Ammonia Let Down Vessel Cracks

[sahsanb]

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

 

[metengr]

The hardness readings that are above 220 HB concern me, especially the 273 HB hardness and indicate one or more of the nozzle attachment welds may not have been subjected to a proper PWHT. The cracks could be caused by one of two conditions - either delayed cracking from hydrogen introduced during the welding process (more likely because of appearance and orientation) or ammonia SCC.

Remove the cracks, re-weld and perform a proper PWHT using UCS-56.

 

[sahsanb]

Thank you for your feedback. We did undertake repairs as stipulated by you. However I am still interested in knowing what does the shape and orientation of cracks indicate regarding the stresses present and their direction. Is it something you have encountered often in your experience?

Regards,
Ahsan

 

[saplanti]

Interesting cracks. It also look like a thermal cycling is available at the nozzle area and around.

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?

 

[sahsanb]

Design code for the vessel is ASME Sec VIII Div. I
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

 

[saplanti]

"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?" YES

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.

 

[saplanti]

In my previous thread "....(might be the shell wall thickness used for the segments at the connected to columns).." should be replaced by "...(might be the shell wall thickness used for the column connected segments)...".

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.".

 

[metengr]

If there is indeed local cooling as mentioned above resulting in thermal fatigue, you will see cracks re-occur at the affected nozzles. Right now with the original hardness reported, I would still be concerned until PWHT from repairs can knock the values down to 220 HB or less.