Power: Business And Technology For The Global Generation Industry

Deciphering Desuper-
heater Failures
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 tur-
bine generator that expands the steam to
produce electricity. Integrating the HRSG
and steam turbine with the combustion tur-
bine is a key challenge for plant designers, as
each system has differing operating profiles,
operational constraints, and design require-
ments. The combustion turbine can rapidly
start, yet the rate of heating the large mass
of metal tubes in the HRSG limits the com-
bustion turbine start-up ramp rate. Some
plants have resorted to “temperature match-
ing” distributed control system software
routines that carefully manage the turbine
exhaust gas temperature during start-up to
protect HRSG tubes and components. Like-
wise, 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 op-

erating 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.

Minimum Design Criteria
A well-designed desuperheater will produce
atomized water droplets that are completely

vaporized before entering the superheater or reheater header and will produce an atomization pattern that completely fills the pipe to eliminate bypassing and formations of thermal gradients. Properly installed desuperheaters should also be installed in a straight section of piping with two to eight diameters upstream and 12 to 20 diameters downstream—not always possible in today’s very compact plant designs, but nevertheless, this remains a good design criteria.

Ensuring that no water droplets remain in the steam is paramount. During operation, 15 degrees of superheat is desirable, 50 degrees is best, downstream of the desuperheater. Also, the thermocouple providing the temperature signal to the desuperheater should be mounted downstream of the desuperheater, perhaps as far as 25 feet away, where all the water should have been evaporated. Finally, good design practice is to include a drip leg to eliminate any condensate buildup in the steam header and to slope the header away from the desuperheater.

Good operating practice is to closely monitor the desuperheater during those periods when sprays should be secured. Few desuperheaters are capable of full shutoff after a few years of service. ASME Section I of the boiler code provides guidelines for these and additional design requirements.

New Desuperheater Option

Tyco Flow Control has developed a new desuperheater valve for combined-cycle power plants that addresses each of these operating concerns, especially for plants when steam temperatures exceed 1,000F with excursions up to 1,150F. The first installation of the Yarway TempLow HT technology desuperheater recently completed a five-year run at Iberdrola’s Santurce Plant located in Vizcaya, Spain.

Tyco Flow Control recognized the challenge emerging in the marketplace as more and more plant managers expressed their frustration with standard construction desuperheaters failing above 1,000F and the costs and operating penalties that created for them. The company decided to take a step back to research a safe, cost-effective, and scalable solution. The company embarked on a three-year study with the goal of bringing a new product to market that could function successfully at 1,150F, and the risk paid off.