Desuperheater Cracking 1

[fwoody2000]

I am currently experiencing cracking in several desuperheaters, and cannot work out why, as they have been ok in the past. The only condition that I can see that does not meet the the data sheet requirements is the spray water pressure, which is 1 -2 Bar high. Is this likely to cause problems in the desuperheater?

Thanks for any help.

 

[metengr]

Cracking in desuperheaters can be caused by either thermal fatigue of seal welds in the nozzle or liner attachment or mechanical fatigue from steam flow induced vibration in service.

Where are the location(s) of cracking? Have you performed a visual inspection of the desuperheater nozzle and liner?

 

[fwoody2000]

The desuperheater has been visually and subject to some NDT and the cracking appears to be in the nozzle and the internal surface (liner?) of the desuperheater.

 

[metengr]

Thermal fatigue. This could explain your performance degradation. Change out the nozzle and replace the liner at first opportunity. The last thing you want to happen is to loose the nozzle downstream or break-up the liner in service.

 

[davefitz]

One technique used to reduce the thermal fatigue damage of the spray nozzle is to ensure the water source is at the highest possible temperature. If teh water has been stagnant in the line, it will cool to room temperaure, so some users provide a warmup bleed that continuously drips some water thru the supply pipe ( dripped to drain or condenser) to keep the water pipe hot.

Another trick is to de-tune the temperature control loop. If the water control valve is cycling open-close many times per startup , and each quench- warmup sequence counts as a fatigue cycle, the best way to extend the fatigue life is to reduce the frequency of the open-close cycles of the valve.

Yet another means of extdending the life of the spray nozzles is to replace the older injection-quil style simple spray nozzles with tangentially mounted , spring loaded , wide turndown spray nozzles. The old- fashioned injection quil style nozzles , with stainless bodies, would develop thte initial crack in the body of the nozzle due to thermal stress. Traces of Chlorides in the spray condensate would attack this initila crack and the crackmay spread due to chloride stress sensitization. Final failure is caused by hi cycle fatigue , due to fluid elastic vibrations of the quil- its profile in the main flow path will excite such vibrations.

 

[davefitz]

The line cracking is similary induced by thermal fatigue, and made worse by chloride stress sensitization ( if the liner is stainless steel) , and final failure is due to hi cycle fatigue from fluid elastic vibrations. The steam will often bypass thru the annular clearance betweent he liner and steam piping, and this will indice the vibration.

Designs that avoid this failure mode include:
a) do not use stainless steel - use an inlcolnel or P91 liner
b) seal weld the inlet section of the liner to steam pipe ass'y. If then is makes the ass'y too rigid for thermal stress reasons, then one can evaluate the use of thermal stress reliefs cut into the liner body.
c) the best design is one that kinetlically defoms the liner to the exact ID of the steam pipe- so called " detna-formed" incolnel liners.

 

[toothless]

What about cracking due to attemperation water impurities such as sodium or chlorides? That causes cracks in steam lines so it seems it could do the same in your desuperheater spray nozzle. Are you sure your attemperation water is pure?

 

[fwoody2000]

Many thanks for the posts. Having had a look at the system, it appears that we have a large dead leg on the spray water system (when not in use) allowing the spary water to drop to ambient temperature. From davefitz post advice, we will add bleeds (themostatic control valve) to the spray system to keep the spray water at temperature and hopefully this should help solve the problem.