What are the Effects of GT Upgrades on HRSG Pressure Parts?

GT upgrades

OEM upgrade packages to gas turbines (GT) can significantly improve performance, operational flexibility and increase outage intervals. In combined cycle power plants, the impact of these upgrades should also be considered for the HRSG where the design was based on the original GT performance.

Common changes that impact the HRSG include :

• More efficient operation of the GT at lower load (lower flue gas mass flows, higher temperatures)
• Modifications to exhaust gas profiles at various loads
• Duct burner firing characteristics
• Changes to fuel fired
• Increased outage intervals

The big question is how these upgrades affect the downstream pressure parts in the HRSG?

Thermal Hydraulic Analysis

By using a boiler simulation model of the HRSG one can simulate the conditions before and after the change in GT performance. The details of the HRSG heat transfer surfaces with regard to tube details (wall thickness, material, fin height and density, etc.) are used as input.

At each measured operating point, key boundary data is used to set the conditions for the design model. They include:

• GT Exhaust mass flow, temperature and composition computed from measured data
• HP, CRH, HRH, LP steam pressures and temperatures from measured data.
• Fuel Gas Heating water flow demand from measured data
• Condensate feedwater temperatures to HRSG
• LP Economizer/Preheater recirculation setpoint (temperature).

A direct comparison is then possible between measured and original design performance at the specific operating points.?

Effects of GT upgrade on HRSG Pressure Parts

In general the limiting factors of HRSGs during upgrades are:

• Operating Pressures (below design limits)
• Tube Metal Temperatures (below design limits)
• Water/Steam Flow Velocities (below good practice limits)
• Water/Steam pressure drops (reasonable values compared to design, adequacy of available pump head for feedwater).
• Exhaust Gas Temperatures below local design limits
• Steam Separator Performance
• Safety Valve Adequacy

Conclusion

By the use of advanced thermohydraulic boiler simulations, the Heat Recovery Steam Generator can be assessed in detail and any potential issues can be addressed before the actual GT upgrade takes place.?

Tetra Engineering Group, Inc. (TETRA) has assisted the owners of natural gas-fired combined cycle plants with effective remedies to HRSG thermal performance and integrity issues since 1993.?Over that time, TETRA inspected hundreds of HRSGs of all types and sizes from nearly every HRSG manufacturer and performed numerous failure analyses on HRSG components.?