October 15, 2007

Focus on O&M (October 2007)

COMBINED-CYCLE RELIABILTY

Why bypass desuperheaters fail
 

Over the past few years, Thielsch Engineering has been retained several times to inspect, evaluate, and/or repair a through-wall failure of a bypass desuperheater at a combined-cycle plant. This kind of desuperheater is operated for short periods of time during plant start-up—to reduce the temperature of steam entering the steam turbine condenser and to maintain a flow of low-temperature steam through the heat-recovery steam generator (HRSG)—until steam conditions are sufficient to start up the steam turbine.

 

1. Poor track record. Bypass desuperheaters with unlined piping and multiple spray nozzles should be inspected often to ensure their continued availability. Courtesy: Thielsch Engineering

Like standard desuperheaters, bypass desuperheaters extract high-pressure feedwater from the HRSG's supply line. The water is sent to a valve control station, which sprays it directly into the flow of main steam, reducing its temperature and pressure. The process enables the steam to be recaptured without doing harm to downstream components.

In most combined-cycle designs, however, the in-line piping of the bypass desuperheater lacks the internal lining commonly found in the piping of a standard desuperheater. As a result, spray water makes direct contact with pressure-boundary piping and produces very high thermal and mechanical stresses that can cause multiple through-wall failures within a short time.

The bypass desuperheater design that has proven most prone to failures has a thick-walled fitting for multiple spray nozzles around its circumference (Figure 1). Failures typically occur on the downstream side of the desuperheater, at the circumferential weld joining thin-walled downstream piping. The cracks that cause the failures usually are found on the inside diameter of the weld (Figure 2).

 

 

Another common type of failure occurs when thermal gradients created by desuperheater operation apply bending stress to the piping system. In these cases, the telltale cracks appear on the outside diameter of the weld (Figure 3). The results of Thielsch's finite-element analyses of the magnitude of the stresses on the desuperheater joint indicate that cracking may be initiated after fewer than 100 operating cycles.

3. Bend over backward. The thermal gradients produced when spray water makes direct contact with pressure-boundary piping create bending stresses. Courtesy: Thielsch Engineering

 

A third type of observed damage has been rupturing of the internal diffuser of bypass desuperheaters (Figure 4). Again, the cause was diagnosed as thermal shock, produced by the contact of cool spray water with hot internal components.

4. Hidden damage. Thermal shock destroyed this bypass desuperheater's internal diffuser. The unit's fitting was removed to show the extent of the destruction. Courtesy: Thielsch Engineering

 

—Contributed by Peter R. Kennefick (pkennefick@thielsch.com) of Thielsch Engineering Inc.

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