Ammonia Let Down Vessel Cracks
Ammonia Let Down Vessel Cracks
(OP)
Dear All,
We recently observed some cracks on one of our Ammonia Let Down Vessel. Mentioned below are brief details of the equipment:
Service: Ammonia
Shell Material: A516 Gr. 60 N
Operating Temperature : 2oC
Operating Pressure: 26 kg/cm2 g
Shell Thickness: 32 mm
PWHT: Yes
Vessel Orientation: Horizontal
While Ammonia SCC is the common cause of cracks observed on Ammonia service equipment, the orientation and shape of these cracks is somewhat different than what we have observed in the past. Depth of the crack varied between 8 mm to 15 mm. Spot hardness was measured on the vessel and while most of the readings were within acceptable range, hardness readings near cracked nozzles was observed to be as high as 273 HB with an average value of 200 HB. Attached is picture of one of the nozzles with these cracks. Need your expert opinion on the possible causes of these cracks and in particularly what does the shape of cracks indicate regarding the stresses present in the vessel.
Regards,
Ahsan
We recently observed some cracks on one of our Ammonia Let Down Vessel. Mentioned below are brief details of the equipment:
Service: Ammonia
Shell Material: A516 Gr. 60 N
Operating Temperature : 2oC
Operating Pressure: 26 kg/cm2 g
Shell Thickness: 32 mm
PWHT: Yes
Vessel Orientation: Horizontal
While Ammonia SCC is the common cause of cracks observed on Ammonia service equipment, the orientation and shape of these cracks is somewhat different than what we have observed in the past. Depth of the crack varied between 8 mm to 15 mm. Spot hardness was measured on the vessel and while most of the readings were within acceptable range, hardness readings near cracked nozzles was observed to be as high as 273 HB with an average value of 200 HB. Attached is picture of one of the nozzles with these cracks. Need your expert opinion on the possible causes of these cracks and in particularly what does the shape of cracks indicate regarding the stresses present in the vessel.
Regards,
Ahsan





RE: Ammonia Let Down Vessel Cracks
Remove the cracks, re-weld and perform a proper PWHT using UCS-56.
RE: Ammonia Let Down Vessel Cracks
Regards,
Ahsan
RE: Ammonia Let Down Vessel Cracks
What kind of nozzle reinforcement is available on the other side of the shell plate? How thick the nozzle itself?
What are the design code and the specification for the vessel?
RE: Ammonia Let Down Vessel Cracks
Each of the nozzle has an external reinforcement pad installed of 8 mm thickness with fillet weld throat of 6 mm.
MDMT of -20 o C and Design Pressure of 30 kg/cm2 g was used for design calculations.
With vessel Operating temperature of 2 o C, are you implying there might be a chance that localized cooling may be the cause of these cracks?
Regards,
Ahsan
RE: Ammonia Let Down Vessel Cracks
It is a surprise for me that an external plate reinforcement was used for this kind of vessel. My expectation was the combination of slightly increased shell wall thickness (might be the shell wall thickness used for the segments at the connected to columns) and self-reinforced nozzle or self-reinforced nozzle only. However the thermal cycling, if available, shall require the nozzle details completely to eliminate the cracking like this.
If there are other similar vessels (or this vessel) you may be able to verify the temperature cycle availability. However you need to measure the metal temperature from inside (may not be possible during operation, but nozzle wall close to the shell wall might give an indication) shell wall, nozzle wall and outside reinforcement at several locations to be more accurate.
RE: Ammonia Let Down Vessel Cracks
The second paragraph last sentence should be read as "However the thermal cycling, if available, shall require CHANGES the nozzle details completely to eliminate the cracking like this.".
RE: Ammonia Let Down Vessel Cracks