objective is to provide a quantifiable system impact assessment, based on the reduced pressure at the LP extraction on Uncle Sam #2.

• Steam Turbine, S/N: 26509
• Generator
• Surface Condenser
• Hotwell
• Condensate Pump
• HP Extraction
• LP Extraction

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 from the 2012 rebuild. The changes in the stage 13 geometry and recalculation of steam pressures resulted in a lower Stage 10 inlet pressure, and subsequently lower extraction pressure. Therefore, two baseline states were evaluated; original design at 50 psig (nominal) LP extraction, and 47.87 psig (nominal, approx.) LP extraction pressure. The existing hardware was then evaluated with a reduced LP Extraction and a fixed condenser vacuum to determine the impact on the stages between the LP extraction and the Last Stage Blades. Pressure drop through the turbine was based on the current nozzle configuration (reference) and a constant exhaust pressure, creating a known condition in the turbine exhaust hood.

Operation of the 2nd controlled extraction pressure below the design point results in excessive turbine losses due to windage of one or more stages. In addition, 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. After modifying plant operations to maintain design extraction pressure, the turbine output increased by more than 1.3 MW.