Heat Recovery Steam Generators

Heat Recovery Steam Generator (HRSG) tubes are the interface where useful energy from the waste heat in gas turbine exhaust is extracted and converted to steam.  HRSGs have undergone substantial changes in design over the past 20 years as the technology matured, starting from small single-pressure units driven by aeroderivative gas turbines at cogeneration facilities to today’s large triple-pressure with reheat units with steam pressures in the high-subcritical region [1].  Besides fundamental design differences, modern HRSGs are also subject to a wider range of duty in power applications.  Baseload operation was normal for earlier HRSGs, but today’s deregulating electric power market imposes that many new large HRSGs operate in cycling or flexible duty.  As with tubes in conventional radiant boilers, HRSG tubes are subject to a variety of service related damage.  Damage mechanisms vary in severity depending on the tube material composition, operating temperatures, magnitude and frequency of stresses and presence of corrosive products.

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 :

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. 

• 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 be for 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

The limiting HRSG issues 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. 

References

[1] D. Moelling, P. Jackson et F. Anderson, HRSG Tube Failure Diagnostic Guide - 2nd edition, Tetra Engineering Group Inc, 2004.