FCC unit, expander inlet flue gas line weld cracks 2

[0707]

FCC unit, expander inlet flue gas line weld cracks

At FCC unit expander inlet, in the weld between 48” flue gas pipe and expander by pass valve, we experienced a few cracks on the weld and heated affected zone. This pipe is subjected to intermittent steam injections for thermal shocks, to remove catalyst-aggregated particles on blades of expander rotor. Steam injection is made 1 meter downstream the cracked weld. Can we assume that those cyclic thermal shocks were responsible for the fatigue cracks on the above weld? Valve body and 48” pipe material is A312 TP304H

Regards
Luis Marques

 

[unclesyd]

Can you give little more information on relative temperatures of the steam and piping?

Can you describe the cracking, direction, length, any pattern?

What does the inside surface of the pipe in the area of the cracks look like?

A good guess at this point would be differential thermal fatigue.

 

[stanweld]

Have you seen any distortion of the pipe near the affected weld?

 

[0707]

We are scheduling a shutdown for the repairing, the line is still working and for this reason SCC, Sigma phase or other fail type can only be evaluated through metalographic replicas, which at the moment are impossible to perform.
Because of thermal socks procedure, to spall catalyst particles on rotor expander blades, we think that thermal fatigue is only a hypothetic cause for the failure occurrence not the definitive one. Cracks were found on a circular weld on top part of the 48-inch pipe at each side of the pipe diameter. Cracks are 600mm length and 1 to 3mm wide. Left crack is on center of the weld. Right crack is on HAZ; a small distortion of about 3mm is seen on HAZ of right crack.
Steam injection is done on top of the 48 inch pipe 1 meter way and downstream of the circular cracked weld. Diameter of steam injection pipe is 6 inch. Flue gas temperature is 700ºC and the cooling rate for the thermal shock is 150ºC/h. When pipe temperature reaches 400ºC is injected superheated steam of 24-bar g misted with boiler feed water to spell catalyst particles of rotor expander blades to bring machine vibrations to limited acceptable levels.

Meanwhile your inputs are always welcome.
Regards
Luis Marques

 

[unclesyd]

I will be willing to bet that it is thermal fatigue. Steam is bad enough but when you throw in water it is a killer. As you take the line down make sure you take a quick close look to see if you can see a spiral pattern in the cracking. Look for a multitude of very small cracks. The surface maybe crazed, that is it will look crystalline.

I personally would start trying to locate some replacement pipe.

 

[stanweld]

I too suspect thermal fatigue or thermal shock as the primary cause of cracking. Have you performed thermal imaging? I had a case similar to yours and thermal imaging revealed a delta temperature of over 800 F within an inch about the pipe circumference due to non-uniform water spray quenching. This led to effective redesign of the desuperheater.