Failure in Ammonia Plant Transfer Line

Two approaches to protection against future failures are discussed:water-jacketing or castable-lining the transfer lines.

J. R. Isbell,Nipak Inc.,

Kerens, Texas

Late in the evening of May 20, 1975, the east tee in theprimary-to-secondary reformer transfer line at Nipak'sammonia plant in Texas ruptured suddenly and violently.

The minor and sudden depressurization required animmediate shutdown of the ammonia unit. The processgases and air valves were closed immediately at the controlboard, and the unit was blocked in as soon as transite, hotceramic balls, and catalyst stopped falling. There were noinjuries, and fire damage was minimal.

Before continuing with the details of the transfer linerupture, the circumstances and conditions which led to thefailure that night should be presented.

Nipak operates a 320-ton/day ammonia plant at itsKerens, Texas, fertilizer complex. This plant, completedand started up in 1964, has gas engine-driven reciprocatingcompressors. The heart of the process is the primaryreformer; and this one is a down-fired furnace containingthree rows of HK40 alloy tubes. Synthesis gas flows downthrough the tubes, horizontally through collection headers,and up through three riser tubes which are connected to thetransfer line.

Purpose of the transfer line, simplified, is to conveyreformed gases at 450 Ib./sq.in.gauge and 1,450°F from theprimary reformer to the secondary reformer. As designedoriginally, the transfer line consisted of a 30-in. O.D.,carbon steel pressure vessel, approximatly %in. thick and27 ft.-3 in. in length, designed to operate at 435 Ib./sq.in.gauge and 500°F. Externally this line was protected by asurrounding jacket through which water was circulated.Internal insulation was provided by five inches of"Insulag" which was in turn protected by a lA-in. thickstainless steel metal liner or shroud. A sketch is shown in.Figure 1.

After approximately five years of operation, the condi-tion of the transfer line was such that a replacement was inorder. The internal stainless steel liner (shroud) was beyondrepair, and the insulation had deteriorated to such an extent

that a 40 to 50° temperature drop existed between theprimary riser outlets and the inlet to the secondary reformer.Design for this section of the process allowed a 16°F dropbetween these same two points.

Figure 1. Water-jacketed transfer line.

Two problems occurred as a result of this deterioratedcondition. The primary reformer was fired excessively in aneffort to maintain temperature for secondary reforming.Methane leakage was high from the secondary reformerbecause of limited temperature control of the secondary. Ahigh rate of jacket-water boil off was also experienced.

Two possible routes to repair

Alternatives for repair of the transfer line were to: ( I )shut down and replace in kind the insulation and stainlesssteel shroud of the existing line; or 2) consider installationof a transfer line of another design. No debate is offeredhere as to the relative merits of water-jacketed vs. castable-lined transfer lines.

Nipak chose to design out the stainless steel shroud,therefore castable lining alternatives were evaluated. Therewas less than 10% variation in fabrication costs between

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Figure 2. Castable-lined transfer line.water-jacketed transfer line repair, bubble alumina castable,and the two-layer castable. The over-riding thought in thedecision, other than for a distaste for shrouds, was the highcost of plant outage which eliminated insitu repair. QuotingMr. C. C. Chaffee of Cooperative Farm Chemicals, Asso-ciation in Lawrence, Kansas who presented a paper,"Transfer Line Failure," to the 1970 Ammonia Symposium:"When problems with transfer line insulation or liners arediscovered, new ones should be installed since they are verycostly to repair or fabricate when in place." As eventsdeveloped, Nipak followed these words of advice and hadfabricated a complete new transfer line.

This new line, 27 ft. long, % in. thick, and 36 in. indiameter, was constructed to accommodate 5 in. of aninsulating castable and 3 in. of a harder protective castableon the interior hot face. The internal diameter remained thesame, at 20 in. The external shell was painted with a heat-sensitive paint to help in the detection of hot spots. Figure 2provides a cross-sectional drawing.

The new transfer line with a two-layer castable internallining was installed in March, 1970. Excellent service wasexperienced for approximately five years with no appreci-able problem or concern.

Another operating company, with castable liners under

Figure 3. Transfer line distortion.

Figure 4. Application of cooling steam.

consideration, visited Nipak in February, 1975. During thisvisit, a bulged area on the transfer line was detected. It isshown in Figure 3. This bulge, about 6 in. wide by 14 in.long by 3 in. high, was noted on the carbon steel pressureshell. It was near the top of the east tee, approximatelyabove where the riser connects to the transfer line. Nodetermination as to the length of time the bulge had been inexistence was possible. The last documented inspection hadbeen in September, 1974, during a plant turnaround.Erosion above each of the risers had been noted, but thedecision was to operate with no repairs, and review the areaagain when opportune.

Temporary steps taken while deciding how to repair

Action taken immediately was to apply cooling steam tothe area of the bulge and operate until a proper course forrepair could be developed. Figure 4 shows the steam beingapplied. The course of action evolved was then to fabricatelocally a new high-pressure shell tee and ship it to Houstonfor installation of a castable lining.

The essentials of a castable-lining installation can besummarized by describing the fabrication of this tee. In thisparticular case, the pressure shell was fabricated so that thehorizontal leg was approximately 5 ft.-6 in. with an internaldiameter of 3 ft. Material of construction was %-in. carbonsteel plate. A vertical leg, 22 in. in diameter, completes theshell. Stainless steel stud bolts then were installed internallyon 12-in. centers.

Next, a cardboard form was inserted and properly spacedbefore the first insulation was poured. This first pour wasthen air-dried approximately 24 hr. before the harder pro-tective layer (3 in.) of castable was poured. After the formwas removed, the tee was connected to a portable directfired heater and the temperatur raised to 250°F for 18 hr.Then the temperature was raised to 1,000°F at about 20°F/hr., and held there for a period of time, which accounted forsome 72 hr. for our particular case.

Finally, heating was discontinued and the tee allowed tocool gradually. The total process consumed approximately

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124 hr. The fabricated item, in this case a tee, can then beset in place and welded. Both faces of the joint must be"buttered." or coated with an appropriate mortar. Thisjoint is cured during the process startup.

Obviously, this description is not specific enough fordesign. These particulars can be developed with a suppliershould a castable lining be desired. The timing involved isimportant to note, especially should a plant outage bedependent on a castable transfer line installation. In ourcircumstance, the original line was totally replaced; thus thethermal curing time was accomplished prior to shutdown.This was also the intent when the single tee was constructedat the time the first castable failure was discovered.

Approximately 13 days after discovery of the bulge a newsection or "tee" of the castable design was on site, avail-able for installation. During this time period, the bulge wasbeing carefully monitored and no changes were detected.

Planned to effect change during shutdown

A major shutdown of the complex had been previouslyscheduled for July, 1975, pending arrival of various mate-rials. The high and low temperature shift catalyst beds andthe secondary reformer catalyst were scheduled for change-out at that time. Due to this schedule, plus the demand for

..product, it was decided to continue operations until July,thereby minimizing -downtime. Continuous monitoring ofthe bulge was of course included''!« the, plan.

The plan worked extremely well. . . untU'lViay 20, 1975.Upset conditions within the ammonia plant process due tothe loss of a boiler feedwater pump apparently caused anenlargement of the crack or cracks in the castable lining,allowing more heat to reach the pressure shell. This isconjecture, of course, because a precise determination ofthe cause of the bulge was never developed. At any rate, thebulge ruptured.

After the debris had stopped falling, and the unit wasshutdown, a temporary patch was welded to the transfer linepressure shell so the catalyst could be oxidized and theammonia unit prepared for an extended turnaround. Cata-lyst and other materials were on hand at this point for theplanned outage, consequently the turnaround began.

During the rapid depressurization, the castable insulationin the middle and west tee's as well as the manway coverwere badly eroded or damaged. These other tee sectionswere therefore removed and sent to an outside contractorfor reinsulation. Again, this was with the two-layer castableinsulation as had been installed in the original castabletransfer line.

While the two old tees and the manhole cover were beingrepaired, the new tee fabricated in February was installed onthe west end of the line. This is the position closest to thesecondary reformer. In this manner, the transfer line was re-constructed. The repaired sections were returned within sixdays and installed by Nipak maintenance. A local contractordid all insulation work as required to join the tee sections.With the installation complete, the plant was ready for startup on June 2, 1975, approximately 13 days from the initial

transfer line failure.Only one other castable failure has occurred and should

be documented here. The lining breakdown was discoveredin February. 1976. approximately seven months after repairof the first total failure. This instance was not spectacularand did not cause damage as did the initial failure. It was,however, considered quite serious and enhanced the im-portance of inspections.

The ammonia plant was down for a normal turnaroundbut the work efforts included replacement of all tubes in theprimary reformer. Insulation was removed from an inlet gasheader at the reformer. There, obscured from view, was adistortion or bubble in the pressure shell of the third risercone section, as seen in Figure 5. A new carbon steel conewas rolled and the castable liner thermally cured andinstalled on site, as illustrated in Figure 6. Thermal curingwas corrdinated with other castable work being accom-plished within the plant.

Figure 5. Riser cone distortion.

Figure 6. Riser cone repair.

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Figure 7. Ammonia plant castable lining.

Conclusions

No precise cause for the initial failure of the transfer linehas been presented. It was theorized that severe erosiondirectly above the riser caused deterioration of the hardprotective castable coating. This was apparently sufficientto expose the softer insulating castable which parted orcracked to expose the supporting studs or the pressure shellproper, to hot gases. Calculations indicate the pressure shellapproached 1,100°F for the pressure shell distortion tooccur at the operating pressure.

Various ways of protecting the castable lining directlyabove the risers have been considered. This is the areawhere the hot gases enter the transfer line and make a 90°change of direction. A metal shroud or target plate could beincorporated into the design. Obviously, the attendant prob-

lems of expansion and contraction of the lining are inherentin a design of this nature. Since the metal shroud problemshad been eliminated by going to the castable, it was notdesirable to re-engineer one into this line. Additional thick-ness of the protective layer could be added for protection.This seemed to add to the problems of thermal curing, thusit was rejected. Thorough inspections and repairs finallydeveloped as the route taken.

When the two-part castable lining is selected for use in atransfer line, the recommendation is that a thorough inspec-tion program be instituted. All major cracks and any erosiondamage should be repaired upon detection to maintain theintegrity of the system. Also, heat sensitive paint should beused on the exterior and inspected frequently.

As stated previously, this article is not presented as ajustification for either water-jacketed or castable-linedtransfer lines. Rather, it allows the reader to compare repairexperiences between the two systems.

Locations within the Nipak ammonia plant which havebeen repaired using the two-layer castable are indicated inFigure 7. Notice that the water jackets have not necessarilybeen removed because castable insulation has been intalled.

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ISBELL, J. R.

DISCUSSION

M.B. STERLING, Amoco Oil: We were one of thepeople who were interested in your grandchild, so Ithink it's appropriate for us to comment on ours. Wehave also replaced our transfer line exactly as you did.The only difference is that we have a five row reformerso we have two additional sections to the line. The linehas been in operation for 21/2 years, and when we leftto come to this session the plant was in turnaround.One of the last things I did was to look into the transferline, and as far as I could see the only problem thatwe had was a small amount of spalling at a riserlocation. The line appears to be in good shape.

We are fond of using heat sensitive paint on thissort of an application - 1 wondered if you use that also?ISBELL: Yes we did. Our original transfer line hadheat sensitive paint on it. Unfortunately we didn'trenew it periodically and it turned chalky white so itwasn't much of an indication on the first one. You

notice on the second one we do have heat sensitivepaint on it. One other thing, I failed to point out on theslides I just showed, the secondary reformer and thewaste heat boiler still have water jackets on them, andI guess we could get into a debate about whetherwe should have removed those or not.Q: Could I ask a point there? Where you've left thewater jackets on, I take it you will circulate it in thewater end?ISBELL: Correct. Certainly be cheaper to do that thanto rip them out, as a matter of fact.Q: You refer to the temperatures you thought you gotto cause this line to explode. What's the normal metalsurface temperature you find on this packing?ISBELL: I believe the design was for 500 degreesand we preserved that. It's considerably cooler thanthat now. You can place your hand on it, if you arevery quick about it.

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