HRSG LOW LOAD OPERATION:

Recognize Your Limitations Before Damage is Done

Today’s power market is pushing some combined cycle plants to operate at low loads for extended periods. Some plants practically give power away during off-peak periods so they are ready for the next day’s peak market. If power is being sold at a loss, the lower the load, the lower the cost of keeping a unit on during off-peak periods. Some turbine suppliers are offering new control logic that permit even lower loads than originally possible, some down to 30% of base load. The HRSG is impacted by combustion turbine low loads, and unexpected problems can occur.

Here is why:

• Economizer fatigue damage caused by low water velocities in sensitive circuits.
• Overspray of desuperheaters because of change in the relationship between turbine exhaust temperature and turbine exhaust mass flow.
• Evaporator problems, again driven by the change in relationship between turbine exhaust temperature and turbine exhaust mass flow.
• Sliding pressure operation of the HP System can affect steam separator performance and can also leave too little heat for proper LP System performance.
• SCR and CO catalyst performance will change at lower gas inlet temperatures. If the turbine operates with stack O2 below 15% (dry) at normal loads, and then above 15% at low loads, satisfying permit requirements will be even tougher.
• Acoustic vibration damage can occur if tube field natural frequencies match low load exhaust flow vortex shedding frequencies.

HRST can help you avoid low load problems by utilizing our performance modeling design engineering and field troubleshooting experience. If problem areas are found, we can identify improvement options. If your plant is part of a cogen, and your HRSG has duct burners, you may have the desire to operate at low turbine load, but yet keep steam production as high as possible, especially if the profits from steam sales are higher than electricity sales. HRST can help you assess this special situation and options, once again using our performance modeling and engineering expertise.

Lester Stanley, P.E.