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Urea Stripper Leakage Identification & Rectification

This article entails the chronic problem of Fertilizer Industry regarding leakage in Urea Stripper. The intent of this article is to highlight the issues faced in identification of Urea Stripper leakage  during plant operation without taking stripper out of service and then to rectify the leakage. It is a million dollar decision when to take Stripper out of service for leakage rectification.
Function of Urea Stripper is to decompose unconverted Carbamate into Carbondioxide and ammonia increasing Urea solution concentration. Usually heat required to decompose Carbamate is transferred from steam which is on the shell side and Urea solution on the tube side. It acts as a falling film evaporator. Due to Urea solution on tube side, the metallurgy of tubes, channel heads and tubesheet is the area of concern requiring material having super corrosion resistance. Normally, tubes are in Titanium, Duplex Stainless Steels, Zirconium or Bimetallic. Tubesheets and channel head covers are cladded or weld overlaid with corrosion resistant materials in order to save cost. Typical schematic for a Urea Stripper is shown in Fig. 1.

Stripper
Figure1: Typical schematic for a Urea Stripper

Urea Stripper is considered as the backbone of Urea Plant performance and efficiency. Its performance determines production and profitability of end product which is Urea Fertilizer. One can be out of Urea business for months if this piece of equipment is not taken care of in a proper way.
Important factors in identifying Stripper leakage
A Process Engineer is the first guy to raise the flag if there is any deterioration in Stripper efficiency and performance. Also if there is any leakage in Stripper tubes or tubesheet, it shall be picked up in steam condensate leaving Stripper shell side. This will appear in the form of high conductivity. Monitoring of fresh steam entering into steam drum and condensate leaving the steam drum is very important and its delta (increase in conductivity of condensate leaving steam drum with respect to incoming steam) will determine the extent of leakage.
First thing that comes into mind while going to look for leakage in such high pressure vessel is that, what are the potential points that can lead to leakage in such thick metal walls. There are three surfaces in contact with highly corrosive Carbamate solution
- Tubesheet
- Tube itself
- Tube to tubesheet joint
Tubesheet thickness is around 250-300 mm (about 10-12 inches thick) with about 10-15 mm (0.5 inch) overlay of corrosion resistant material when the pressure difference across the tubesheet is around 2000 Psi. Leakage would only develop once that overlay is eaten up due to bad operating practices like improper development of passivation layer, high operating temperature (as rate of corrosion is directly related to temperature) etc. Once that layer is damaged then the damage of tubesheet beneath that overlay is a matter of days and you would end up with leakage in your Stripper. However, if proper operating conditions are maintained and thickness checks done at specified interval, then it is the least bothering spot regarding leakage until unless there is a design flaw.
Tubes are usually manufactured from corrosion resistant material, and are under protection of good passivation layer as gases are closely in contact with tubes and keep building passivation layer, and if the thickness measurement thru ECT (Eddy Current Testing) is done periodically, tubes will never give a surprise leak.
Tubes in stripper are closely located, and there is no welding between tube and CS part of tubesheet. The only part of the tubesheet that is welded with the tube is the overlaid portion of the tubesheet which is normally 10-15 mm thick. This is the tube-to-tubesheet joint which has the maximum potential of getting corroded during service due to the HAZ area formation during welding. Welding of tube to tubesheet in strippers requires very high quality work. An air bubble trapped in weld or any other defect would ultimately lead to stripper leakage.

Stripper
Figure2: Typical operating conditions in a Urea Stripper

The extent of stripper leakage in different modes of leakages:
In case there is a leakage because of decrease in thickness of overlay material of tubesheet or tube metal, the leakage would be gradual and would cause quantifiable decrease in stripper performance like pressure rise in stripper chest side, decrease in steam consumption, slippage of unconverted Carbamate to D/S section. This is because the decrease in thickness would be uniform and will keep on increasing, however in case leakage is because of weld defect, the extent of leakage would be very small, sometime it would appear as if there was no leakage. It is a common observation, in such kind of leakages that leakage symptoms almost disappear once the plant is restarted after shutdown and then slowly increase to its earlier extent or worsen sometime. This proves that the leakage is so small that even thermal expansions caused by shutdown and restart of the plant are affecting the opening. This is quite possible that leakage may not be detected in such cases if the plant is shut down.
Conductivity meter (to measure conductivity of the steam condensate returning from stripper chest) is considered the most effective and economical way to detect a leakage, however sometimes it becomes misleading because of the factors like
- High conductivity of incoming stream
- Impurity in the incoming steam that may get hydrolyzed inside stripper shall side to cause increase in the conductivity at outlet.
Once the leakage is there then steam system (if total recycled) comes to equilibrium and delta of conductivity across the stripper does not show an appreciable rise in the conductivity. The important thing to consider in such kind of situation is to know where to start for leakage detection; otherwise you will keep looking for ghost leakage all around the steam circuit.

Observations regarding conductivity behavior once there is leakage in the Stripper:
There is always a difference in the conductivity of steam & its condensate if there is NH3 mixed in the steam.
Steam going to Stripper chest would indicate higher conductivity than condensate coming out of stripper chest, even with leakage in stripper, this thing is also visible in conductivity table for ‘steam and condensate circuit across the stripper’ given below.

 

17-Dec-07 20-Dec-07
(before Shutdown)
21-Dec-07
(After Startup)
18-Aug-08
(before Shutdown)
20-Aug-08
(After startup)

  

Cond (uS)     Cond (uS) Cond (uS)
600# steam to Stripper Steam Drum 103.5 101 48 68 69
Stripper Chest I/L steam 3222.5 2140 346 1566 377
Stripper Chest O/L (Vent point) 2212.5 1824 420 2390 722
Stripper Steam Drum outlet Condensate 117 110 62 114 92
Stripper Chest O/L (Drain point) 2247.5 1920 520   1568

Conductivity Table:
This data refers to a TOYO Stripper it was operated over a year with leakage. Leakage trend was wavy revealing that the leakage was due to some flaw in the weld in tube to tubesheet weld. Even operating with this leakage over a year, there was no damage done to the stripper tubesheet. It was confirmed with NDT technique applied to check the health of tubesheet.

Conclusions from the table:

- Actual conductivity pickup is shown by the difference in conductivities of steam drum inlet and outlet streams.
- Stripper chest inlet/outlet conductivity delta is always misleading.
- Conductivity delta across steam drum is decreasing once the plant is started after a short shutdown.
- If there is a leakage, than the conductivity between steam drum outlet and inlet would always be positive.

Welding issues
Figure 3: Corrosion and exposure of hidden defect
Burning
Figure 4: Burn thru area (HAZ) due to high welding heat
Pin hole leakage
Figure 5: Pin hole leakage from tube-to-tubesheet joint
stripper leakage
Figure 6: Corroded tube ends

Detection of Leakage:
After the confirmation received from Process Engineer, it’s the responsibility of Inspection group to look for the leakage in Urea Stripper. Both the channel sides of Urea Stripper have to be observed using the applicable leak detection methods.

Ammonia Leak Test
In this test, the mixture of ammonia and nitrogen gas is used to detect the leakage. The shell side of Urea Stripper is pressurized normally at 2-3 kg/cm2 and the tubesheet is covered with ammonia paper the color of which changes upon contact with ammonia.  Total time for conducting this test is almost 20 hours out of which 08 hours is the holding time. Partial pressure of NH3 is 15~30% of the total pressure inside the shell. Excess Partial pressure of NH3 would make difficult to point out the exact location of leakage.
After the holding time, entry is made in the channel sides for leak detection. Rigorous safety measures are required to be taken before entering. A proper job safety audit must be made with all concerned interfaces under the supervision of Safety Advisor. If the leakage is small then the area of decoloration of ammonia paper would not be large and detection of leaking point would be easy. On the contrary, if a large leakage is present, it would be a mess to detect the leaking point. Sprays are then used to minimize the detected area of leakage and to narrow it down to the defect. Ammonia test has following features which must be considered:
- It’s a reliable test and provide you with all possible leakages even smaller ones.
- Being used abundantly across the industries.
- Higher safety risk associated with this test.
- It’s a color change test – easy to detect and find leakage.
- With large leakages, it would not be much helpful.
- Final test after carrying out repair must be done with ammonia as it’s the most reliable and authentic.

Water Leak test:
This test is conducted with water at about 8 kg/cm2 from the shell side but would not be coming up with detection of small defects. Sensitivity of this test is quite lesser as compared to other tests. If you have much of time available and you assume that the quantum of leakage is higher than you may opt for this test otherwise considering other tests would be a nice idea. Normally, due to lower sensitivity this test is not used until or unless high quantum of leakage is foreseen. The salient features of this test are:
- Less reliable
- Safe and less risky than ammonia test
- Less sensitive
- Good for heavy leakages, smaller leakages may not be found always

Air Soap Leak test:
This test is the most effective and safe for detection of leakage. Although it takes much and much of time for detection of the leaking point and finding the defects but increasing the inspectors during the test serves the purpose. It is normally carried out by pressurizing the shell side with air at 6.5 kg/cm2.
After pressurization, inspectors can readily enter into the channel sides with soap solution and can start finding the leaking points. It requires much of attention and skill of inspector to look for all the defects. Full scanning is must and a cumbersome job. More than one inspector shall be used for the same so that carefulness and consistent attention be achieved all the time during inspection. The salient features of this test are:
- Reliable and effective
- Time consuming
- Good for all small and large leakages
- Safe and less risky than ammonia leak test
- Can be used a final test also in place of ammonia test but time is a big factor
- Skill dependant test, not easy to detect all the defects until or unless proper care and attention be given during detection process

Reasons of leakage
The main reasons of Urea Stripper leakage are:
- Corrosion and exposure of hidden defect in tube-to-tubesheet joint (see Fig. 3)
- Burn thru of tube from HAZ area (see Fig. 4)
- Crater corrosion of tube-to-tubesheet joint

Major threats related to leakage

1. Damage to ligament area
2. Tubesheet integrity as a pressure part
3. Nearby tubes

Stripper leakage
Figure 7: Leakage path from burn thru area

Rectification of defects
The rectification of defects is to be carried out very carefully as it involves welding adjacent to the healthy tubes and tube-to-tubesheet joints. It is recommended to follow the guidelines listed below:
1. Grinding shall be done effectively in order to flush the pin hole completely. Ineffective grinding would never rectify the leaking point.
2. Welding shall be made after dye-penetrate testing of ground off area.
3. It is recommended that TIG welding with lower heat input shall be used for rectification.
4. Adjacent tubes and tubesheet weld joints shall be avoided. Certain jigs (see Fig. 8) may be used for the same in order to concentrate the welding heat where required.
5. Qualification of welder shall be made before welding in this sensitive area. Welders may be tested on test pieces identical to the situations inside the stripper keeping the same dimensions.
6. High heat input will cause burn thru / HAZ area which would eventually be a potential point of leakage (see Fig. 10)
7. Arc termination would form a crater which would be a source of corrosion initiation.

Picture Gallery
Below find some self explanatory figures which would also help you in understanding the rectification process and the associated risks if not done properly.

rectify-1
Figure 8: A sample jig for welding
recity-2
Figure 9: High heat input
recoty-3
Figure 10: Burn thru tube
recitify4
Figure 11: Arc strike on adjacent tube

 

       
  Imran Idris
Working as a Process Engineer in Engro Fertilizers Pakistan on a new Fertilizer Project.Earlier on, he has been working with other Fertilizer Industries of Pakistan. He has vast working experiene with advanced Urea technologies like Stamicarbon, TOYO and Snamprogetti.

 

  Ashfaq Anwer
Working as an Inspection & Reliability Engineer in Engro Fertilizers Pakistan on a new Fertilizer Project since 2007. Earlier on, he had been working as a Project Engineer with Fauji Fertilizer Company Limited, Pakistan since from 2005 to 2007.

 

       
 
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