Tube Overload Failure

Posted in Failure Blog

During the startup of the triple-pressure HRSGs, plant staff noticed steam coming from the stack and a discrepancy between feedwater and steam flows.  On shutting down the unit a T23 superheater tube was found with a complete circumferential failure.  The failed tube was removed and the bore was plugged with some difficulty due to poor access and the uncommon material.

Tetra Engineering performed a metallurgical analysis of the failed tube finding that the failure was due to pure mechanical overload (see shear lips on the fracture surface).  It was suspected that liquid water would have entered the tube while it was still warm.  The difference in average temperature of the failed tube compared with adjacent tubes would have led large thermal loads in the tube.  The figure below shows how the failed tube was situated directly underneath the header outlet nozzle, upstream of the final attemporator spray.  Tetra recommended to check if the valve on the attemporator spray was passing water.  Investigation by plant staff found that it was indeed passing significantly.  An investigation is under way to determine why the water did not drain correctly.

Mechanical overload of superheater tubes is a common occurrence in combined cycle plants and requires a two-pronged approach to prevent recurrence: identifying the source of liquid water (spray water leaks, condensed steam, etc) and then understanding why the fluid did not drain correctly (pipe gradient, drain system capacity, etc).  Typically thermal loads are less likely to result in failures by relaxing once the steel has yielded.  This is one example of a failure type where the loads can exceed the tensile strength of the material.  Note that leaking of spray water into the steam pipe during operation can cause thermal shock of the pipe steel, leading to cracking and failure. Non-destructive examination (NDE) methods can help detect defects before they become problems!

 

Condensate in Superheater from Attemporator Spray Leak

 

Sootblower Erosion

Posted in Failure Blog

Sootblower Erosion Oil-fired boiler

Several economizer tubes in an oil-fired boiler failed near the passage of a soot-blower lance.  Mechanical wear was found on the external tube surface.  Possible causes including fretting by lance on tube and droplet or steam impingement.  Sootblower design and the boiler operating history were analyzed in conjunction with a metallurgical analysis of one of the failed tubes.  The cause of the erosion was identified as wet steam droplet impingement, the root cause being a combination of insufficient clearing of condensate from sootblower piping and excessive cycling of sootblower during operation.

Overpressurisation Failure

Posted in Failure Blog

The LP Economiser Pressure Relief Valve had been incorrectly installed on a horizontal gas path / vertical tube HRSG. Heating of the isolated volume of water resulted in excessive pressure and rupture of one of the lower headers.

Access for repairs was difficult as the header was within a bundle. It was eventually repaired by lowering the whole harp, cutting out the damaged section (see photo), and welding in a new section of header and tube stubs.

Under Deposit Attack

Posted in Failure Blog

A radiant boiler having problems with chloride ingress began having many multiple failures of waterwall tubes, all with similar characteristics.  The underlying mechanism was understood to be buildup of deposits containing chlorides (Under-deposit Corrosion) which would insulate the steel from the cooling water flow.  The metal underneath the deposit would have been additionally weakened due to hydrogen embrittlement.  The embrittled metal would fail either by corrosive attack or in this case by localised overheating. 

HP Main Steam Pipe Failure

Posted in Failure Blog

The sudden rupture of the main steam outlet pipe at one of the HRSGs providing steam to the steam turbine was reported to have seriously damaged piping upstream and downstream.  Luckily no injuries were reported.  The fracture occurred suddenly (no leak seen prior) at the weld between P91 pipe and Stainless Steel Flow Meter.  The rupture was brittle and clean occurring at the interface between the Inconel weld and the P91 pipe.  Dissimilar Metal Weld failures for these materials are not uncommon, especially in a cycling regime.

 

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Established in 1989, Tetra Engineering has more than 25 years experience providing solutions to the power industry. We specialize in solutions for HRSGs, conventional boilers & steam-cycle balance of plant.

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