HRSG Bypass Dampers – Eliminating the Flap


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If you want a more efficient turbine/heat recovery steam generator combination, look no further: Forney Corporation offers an innovative solution to standard bypass damper systems.

HRSG Bypass dampers are installed between a gas turbine and the heat recovery steam generator (HRSG) on a combined cycle plant. Therefore, the dampers must be designed for the extreme conditions they are subjected to. Forney’s advanced damper system includes a robust design, efficient operation, and improved reliability. The damper was designed with safety in mind and reduces operational hazards and maintenance costs while increasing uptime for more direct power generation.


As depicted above, Forney utilizes 2 Bi-Plane dampers instead of one large flap damper. The Diverter Damper system controls the flow of turbine exhaust gas (TEG) between a bypass stack or the HRSG.  Forney’s HRSG isolation damper allows exhaust to enter the HRSG while the bypass damper is closed, and upon closing the isolation damper and opening the bypass damper, forces the exhaust gases up the bypass stack. Forney’s system also has a mechanical interlock that prevents both dampers from closing simultaneously, reducing the risk of inspection and maintenance hazards.

Forney’s dual damper arrangement, is designed for peaking or standby systems for covering peak electrical needs. It allows the HRSG to go online quickly, and shuts down the flow to the HRSG when peak requirements are accomplished.

bypassDamper_flowStartupThe common single-blade, or flap damper system, at startup directs exhaust into the HRSG at the bottom section of the inlet duct, creating a thermal gradient across the duct and HRSG. However, the poor air flow from the slightly open blade at start up cause’s a high temperature difference which induces stresses in the structure that shortens equipment life. The flap damper, with its shaft mounted at one end, is very difficult to maintain a straight blade for proper sealing. Due to the lack of reliability for a flap damper, a guillotine damper is then provided behind the flap damper to insure safety. Plus, heat will be escaping into the stack when the HRSG is in service, lowering overall efficiency.

Unlike the single-blade flap dampers, Forney’s dual, Bi-Plane damper configuration brings exhaust to the stack and HRSG more evenly, greatly reducing the temperature gradients across the structure. In the same vein, dual multi-blade dampers allow for a ‘soft start,’ a gradual transfer of turbine exhaust from the bypass stack to the HRSG, which significantly lessens the thermal shock on both. Less stress means longer equipment life and reduced maintenance costs.  Since the Bi-Plane design incorporates an air seal, no guillotine damper is required.

The multi-blade design also enables more operational control, so the operator can better match the steam generator’s output to the existing load, increasing efficiency. Without a damper system, the only control parameter available is to constrain the gas turbine output, causing the loss of saleable power.

Safety is built into Forney’s design in two ways.

The first system prevents simultaneous closing of the HRSG inlet and the bypass damper, by mechanically tying the two dampers together. The design ensures the turbine exhaust cannot be completely blocked by requiring one damper is 80% open.  A mechanical interlock between the HRSG inlet damper and the bypass damper keep both dampers open, locking the open damper by over 80o while the mating damper closes by over 10o.  This linkage ensures padlock protection for maintenance personnel with the HRSG isolated.

The second safety method uses an air seal system that pressurizes the blade cavity higher than the exhaust pressure creating an air curtain that allows entry into the HGSG inlet for inspection or maintenance. It is designed to maintain temperatures below OSHA requirements (140 degrees F), necessary for areas classified as confined spaces.


The damper blade itself is a truss design (or bridge structure) that creates a cavity for the air seal system. In this system, two 200% capacity fans help maintain the seal air to the damper when in the closed position. To reduce seal air requirements, our damper design uses a flexible seal pack to provide sealing for the blade tips.  These seals are used on the upstream and downstream edges of adjacent blades and are bolted to the blade edge.

The truss design also maximizes the rigidity of louver blade allowing the damper to operate in rapid temperature changes experienced in this application. The blades are also insulated on the upstream side, reducing thermal losses across a closed damper.

Carbon packing with a purged lantern ring is also used in this application to prevent exposure of the carbon components to corrosion and temperatures above the 753oF (400oC) oxidizing temperature.  It is equipped with a close-coupled self-aligning flange block bearing assembly with a high temperature insert. While it allows shaft movement in both rotational and axial directions, it does not require external lubrication that can attract dirt and abrasives. The shaft itself is made of 17-4PH ‘high strength’ stainless steel.

When evaluating a HRSG Bypass Damper system remember the basic requirements it should meet:

  • The damper must guarantee zero damper leakage when closed to either the bypass stack or the HRSG under maximum gas flow conditions.
  • The HRSG should be safe for entry by maintenance personnel with the combustion turbine running at full load, when the dampers have the HRSG isolated, and the seal air system operating with proper ventilation.