failure and repair of the liner of a high pressure carbamate … · 2018. 8. 22. · failure and...

Failure and Repair of the Liner of a High Pressure Carbamate Condenser The leak detection system of the High Pressure Carbamate Condenser at Yara Belle Plaine indicated a leak in the top portion of the vessel. The plant was stopped and after inspection, the small leak was located. A local repair was performed with consideration that the liner would need to be replaced in the near future. Approximately three weeks after the local repair, another leak was detected and the

plant was stopped to replace the liner.

Shane Roysum Yara Belle Plaine Inc.

Johan Thoelen Yara Belgium

Vincent Duponchel Yara Belgium

Introduction

ara Belle Plaine is a 2000 metric tons per day Stamicarbon urea plant with stripping technology revamped twice (double stripper and Medium Pressure

add-on) to reach a production capacity above 3300 metric tons per day. In February of 2013 the leak detection system of the High Pressure Carbamate Condenser (HPCC) indicated a leak in the top portion of the vessel. The vessel was from original construction and therefore had been is service for 22 years. The plant was stopped when the leak was locat-ed in a particular zone (Figure 1) of the vessel. The gas in the leak detection system was bub-bled through water with an indicator to deter-mine presence of ammonia in the system.

A thorough internal inspection was conducted in order to locate the leak. The inspection pro-cess took approximately 48 hours in order to ac-curately locate the leak. A combination of liquid penetrant inspection (LPI) and helium leak test-ing were used to locate the leak. An ammonia leak test was tried, however due to high levels of residual ammonia in the building in which the HPCC is housed, the results were inconclusive. After the leak was located, it was determined to be a combination of a manufacturing defect and increased passive corrosion of the liner weld. The leak was located where the liner was weld-ed to the overlay weld above the tube sheet. A localized repair was made. This repair was considered to be only temporary because the condition of the liner at the time of inspection was poor.

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Upon completion of the repair, planning for the complete replacement of the liner in the event of another leakage, was started. Approximately three weeks after the local repair was completed, another leak was detected. The plant was stopped and the repair work began immediately on removing the old liner and pre-paring to install a new liner. In order to maintain the ability to detect liner leaks, it was decided to replace the liner in such a way that the existing leak detection grooves could be used. Liner plates were cut appropriate to be able to connect the backing strips with the existing overlay welding. This procedure was chosen to eliminate the need for welding on the carbon steel, which prevented additional heat treatment. It also allowed the original leak detection grooves to be utilized. No repair on the carbon steel was required because corrosion was pre-vented by taking the plant out of service soon after detection of the leak.

First leak, detection and temporary repair

For the first leakage, an internal inspection was performed on the top portion of the vessel. Figure 1 shows the top portion of the vessel with the zones identified.

Figure 1. Top portion of the vessel. Helium was applied to the leak detection system through the ports to the backside of the liner at a controlled pressure. The helium test was kept at a pressure of 30 kPa and was protected with an inline pressure relief device that was set at 50 kPa. This pressure was determined to be the maximum external pressure of the liner in the corroded state including a safety margin. Initial inspection was performed, as well as helium leak detection, on all of the weld seams with in-conclusive results. The existing weld material was 25-22-2 while the liner plate was 316L Urea Grade which ex-plained the large variation of the corrosion rate. The liner plate had corroded at a higher rate than the weld seams because of the two different metallurgies, therefore the weld seams protrud-ed approximately 3mm out of the liner plate. It was then determined that in order to properly in-spect the weld areas the welds would need to be removed by grinding. The vertical welds were ground flush, and top and bottom circumference welds were ground to remove the corrosion cav-ity behind the welds. After the above tasks were completed, another inspection was performed on all of the welds us-ing both the helium test and Liquid Penetrant Inspection (LPI). A leak was located on the bot-

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tom circumference weld in the northwest quad-rant of the vessel. Figure 2 shows the leak loca-tion after grinding, which exposed lack of weld penetration during original manufacturing. The area was ground out and the leak appeared to be at the interface of the bottom of the liner and the overlay weld above the tubesheet

Figure 2. Leak location after grinding. An ultrasonic thickness grid scan was performed externally in the leak location with no major wall loss noted. The repair plan was to run two new passes of weld on the circumference welds above and below the overlay and to repair any indications noted in the ground vertical welds. The tray clips were replaced as well due to nu-merous indications present in the clip to liner welds. The completed repairs were inspected with LPI, a helium leak test was performed suc-cessfully and the HPCC was returned to service. At this time it was decided to prepare for a full liner replacement in the event of a new leak, since the condition of the liner was poor.

Second leak and full liner replacement

After approximately 3 weeks of service, the lin-er leak detection system indicated another leak. At this time the final decision to replace the en-tire loose liner in the top channel (Zone 1 and 2) was made. Schoeller-Bleckmann Nitec (SBN) was contracted to perform the liner replacement at site.

As soon as the vessel was opened for entry, work began to remove the old liner and prepare for the new liner installation. Figure 3 shows the carbon steel shell after the old liner was re-moved. There is evidence of carbamate staining but no wall loss due to corrosion.

Figure 3. Carbon steel shell behind old liner Figure 4 shows the location of the new liner in-stallation. The top weld in the figure is the be-ginning of the head overlay weld and the bottom weld ties into the tubesheet overlay weld. The liner was replaced with 25-22-2 material.

Figure 4. New 25-22-2 liner installed

Root causes

Over time, the thickness of the liner had been reduced due to passive corrosion. In the areas of condensation corrosion the corrosion rate is


higher. The weld between the liner and the weld overlay was not full penetration as required for this configuration of weld in urea service. Con-sequently, a small gap appeared between the liner and the weld, leading to a leak of carba-mate to the carbon steel structure and thus to the leak detection system (Figure 5)

Figure 5. Origin of the first leak in the HPCC Vessel damage may have been worse if the plant had not been shut down based on the li-censor recommendation and the internal proce-dures of Yara. The second leak is likely related to the liner cor-rosion and a defect in a liner vertical weld. This defect is from the manufacturing process but the thinning of the liner led to a leak path through the weld. With the liner failure, the carbamate reached the carbon steel (and thus the leak de-tection system) and the plant had to be stopped.

Figure 6. Probable origin of the second leak in the HPCC

These two leaks are consequently due to manu-facturing defects and corrosion of the lining. The choice of using 316L Urea Grade as liner is the minimum requirement material specification for this urea synthesis section equipment, from a corrosion point of view. By choosing 25-22-2 material the lifetime of this equipment may have been extended.

Condensation corrosion

While looking at the thickness measurement of the liner since the in-service date, we do not ob-serve an increase of the corrosion rate, but it remains locally at 0.2 mm/year, which is a high value for passive corrosion probably due to condensation corrosion. Figure 7 graphs the minimum thickness measured during inspec-tions.

Figure 7. Minimum thickness measured during inspection Inadequacy of insulation of this HPCC has been pointed out in several inspection reports without corrective actions: - 2003: high corrosion observed in gas phase

top HPCC: 0.5mm or 1mm/yr. - 2007: local high corrosion in top liner gas

phase: 0.3 mm/yr In 2013 inspection, the insulation of the dome part and the top channel have been found in good condition and as per original design. Something else should explain this local high corrosion. While reinstalling the liner plates in the workshop, we figure out that the highest cor-rosion rate was on the west and east side of the

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HPCC (see Figure 8) on liner plate n° 5/6 and 11/12. In this area the remaining thickness is about 3 mm (meaning 3.5 mm corrosion in 20 years). The top part of the liner plates (with blue oxide) shows thickness of 5.5 mm which corre-spond to 1 mm loss only in 20 yrs.

Figure 8. Dismantled liner plates of the HPCC. The area with the highest corrosion is linked to the location of the lifting lugs on the outside.

Both lifting lugs have been removed after the installation of the HPCC in 1992 as required for high pressure urea equipment. But, after remov-al of the lifting lugs, a piece of metal remained on the external surface of the vessel. The thick-ness of this piece is 90 mm (see Figure 9) for an insulation thickness of 100 mm, which is insuf-ficient to avoid condensation. In the licensor mechanical data sheets, these lifting lugs are di-rectly bolted on the shell of the HPCC in order not create any thermal bridge. The position of the spool pieces (former lifting lugs) is repre-sented in red on Figure 8.

Figure 9. Lifting lugs – comparison with re-quirement from Mechanical Data Sheet.


The complete removal of the lifting lugs could have increase the lifetime of this equipment by reducing the corrosion rate of the liner.

Early detection and Probability of Failure on Demand (PFD) of leak detection systems

In the two leaks mentioned above, the leak de-tection system gave an early alarm, which lim-ited damages on the carbon steel structure and never jeopardize the safety of the plant. Following this incident, internal discussion starts within Yara Urea specialists to determine the Probability of Failure on Demand (PFD) of a leak detection system. The PFD is calculated with the lDU, the Rate of Dangerous Undetected failures, commonly ex-pressed in yr-1 For a single component it can be expressed with the following expression (only valid if lDU x T << 1): where T is the inspection interval. For components in series, the global PFD is the sum of all individual PFD.

Requirements for leak detection system

The consequences of a carbamate leak through the liner can lead to a complete failure of the HP equipment with major damages on the produc-tion facilities and several fatalities possibly out-side the fence. Using the F-N curve (frequency – Number of fatalities) as societal risk ac-ceptance criteria, the probability of such event must be lower than 10-5 per year to be in the ALARP (As Low As Reasonably Possible) if we consider the Netherlands (Figure 10).

Figure 10. Different FN curves for different countries [1] The initial frequency to have a leak through a reactor liner made of 316L Urea Grade is every 20 years if no preventive maintenance is made (based on Yara experience). We have to men-tion that the first leak is the signal that other leaks are likely to happen and one should recon-sider this initial probability with much higher values (the overall passive corrosion has reached manufacturing defects and the probabil-ity to face other leaks is becoming very high like in our HPCC case). It shows that the quality of the manufacturing process of HP equipment could be a first reduction of the above probabil-ity. To have the scenario of the complete rupture of the equipment, the probability could be miti-gated by: - Probability to find the leak during an in-

spection before the rupture - Follow-up of the corrosion and preventive

relining (only considered if the job is per-formed by a qualified company with a pro-cedure approved by a licensor.

- Probability to have a leak before break

An example of Layers of Protection Analysis (LOPA) (Figure 11) is showing that the PFD requirement of the leak detection system must be 10-2 to reduce the probability of catastrophic failure with fatalities outside the fence below 10-5 per year in that given example.

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T x DUPFD


Figure 11. Example of Layers of Protection Analysis (LOPA) for the failure of the complete liner of High Pressure Reactor due to carbamate leak

PFD evaluation of leak detection system

How the unreliability of the leak detection sys-tem can be calculated? A summary of undetected failure of the leak de-tection system is presented in figure 12. Using this figure, the PFD of the leak detection system can be express as

Consider the leak detection system presented in figure 13. This system uses a pressurized gas vector with individual flow measurement with low alarm switch on all lines. The alarm is re-ported in the control room; the rate of dangerous undetected failure of these flow switches is 0.03 per year. The gas leaving the grooves is contin-uously analyzed by a sensor with the ammonia concentration reported in the control room; the rate of dangerous undetected failure is 0.3 per year.


Figure 12: Failure possibilities of leak detection system

Figure 13. Example of leak detection system

We can then determine the probability of fail-ure on demand of this system: PFDtot = ½ * (lDU_Flow switch + lDU_NH3analyser) * T PFDtot = 0.165 * T where T is the test interval. The PFD due to wrong measurement is not pos-sible by using a pressurized system. The system is properly commissioned to avoid any missing information due to unconnected grooves.

Testing and frequency of a leak detection system

If we consider a PFD of the leak detection sys-tem must be then less than 0.1, by testing the


system every 6 months, we reach a PFD of 0.0825. If the LOPA gives a PFD requirement of the leak detection system less than 0.01, With the above system, test interval must be every 3 weeks to reach a PFD of 0.0095. This interval can of course be increased by installing a redun-dant ammonia analyzer. The last step is to perform the test within the de-fined frequency. In order to test the ammonia analyzer, a possibility could be the injection of a diluted ammonia gas upstream the groove and to see the effect on the final analyzer. Flow switch can be easily tested by isolating the sensor to check for the alarm.

Conclusion:

Based on the described incident, when the in-formation of a carbamate leak is confirmed by the leak detection system, the decision to stop the plant for repairs must be made. It is not recommended to run with a leak in the liner system, which will result in very extensive repairs to the carbon steel of the high pressure equipment of Urea plants and could lead to a potential catastrophic failure The reliability of the leak detection system is es-sential and should be considered as a critical protection device, with its own probability of failure. A dedicated testing program must be put in place to guaranty its reliability.

References:

[1] “An overview of quantitative risk measures for loss of life and economic damage” S.N. Jonkman, P.H.A.J.M. van Gelder, J.K. Vrijling. Journal of Hazardous Materials A99 (2003).


failure and repair of the liner of a high pressure carbamate … · 2018. 8. 22. · failure and...

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