Information request on StressCorrosionCracking in Anhydrous Ammonia se 1

[llls]

Currently I am reviewing the need for PWHT of anhydrous ammonia and ammine piping systems as a service requirement. Therefore I would greatly appreciate your feedback to following questions:
1. Is it the best industry practice to heat treat weld joints in anhydrous ammonia & ammine process pipe lines as a service requirement to minimise the risk of SCC damage?
2. Is there any reported incidents related to SCC damage in process piping in ammine or anhydrous service? If so how was the defect found / Did the failure cause damage to plant, people / Service life of the line etc?
3. What type of non-intrusive (external) inspection methods do you use to look for SCC in, say 50mm to 100mm NB piping systems? (We are planning to use UT crack testing, as it is not practical to use MPI)

 

[deanc]

http://www.nationalboard.org/Classics/articles.html

 

[unclesyd]

Here is another good paper. There are quite a few papers out there. I think the NH3 producers have one.

http://www.ncedaust.org/peter3.htm

You can rest assured it happens as we had two Horton spheres crack.

 

[llls]

DEANC & unclesyd thanks for the replies. I agree there is a lot of SCC related papers published on storage vessel. But todate I have not found any on that relate to process plant piping systems.

 

[stanweld]

As a Contractor installing anhydrous ammonia piping systems in accordance with Owner/Engineer's specifications, we have seen carbon steel without PWHT, carbon steel with PWHT and TP 304/304L and 316/316L stainlees steel specified. I have seen almost no uniformity in Engineering specification requirements. Note that all such sysyems operate in similar temperature/pressure ranges.

 

[llls]

Standweld,
Yes I agree. That is why I am doing this review as it is crazy to see some companies do specify that PWHT of anhyrous ammonia service lines but then do agree to waiver the PWHT of weld joints closer to I/V's (in order to protect the valve been damaged during the heat cycle).

 

[unclesyd]

This is why you can see varying requirements on PWHT of pipes and vessels.

At one time until around 1980 we had tons of Anhydrous NH3 whizzing allover the plant site mainly in A53 piping. We pumped it to 6250 psi, distilled it at 450 psi, vaporized it at 300°F and so on. Nothing was ever underwent PWHT. In fact we welded the High Pressure (6250) tubing with OAW until the process was abandoned.
Over the Holidays in December a line cracked and leaked in a elbow on a 2" Sch 40 line. Against company policy it was welded up as they didn’t want to bring someone in to analyze the failure. In February this happened again, no failure to analyze. My boss became very disturbed at the these incidents and we started to check some other NH3 piping. We found several small cracks that were determined to SCC, the exact mechanism was unknown at the time. About this time we got our first look in one of 2 Horton spheres. This so happened to be the newest one made from the stronger, better, material than the original. It was cracked in and around the welds from top to bottom. After repairs and this one returned to service and the oldest, weakest, was opened and checked. It had the same type of cracking, probably 10% of the amount found in the newest sphere. After repairs it to was return to service. Around 1990 NH3 storage was changed to cryogenic and both spheres dedicated to other services. All C/S piping in this system is PWHT. S/S piping as welded.
We made amines and used amines, MEA. The MEA vessels and piping were not PWHT. All the amine process piping that operated above 150°C was stress relieved. The older columns, prior to 1974 were not stressed relived. Any column after that was.
For as much piping and equipment we had in service I say we had very little trouble other than the Spheres. Until taken out of NH3 service the spheres were checked yearly with AE.

 

[deanc]

A good source could be the Jurisdiction of installation and the Insurance carrier for the equipment.

 

[BenThayer]

I worked at an ammonia producing site for many years. SCC in vessels was very common place and some Horton spheres were removed from service because of this. Particulary the high purity (99.999%, <10 ppm H2O) vessels were susceptible to this. The articles referenced sound very much like our experience. We did not see a lot of failures that were specifically identified in pipe that was attributed to SCC but a lot of peculiar failures in the past were suspected as SCC as we became more familiar with it due to the vessel inspections.

For piping we first adopted A-106 as an upgrade from A-53 (for -20°F and greater) and started doing PWHT on all welds. Then some random x-rays showed that we were getting cracks after the PWHT. We then adopted 100% xray after 100%PWHT. Another problem we fought was under lagging corrosion due to the systems operating at -28°F and 34°F and the piping was wet or iced due to old insulation. We finally decided to stop fighting carbon steel and exterior corrosion and PWHT and adopted 304SS. Installed costs for B-31.3 x-ray requirements, no painting, no PWHT contractors made it less than expensive than the A-106.

Another advantage with the 304SS is that we did not have to worry about availability for the A333 for the -28°F service and always had material available for repairs. Try to find low temperature carbon steel equipment on short notice. It may be better now, but I doubt it.

It may be cost effective for a major grass roots project to install the A333 and A106 and PWHT, but it is a major pain to maintain once the project team leaves the site.