Syn-Loop Waste Heat Boiler Exit Line Failure · The Syn-Loop Waste Heat Boiler [WHB] also known as the Ammonia Converter Effluent Waste Heat Boiler is located downstream of the Ammonia

2003 AMMONIA TECHNICAL MANUAL 3

Syn-Loop Waste Heat Boiler Exit Line Failure

This paper will describe two failures that occurred two years apart on the same exit line of a Syn-loop waste heat boiler. The failures occurred at the weld joint between two different Chrome – Moly alloys

at almost the identical location. At the time of the second failure the downstream inlet nozzle to the Syn-loop feed preheat exchanger also failed. This paper will provide the analyses from a

metallurgical investigation, and pipe Finite Element Analysis and provide some insight into the problems encountered in making a sound repair at these weld joint locations.

D.H.Timbres Agrium, Inc.

Introduction

grium Inc. is a leading global producer and marketer of fertilizer and a major retail supplier of agricultural products and ser-

vices in both North America and Argentina. The Cor-poration produces and markets four primary groups of fertilizers: nitrogen, phosphate, potash and sulphur.

Agrium, Fort Saskatchewan Nitrogen Operations [Figure 1] was commissioned in 1983 and produces am-monia and urea fertilizer for the Western Canadian and export markets. The site comprises a 1000 metric tonne per day nameplate Kellogg ammonia plant and 907 met-ric tonne per day nameplate Stamicarbon urea plant. Cur-rent operating capacities are 1350 metric tonnes per day ammonia and 1280 metric tonnes per day urea.

The Syn-Loop Waste Heat Boiler [WHB] also known as the Ammonia Converter Effluent Waste Heat Boiler is located downstream of the Ammonia Con-verter. The Syn-Loop WHB is the first of several ex-changers used to cool the converter effluent prior to re-frigeration as well as act as a preheater/steam generator of 1800 psig [12,400 kPa] steam.

In November 1998 the plant experienced the first of two failures at the exit nozzle of the Syn-Loop WHB,

and a second failure was experienced at the same joint almost to the day two years later in November 2000.

Syn-Loop Waste Heat Boiler Description

The subject exchanger is not the original unit in-stalled when the plant was commissioned in 1983. The initial unit was replaced after three years of operation with the subject unit in 1985.

The subject unit is a vertical U-tube removable bundle design. See Figure 2.

The unit takes ammonia converter effluent and in the process of cooling, the ammonia converter effluent generates 1800 lb. [12,400kPa] steam, and preheats boiler feed water from 260oF [125oC] to 625oF [330oC].

Operating History

The subject unit ran reasonably well from 1985 un-til 1998, when in November 1998 the first failure oc-curred at the outlet nozzle.

Figure 3 shows the location of the failure.

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AMMONIA TECHNICAL MANUAL 2003 4

The failure occurred in the weld joint between the 20 inch self reinforced vessel nozzle and the exit piping. The vessel nozzle is made of 2 ¼ Chrome + 1 Mo, and the exit piping is made of 1 ¼ Chrome + ½ Mo. The pipe schedule is 120 [nominally 1.5 inches (38 mm) thick].

On the inlet side to the Syn-Loop WHB there are two dissimilar metal welds. An Alloy 800H pup piece is welded between the 2 ¼ Chrome + 1 Mo WHB nozzle, forming one dissimilar weld, and the second between the Alloy 800H pup and inlet pipe which is 304SS pipe. Nei-ther of these joints has presented a problem in this unit to date. Each of the latter dissimilar metal weld joints are non-destructively checked each plant turnaround.

Description of First Failure

Operations personnel on a routine tour of the plant on November 14, 1998 at about 0200h heard an ex-traordinary loud noise coming from the Syn-Loop WHB. Further investigation revealed the noise coming from the outlet nozzle and the insulation was showing discoloration.

No fire was observed at the nozzle, but the opera-tors considered the situation very serious.

Once operations reviewed the severity of the situa-tion, management, engineering and maintenance staff was then called out to further assess the situation. All personnel were on site within an hour.

The plant was reported to be in a normal operating mode at the time and the plant had not experienced any abnormal cyclic operations.

The conclusion of the group was continued opera-tions were untenable and the operators commenced shut down procedures. By this time it was about 3 am.

Other Maintenance personnel were alerted and re-quested to be on site by 0800h the same day to set up scaffolding and remove insulation, so a better assess-ment of the damage could be undertaken.

Due to numerous dissimilar weld failures experi-enced in the plant since start up, most particularly with stainless steel to low alloy chrome steels, this particular dissimilar weld joint was flagged for routine UT shear wave inspection during plant outages. This joint had been checked several times since installation in 1985, with the most recent being August 1996; two years prior to the first failure in November 1998.

After the insulation was removed, and the unit had cooled down, both visual and wet fluorescent mag-netic particle inspection [WFMPI] confirmed two cracked areas.

Figure 4 shows the location of the two cracks, which on the external surface were oriented in the circumferential direction, and between the final weld cap passes.

Both cracks were about 2 inches [50 mm] long ori-ented at approximately the 1 o’clock position and 2-3 o’clock position facing the vessel.

An ultrasonic [UT] shear wave scan was conducted over the entire weld seam to determine the depth and extent of both surface cracks. The surface cracking was confirmed to be through wall, and the scan revealed ad-ditional subsurface cracking from the 8 o’clock to 1 o’clock position, peaking in depth at 1 ¼ inches [32 mm] measured from the inside surface.

So basically there was cracking close to 60% of the circumference, and almost all cracking was through wall cracking.

The consensus was that there was no alternative but to remove the entire weld and replace the weld in total. A partial repair was not viable.

For the most part the crack was in the weld metal. But during excavation two smaller transverse cracks were seen at the 1 o’clock position. One about ¾ inches [19 mm] long running into the 1 ¼ Chrome material, and a second transverse crack about ½ inches [13 mm] long running into the 2 ¼ Chrome material. See Figures 5 & 6.

Two “boat” samples were removed from the cracked weld for metallurgical analysis. A large piece was taken from the external surface [weld cap area], the second, and smaller boat sample, was recovered during excavation of the weld near the internal diameter. See Figure 7.

Examination of the larger boat sample revealed that the cracking on the outside diameter occurred in a circumferential manner in the area between the two weld passes as shown in Figure 8. No other significant external features were noted.

The fracture surface of the crack from the large boat sample was then exposed and is shown in Figures 9 and 10.

In general, the fracture surface had a rough brittle appearance consistent with a brittle cleavage type fail-ure. No evidence of any pre-existing defects or fatigue type cracking was noted. The pattern of staining on the surface would possibly suggest that the cracking initi-ated from the inside diameter.

The smaller sample also contained a fracture sur-face along one side as shown in Figure 11. The fracture also exhibited a brittle cleavage type fracture with no evidence of any fatigue type cracking.


The fracture surface of the larger sample was cleaned and examined under a scanning electron micro-scope. Observations supported the optical metallogra-phy that fracture occurred in a transgranular manner by a brittle cleavage mechanism. No evidence of any inter-granular cracking or ductile fracture was noted.

Scanning electron observations on the fracture sur-face of the smaller sample also exhibited a brittle cleav-age type fracture, again supporting the optical metal-lography observations.

Sections through each boat sample were prepared for microscope examination and hardness testing. Fig-ure 12 illustrates the section through the large sample. Examination of the cracking revealed it occurred in a brittle manner and exhibited some secondary cleavage cracks. See Figure 13.

The orientation of the secondary cracking would also suggest that the main direction of crack propaga-tion was from the internal diameter to the outside di-ameter. The weld at this location exhibited an accept-able as-weld microstructure with a hardness of 204 – 216 HV500 [94-96 HRB]. No unusual microstructures or excessive hardnesses were noted.

The section through the small sample is shown in Figures 14 and 15. These sections exhibited three weld layers with the center layer having a different micro-structure and etching characteristics [i.e. this layer did not etch in Nital]. It is suspected this layer is a higher alloy weld metal pass. The hardness of all three layers was in the range of 237-258 HV500 [20-24 HRC]. Aside from the above, no welding defects or excessive hard-nessses were noted. The cracking observed on this sec-tion was consistent with that caused by a brittle cleav-age mechanism.

The chemical composition of the samples was de-termined using a Texas Nuclear Alloy Analyzer.

The larger boat sample was identified as 2 ¼ Cr + 1 Mo material [i.e. AWS E8018 B3/B3L filler metal].

The smaller boat sample was identified as 4 ½ Cr + 1 Mo material [possibly a mixture of E8018 B3/B3L and E502 (5 Cr) filler metals].

Metallurgical Conclusions

The conclusion from the metallurgical work on the two boat samples indicates the cracking occurred by a brittle cleavage mechanism. Based on the orienta-tion of the secondary cracking next to the main frac-ture, it is suspected the cracking most likely initiated from the internal diameter and propagated towards the outside diameter.

This metallurgical work did not preclude that a pre-existing defect or crack was present as some other loca-tion in the weld. While the material in the smaller boat sample may have consisted of two different types of filler metal, this was not considered a major factor in the cracking since the weld metal hardnesses were not excessive. Based on the hardness values, we suspect the weld had been stress relieved.

Repairs Following First Failure

The two transverse cracks in the WHB nozzle and pipe were first repaired. The 1 ¼ + ½ Mo pipe with E8018-B2 and the 2 ¼ C + 1 Mo nozzle with E8018-B3L. The nozzle and pipe was then machine beveled before commencing the main circumferential welding repair.

The first attempt at making a repair was to use a Gas Tungsten Arc Welding Shield Metal Arc Welding process [GTAW/SMAW] with ER80S-B3L and E8018-B3L respectively. No weldability tests were performed on the nozzle or pipe, and no hydrogen out gassing per se was conducted, although the nozzle and pipe were preheated as noted below.

Shortly into the repair the purge gas dams placed in the nozzle and pipe failed. Consensus was the dams were too light. It must be pointed out that there was a natural draft through the exchanger and piping. Basi-cally there was no way to block in the system.

Due to pressure to get the plant up and running again, we then reverted to using a backing ring and welding only with the SMAW process. A preheat of 400oF [204oC] was applied to the joint for about 4 hours prior to actual commencement of welding. The weld was completed without incident. No intermediate non-destructive testing was conducted in order to maintain the preheat temperature.

The completed weld was examined by X-ray diffrac-tion prior to and following a post weld heat treatment. Cobalt 60 was used due to the thickness of the joint.

The post weld heat treatment was performed ac-cording to the ASME Code with a holding time at 1350oF [732oC] for 2 hours.

Hardness measurements were made on the weld metal, and both heat affected zones after post weld heat treatment. The results averaged 223 HRB on the P11 side, 237 HRB on the weld metal, and 237 HRB on the P22 side. The results conform to ASME B31.3 limits for P4 and P5 materials, albeit close to the upper limit.


Description of the Second Failure

Almost to the same day the aforementioned weld joint failed for a second time in November 2000.

Operations personnel on a routine walk about in the plant heard an unusual noise around the WHB. On closer examination the noise was found to be emanat-ing from the outlet nozzle. As well the operators no-ticed the aluminum sheathing protection cover was starting to discolor.

There was an immediate callout of management, engineering and maintenance personnel and by the time the aforementioned personnel arrived on site [within one hour] the outlet nozzle was engulfed in flames as seen in Figure 16.

As per the operator findings in the first failure, the plant was in steady state operation and had not experi-enced any cyclic operations for a period of months.

Operations commenced shut down action and backed gas out of the loop. While the loop was depressuring the fire was allowed to burn itself out which took place over a two hour period. Once the gas was backed out, and ni-trogen applied to the process, the system cooled down sufficiently to allow the insulation to be removed.

Visual observations showed a primary crack about 13.5 inches [343 mm] long in the circumferential direc-tion, and a secondary crack ½ inch long [13 mm] trans-verse to the primary crack oriented to the 2 ¼ Cr + 1 Mo nozzle. See Figures 17 & 18.

Again, as per the first failure, and from external ob-servations, both cracks were confined to the weld metal area.

An ultrasonic shear wave examination revealed the primary crack to be approximately 34 inches long [864 mm] running internally from about the 1 o’clock posi-tion to the 8 o’clock position looking towards the WHB vessel [approximately 50 % of the weld circumference]. No other subsurface cracking was detected by the shear wave UT examination.

The decision was to remove all the weld metal that had been replaced in the first failure in 1998.

When the machined surface was checked for de-fects, it was discovered that a 1 inch [25 mm] transverse crack had propagated into the 1 ¼ Cr + ½ Mo material [P4]. This crack was found at the tip of the primary crack at the 1 o’clock position. See Figure 19.

Inlet Nozzle Crack in Syn-Loop Feed Preheat Exchanger

One new observation was found following the sec-ond failure and that was longitudinal crack at the inlet

nozzle to the first downstream exchanger from the WHB [the Syn-Loop Feed Preheat exchanger]. In a Kellogg designed plant this exchanger is called the Ammonia Converter feed effluent exchanger or 121C.

Operations had reported an ammonia smell in the area of the 121C exchanger prior to the crack failure on the exit nozzle of the WHB but were unable to deter-mine the exact location. The crack was subsequently located by a wet fluorescent magnetic particle examina-tion. See Figure 20.

A 1.5 inch [38 mm] long transverse crack was noted in the 20 inch [508 mm] circumferential nozzle butt weld, located below the outside repad weld. The nozzle material is a forging of SA 182- F1 [C + ½ Mo] and the pipe is SA335-P11 (1 ¼ Chrome + ½ Mo).

A UT shear wave inspection was performed to de-termine the length and depth of the crack indication. The crack was determined to be through-wall and did not propagate into the pipe material any further than what was visible on the surface.

A boat sample with the subject crack was taken from the spot. However, the sample did not contain the entire crack depth. Removing the entire crack would have resulted in the creation of a hole that was much too large to repair from the outside only. As it was a fairly large excavation was made in the nozzle pipe connection as shown in Figure 21.

The relevant information found from the metal-lographic examination of the boat sample was the crack-ing propagated through the various weld passes in a rela-tively straight brittle manner. Based on the crack appearance, the fracture appeared to initiate near the in-ternal diameter and propagate towards the outside diame-ter. A crack arrest line near the outside diameter sug-gested the cracking might have propagated in two stages.

There was no evidence to suggest that high tem-perature hydrogen attack or weld related defects con-tributed to the cracking. Nor were there any abnormal microstructural features or excess hardnesses noted that would contribute to the failure.

A piping stress and finite element analysis [FEA] concluded that the cause of the weld failure was primar-ily mechanical loads. The analysis suggested that high-sustained stresses, from pressure and piping loads, were the most likely cause of the cracking. Localized yield-ing was concluded from the FEA during normal opera-tion and that when the system cooled down to ambient temperatures, brittle fracture might result.

The consultant who conducted the analysis recom-mended the piping system be reviewed and upgraded as required to meet the system requirements.


At present this recommendation has not been acted on.

Repairs Following the Second Failure

For the second failure repairs, the decision was made not to remove any boat samples. Instead the joint was prepared for immediate repair.

Checks were made for hardness on the machined beveled surface, and the values ranged from 131 to 174 BHN on the 1 ¼ Cr + ½ Mo elbow, and from 145 to 164 BHN on the 2 ¼ Cr + 1 Mo nozzle.

Both the pipe and nozzle were “outgassed” at 600oF [316oC] for 16 hours. Weldability tests were conducted on both the 1 ¼ Cr and 2 ¼ Cr materials. The 1 ¼ Cr side passed. The 2 ¼ Cr side failed. An additional 16 hours bake out at 600oF [316oC] was performed on the 2 ¼ Cr material and the weldability test failed again.

Prior to welding, precautions were also taken to en-sure all weld materials were meeting the Bruscato Χ factor. All weld materials had a Χ factor of less than 15.

The decision was then taken to “butter” the 2 ¼ Cr material with E8018-B3L, rebevel and then take an-other weldability test. This time the weldability test was satisfactory and the repair proceeded to fit up. All weld-ing was conducted with a preheat temperature of 400oF [204oC].

First the transverse crack in the 1 ¼ Cr + ½ Mo pipe was repaired using E8018-B2 electrode.

Purge dams were a reinforced variety compared to the first failure. The dam on the pipe side was vented and a thermowell downstream was removed for addi-tional venting.

Welding proceeded to one GTAW pass and two SMAW passes where after an X-ray was performed, which was acceptable. The GTAW was done using ER80S-B3L and the SMAW with E8018-B3L.

Welding then proceeded to ½ thickness, where a further X-ray was taken, which was also acceptable. The joint was then completely welded out. A third X-ray with Cobalt 60 proved acceptable.

A post weld heat treatment was conducted for 5 hours at 1350oF [732oC] to compensate for the thicker nozzle on the vessel. A fourth X-ray was taken and proved acceptable with no relevant indications.

Hardness checks on the two heat affected zones and weld cap showed: 130-145 BHN on the 1 ¼ Cr side [HAZ], 155-174 BHN on the 2 ¼ Cr weld cap, and 160-163 BHN on the 2 ¼ Cr side [HAZ]. These numbers were far lower than recorded after the first repair.

The plant has run somewhat intermittently until the present without any further failures. Ultrasonic shear wave checks have been made on three separate occa-sions since the November 2000 repair without further detection of any cracking.

Additional Investigations

A number of unknowns persist with respect to the root cause of the failure at the subject weld joint. The metallographic data from the first failure was not con-clusive; the surprising second failure can perhaps to some extent be focused on a less than perfect weld re-pair procedure and method.

Agrium then undertook piping stress analysis on the down stream pipe system to check whether there were any inadvertent changes made over the years since start up in 1983.

A thorough review of the piping stresses on the out-let nozzle of the Syn-Loop WHB showed no undue pip-ing stresses, and all support hangers and support points were more than adequate.

Attention was then turned to a finite element analy-sis of the nozzle to pipe joint.

Finite Element Analysis

The WHB nozzle to pipe connection transitions from a thickness of seven inches [178 mm] for the noz-zle down to one and half inches [38 mm] thick for the pipe over a span of approximately five and one half inches [139 mm] giving a transition ratio of 1:1. This joint does not meet current ASME standards of 3:1.

This sharp transition results in a significant increase in mass of steel over a very short distance, from a ther-mal standpoint. Thus for a given heat transfer coeffi-cient, the wall of the thinner pipe [1 ¼ Cr pipe] will reach its equilibrium temperature a lot quicker than the nozzle which is much thicker.

A finite element analysis [FEA] model of the noz-zle junction was produced to evaluate the possibility of a transient peak stress condition.

The results indicate that a peak stress of about 80,000 psi [552 MPa] would occur in about 200 sec-onds into the start up condition if the process gas goes from ambient to operating conditions. The strain asso-ciated with this high stress exceeds the allowable value by a factor of 1.75.

The change in temperature transients are shown in Figures 22 to 26, the change in stress transients in Fig-


ures 27 to 31 and the change in strain transients in Fig-ures 32 to 36.

Normally the process temperatures changes from ambient to operating conditions over a heat up period of 2 to 3 hours or more. Given this slower temperature rise the thermal gradients would be much slower.

Conclusions

The various metallurgical and finite element analyses have not reached a consensus as to the root cause of the two failures at the exit nozzle to the Syn-Loop WHB.

There is the suggestion that the high transient stress and strains over time may have contributed to the initia-tion of the crack, but in both incidences the plant was in a steady state operation at the time of the failures.

Based on the first failure and the metallurgical re-sults the crack surface did not produce any evidence of fatigue type failure, so it can only be surmised through wall cracking was a short time occurrence.

The second failure also occurred while the plant was in steady state operation with no reported cyclic operations. Likewise to the first failure the facture sur-face had a similar visual appearance to the first failure and was sudden in occurrence.

One might argue the case the second failure may have resulted because less than favorable repair meth-ods were used in making the first repair, and while the weld hardness [surface] was close to maximum accept-able hardness for the materials involved, the conditions at the root may have been another matter and one could speculate the hardnesses were much higher.

The latter speculation is based on the inference the larger vessel nozzle was not properly soaked through

the thickness and coupled with a natural draft in the in-side of the pipe, may have resulted in lower than opti-mum heat treating temperatures on the internal diame-ter. With the high transient stress and strains over a number of start ups the conditions may have been set for the crack initiation and failure.

The various transient temperatures, stresses and strains found with the FEA should not normally hap-pen. However, speculation again suggests changes to the operating conditions of the ammonia converter ef-fluent through the WHB and by pass line may have set up conditions for the high transient conditions. Like-wise, an interruption in the water flow to the WHB will offer conditions for the high transient temperatures, stress and strains at the weld joint.

Suggestions to lessen the likelihood of future fail-ures at this location have been to:

1. Ensure a gradual start up process so that the

stresses and strains are not in excess of yield values.

2. Alter the nozzle transition from a 1:1 taper to a 3:1 taper,

3. Install on line recording sensors to measure skin temperatures and strain.

To date only item 1 has been implemented along

with continued ultrasonic shear wave inspection of this weld joint.

It maybe with the more rigorous repair performed on the weld joint after the second failure and the lessen-ing of plant cycles along with the more gradual start up process, this joint may last similar to or longer than the time between installation and the first failure.


Figure 1. Plant Overview.

Figure 2. Syn-Loop Waste Boiler.


Figure 3. External Location of Subject Failures.

Figure 4. External Crack Locations. Crack is Oriented Circumferential and has Ex-ited through the Wall in Two Locations.


Figure 5. Transverse Crack Running into 1 ¼ Chrome + ½ Moly Pipe Material.

Figure 6. Transverse Crack Running into 2 ¼ Chrome + 1 Moly Vessel Nozzle.


Figure 7. Boat Samples Removed from Weld Area.

Figure 8. Large Boat Sample Viewed from Top Showing the Main Crack is between Weld Passes.


Figure 9. Fracture Surface Appearance – Large Boat Sample.

Figure 10. Fracture Surface Appearance – Large Boat Sample.


Figure 11. Fracture Appearance of Small Boat Sample.

Mag. X5, 2% Nital Etch

Figure 12. Cross Section through the Large Boat Section at the Top of the Weld.


Mag. X 264, 2% Nital Etch

Figure 13. Brittle Cleavage Cracking through the Weld.

Mag. X10, Villella’s Etch

Figure 14. Cross Section through the Small Boat Sample. Note the Three Different Weld Layers, A, B, C.


Mag X132, Villella’s Etch

Figure 15. Change in Microstructure across the Small Boat Section.

Figure 16. Observations following the Second Failure.


Figure 17. Overview of Crack Location following Second Failure.

Figure 18. Close Up View of External Crack following the Second Failure.


Figure 19. Transverse Crack Oriented into the 1 ¼ Cr- ½ Mo Pipe.

Figure 20. Crack in Inlet Nozzle to 121C.


Figure 21. Excavated area on Inlet Nozzle to 121C.

Figure 22. Temperature Transient after 120 Seconds.








Figure 27. Stress Transient after 120 Seconds.







Figure 32. Strain Transient after 120 Seconds.







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Syn-Loop Waste Heat Boiler Exit Line Failure · The Syn-Loop Waste Heat Boiler [WHB] also known as the Ammonia Converter Effluent Waste Heat Boiler is located downstream of the Ammonia