High Pressure Carbamate Condensor Leak

 

 

High Pressure Carbamate Condenser Leak Detection and Control  SA F CO h  SAF ha as exp xpe eri ence nced a le lea ak in the H i gh Pr essure ssure Carb Carba amate Con Cond dense nser o off tthe he Ur Ure ea pla lant nt,, twi ce. ce.  As  A s a r esult sult,, a technica chnicall i n-ho n-hous use e team ha hass stud studii ed and develo lop ped a metho hod dolo logy gy fo forr d de etecti cti ng and moni onito torr i ng the lea leakk unti untill iitt ca can n be fi xed. T he d de eve velope loped dm met ethod hod has he helpe lped d in enabli nabling ng op ope er ati ati on o off the H P C C equi quipm pme ent iin n the fi r st iinsta nstance nce,, ffor or mo morr e tha than n 14 mo months nths and i n the seco second, nd, ffor or mo morr e for 10 months. months.

Khaled Abdul Aziz Al-Khuraimi SAFCO

Introduction

Problem Statement

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In June 2012, a leak was detected in the HPCC. It was kept in service under very close monitoring until its scheduled replacement during the turnaround in April 2013.

AFCO is one of the leaders in the fertilizer sector, worldwide. It has four urea plants - three of them are Stamicarbon licensed with a total design capacity of 8,700 MTPD. The SAFCO-2 SAFCO-2 urea urea  plant is a Stamicarbon licensed plant built by Chyoda. The name plate capacity of this urea  plant is 1,800 MTPD, and the normal plant load

Process Description The SAFCO steam/ condensate system is a semi-closed loop, as shown in Figure 1. The source is from extraction of steam turbine (TS102). Steam is generated in the HPCC (E202) steam drum vessel (V904A/B) and condensed in the stripper (E201) steam drum (V905), rectifying heater (E302), and evaporator (E401) steam drum (V903). The SAFCO-2 urea  plant is the only one of the SAFCO plants that uses condensate from the desorber as feed to the steam drum, V-904A/B.

is 108% to 111%. The high pressure carbamate condenser (HPCC) of SAFCO-2 has been in service since commissioning in 1992. The total total number of on-stream time since commissioning is close to 21 years.

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 Figure 1. SAFCO-2 steam/condensate circuit Conductivity in the system is measured online in two locations – the turbine condensate and the desorber outlet (C802). (C802). Routine monitoring monitoring of the system is done using the turbine condensate meter, the desorber outlet meter, and the steam drum, V-904A/B blowdown lab analysis.

condensate had a direct relation with the desorber outlet conductivity.   From December 2011 until May 2012, the turbine condensate conductivity and desorber conductivity increased above the limit of 15 and 20 μS/cm·s, respectively.   On June 18, 2012, the hydrolyzer level transmitter was replaced. The desorber conductivity returned back to its normal

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Background History and Observation

value of less than 15 μS/cm·s, however the turbine condensate conductivity did not decrease.

The background and history can be summarized in the following points:   In December 2011, the hydrolyzer level transmitter malfunctioned, resulting in turbine and desorber conductivity increasing above the limit of 10 and 15 μS/cm·s, respectively (turbine condensate and desorber conductivity are normally less than 10 and 15 μS/cm·s, respectively).   Until May 2012, it was observed that

Leak Detection Methodology

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All observations in the previous paragraph indicated a leak in the system. The following steps were taken to identify the leak: 1.  List all the possible causes. 2.  Trace the leak. 3.  Prepare an action plan.

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increasing

conductivity

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the

turbine

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CO2. Results were normal normal as pH was 8.2, conductivity was 4.6 μS/cm·s, and no NH3  or CO2 was detected. 2.  Tube side leak from rectifying column – E-302 steam inlet and condensate outlet samples were analyzed for pH, conductivity,  NH3, CO 2 and urea. urea. Results at the inlet and outlet were the same. Some results showed

Possible Causes

All possible causes were brainstormed and listed. The list list was classified into minor minor and major points. As the leak amount was small small and only slowly increasing, it was assumed that it was from a minor point such as non-return valves (NRV), steam jacket, the nitrogen:carbon

higher NH3  at the inlet compared to the outlet, which was due to temperature difference. To double check, the outlet condensate from the rectifying column was isolated from V-903 and diverted to the waste water tank (T-901), but conductivity,  NH3 and CO2  measured in V-903 remained in the same range. 3.  Tube side leak from evaporator – The evaporator tube side operates under vacuum, so in the case of a tube leakage, steam will go into the process side diluting the urea melt. Thus, there is no chance for urea to

ratio (N/C) that meter, steam traps. It was also considered the or leak couldtraps. be from a major source such as equipment or the feed itself. The minor leakage points are as follows: 1.  Urea melt header into the steam jacket, 2.   N/C meter into the steam jacket, 3.  Carbamate pump into the steam jacket, and 1 4.  Urea solution transfer line  steam jacket. The major leakage points are as follows: 1.  Make up water quality, 2.  Tube side leak from the rectifying column, 3.  4.  5.  6. 

come out to the shell side under normal operation. To double check, a sample was taken and the results showed pH, conductivity, NH3, and CO2  were the same as the rectifying column and no urea was found in V-903. Also, urea melt going from from the evaporator to the granulator was normal and there was no change in urea solution concentration. 4.  Desorber outlet condensate quality – Desorber outlet condensate was affected to some extent, but when the desorber condensate supply to V-904A/B was isolated, the conductivity reduced from 21 to 8.6 μS/cm·s and then remained constant. 5.  Tube side leak from HPCC – Condensate inlet and steam outlet were analyzed for pH, conductivity, NH3, and CO2. Concentrations of CO2  and NH3  in V904A/B were higher than the other places. Repeated samples for HPCC inlet and outlet revealed a minor leak in the equipment. 6.  Tube side leak from stripper – Make up steam condensate coming from V-905 via the stripper was analyzed for pH,

Tube side leak from the evaporator, Desorber outlet condensate quality, Tube side leak from the HPCC, and Tube side leak from the stripper.

Tracing the Leak

Leak source detection was done, starting from the minor points, by checking the suspected area  physically, isolating the suspected source to check the effect on the system and taking lab analyses. The investigation was conducted as follows: Minor Points: 1.  The health of steam jackets and N/C meter were checked and found to be in normal condition. Major Points: 1.  Make up water quality – Make up steam condensate from V-905 via stripper was analyzed for pH, conductivity, NH3, and 1

  In SAFCO there is a grid connection between all urea

conductivity, NH3  and CO2.

 plants that is used used for transferring transferring urea solution in in the case of a shutdown.

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The results

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2.  A leaking tube-to-tube sheet (TTS) weld, caused by stress corrosion cracking due to chloride attack. 3.  A leaking welded tube plug from a  previously plugged tube.

were normal and no NH 3  or CO2  was detected. Inspection Report

The last inspection for the HPCC was done in  November 2010. The inspection found stress corrosion cracks in the material of the tube due to chloride. condensate occurred in the crevice. 2 and 3.

In order to trace the source of the leak an on-line inspection of the HPCC top and bottom tube sheet’s outer periphery was conducted by Stamicarbon. The inspection result was found to be acceptable.

The chloride was from from upsets which in the coming from utilities, the past, causing chloride to stay in A tube crack is as shown in Figures

Impacts and Mitigation Action Plan

The following are the impacts of operating under the conditions described above:   Corrosion in steam and condensate circuit.   Corrosion in the turbine.   HPCC failure before turnaround. 

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Based on the existing situation and the previous experienced with a HPCC leak in SAFCO-3, it was agreed to operate the plant under close monitoring and control of the conductivity and ammonia content. The measured values of conductivity and ammonia indicated a very small leak, so it was believed that there was a minor risk for corrosion and further damage.

 Figure 2. HPCC tube crack

The following action plan was developed and implemented to manage the potential impacts: 1.  Monitor the top and bottom HPCC lab analysis for NH3  and CO2  pick-up, pH and conductivity.  2.  Monitor the turbine condensate conductivity analyzer and lab analysis for NH 3  and CO2   pick-up, pH and conductivity. 3.  Divert the admission steam to vent when the cation conductivity crossed the limit of 5 μS/cm·s.  4.  An on-line inspection of the HPCC top and  bottom tube sheet’s outer periphery was done. The inspection result was found to be acceptable. 5.  Clear limits were also agreed upon, so that a shutdown has to be taken in case those limits

 Figure 3. Microscopic picture of the crack Based on the inspection report, the leak source was assumed to be from the following: 1.  A leaking tube, caused by stress corrosion cracking due to chloride attack.

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were exceeded. These limits were as follows:   A sudden change in conductivity (increase or decrease).   An increase in the NH 3 from the HPCC above 50 ppm (based on experience with a high pressure scrubber leak where the leak could not be detected below this

equipment was ordered after the crack was found in the last inspection). 

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Monitoring and Control

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It was agreed to operate the plant under close monitoring and control of the conductivity and ammonia content. The measured values of conductivity and ammonia indicated a very small leak, so it was believed that there was a minor risk for corrosion and further damage.

value).

  Keep the pH of the steam condensate

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system above 8.5 (preferably above 9) to minimize the risk for corrosion in the steam condensate system.   In order to save the turbine, the cation conductivity limit was 5 μS/cm·s (also  based on experience). 6.  In case the leak crossed the limit, the equipment has to be repaired or replaced if the new equipment was delivered (new

Equipment leakage was increasing at a very small rate, reaching a value of 60 μS/cm·s. The leak continued in the same increasing rate until October 2012 when a crash shutdown occurred. After the shutdown, the leak quantity declined,  but still had the same increasing rate, as shown in Figure 4.

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HPCC Top pH

Conductivity

NH3

CO2

300

Low Load

250     y      t      i 200     v      i      t     c     u       d     n 150     o      C        &

Poly. (Conductivity) 250

200

S/U after  unplaned S/D 150 Normal Operation 100

     H     p 100

    p     u       k     c      i     p      2      O      C        &      3      H      N

50

50

0

0

 Figure 4. HPCC leak trend after Oct 2013 crash shutdown conductivity was possibly due to presence of corrosion products in the crevice, which  partially blocked the leak point.

Conductivity had increased sharply during the low load period due to fluctuation in the differential pressure across the HPCC. After increasing the plant load, the reduction in

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SAFCO-3 Case Benchmarking

Regardless of the source of the leak, action  plans to fix the problem were developed as follows: 1.  On short notice until a plant shutdown, monitor the conductivity, NH3, CO2  and Fe in the HPCC. 2.  It is not mandatory, but advisable to treat the unit with 2% trisodium phosphate (TSP), to

The HPCC leak in SAFCO-2 was the second case of such a scenario within SAFCO plants. Therefore, the SAFCO-2 HPCC leak case was  benchmarked with the previously experienced leak in the SAFCO-3 plant. The SAFCO-3 HPCC exchanger leak started on September 9, 2010, and contaminated the entire steam and condensate circuit. About 30 MTPH (33 STPH) of steam was vented to atmosphere as admission steam, while about 40-45 MTPH (50 STPH) of desorber condensate was dumped. Even so, the leak amount had crossed 2000 µS/cm·s and NH3  pick up across the HPCC was more than 700 ppm. The plant load was maintained at full load (114%).

slow down stress corrosion. 3.  During a shutdown, pull a suspected tube for further investigation to confirm the presence of chloride stress corrosion cracking. 4.  During a shutdown, find the leak and repair as there is a risk of corrosion of the C-steel tube sheet. Results after the shutdown and repair were as follows:   A total number of 16 tubes were plugged with a reduction in the heat transfer of only 0.5%.   The vented admission steam of 30 MTPH was recovered.   30 MTPH (33 STPH) of desorber water contaminated with ammonia was recovered.   Total recovery of steam as well as process condensate was 60 MTPH (STPH). 

The plant was closely monitored for pH, conductivity, NH3 pick up from the HPCC, and iron (Fe) content. As the leak was high but only increasing by a small rate, the decision was made to keep the plant running with close monitoring for the mentioned aspects. The leak  behavior was fluctuating up and down d own following the system pressure fluctuation. By the end, equipment ran around 14 months until a planned shutdown was taken to inspect and fix the  problem.

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The main cause of the leak was the TTS weld  joint pin hole (Figure 5). Carbamate passed through this pin hole with high pressure, initially eroded the stainless steel layer (8 mm, 5 /16  in thick) and then corrosion started at the

Based on the last inspection report, it was expected the leak source was a leaking tube. Even so the following scenarios were assumed: 1.  A leaking tube caused by stress corrosion cracking due to chloride attack. Chloride analyses of steam condensate (V-904) for the last three years remained less than 0.05 ppm, except on September 11–12, 2010, when it reached as high as 3.3 ppm due to an upset in the utility demineralization unit. 2.  A leaking tube to tube sheet weld due to thinning of tubes was unexpected to happen as the tube to tube sheet weld was found to  be satisfactory.

carbon steel portion (475 mm, 18.7 in thick). The cavity was up to 350 mm (13.8 in) deep and 125 mm (4.9 in) wide (Figure 6). The remaining portion of the CS intact was only 125 mm (4.9 in).

 

3. A leaking tube plug.

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TTS pinhole leak from tube 36 N 1W 

 Figure 5.  started

  Initial pinhole leak where cavity

 Figure 7. The welded repair area

Findings and Results The equipment in SAFCO-2 has been operating under close monitoring with a known leak, keeping leakage as a benchmark. The continued operation and planning has been based on a similar experience with the SAFCO-3 HPCC. Unfortunately, the plant experienced a shutdown twice during the leak period and the leak  behavior changed. Even so, the plant was kept running until a forced shutdown was taken when the NH3  pick up increased sharply crossing the value of 500 ppm. This amount of ammonia had an effect on the evaporation section vacuum and the plant was shutdown.

 Figure 6. Corroded area after grinding The present repair (Figure 7) is only a temporary measure. SAFCO inspection and Stamicarbon indicate the HPCC is to be replaced at the the soonest possible time. time. Based on

Conclusion

the inspection report issued, procurement actions have started for a new exchanger.

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SAFCO has good experience in plant troubleshooting, even though each case is unique. The monitoring and control plan succeeded in the SAFCO-3 HPCC but it failed in the SAFCO-2 case because two crash shutdowns changed the leak behavior. Fortunately, the SAFCO-2 shutdown occurred 10 days before the turnaround and the new equipment was already on-site and turnaround resources were simply rescheduled.

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