Leakage from Urea Reactor Nozzle 3

[franklin55]

There is a leakage from the nozzle of our urea reactor through weep holes (02 weep holes in the nozzle body), which indicate the liner inside is somewhere damaged. I am attaching its drawing too. The nozzle is attached to the top hemihead. Can somebody guide how to repair?

 

[vesselfab]

the first thing you have to do is

drop the pressure

gas free the reactor

do a LPT examination all around attachment weld for sleeve liner weld to shell and sleeve liner to flange.

find the bad spot if it is not visible

then write out your repair procedure depending on the type of repair to be done and notify your AI for NBIC

repair the weld or bad spot in liner

pt the welds

bolt it back up

 

[iqbaliqbal]

The nozzle inside diameter is 92 mm and O.D is 237 mm (hence t=72.5 mm). The problem is: 1. How to do LPT inside the liner sleeve; we can only do the LPT at the liner inside joint (not the sleeve because of difficulty in accessibility)). 2. How can we find the extent of corrosion in the nozzle body behind the line (I mean corrosion of C.S portion). 3. If the damage is located by some means inside the nozzle (the portion between nozzle flange and joint with vessel hemihead), How to repair it. I am asking this because the nozzle inside will not be accessible by welding repair.
Btw the liner material is 316L UG and the nozzle body material is C.S.

 

[iqbaliqbal]

Thanks for reply ..I want to know:
1. How we can do LPT inside of nozzle (550 mm length from vessel inside to nozzle opening).
2. Besides repairing the liner we also need to find the extent of damage done to C.S portion of the nozzle; so that it may be repaired if badly damaged due to corrosion by carbamate solution). (The O.D of nozzle extended portion is 237 mm and I.D is 92 mm so thickness is 72.5 mm)

Note that material of liner is 316L UG and nozzle body is of C.S)

 

[roca]

cut out the complete nozzle and replace with a new clad nozzle - unless it is absoloutely required to use a loose liner

 

[ak1965]

Dear franklin55

I have seen some difficult to find urea reactor leaks...and attended them..!


If the leak rate is high and it warrants and immediate shut down....you must shut down and evacuate the reactor...! Take care the leaky area must be kept hot with the help of steam hose to avoid choking of passage due to soildification of liquid carbamate....! Post opening the reactor & proper purging....Enter the reactor with proper scaffoldings and ladders....! Please do not allow any one to go inside with shoes with Ferritic nails....dangerous to reactor.....!

The air @0.5 kg/cm.2 should be injeted through the leaky weep hole & the corresponding weld seams on the liner inside, should be checked with soap solution...you will see the bubbles coming out from leaky point.....this must be done with extreme care and attention bacause the leaky points get squeezed up after cooling of reactor and it becomes difficult to locate the leak....! It is very much likely that you may get a delayed response from leaky point after pouring soap solution on it due to a very tight opening....so wait patiently after pouring Soap Solution....! Do not rush...!


Once your are sure of the leak location, you need to grind the affected point with a non magnetic griding wheel...! Carry out DP Test on affected area to confrim the extent of defect.Dont forget to clean the soap solution before DP Test...!

In addition, carry out a visual inspection of weld seams of whole reactor and mark out the points of undercuts , visual porosities, pin holes etc. Also mark the weld seams that has appreciably coorroded. It is a must because your company may not afford to open the reactor again and again...!

Also DPI should be done on all suspected weld seams.

Once you are through with marking of defects. make a weld repair plan....I believe there could be 10 trays in your reactor....!

Repair all weld defects sequentially...using ER316L..i believe the Liner material SS316L. Ask your welder to
fill the ground portion at leaky portion carefully using low currents and fuse the small porosities found in visual inspection and DP test without using a filler wire...! he has to just use the heat and fuse the parent material.

After you are through with weld repair.....Check the ferrite content on weld deposit...it should be less than 1....Carry out DP Test again on the leaky portion....!

Clear all garbage from the reactor....and box it up...!

All the best to you.....!

 

[franklin55]

Thank you very much Mr. ak1965 for your detailed reply.
You have elaborated the repair procedure for the shell specifically. The problem with our reactor is that the leakage is from the weep hole of nozzle only ( not from the reactor shell weep holes). This nozzle is attached to the top hemi-spherical head. The throat of the nozzle is approximately 600 mm and inside diameter is 92 mm. Can I draw conclusion from your discussion that: the liner can only leak from the weld joints (i.e joint with the adjacent piece of liner)? and if for example the leakage is from the inside portion of the nozzle , how can it be repaired? Also I want to ask that the nozzle body (whic is of ASTM A266 Cl.2 material) may have been damaged by coming in contact with carbamate solution. Is there any method to find the extent of damage to the nozzle body from inside???(i.e. behind liner)

Note that the liner material is 316L UG
Please also check the picture attached at the beginning of thread for details.

 

[ak1965]

Dear franklin55

Nozzle also should have a liner...my experience says that reactors always leak from the weld joints of liner initially....however if the leak is not attended and carbamate comes in contact with shell....the damages are far beyond imagination....!

It is rare that liner parent metal fails...as you know...weld joints are always more vulnerable to fail....! I have always found the leakage from the weld joints in a Urea reactor. The places having notches like corners of support cleats or crevices in the weld joints are more prone for SCC...! that is why I insisted for thorough inspection of all weld joints in reactor...! be assured..may look tough...but it would improve the reliability of your reactor...!

I am not aware the prodcution criticality in your case...because it takes 2 or 3 days for reactor to cool down enough for entry..& around a week to complete inspection , repair and box up....you to decide on this...

It is very relevant to ask about damage on nozzle body....!! Nozzle parent area being a small one and without multi layering body...you can easily scan the area with Ultrasonic flaw detection...that would reveal any thinning or damage if had taken place in that region....! you must do this...this is very critical....! However, before doing UT, do a simple experiment...inject steam from one weep hole...steam should exit freely from the other weep hole confrming that steam is not going elsewhere...

Normally the neighbouring weep holes are interconnected....so refer the weep hole drawing to know the exact lay out of weep holes. In your case 02 weep holes are leaky so they must be interconnected..through internal channels in engraved on nozzle body.

I used an innovative technique in our urea reactor with multilayered sehll construction to know the extent of damage caused during leakage from 02 weep holes in the shell.......I did the radiography of affected area..results were quite revealing...!and no damgewas seen on the sehll ...reactor is still doing well even after 6 years of repair....though i have left that organisation 2 yrs before...I still derive great satisfaction from the success of that job.

Hope above info should further help...!!

I know the urea reactors act naughty and fuzzy..once leak...so be extra careful...!

All the best.....

 

[franklin55]

Thanks again for your prompt reply.
We had thought same thing here ( I mean Ultrasonic Flaw Detector) as a means of detecting the damage to C.S portion material of the nozzle. But the radiography technique as you mentioned is really an innovative idea (In the sense that it can only give the comparison of the metal thickness at different points, and not the exact thickness)..Now if the nozzle thickness is reduced due to corrosion at some area..what should be the next step, (thickness of the forged C.S portion is 72.5 mm,how much reduction in thickness will be alarming
By the way your advice is directing us gradually towards the right direction.
Thanks in advance.

 

[ak1965]

It would be my pleasure if i could be of some use in reviving your the reactor...!

Radiography is indeed innovative, however, you would need a cobalt60 radioactive source for this thickness (72.5mm) but it ismore useful for urea reactors with multilayered shell. so better to go for an easier method i.e. UFD.

In my opinion, the chances of corrosion on forging is pretty less...& the leaky reactor can also run long unless
carbamate solution comes in contact with the shell...! however if carbamte comes in contact with shell material.. it can eat away CS at unbelievable rate.

Consider a case when the repair of the damaged nozzle forging is unavoidable...however, that would depend on the extent of the damage...ideally a nozzle with any appreciable thickness reduction should be replaced..but replacement being a big task, a local repair with metal deposit by welding can also be a good solution. However, you would need a nicely prepared WPS and a Post weld heat treatment for the repaired portion. The liner of nozzle may also be replaced, if found damaged.

So first inspect reactor carefully fix the extent & type of of damage......next action plan would come up accordingly.

Be prompt now and move on....by the way... which part of the world your plant is located is it Kellogg or Snam technology......?

 

[franklin55]

Thanks once again. we have planned the activity and I shall update as the course of events take place. Our plant is situated in the indian sub-continent and our urea plant is designed by TOYO of Japan (Its TOYO ACESS Process)

 

[dave5599]

Dear ak1965,

You have mentioned that things with ferritic content should not be taken inside the reactor. What is the possible damage due to ferrite to urea reactor???

 

[ak1965]

Dave5599

The presence of iron or ferritic materil can easily contamintae welding being done in the reactor......!

You may be aware that a contamintaed weld joint would easily corrode under seever corrosive conditions with carbamate.....! This is alsois a reason for using SS316 L with low carbon to avoid carbide preciptation due to presence of ferrite....!

 

[dave5599]

Dear ak1965,

You have mentioned that things with ferritic content should not be taken inside the reactor. What is the possible damage due to ferrite to urea reactor???

 

[dave5599]

Thank you ak1965 for explaining and sorry for repeating my question. (i repeated the question by mistake)
Anyhow I had asked the question because we used to install C.S pipe scaffolding during inspection work in reactor, but no welding job has been performed uptill now ( after 11 years of its operation)..I think that will not be harmfull..if there is no welding activity inside.

 

[ak1965]

dave5599

yes....normally scaffolding material should not cause the problem unless ferrite gets into the weld metal....risk is less....however...ferritic material should be avoided especially during welding inside the reactor...because any loose iron particle may prove catastrophic to welding repair.

I suggested franklin because they plan to weld inside...!