Failure of Methanator Feed Effluent Exchanger Tubes due to

Failure of Methanator Feed Effluent Exchanger Tubes due to Benfield

Solution Carry over from CO2 Absorber

Two new Methanator Feed Effluent Exchangers (E-209 A/B) were installed during an ammonia plant revamp in 2007. After only five years of service, leakage was observed from one exchanger

(E209A), where conditions were found promoting under deposit corrosion of tubes due to Benfield solution carry over with process gas. This paper describes the detailed inspections carried out to

assess the extent of tube damages, to establish the failure cause and to determine recommendations for successful resolution of the problem.

Anwar Mahmood Shahid

Fauji Fertilizer Bin Qasim Limited

INTRODUCTION

auji Fertilizer Bin Qasim Limited (FFBL) operates an ammonia, urea and DAP fertilizer plant located at Bin Qasim

Karachi, Pakistan. The 1270 MTPD ammonia plant was designed and constructed by Bechtel Corporation in 1965 in Lake Charles USA, where it was operated by Olin Chemicals Company until 1992. It was later relocated to Karachi and commissioned in 1998. FFBL carried out a major Balancing, Modernizing and Revamp (BMR) project of the ammonia plant in 2007 to increase the production capacity from 1270 MTPD to 1570 MTPD and address major integrity issues associated with old plant equipment which caused frequent downtime and operation of plant at reduced load. As part of BMR, old Methanator Feed/Effluent (F/E) Exchangers (E-209A/B) that were having severe leakage problems were replaced with new exchangers of larger capacity and 1Cr-0.5Mo tubes and tubesheets. Also the design was modified from floating tubesheet to fixed

tubesheet with strength welded tubes. The new exchangers were taken in operation in 2007.

PROCESS DESCRIPTION

In the ammonia plant raw synthesis gas is produced by reforming natural gas in the primary and secondary reformers. Reformed gas passes through purification sections of shift conversion, CO2 removal, and finally through methanation that removes unconverted CO and unabsorbed CO2. Pure synthesis gas is then sent to ammonia synthesis. The process flow diagram of the exchangers E-209 A/B is shown in Figure 1. Methanator Feed Effluent exchangers (E-209A/B) are installed in series downstream of Absorber KO Drum (C-209). These exchangers preheat synthesis gas going to Methanator (C-210) by using heat of Methanator outlet gas. Process gas coming from CO2 Absorber (C-208) and C-209 enters shell side of E-209A at 97oC (207oF) and is heated to 281oC (538oF) when it leaves shell side of E-209B and enters into

F

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Methanator (C-210). At tube side, process gas effluent from C-210 enters tube side of E-209B at 310oC (590oF) and is cooled to 127oC (261oF) as it leaves tube side of E-209A.

Figure 1 – Process Flow Diagram of E209A/B. EQUIPMENT HISTORY

Lake Charles, USA

The exchangers were originally constructed with ¾ inch OD and SA-214 (carbon steel ERW) tubes and SA-105-II (forged carbon steel) tubesheets. In original scheme, the exchanger was TEMA Type BET.

As per history, both Methanator exchangers (E-209A/B) developed tube leaks which were suspected by corrosion from Benfield solution carry over to shell side with process gas coming from C-208 and C-209. Tube leaks caused synthesis gas to bypass around Methanator and reduce plant load by forming carbamate in synthesis gas compressor loop exchangers. The tube bundles were replaced in 1987 with tubes of SS-316 (seal welded to tubesheet) and with fixed tubesheet of C-1/2Mo and floating tubesheet of 1-1/4Cr-1/2Mo. However ammonia plant was later stopped and mothballed in 1992.

Inspection by Industrial Technical Company (USA) before Relocation

Industrial Technical Company (ITC) carried out inspection of E-209A/B at Lake Charles, USA before relocating to FFBL. Eddy current testing of 20% tubes was carried out and revealed significant thickness loss in SS-316 tubes:

1. In E-209A, 27 tubes were already plugged, while 100 tubes were found with moderate damage (≤ 50% loss) and 21 tubes found with major damage (> 50% loss) and were recommended for plugging.

2. In E-209B, 9 tubes were already plugged, while 123 tubes were found with moderate damage (≤ 50% loss) and 58 tubes found with major damage (> 50% loss) and were recommended for plugging.

The results of eddy current inspection clearly indicated that replacement of tube material from carbon steel to SS-316 did not resolve the corrosion / leakage problem in the exchangers. Inspection at FFBL (1998 to 2007)

E-209B:

Upon receiving at FFBL site in 1998, solution film leak testing of the exchanger E-209B was first carried out which revealed leaks in 51 tubes and numerous tube-to-tubesheet seal welds (TTS). Plugging / welding of leaked tubes and repair of TTS welds followed by post weld heat treating (PWHT) was done, however, cracks and weld defects were observed in tube to tubesheet welds (possibly due to H2 embrittlement). Multiple attempts made to rectify weld defects including PWHT of inlet tubesheet, remained unsuccessful. Therefore in 1998 it was decided to remove the tube bundle and install a bypass pipe inside the shell that would allow operation as catalyst manufacturer confirmed that their Methanator catalyst can operate continuously down to a temperature of about 200°C (392°F). In 2000, the exchanger was taken back in service after installation of its repaired tube bundle.

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E-209A:

Based on unsuccessful repair experience with E-209B, no inspection / testing was carried out and the exchanger was taken into service. In May 2001, the tube bundle was pulled out which revealed heavy sludge accumulation on shell side (probably due to Benfield solution carry over with process gas). A pneumatic leak test from shell side was carried out which revealed 13 psig drop in pressure in 10 minutes indicating tube leakage, however, detailed testing was not done to identify leaked tubes. The problem was referred with process licensor Haldor Topsoe (HTAS) who estimated that the leakage was 6 to 8 ppm of CO in synthesis gas which was well below what the ammonia synthesis can tolerate and was not enough to damage ammonia converter catalyst. No repair was attempted based on HTAS advice and due to previous bad experience with E-209B. In turnaround 2003 (TA-2003), a pneumatic test again revealed a drop in pressure and leakage in exchanger. Again no repair was carried out and exchanger remained in service. Inspection results by ITC USA and at FFBL site both indicated that tube material up gradation with SS-316 in Lake Charles did not resolve tube corrosion / leakage problem in these exchangers.

INSTALLATION OF NEW METHANATOR F/E EXCHANGERS (E-209AB) In year 2007 as part of Ammonia plant BMR, two new exchangers of larger size (3613 tubes) with SA-213 T12 (1Cr-0.5Mo) tube material and SA-387 Gr.12 Cl.2 (1Cr-0.5Mo) shell material were installed in place of old exchangers in order to accomplish increased production capacity of 1570 MTPD of ammonia plant. The new exchanger design was reviewed by Haldor Topsoe and the exchangers were manufactured by Descon Engineering Works (DEW), Lahore. The TEMA type was modified to BEM as

practiced on other Haldor Topsoe plants and tube joints were strength welded. Also, tube size of 5/8 inch was used instead of 3/4 inch OD to meet area constraints and pressure drop. These exchangers remained in service for 5 years until leakage was observed from E-209A in year 2012. Leakage in New Exchanger E-209A

In July 2012, a flow limitation was experienced in the Synthesis Gas Compressor (K-306A) at back end of ammonia plant. Upon investigation during back end shutdown on August 1, 2012, a boroscope inspection of channel side of Ammonia Cooler Condenser (E-303A) was carried out which revealed heavy formation of ammonium carbamate deposits on inlet side of tubesheet, resulting in blockage of approximately 80% of the tubes. A similar load limitation was also experienced in second loop Synthesis Compressor (K-306B), so an inspection of E-303B was carried out on August 9, 2012 which also revealed similar ammonium carbamate deposition and etching corrosion on channel side. Due to formation of ammonium carbamate at the tubesheets of these exchangers, leakage was confirmed in Methanator FE Exchangers (E-209A/B). Laboratory analysis confirmed up to 20 ppm of CO / CO2 leakage in E-209A. Preliminary Inspection in September 2012

Availing a gas curtailment opportunity in September 2012, E-209A was dismantled for inspection. Black solution was found accumulated inside the channel cover and tubes which was analyzed and confirmed by laboratory as Benfield solution. Both tubesheets were found completely covered with Benfield scale and tubes were also found choked due to crystallization of this solution at the tube ends.


Figure 2 – Benfield solution scale on tubesheet

Figure 3 – Crystallized Benfield deposits at tube ends. Solution film testing and hydrostatic testing from shell side of the exchanger was carried out at design pressure and total 119 tubes were found leaked. All the leaked tubes were plugged by hammering 1.25Cr-0.5Mo plugs in tubesheet instead of welding the plugs (to avoid PWHT of tubesheet of 1Cr-0.5Mo material). Also two tube-to-tubesheet strength welds were found with seepage which was repaired by hammer peening TTS welds to avoid welding. Final hydrostatic testing was performed and the exchanger passed. Based on above, probable reasons for tube leakage were suspected as either carbonate stress corrosion cracking (SCC--however, this was less likely as the new exchangers were fully stress relieved) or under deposit corrosion due to

abnormal Benfield solution carry over with process gas to shell side of heat exchanger. However, detailed inspection was required to confirm exact cause of failure. Therefore tube extraction and eddy current inspection of the exchanger was planned in next turnaround scheduled in 2013. Detailed Inspection in Turnaround 2013

Visual Inspection

In TA-2013, visual inspection again revealed Benfield scaling on tubesheets and tubes. The plug of one failed tube was removed and the shell side was partially filled with water in order to facilitate leak detection along the tube. The tube was inspected by endoscope and leak was found at 1.55m from the cold side of the E209A.

Figure - 4 Benfield deposits and leak found at 1.55 m from cold end tubesheet. Inspection after Tube Pulling

Another leaking tube of E-209A was pulled out for investigation which showed a hole (diameter 3-4mm) at 1.10m from the cold end tubesheet. Benfield deposits and pitting corrosion were present on outer tube surface. It was concluded that the corrosion was influenced by the deposits present on tubes (at shell side). Stress corrosion cracking was therefore excluded as the root cause.


Figure 5 – Corrosion of leaked tube at outer dia. showing local thickness loss and 3-4mm diameter through hole.

Figure 6 – Corrosion of another leaked tube at outer dia. showing local thickness loss and leakage

Remote Field Eddy Current Testing of E-209A

Remote Field Eddy Current Testing (RFET) of all tubes of the exchanger was carried out over full length. It became clear that the damage was limited to the following areas of the heat exchanger:

only at the cold side of the heat-exchanger before the first “real” baffle plate near the inlet nozzle at shell side

at the outer perimeters (both left- and right-hand side).

The thickness loss was measured by remote field eddy current testing and the results categorized in 10% ranges. A total of 927 tubes having wall loss over 50% were recommended for plugging by Inspection Department as given below: Defect Category

No. of tubes to be plugged

%age of total tubes

>50% 927 29 >60% 759 24 >70% 488 + 14 16 (Red only)

Figure 7 – Tubes having severe wall loss in RFET of E-209A (marked blue + red) However, after thorough discussion, it was decided by Plant Management to plug tubes having over 70% thickness loss. Total 502 tubes were plugged. Solution Film Testing (SFT) and hydrostatic testing of the exchanger was also carried out in which 9 more tubes were found leaked. Based on above total 613 tubes were plugged tubes in exchanger. Remote Field Eddy Current Testing of E-209B

Because E-209B operates at higher temperatures where all Benfield droplets are already evaporated, the potential for Benfield deposits was minimal and therefore no corrosion was suspected. This was confirmed by Remote Field


Eddy Current Testing of 21% of the tubes. No plugging was required in the E-209B.

EXPLANATION OF FAILURE CAUSE

The shell side of the E-209A receives feed gas from the CO2 Absorber column (C-208) and the Knock-out drum (C-209). The operating temperature is 95-105oC (203-221oF) and the operating pressure is 25-27 bar-g (363-392 psig). The gas contains about 72% H2, 25% N2, ~1000 ppm CO2 and is saturated with water. The amount of water is about 2.3%. Further, about 5-6 ppm K2CO3 (Benfield solution from the CO2 absorber) is present in the gas due to carry-over from the C-208 and C-209. According to calculations the dew point of the gas is 90oC (194oF) at 27 bar (392 psig) and slightly higher at lower pressures. The heat exchanger is operated above the (theoretical) dew point. Also, the tubes are at a slightly higher temperature than the gas. Thus, condensation is not possible. However, evaporation of droplets is possible. This also implies that droplets must be present in the gas, and thus are entrained from the CO2 absorber (C208) and knock-out drum (C209). Subsequently, this implied that the demisters in both the CO2-absorber and knock-out drum were not acting as a barrier against droplets. The droplets contained mainly CO2 and Benfield solution. From laboratory analyses it is known that the Benfield solution also contains chlorides. When the droplets evaporate, the Benfield solution will deposit as salts on the tubes. These Benfield salts are electrically conductive and will contain chlorides. This is corrosive for ferritic steels like the 1Cr-½Mo steel (Grade P12) of the E-209. Also, the corrosion products will promote deposition and corrosion. As soon as all droplets have evaporated from the gas stream, no more corrosive deposits can be formed and the corrosion will stop. Based on the

RFET measurements this was the case in the upper half of the tube bundle before the first baffle plate of the E-209A Also, this explains why no corrosion was measured in the E-209B.

Figure 8 – Side view of corroded area near inlet nozzle in E-209A (marked in red) FACTORS AFFECTING DEMISTER EFFICIENCY

Wire mesh demisters are used in the CO2 absorber and knock out drum for droplet separation. The separation process in the wire-mesh mist eliminator of this type intended for higher velocities includes three steps; first ‘inertia impaction’ of the liquid droplet on the surface of wire. The second stage is the coalescence of the droplet impinging on the surface of the wires. In the third step, droplet detach from the pad.


Figure 9- Mechanism of entrainment removal The primary parameters affecting demister droplet removal efficiency are; gas velocity, surface area, free volume, packing, wire diameter used in mesh and thickness of a demister.

Demister efficiency is low at lower gas velocity. This could be caused by the fact that only smaller droplets pass upward at low velocities and that many of these are carried along with gas path around mesh wire. However, as the velocity increases, even the smaller droplets are less likely to be carried around the wires in the gas stream lines, owing to the inertial forces overcoming the tendency of these droplets to follow the path of the gas stream lines. Hence demister efficiency increases at higher gas velocity or increased plant loads.

POSSIBLE REASONS FOR BENFIELD CARRY OVER

Possible reasons for Benfield solution were suspected as follows:

a. New demisters and high performance wash trays were installed in C-209 during TA-2007 (BMR) as per Koch Glitsch design to reduce entrainment of liquid droplets. However, wash trays were taken intermittently into service after 2007 instead of being taken into

continuous service and Benfield carry-over of 1-17ppm was found in outlet gas from C-209.

b. Old demisters were reused in CO2 Absorber (C-208) during revamp in TA-2007. Reduced efficiency from old demisters in C-208 could also possibly contribute to droplets carry over with process gas from the absorber column.

c. After 2011, FFBL was facing gas curtailment and low load operation of ammonia plant CO2 Absorber (C-208) and Absorber KO Drum (C-209) which could result in liquid droplets by passing in mist eliminators and entrainment with process gas.

SOLUTION FOR CARRY OVER PROBLEM

The main solution identified was the prevention of entrainment of droplets of Benfield solution coming with process gas from CO2 absorber (C208) and Knock-out drum (C209). As immediate action to curtail this problem, gas washing trays in C-209 were taken into continuous service ensuring 5 ton water was sprayed for leaching out Benfield. During turnaround in January 2013, the old demisters in CO2 absorber were fixed to eliminate any gaps during installation. Later on, the old demisters of C-208 were replaced with York mesh type 194 demisters during turnaround in January 2014. As a result of these measures, Benfield carry-over to E-209 was found significantly reduced to 1.5-2

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ppm. This was the intrinsic solution of the problem (“prevention of the disease”).

CONCLUSION

1. The leakages in E-209A are caused by thickness loss due to corrosion from external side of tubes. The corrosion is a type of under deposit corrosion due to formation of corrosive salts from Benfield carry over by synthesis gas from C-208 and C-209.

2. The corrosion has occurred only at the cold side of the E-209A where such evaporation of droplets in the gas is possible. The hot-side of the E-209A and the complete E-209B were not affected by corrosion, because all the droplets had evaporated.

3. Possible reasons for Benfield solution entrainment included:-

a) The gas washing system of C-209 not in continuous service after 2007.

b) A low load operation of ammonia plant resulting in bypassing in demisters.

c) The old demisters installed in C-208.

4. The thickness loss of the tubes was measured by Remote Field Eddy Current testing (RFET) and classified in 10 defect categories of 10% of the nominal thickness. To reduce the efficiency loss, tubes were plugged that had a 70% loss of nominal thickness or greater.

5. The Benfield solution ingress on shell side of E-209A is abnormal and not as per design conditions. As the hot side of E-209A and complete E-209B is not affected by corrosion, the existing tube material Grade T12 (1Cr-1/2Mo) is compatible with service conditions, provided Benfield carry over with gas is prevented.

FUTURE RECOMMENDATIONS

Following remedial actions have been recommended for future:

1. It was recommended to ensure prevention of Benfield solution entrainment with process gas by keeping in service high efficiency wash trays of C-209 and by replacing the old demisters of C-208 (these demisters were replaced in January 2014).

2. Regular process monitoring and analysis of synthesis gas to E-209A was advised to be carried out for detection of Benfield solution carry over with gas to exchanger shell side.

3. It was recommended to replace the E-209A within 1 year. In the mean time it was recommended to plug the tubes that were most affected by corrosion in order to prevent leakage (tubes having over 70% wall loss were plugged)

4. It was recommended to replace the affected E-209A with new exchanger of same material Grade T12 (1Cr1/2Mo) as per existing exchanger design during next turnaround.

5. Austenitic stainless steel (SS-304, SS-316) could be attacked by chloride stress corrosion cracking because of the chlorides present in the Benfield solution. Also SS-316 tube replacement as per exchanger history did not resolve corrosion problem. Therefore austenitic stainless steel was not recommended. Duplex stainless steels were also found not suitable for design temperature of 400oC of exchanger tubes (use of duplex stainless steels is generally

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restricted to 300oC / 572oF. Also as per ASME Sec II Part D maximum allowable stress values are provided up to 325oC).

6. The shell side of E-209A was recommended to be washed during turnarounds with detergents to remove deposits as far as possible to avoid formation of Benfield scale on tubes outer surface.



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Failure of Methanator Feed Effluent Exchanger Tubes due to