1999: Air Compressor Interstage Cooler Leakage

Air Compressor Interstage CoolerLeakage

A detailed account is given of the failure of the interstage cooler and its effects on the process air com-pressor which resulted infrequent interruption of production.

Jagmohan Singh, H. M. Patel, and P. Khetarpa!Kribhco, India

Introduction

Tere are two ammonia plants of a capacity of,350 MTPD each based on the process tech-ology of Kellogg. The urea plants are having

four streams of 1,100 MTPD each employing theSnamprogetti process technology. The plants were com-missioned in Nov. 1985 and commercial production start-ed on March 1, 1986.

Natural gas at 42 kg/cm2 (597 psi) and 30°C (86°F) isfirst preheated to 350°C (662°F) in a desulfurizer pre-heater. After desulfurization, the gas is mixed with steamand fed to the primary reformer. The steam-driven aircompressor supplies process air for the nitrogen require-ment in the secondary reformer. Shift reaction is in twostages. Carbon dioxide is removed in the Benfield hotcarbonate system. After methanation synthesis, gas iscompressed to 185 kg/cm2 (2,631 psi) in a synthesis gascompressor and mixed in synthesis loop before enteringinto ammonia converter. Ammonia synthesis takes placein an axial-radial converter with three catalyst beds andan interchanger.

Air compressor Intercooler

Air to the secondary reformer is supplied through asteam driven air compressor. Air after the first stage ofcompressor is cooled in the first interstage cooler before

entering the second stage of compressor (Figure 1)Air compressor first-stage intercooler (Figure 2) is a U-

tube exchanger with four passes on the tube side having596 U-tubes per shell. A brief description of the exchang-er is given in Table 1.

History of Intercooler Leakage

Cooling water treatment was chromate based (changedto nonchromate based since March 1999). Leakage wasobserved in this cooler in ammonia plant 2 in 1993. Colorchange and subsequent chromate analysis of water beingdrained from the first-stage intercooler trap and down-stream separator trap showed leakage of tubes. Tubeswere plugged and the plant was restarted. Leakageoccurred in the ammonia plant 2 and also started in theammonia plant 1 air compressor first-stage intercooler.

The recurrence of the intercooler leakage in ammoniaplant-1 and 2 continued at regular intervals and this ledto shutdown of ammonia plant-1 three times and ammo-nia plant-2 five times. Each time, the leaking tubes wereplugged and the plant was restarted. The total number oftubes plugged was 39 and 55 in ammonia plants 1 and 2,respectively.

This interstage cooler of ammonia plant 2 was openedduring the annual turnaround in March 1996 and thor-oughly inspected. Based on a visual observation and anondestructive testing technique (NDT), it was decided

AMMONIA TECHNICAL MANUAL 313 2000

AIR FILTERVEI1Ï

STEAMTURBIME

GEAR BOX

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Figure 1. Air compressor and interstage coolers.

AIR OUTLET AIR INLET

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Figure 2. Air compresser first interstage cooler.


Table 1. Brief Description of the U-tube Exchanger

FluidTemperature in/outFlowPressurePasses

Shell lube

TEMA type BEUDuty 3.5 Gcal/hr.

Air Cooling Waterdeg°C (deg°F)kg/h (Ib/h)kg/cm2 (psi)

Material of ConstructionBaffle plates, tie rods and guide pipes are of Carbon

190/45 (374/113)79,850 (176069)2.15(30.6)1Carbon Steel

Steel.

35/45 (95/113)3,32,000 (732060)4.5 (64.0)4SS 304, Low Fin.

to replace the tube bundle with a new one, incorporatingimproved configuration and metallurgy.

Effects of Cooler Leakage in AirCompressor Operation

A leakage was reported in ammonia plant-2 early infirst week of November 1996 from this exchanger. OnNovember 7, 1996, the air compressor discharge flowwas reduced. Even on increasing the speed of the aircompressor, air flow to the secondary reformer did not

increase. H2 to N2 ratio in the synthesis loop increased.The air compressor first-stage discharge pressure

increased from 2.1 kg/cm2g (29.9 psig) to 2.4 kg/cm2g(34 psig). This resulted in compressor surge leading toshutdown of the plant.

The air compressor LP case was opened. Leaking cool-ing water along with air had been carried over to the aircompressor low-pressure case second-stage impellersand diaphragms and caused deposits of chemicals used incooling water treatment. The cyclone separator hi thedownstream of the interstage cooler was not effectiveenough for the increased water load. Reduced clearancein impellers and diaphragms led to the surging of thecompressor. This was caused by prolonged running ofthe plant with the leaking exchanger. The choking ofcompressor impellers and the diaphragm forced the plantto be shutdown for a week for cleaning by hydrojetting.

Cooler inspection

Visual inspection

This interstage cooler of Ammonia-n was opened dur-ing the shutdown in March/April 1996 and inspected.The following are the main observations.

• It was found that a few tubes were broken and piecesof some of them were lying in the shell.

• Bolts, tie rods, and guide pipes of carbon steel werefound in badly corroded condition.

• Some tubes were found cut and sheared near thebafffle plate near U-bend.

• In some of the tubes feather like cracks wereobserved.

• Most of the damaged tubes were in the upper periph-eral area near the hot air entry point.

• Heavy deposits/scale were found inside the tubeswhich were mostly confined to the U-bend area near thehot air entry point.

• No corrosion was observed on the shell or channelcovers. The following causes were attributed to the fail-ure.

• Flow induced vibrations.• Corrosion of tubes.• Tube/tube sheet joint proper re-rolling was not done.

Detailed inspection

The bundle showed 10 tubes, six in the top row andfour in the second row from the top having longitudinaland transverse cracks clearly visible on the outside sur-


face, on and near the U bend portions. These sections oftubes could be easily removed from the baffle close tothe U bend by simply shaking them out (Photo 1). Heavydeposits were seen on and around the baffle. In addition,corrosion of the baffles was also seen. Further, theplugged tubes which had cracks were found to be chokedwith deposits.

On close inspection, cracks were seen on samples cutfrom U bend portions of the plugged tubes. Photos 2 and3 show close views of one of the cracked tubes as seenfrom outside. Multiple branched longitudinal and trans-verse cracks were seen. Photo 4 shows choking of theinside of the tubes in two samples. Photo 5 gives a closeview of the choking. The choking was due to depositsfrom the cooling water chemicals.

Inspection of the tube side was carried out to the extentpossible from the tubesheet end. The tube ends were freefrom corrosion but had some roughness indicative oflocalized erosion. The typical light greenish passivationlayer due to the chromate present in the cooling watercould be seen everywhere. The tube sheet was free fromany visible corrosion.

Sample test

Samples from the cracked tubes from U bend areas ofthe first-stage cooler, a few samples from the straightportion of the first-stage cooler and one sample from thecracked top row tube of the second-stage cooler (con-taining integral fins) were subjected to various tests.

Microscopic examination of tube cross-sectionrevealed several cracks, all originating from inside sur-face of the tube. Photo 6 shows a typical crack which hadinitiated on the inside surface and propagated towardsthe outside surface. The branching of the crack couldalso be seen. Photo 7 shows similar cracks again startingfrom the inside surface of the tubes at several places(multiple location cracks). Photo 8 is a magnified view ofone of the cracks showing branched transgranular cracks.

Probable Cause

From the visual inspection, it is clear that tube damagedisplays two distinct patterns of tube breaking.

(1) Cutting/shearing of tubes near the baffle platetowards the U-bend.

(2) Feather like cracks on the tubes.

(1) Cutting / shearing of tubes

Vibration analysis was carried out, even before open-ing of the exchanger showed major causes of vibration asunsupported U-bend and large tube spans of 2.3 meters.The vibration of the unsupported U-bend area theory issupported by the fact that most of the tubes were foundcut near the baffle at the U-bend. Baffle holes wereenlarged due to such vibration coupled with corrosionwhich further aggravated the situation and helped inshearing of the tubes near the baffle plate.

Vibration of free U-bend seems to be one of the rea-sons that most of the tubes were cut near the baffle plate.

Stress corrosion cracking of tubes

Other type of cracks, as stated above, seemed due tostress corrosion cracking (SCC). The cooler wasdesigned for water velocity of 1.4 m/s (4.6 ft /s) whichseemed to be on the lower side. The actual velocity inoperation could be even lower. In the U-bend area nearthe air inlet, due to high temperature, chances of deposi-tion were high. Once this deposition started, flow getsfurther restricted leading to concentrated deposits rich inchlorides which might have caused SCC.

Another cause of SCC may be that some of the tubesmight have broken because of vibration and, with hightemperature at this end, leaked cooling water kept on get-ting concentrated and causing chloride rich deposits onthe tube and near the baffle plate area.

Chances of stress corrosion cracking were more in theU bend region because this area of tube might have unre-lieved stress and remained under stress because of theunsupported bend.

Leakage of cooling water inside the shell formed astagnant water pool and caused the electrochemical cor-rosion of guide pipes, tie rods and a baffle plate. This issupported by the fact that the corrosion of tie rods and thebaffle plate was most pronounced in the lower portion ofthe shell.

Analysis of Failure

The cracks seen in Photos 6,7, and 8 are similar to the(SCC of austenitic stainless steels induced by chlorides.

For SCC to occur, four conditions have to be metsimultaneously:


• A susceptible material;• Tensile stress;• Continuous presence of chloride; and• Temperature about 55-60 °C ( 131-140°F ).These conditions were present on the tubes of the first -

-stage cooler, particularly at the U bend locations facingthe shell side inlet. Austenitic stainless steel type 304 is amaterial susceptible to SCC by chlorides. Residual tensilestresses were present on the tubes at and near the U bendsdue to bending of the tubes. (The threshold tensile stressrequired to cause SCC by chlorides in Type 304 SS is onlyabout 5 to 10% of the ultimate tensile strength.) The cool-ing water makeup is a treated river water after filtrationand chlorination which contains 25-30 ppm of chlorides.Circulation water chloride is maintained to a maximum ofabout 250 ppm. Because of heavy scaling and porousdeposition on the inside of the first-stage cooler tubes(Photo 5), the chloride-laden water remained stagnantcontinuously in contact with the inside surface of thetubes. The shell side inlet temperature is 190°C (374°F).The first one or two rows of the tubes would have theinside wall temperature close to the above value higherthan the minimum temperature of about 55°C (131°F)required for chloride SCC.

Once the above mentioned four conditions were simul-taneously present, the cracks could be initiated after a cer-tain period of continuous exposure to the chloride water.The deposits on the tubes had been slowly occurring overthe initial years leading to choking and subsequent stag-nation of water causing cracking a few years after com-missioning.

Remedial Measures

The condition of the tube bundles was very bad andalready 39 tubes had been plugged in ammonia plant-1and 55 in plant-2. Replacement of the tube bundles at theearliest became essential. However, to keep the plant run-ning, as a temporary measure, stiffening strips were insert-ed in U-bend area. However, it was possible to insert suchstrips in only three rows as accessibility was not there forthe rest of the rows.

The following three problems were identified and it wasdecided to take care of these in the design of new tubebundle.

( 1) Support to the free U-bend

Support to the free U bend was provided with the helpof proper insertion of the stiffening plate, so that the vibra-tions in the U-bend region was reduced.

(2) Velocity increase in the tubes

As stated in the analysis part, the main cause of SCCseemed to be lower velocity of cooling water inside thetubes. For this purpose, the exchanger was simulated andit was found that 35% of the total pressure drop, whichwas 0.6 kg/cm2 (8.5 psi), Mes in the inlet/outlet nozzles.The existing design velocity in the tubes was 1.4 m/s (4.6ft/s). This was increased to 1.7 m/s (5.6 fiVs) as per theprocess licensers specifications. This would help in reduc-ing the tendency of deposition inside the tubes.

Water flow corresponding to 1.7 m/s (5.6 ft/s) wasachieved by increasing inlet and outlet nozzle size from 8in. to 12 in. keeping total pressure drop across tubes andnozzles at the same level, that is, 0.64 kg/cm2 (9.1 psi).

(3) Change of material to stress corrosion resistantmaterial

If one or more of the factors mentioned above are elim-inated then SCC can be prevented. Obviously temperaturecannot be reduced below 55°C (131°F). Chlorides in thepresent make-up water are concentrated to about 250 ppmin the circulating cooling water since the cycle of concen-tration is maintained at about six to eight.

A small amount of tensile stress is always present inheat exchanger tubes in the installed position/under oper-ation. The residual stresses may be those introduced dur-ing the straightening after annealing and after U bending,tube-rolling/expansion, tube-to-tube sheet welding, orduring operation due to differential expansion and thermalcycling, and so on. It is almost impossible to remove allstresses in an operating heat exchanger tube.

It was decided to change the material of construction ofthe tube from 304 SS to duplex stainless steel ASTMA789/UNS S31803. It has better resistance to chlorideSCC. Material having ferritic and austenitic properties(22% Cr, 5% Ni, and 3% MO) have better mechanicalproperties and fabrication characteristics.

Further, this material had good ductility (25% minimumelongation), and the seamless tubes could be used for


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Figure 3. New tube bundle-air compressor first interstage cooler.

rolling integral fins on the outside surface. (Photo 9 is across-section of the fins present on the type 304 SS tubesample. The fins were rolled out of the 1.65 mm thicktubes.)

New Interstage Cooler Tube Bundle

A new interstage cooler tube bundle assembly tak-ing care of above design requirements was installed inboth ammonia plants 1 and 2 in June and July 1998with the following features (Figures 3 and 4).

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OTL"-1252DIA BAFFLE-1294

I.D.SHELL*»1300

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• Tube material changed from SS304 to duplexstainless steel ASTM A789 / UNS S3 1803 which hasbetter resistance to chloride SCC.

• The number of baffles are increased to eight fromfour, as flow induced vibration was one of the causesof tube failure. Baffle material is also changed fromCS to SS.

« Tie rod material is changed from CS to SS.• Additional baffle support is given in the U-bend

area to arrest vibration.• Replaced tubes are seamless instead of welded ear-

lier.• Water velocity in the tube is increased to reduce

the deposition inside the tubes from 1.4 m/s (4.6 ft/s)to 1.7 m/s (5.6 ft/s) by changing the nozzle size from8 in. to 12 in.

• The impingement plate is provided at the air inletnozzle to arrest vibration and provide proper distribu-tion of air. This will also prevent local overheating oftubes which increases deposition in the tubes.

Since installation, the performance of the system issatisfactory. These exchangers will be removed forinspection during the next annual turnaround ofplants.

Conclusion

The leakage in the interstage cooler was due to flowinduced vibration and SCC by the chlorides present inthe cooling water.

During design of the heat exchangers, process con-dition, cooling water circulation rate, and type oftreatment need to be critically reviewed with respectto mechanical design of the equipment before finaliz-ing the specification or material of construction formanufacture.

In the case of cooling water leakage from anexchanger normally the tendency is to plug the leak-ing tubes and line up the equipment to minimize thedowntime. However, in the aforementioned situation,it is worthwhile to check the upstream equipment (in

Photo 1. View of the U-bend portions of the tubesof first interstage cooler just below the shell side

hot air inlet.

Photo 2. A view of the cracks in the U-bend portionof a cracked tube of first interstage cooler.

the present case, it was process air compressor) alsofor any process parameter variations.

If the process conditions show abnormal variations,it is advisable to stop the equipment and open up forinspection. This will ensure safety of the equipmenteven though stoppage of the equipment for inspectionmay result in some production loss.


Photo 3. Close view of the cracks of the tubes offirst interstage cooler.

Photo 4. Choking of the inside of two crackedtubes with water brone deposits.

Photo 5. Close view of choking in one of the cracked tubes.


Photo 6. Branched crack initiating fromthe inside surface of the tube and propa-

gating to outside surface (50x).

Photo 7. Multiplied branched cracks initi-ating on the inside surface of the tubes

(50x).

Photo 8. Branched transgranular SCC intubes (220x).

Photo 9. A cross-section of one of the finsbottom portion along with the wall of the

tube (200x).


1999: Air Compressor Interstage Cooler Leakage

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