High temp cracking in steam pipework

[JonD71]

Hello all

Am having trouble with high temperature cracking of a gas turbine steam dump line. The 600mm dia line is made of CrMo and C/S welded together, though cracking is evident circumferentially in both materials. Cooling water is injected into the pipe when in operation at around 40 degC and steam is run under vacuum at approx 440 deg C.

Problem is I have to repair / replace this pipe and need to come up with refabrication in a different material or replacement of the C/S section with full length CrMo, though with the established cracking in the CrMo am not sure a replacement with the same material is feasible.

Hope this makes some sense?

Regards

JonD71



JonD 71
Australia

 

[unclesyd]

Without see the failure I would be inclined to believe that you are experiencing Differential Thermal Fatigue cause from the water spray hit the hot pipe especially if you are seeing multiple cracks.

This may not be your problem but is the first thing that pops up in a mixing situation.

Can you come back with a little better description of the cracking.

What is the configuration of the water spray nozzle?

 

[JonD71]

Unclesyd

Thanks for the prompt response.

Cracking has been identified by a metalurgist as temperature related and is multi facited circumfernetially around the pipe. The water injection nozzles are present as this is the steam bypass line and spray marginally constantly to improve their reaction time during steam discharging. My main problem is selection of a material for replacement or repair.

Thanks again

JonD71

JonD 71
Australia

 

[unclesyd]

The main thing you need to accomplish is keeping the water off the walls of you pipe regardless of the MOC. You need to redesign your water injection nozzle to make sure you atomize the water and keep if off the wall. As stated in previous post I've seen Differential Thermal Cracking with less than 100?F varying temperature differential on the metal surfaces.

With your present setup a change in material will not help. You have to stop the thermal cycling on the metal surface.

Your system should work like desuperheater in a steam line.

 

[metengr]

Agree with unclesyd - the water spray attemperation needs to be designed properly. Have you considered installation of a liner within the pipe run? A liner would prevent the atomized water spray from contacting the "hot" pipe wall.

 

[stanweld]

Yours is not an uncommon problem and has been solved by installation of a liner as metengr has stated. It may also be solved with resesign of the desuperheater/attemperator spray system or a combination of the two. If you immediately change material spec at the attemperator to carbon steel, you may also wish to extend the CRMO alloy downstream of the attemperator before making the material spec break.