April 1, 2010

Deciphering Desuperheater Failures

Tyco Flow Control

The "combined" portion of a combined-cycle plant is the heat-recovery steam generator (HRSG) that generates high-pressure and high-temperature steam and the steam turbine generator that expands the steam to produce electricity. Integrating the HRSG and steam turbine with the combustion turbine is a key challenge for plant designers, as each system has differing operating profiles, operational constraints, and design requirements. The combustion turbine can rapidly start, yet the rate of heating the large mass of metal tubes in the HRSG limits the combustion turbine start-up ramp rate. Some plants have resorted to "temperature matching" distributed control system software routines that carefully manage the turbine exhaust gas temperature during start-up to protect HRSG tubes and components. Likewise, the temperature, pressure, and rate of HRSG steam production is often limited by the start-up ramp rate of the steam turbine.

Many Operating Problems

Most commercial combined-cycle plants were designed and constructed for baseload service where rapid start-ups and shutdowns were infrequent. When natural gas prices rose, many of these plants were relegated to summer cycling service — some twice a day — followed by months of inactivity during the winter when demand for electricity eases. During the 1990s, the design and operating temperature requirements for desuperheaters used in HRSGs to condition the steam supplied to the steam turbine increased from below 900F to over 1,050F. In most cases, probe style desuperheaters (also called attemperators) were not designed to deal with these elevated steam temperatures, much less the added thermodynamic stresses that came with cycling service.

Component problems in the main and reheat steam systems were some of the first to experience potentially catastrophic operational problems in these high-temperature steam systems. The favored design approach for matching steam temperatures with those required by the steam turbine is to insert a desuperheater between primary and secondary superheater and reheater sections in the HRSG. When the desuperheater is not operating correctly, prolonged exposure to the higher-than-specified steam temperatures in the reheater and superheater can damage expensive equipment and lead to unsafe operating conditions for tubes and surrounding components. Not only are bent or cracked pipes extremely dangerous, but plants forced to shut down for costly repairs will lose electricity sale revenue and may be required to purchase expensive replacement power.

The desuperheater operates by injecting condensate into high-temperature steam to precisely match the steam temperatures required by the steam turbine. This temperature-matching function is most important during system start-up and shutdown to prevent large temperature gradients in the HRSG or steam turbine steam supply. When the desuperheater fails to temper the steam correctly, even a single large overspray excursion can damage steam turbine internals, cause costly tube leaks, and significantly reduce the steam turbine efficiency. Wet steam can also quench regions of the steam pipes, causing additional, long-term problems. Heavy desuperheater sprays during start-up can, over time, initiate cracking in HRSG tube joints or even distort the shape of tube banks. Prolonged operation outside the design-operating envelope will produce accelerated fatigue damage to the pressure parts.

Some plant designs have also incorporated a desuperheater after the secondary superheater pass to reduce heavy sprays by the first desuperheater and to trim the steam temperature prior to entering the steam turbine. These superheater and reheater attemperators are exposed to temperatures routinely over 1,000F and have exhibited seat leakage and nozzle cracking; in some cases they have even contributed to premature failure of the surrounding pipe.

Low-load operation is also problematic. A typical industrial combustion turbine’s exhaust temperature remains well above the design steam temperature even at low loads. Also, with lower gas flows, the heat transfer usually moves forward in the superheater section, where the exhaust gas first contacts heating surface. This design peculiarity means that heavy spraying of the superheated steam to maintain design steam conditions has on occasion allowed "wet" steam to enter the steam turbine. The same is true for the reheaters, assuming they are not designed to operate dry during start-up. Wet steam can enter the cold reheat lines of the HRSG, causing the same type of steam turbine problems.