Ammonia Separator AccidentA change in design of the reentry nozzle appears to have been agoodmove, although it has caused other operational problems.

L.B. PattersonHawkeye Chemical Co.

Clinton, IA

On March 30, 1970 an explosion occurred at the HawkeyeChemical Co. plant at Clinton, Iowa. No fatalities occurred,but six people were taken to the hospital. Two were treatedand released immediately; others were released a day or twolater. Subsequent investigation showed that a dual purposevessel, referred to as a cold exchanger/secondary separator,failed—causing considerable damage to adjacent equipment.Litigation has prevented public discussion of the incidentprior to this time, but a settlement has now been reached.

This article describes the related events and is pres-ented in the interest of helping others avoid similar prob-lems. I am not qualified to discuss the metallurgicalaspects and so, will rely on what others have said. I can-not elaborate upon their statements nor vouch for theiraccuracy.

Hawkeye Chemical Co. built and began operating itsfacility at Clinton, Iowa, in 1963. It is a conventionalreciprocating compressor type plant designed to produce400 short (362.8 metric) tons per day of ammonia. Thecold exchanger/secondary separator is a common pur-pose multiwall vessel. The plant uses steam methanereforming with two high temperature shift converters,ME A COa absorption and a methanator for the finalcleanup of the gas. The synthesis loop operates at 5,000psi (34,475 kPa). Upgrading facilities associated with theplant also exist, but they are located some distance fromthe plant itself. This spacing probably saved the facilitiesfrom more serious damage.

Discovery of a weep hole leakThe plant originally used a water-cooled condenser to

remove the ammonia from the synthesis gas, but due toleaks and subsequent scaling, the water-cooled condenserwas replaced with an air-cooled condenser on March 10,1970. The plant operated well for 10 days until a gas leaknear the bottom of the cold exchanger vessel was noticed.Later in the day of March 20, an ice blister formed abouthalfway up the vessel on the opposite side. When we

removed part of the insulation near the bottom we con-firmed that the gas flow was coming from the weep holeon the northeast quadrant of the vessel, near the bottom.When the insulation was removed, the ice blister dis-appeared.

Our reading of the equipment literature led us tobelieve that as long as the leaking gas could be relievedthrough the weep holes, it would be safe to operate the equip-ment. We called a number of knowledgeable people anddiscussed the safety issue with them. Consensus at the timesupported our conclusion, but after the explosion, there wassome dispute over exactly what was said and what wasmeant.

Shortly after March 20, we started having an entirelyunrelated problem with a compressor and had to operatethe plant at half rate for several days. The pressure in theloop decreased and the leak from the weep hole appearedto remain about the same. I continued to check the leakintensity at least once a day until the day of the explosion.At no time did I feel that the intensity had increased.However, on two occasions, shift supervisors noted in thelog that they felt the leak rate was growing. Upon readingthese notations, I rechecked the leak but could see nochange. I attributed the supervisors' observations to thefact that the compressor under repair was being tried andrun in, for the notations coincided with the time of anabortive attempt to put the compressor back on line andreturn to full production rates. Repairs on the compres-sor were made during the Easter weekend and completedearly Monday morning. After a ten-minute run in andcheck, followed by a one hour run in and check, the com-pressor was put back on line. This was done in my pres-ence at approximately 9:00 Monday morning, March 30,1970.

I only had time to get from the compressor building tomy office when the explosion occurred. I returned to theammonia area immediately and helped some people tothe first aid room. The injured later were taken in a pri-vate car to the hospital for treatment.

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Figure 1. View of compressor building. Figure 3. Interior of control house.

The gas had burned out by the time I arrived back atthe operating area, and only a few small fires of paper andother combustibles were still burning. The control housewas severely damaged. Practically all the personal injur-ies were caused by flying glass from the wire reinforced"picture" windows of the control house. One of the high-pressure lines was ruptured and bent in such a way thatthe burning gases hit a tower approximately 50 ft (15 m)away and burned the aluminum covering off the insula-

tion. The insulation protected the tower from any dam-age. Several heat exchangers were perforated by flyingdebris.

The entire compressor building was denuded of shea-thing, but the structural members were undamaged. Theforce of the concussion severely damaged the steam plantbuildings, the cooling tower, the nitric acid building andthe storage tank refrigeration compressor building. Theconcussion blew off all of the transite covering on thenear side of the cooling tower 450 ft (140 m) away, passedthrough the cooling tower and damged some of the tran-site covering on the far side of the tower.

Although we were not aware of it at the time, the cou-pling between the nitric acid centrifugal compressor andits driver was misaligned by the explosion, which causedit to be damaged when we restarted it. The coldexchanger and secondary separator ruptured, scatteringshrapnel around the area and bending the heat exchanger

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Figure 2. Lowering converter to ground. Note burnedinsulation covering on adjacent tower.

Figure 4. View of shattered cold exchanger and fallenconverter.

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Figure 5. Steam plant In foreground and coolingtower in distance. Figure 6. General view of explosion site.

tube into a spaghetti-like mess. The heat exchanger wastoppled on its side; The crane structure, which supportedthe heat exchanger and the synthesis converter, was bentoutward at the bottom, causing the converter to fallabout 10 ft (3 m) straight down with enough force to drivethe tailpipe through a six-inch (15 cm) concrete slab intothe ground. The converter was canted but remainedupright.

We found missiles as far away as 1,500 ft (460 m).Buildings, which were mainly sheathed with corrugatedaluminum siding, were damaged 500-600 ft (150-180 m)away.

The most serious problem we had at the time wasaccounting for all personnel. One individual left the plantunder the fence and went home without checking out. We"lost" one man by not getting his name on a list when wesent him to the hospital by private car for checking. Thelocal emergency number was called immediately from theplant and the fire trucks and police cars were dispatchedto the scene by radio. Their response was very good, butfire equipment was not needed, because the gas valveswere shut off automatically by the blast and later blockedin manually. The sound of the explosion was heard intown six miles (10 km) away. The subsequent phone callsoverloaded the local telephone exchange to such anextent that no calls in the city could be made for a consid-erable period of time.

The following day we restarted the steam plant, theacid plant and the ammonium nitrate prilling. The mis-alignment of the acid plant compressor caused damage tothe coupling and required repairs a few days later.

Damage repaired in four monthsWe had a contractor team flown in to survey the dam-

age and determine our ability to repair the plant. Theydid a very fine job in the survey, expediting the repair, andgetting us into a position to restart the plant. This was accom-

plished in four months, which seemed to me a very shorttime, considering the extent of the damages. The worstproblem was the ruptured vessel, but the forgings weresound and we were able to get them cleaned up andrewound in time for reconstruction.

The multiwall vessel was made of a 0.5-in ( 1.27 cm) lin-ing with 11 wraps, each of quarter-inch (0.635 cm) insidediameter and about 35 ft (11 m) long. The inner shell, ofcourse, was solid. The next two layers of wrapping weredrilled with a weep hole at one point and the last nine wrapswere drilled from the outside at a slightly different location.The vessel was made of two courses of wraps weldedtogether in the center.

Because of its precarious position, the converter, com-plete with shell, basket, and catalyst, had to be lowered tothe ground before any work on it could be done. Thisrequired that a special crane be shipped in for the pur-pose. After lowering the converter to the ground, it waspossible to disassemble the unit, and in time, dump the

Figure 7. Exterior view of control house.

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Figure 8. Closeup of heat exchanger tubes after coldexchanger had been removed.

Figure 10. End view of multiwall vessel, showinglayers of metal.

catalyst from the basket. The condition of the catalystconfirmed our suspicions that air had gotten into it afterthe explosion and ruined it. There was evidence of clin-kering of the catalyst, indicating overheating after theplant was down. All catalyst in the plant was replaced atthis time. The compressors were not damaged except for apuncture in a couple of the bottles and a few broken waterlines. The control house was rebuilt with reinforced concretewalls and roof and very small tempered glass windows. Theinstrumentation was completely replaced, exchanged orrebuilt.

The original cold exchanger and separator had a noz-zle in the side of the multiwall vessel. While rebuilding,we decided to eliminate that nozzle and return the lineinto the vessel through the forging. That was about theonly design change made in the plant. About the only dif-ficulty in restarting the plant resulted from permitting thecooling water lines to sit empty for a prolonged period oftime.

Figure 9. End view of bent layers of cold exchangerafter removal from explosion site.

The scale and dirt from the inside of the lines dried outand flaked off during its inactivity. We washed the linesout by flowing water in both directions a number oftimes, but there was enough scale washed into the com-pressor packing to block them. This required repeatedshutdowns and cleanouts before we eliminated all of thetrash.

As can be seen from the illustrations, the superstruc-ture around the converter was severely damaged at thebottom. An interesting sidelight to the repairs was thatthe top of the structure was removed intact and set asidewhile a new structural steel bottom was installed. The topwas then put back in place on the new bottom.

The nozzle that entered the side of the multiwall vesselwas split cleanly in two by the explosion, almost as if ithad been cut with a saw. Half of it landed on one side ofthe compressor building and the other half ended upbeneath a heat exchanger on the other side of the build-ing. Perhaps that was what caused the puncture of one ofthe heat exchangers.

Whenever something of this sort happens, we shouldexamine the circumstances and determine what needs to bedone to prevent its recurrence. I hope this paper can giveother operators information on what happened so that similaraccidents will not happen in another plant. What did we learnin our plant? A presentation in a Safety in Air and AmmoniaPlant Symposum described forging failures, but at the time ofthe incident, no previous failure of this sort had occurred witha multiwall vesse. The big question in everyone's mind is"What caused the explosion?" From investigations made,statements given, and tests made, I can report the following:

Chevron marks on the shell fragments, according toone metallurgical expert, suggest that the reentry nozzlefrom the chiller was the source of the original failure.There is further indication that a weld overlay was madeon the ID of the nozzle before the deflection shield weldwas made. There was testimony that this weld overlay

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Figure 11. General view of ammonia area, showinglocation of control room in reference to explosionsite.

created stresses which ultimately led to the failure of thenozzle. If this original fault, in fact, was the cause of theexplosion, what caused it to withstand all of its tests andoperate satisfactorily for seven years before failing? Theanswer to that question remains unknown, as does whythe explosion occurred when it did rather than 20 minutesearlier, when a relatively large group of people wascrowded around the compressor observing the results ofits repair.

Knowing what we know now, there can be no othercourse in the future than to shut down operations in the

event of a leak from a weep hole under similar circum-stances. Without completing our investigation as to thecause of the failure, as a precaution, we chose to eliminateall possibility of a failure of the multiwall vessel by elimi-nating the nozzle going in through the side. The change indesign of the reentry nozzle appears to have been a goodmove, although it has caused other operational prob-lems. A good seal between the two sections of the vesselhas not been achieved. As a result, a carryover of ammo-nis, water, and oil has created problems in the convertercatalyst. The reinforced concrete blockhouse-type con-trol house was another result of the explosion.

AcknowledgmentsI want to thank Hawkeye Chemical Co. for the oppor-

tunity to present this paper. The staff of Hawkeye,including L.C. Knockel and Karl Berg, have been mosthelpful in its preparation. I am also indebted to my mosthelpful slide préparer, my wife Jean Patterson. #

PATTERSON, L.B.

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