1995: Catastrophic Failure of Ammoni/Air Mixture

Catastrophic Failure ofMixture

Kemira B.V. has been operating a 1,200-metric-tpd (100%) high-pressure nitric acid plant normallysince 1968. Operation had been uneventful, until August 1993 when the ammonia/air mixer sudden-ly burst open, releasing large amounts of air and ammonia to atmosphere. Fortunately, two workersin the plant miraculously escaped the gas jet and the flying debris. The incident is described, along

with its causes and measures taken to prevent recurrence.

W.D. VerduijnKemira B.V., 3180 AA Rozenburg, The Netherlands

Rozenburg Site

Kemira is a multinational company with itsheadquarters in Helsinki, Finland. Active invarious branches of the chemical industry, the

company's Agro Division is by far its largest andresponsible for about 40% of the turnover.

Agro Division, an affiliate of Kemira B.V., operatesa 1,600-metric-tpd ammonia plant, a 1,200-tpd (100%)nitric acid plant, a 700-tpd urea plant, a 1,600-tpdCAN plant, an 800-tpd UAN plant, a 60-tpd liquid fer-tilizer specialties plant, and a 300-tpd liquid CO2

plant. Other Kemira Divisions on the same Rozenburgsite operate a 60-tpd hydrogen peroxide plant and a400-tpd water-treating chemicals (40 wt. % FeCl3solution) plant (Figure 1).

Nitric Acid Plant

The nitric acid plant was designed by C.F. Braun andCo. (now Brown and Root) of Alhambra, CA, basedon a Du Pont concept. Its original capacity was 700metric tpd (100%); product strength is 58 wt. %. The

plant was started up in 1968. The original operatorwas Exxon Chemical Company. Kemira took over in1985.

It is obvious that an old plant, like the one justdescribed, cannot survive in a highly competitiveatmosphere. Therefore, over the years, various revampprojects were executed. The plant's NO and NO2

emission rate, for instance, was significantly reducedin 1978 by installation of a nonselective catalytic NO^abator. The plant's energy efficiency was improved byinstalling a boiler feedwater heater upstream of thecooler condenser (1980) and by installing an addition-al stack heater (1986).

Its production capacity, already increased by smallmeasures in the 1970s from 700 to 850 metric tpd, wasfurther increased in 1982 to 1,200 tpd by installing alarge booster upstream of the process air compressor.The operating conditions as a result of that in themixer/converter area are shown in Figure 2. Filtered,compressed and refiltered process air is heated inprocess air heater E-204. It subsequently passes finalair filter fil-211A. An ammonia sparger introducesreactant ammonia. The ammonia/air mixture subse-

AMMONIA TECHNICAL MANUAL 67 1996

Figure 1. Aerial view of Keimra's Rozenburg site.

ammonia/air mixer pipe

MC-201C MC-201Bgaseous NH^(126C-260F)

TM374(220C-428F>

guidevanes

I P-11kg/cm2g J 4static mixer. -156psiq !L

installed 1983

ammonia/airmixer MC-201A

TM-382 (225C-437F)

reactorcone

/(10.6vol/NH3)

I Pt<10/Rh>gauze;940C-1724F

waterjacket

process airfrom filter FIL-203

(T-160C-320F)(P-l1.4kg/cm2g-162psig>

Figure 2. Flowsheet of the mixer/converter area.Normal operating conditions indicated.


Figure 3. Flange leak observed prior to incident.

NH3

hole of 60x60cm(2x2ft) in mixerpipe MC-201B

Figure 4. Hole in pipe wall observed after incident.


Figure 5. Failed mixer pipe after removal ofremaining insulation.

Note that pipe opened up on weld seams and two rectangularhatches cut out for material tests.

quently passes a long mixer pipe and flows via thereactor cone to the Pt/Rh gauze, where the reaction toNO takes place.

One of the measures following this 1982 revamp,with the aim to restore the original gauze efficiency,was installation of a static mixer in the large horizon-tal ammonia/air mixer pipe in 1983 (Figure 2). Theimproved mixing on microscale indeed enhanced thegauze efficiency. It was, however, this static mixer thatwould prove to be the root cause of a serious incidentten years later.

The Incident

On Saturday morning, August 14, 1993, the nitricacid plant was started up after an outage of two days.At 10:00 a.m. when the gauze was lit, the productionrate was rapidly increased.

Around 11:30 a.m. the operator on duty observedthat the flange between air heater E-204 and air filterFil-211 A was leaking: it released a powerful stream ofhot air in the direction of the converter (see Figure 3).The operator, convinced that he was dealing with adamaged packing, alerted the shift supervisor. Theshift supervisor who also observed the air leak con-

cluded that it was growing rapidly and returned to thecontrol room to shut down the plant.

Almost having left the nitric acid plant, he suddenlyheard what he later described as "a heavy blow, fol-lowed by an enormous roaring noise." An employeedoing overtime in the plant's administration building,about a kilometer away from the nitric acid plant,jumped up from his chair and described the sound as"the noise of many 100-barg (1,500-psig) steam safetyvalves popping at the same time." Small amounts ofunidentified debris flew through the plant, but did nothit the operator who at the time of the incident hadmoved to the compressor platform nor the shift super-visor who had almost left the nitric acid plant. Theplant was tripped immediately.

Damage Observed

When returning to the plant after shutdown, it wasfirst observed that the debris found in the area consist-ed of metal insulation sheeting and insulation material.The leaking flange appeared to be intact, however, inthe mixer pipe high up in the air, a hole of about 60 x60 cm (2 x 2 ft) was observed (Figure 4). Amounts ofinsulation and insulation sheeting around the hole hadalso disappeared. The hole itself was of the fishmouthtype (Figure 5): the pipe apparently opened up on theweld seams.

Safety Hazards

In an incident like this, generally three safety haz-ards exist for operating personnel: being hit by thepowerful hot gasjet, suffocation by the hot vaporammonia released to atmosphere, and being hit by fly-ing debris. These hazards will be discussed brieflybelow.

Hit by gasjet

The location of the hole, already high up in the air,made it clear that the ammonia/air jet must have point-ed definitely away from the position of the operatorand supervisor at the time of the incident.

Suffocation by NH3

The jet also carried the ammonia away from, rather


than toward the operator and supervisor. Moreover,the eastern wind carried it further away from them intoa safe direction where no other personnel was present.

Flying debris

The flying debris turned out to be just insulationsheeting and insulation material: the failed mixer pipeitself had not lost material of any substance. Althoughinsulation sheeting is sharp and will follow an unex-pected path when being blown away with high force, itdid not hit anybody. The insulation material blownaway was fortunately asbestos-free.

All in all, Kemira feels that from a personnel safetystandpoint we have been miraculously fortunate.

Theories

Explosion

We first thought of an explosion. The theory was thatfor some reason so much ammonia or so little processair (leaking flange!) had ended up in the mixer pipethat the explosion limits were reached.

However, this theory was rejected as process comput-er history data showed that there was no sharp jump inmixer temperature at the moment of the incident; fur-thermore, the pressure in the system had remainedessentially constant.

Preignition in mixer

It was subsequently found that the temperature of theammonia/air mixture, as measured by TM-374, had in7 min increased from the normal value of 220°C(428°F) to over 1,000°C (1,832°F):

ll:25a.m.11:31 a.m.ll:26a.m.ll:32a.m.ll:27a.m.ll:33a.m.ll:28a.m.ll:34a.m.ll:29a.m.ll:35a.m.ll:30a.m.

221°C (430°F)998°C (1,828°F)484°C (904°F)

1,009°C (1,848°F)751°C (1,384°F)996°C (1,824°F)853°C (1,568°F)598°C (915°C (269°C (516°F)965°C (1,769°F)

The process computer history sheets showed that theprocess computer properly provided the high-tempera-ture alarm (243°C, 470°F) at 11:25:56 a.m. The paneloperator, however, had elected to turn that alarm offand to not react on it: being an experienced nitric acidman, he knew that high mixer temperature alarms oftenshow up and disappear without problems during a start-up situation. Gauze temperature and process pressureremained essentially constant during the period 11:25to 11:32 a.m. The mixer pipe failed at 11:32 a.m. It wasconcluded that a classical case of preignition was thecause of the failure.

Further Investigation

Leaking air flange

We had first thought that the leaking air flange hadinitiated the failure (see under "explosion"); it nowbecame clear that it was rather the other way around.The gradually increasing mixer temperature mustnamely have led to quite some thermal expansion ofthe mixer tube, leading in turn to a substantial force onthe vertical Fil-211A/MC-201 assembly. The situationis shown in Figure 6.

Assuming a temperature rise of 220 to 1,000°C (428to 1,832°F) over the entire 10 m (30 ft) long mixerpipe, one can calculate a thermal expansion of 150 mm(6 in.). Assuming that 1/3 ofthat thermal expansion (50mm) is taken by the strong reactor cone, point A takesthe rest and moves 100 mm (4 in.) toward its new posi-tion Al. Our mechanical engineers have calculated thatthe force necessary to do that is equal to 12 tons. Themoment on the fil-211A/E-204 flange must thereforehave been 7 x 12 = 84 tonmeter: no wonder that arapidly growing flange leak was observed in the direc-tion of the reactor cone.

Iron oxide

In the failed mixer pipe, in and close to the staticmixer, hundreds of extremely thin flakes were found,with a total weight of about 20 g. Laboratory analysisshowed that they consisted of 66.0 wt. % iron, corre-sponding to 95.0 wt. % Fe2O3. Chromium, nickel, andnitrate/nitrite were essentially absent. We have there-fore concluded that these flakes did not originate fromthe hot 304L mixer material, but from "foreign foul-ing." It was also concluded that the mechanism of our


preignition incident must have been catalysis by rust.The source of the rust was probably carbon steel cor-rosion products, slipping through the ammonia gas fil-ters of the nitric acid plant (that showed signs ofbypassing) and accumulating on and near the staticmixer. Although the mixer pipe was always carefullycleaned during stops, the static mixer itself had notbeen cleaned for years, since "it was so time-consum-ing to remove it and, after all, it could not be verydirty as long as the mixer pipe was cleaned regularly."This time, of course, we could not refrain from pullingthe static mixer out: it turned out to be unacceptablydirty.

Backflow platinum

Finally, a few grams of platinum dust were found onthe guide vanes of the reactor cone. It was obviouslycarried along with the hot NO.,, containing backflow

from the gauze to the hole in the mixer during the inci-dent.

Conclusion and Actions

Conclusion

It was concluded that the incident was caused bypreignition of the air/ammonia mixture, leading tosuch a temperature rise that the mixer pipe finally col-lapsed. Cause of this preignition was catalysis by rust.The rust probably originated from carbon steel corro-sion products slipping through the ammonia gas filterand accumulating on the static mixer that was seldomcleaned.

Action

Actions to prevent reccurrence included replacementof mixer pipe MC-201B; the pipe was damagedbeyond repair and only the existing flanges could bereused. The (elongated) bolts of the fil-211A/E-204flange were replaced. All other equipment involvedwas inspected in cooperation with the authorities andfound it fit for reuse: even thermowell TM-374 turnedout to be still intact and was reinstalled. The ammoniagas filters were repaired. The high-temperature alarm

on the mixer was equipped with a high-temperaturetrip. Static mixer and guidevanes were removed andchemically cleaned. All other equipment involved wascarefully cleaned by hand. The decision was taken topay much better attention to cleaning in the future; thestatic mixer was to be removed for chemical cleaningat least once a year.

Startup

About a week after the incident the nitric acid plantwas started up without problems. No single sign ofpreignition was observed. Proud of our problem analy-sis effort we returned to the duties of the day. Weknew the cause; this would not happen to us again!

Incident 2

A year later, on the first day of July, 1994, around 2a.m., the nitric acid operator saw a red glow on mixerpart MC-201C. It turned out to be the weld attachingnozzle N16 to MC-201C (see Figure 7). The weld wasglowing over its full circumference. Estimated weldtemperature was 600°C (1,112°F). The other threenozzles of MC-201C did not show the phenomenon.Also, thermocouple TM-374 showed a normal andconstant value of 224°C (436°F). Finally, there wereno signs that the hot spot was growing.

The operators, still remembering the trauma of inci-dent 1, refused to approach the hot spot for furtherinspection and took the wise decision to shut the plantdown. Because of the small size of the hot spot, it wasdecided not to dismantle the mixer. Instead, nozzleN16 was opened from the outside. Some minusculeblack particles were found there and recovered.Because of this minimal quantity, the laboratory couldonly roughly determine the metal content ratio:Cr:Ni:Pt:Fe= 1:1:2.5:10.

We concluded that residual platinum and rust, appar-ently not sufficiently removed from the narrow hori-zontal nozzle pipe the year before, were the cause ofthis limited preignition phenomenon. It could not beexplained, though, why it took the phenomenon a yearto strike again. The plant was restarted the same day.


1Qm(30ft)

process airheater E-204

Figure 6. Cause of flange leak prior to mixer failure.Mixer pipe: Material SS-304L; wall thickness 10 mm (0.4 in.; length 10 m (30 ft); ID 915 mm (36 in.). Thermal expansion mixer

pipe: point À moves to new position A'.

MC-201C

TM-374

V this weld wasglowing

Fe/Pt particles

Figure 7. Hot spot of incident 2.


MC-201C

Figure 8. Hot spot of incident 3.hot spot on south side; 'hot spot on north side and bottom side.

SECTION 4 SECTION 3 SECTION 2 SECTION 1"V

{LPAù

fe:

HIPA)

<

-(LPA)

f::::ï£H

^$

PRESSURIZED TEFLON TUBE

Figure 9. Teflon hot spot detection system.LPA = low pressure alarm (in control room); air pressure in tubes kept at 5 barg through small air supply.


MC-201C

Figure 10. Hot spot of incident 4.• Location hot spot on north side.

Incident 3

This time, unfortunately, it was not an uneventfulstartup. Right after it, still on July 1, hot spots devel-oped rapidly on the upper end of the reactor cone,close to the place where grams of platinum laydownwas observed after incident 1 (Figure 8).

The plant was immediately shut down again. Mixerpiece MC-201C was removed, and the affected reactorcone area was carefully cleaned. Insufficient materialfor sample taking was available, but it was postulatedthat again minuscule particles of Pt and rust hadplayed a role. New filters were installed where neces-sary;

We now fully realized that we had a structural preig-nition problem at hand, probably due to the presenceof very minor residual platinum and rust pockets,which might or might not all have been removed now.To test our luck, we decided to start up again; if thingswould go wrong again, we would turn to a major plantcleaning operation.

Hot spot Detection System

In line with the above, it was also obvious that wehad a structural safety problem at hand: large areas ofmixer and cone were externally insulated and couldtherefore not be visually inspected for hot spots. Sincea repeat of incident 1 was the last thing that we wishedto happen, all insulation of mixer MC-201ABC andreactor cone R-201A was removed; the bare metal wasprotected against the weather by a rain roof wherenecessary.

On top of that, a hot spot detection system wasdesigned and installed. It consisted of four sections ofhollow teflon tube, tightly wound around mixer tubeand reactor cone top (Figure 9). The teflon tube waspressurized with air to 5 barg and kept at that pressureby a small air supply. In case a hot spot would devel-op, the teflon tube would warm up and at 250°C(482°F) melt. The corresponding loss of air pressurewould subsequently activate an alarm in the controlroom. Armed with this safety device, the plant wasstarted up again on July 3, 1994.


Incident 4

Only a month later, on August 11,1994, the control-room reported a teflon-tube low-pressure alarm in sec-tion 4, the upper end of the cone. An operator was sentto the spot and indeed observed there a molten teflontube. He measured at the breakpoint an outer wall tem-perature of 240°C (464°F), whereas all other placesaround it showed 180°C (356°F): it was obvious thatthe simple warning system had very accurately identi-fied a hot spot in development! Figure 10 shows itslocation. Within 10 hours, the hot spot temperaturerose to 520°C (950°F). It was decided to shut it down.During the entire period, TM-374 showed a normal,constant temperature of 218°C (424°F), not surprising-ly, taking into consideration the location of the hotspot. It was therefore again concluded that thermocou-ples in the mixer/cone area are not always able to pickup a preignition signal. It was also concluded that theteflon hot spot detection system provided an adequatewarning at an early stage.

A small brown grey spot was found inside of thereactor cone at the hot spot location. It was positionedin a difficult-to access point where the vanes werebolted to the cone and was too small to take a repre-sentative sample. Yet, we believe it was a minuscule Ptand rust residue.

Therefore, it was decided to end our preignition eraonce and for all. The entire air and ammonia systemswere dismantled and precision-cleaned. All filter ele-ments were renewed. An extra manhole was made inthe silencer compartment of the air suction filter toimprove access for detailed cleaning. The ammoniasystem was waterjetted, and its stainless steel partschemically cleaned. The mixer system, including thestatic part, was blasted with glass beads, chemicallycleaned and passivated. Two obsolete nozzles in MC-201C were blind off at the inside to remove smallareas that are difficult to clean. All cleaned and passi-vated parts were wrapped in plastic until reinstallationto ensure that no intermediate contamination couldtake place. Subsequently, the plant was started upagain on August 21, 1994. We were sure all rust andplatinum were removed; things could no more gowrong!

Incident 5

It was therefore quite a surprise that another hot spot

developed just one month later. On September 23,1994, the temperature indicated by TM-374 started tocreep up very slowly. Its normal value was 223°C(434°F). Two days later it reached 270°C (518°F) andthe next day 300°C (572°F). However, the thermowellnozzle and all other mixer wall parts, tested with thecontact pyrometer, remained at 180°C (356°F).Consequently, the teflon warning system was not acti-vated either. These data led us to believe that it wasjust the thermowell itself that was promoting preigni-tion. To our shock we realized that it was still the oldoriginal thermowell from incident 1 that was replacedin the precision-cleaned mixer. This thermowell hadseen temperatures over 1,000°C (1,832°F) and its sur-face was visually burned!

The plant was shut down, the well removed andinvestigated by a University laboratory: large surfacesof burned stainless steel (iron oxides) and small sur-face areas consisting of almost pure platinum weredetected. It was clear that this explained the problem.The thermowell was replaced by a new one; sincethen, the plant is operating without further problems.

Outside Help

In an effort to find the cause of the five preignitionincidents in 1993 and 1994, various samples weretaken at that time from all over the plant and analyzedby our laboratory. Also, help was sought from otherlaboratories, like TNO in Delft (scanning electronmicroscope), Haldor Tops0e in Copenhagen (X-raypowder diffractometer), and the University of Twente(various tests). Based on the combined results we havedrawn the following conclusions:

(1) Preignition incident 1 (the tube rupture) wascaused by accumulation of rust in the seldom-cleanedstatic mixer area. The rust was carbon steel corrosionproduct (ccFe2O3) and not ammonia synthesis catalystor dust from a nearby ore processing plant (magnetite)that was started up in 1993.

(2) Preignition incident 2 (the hot nozzle weld) wascaused by backflow platinum from incident 1, precipi-tated in an area difficult to clean.

(3) Preignition incident 3 (the hot vane section) wasprobably caused by backflow platinum from incident1, precipitated on the vanes, which were thereafterinsufficiently cleaned.

(4) Preignition incident 4 (the hot vane section) wasprobably caused by a mixture of rust and Pt in a diffi-cultly accessible vane bolting area.


(5) Preignition incident 5 (the thermowell) was defi-nitely caused by precipitation of backflow Pt on aburnt and therefore rusty thermowell surface. Notethat the causes identified for incidents 2, 3 and 4 stillcontain an element of speculation. Note also that themoments at which the incidents took place have notbeen explained yet; for instance, why has the ther-mowell of TM-374, obviously in the same conditionfrom August 1993 through September 1994, waited toignite until September 1994?

Conclusions

Preignition in nitric acid plants is a treacherous

phenomenon that can strike unexpectedly. That is alsotrue in plants that "already for 25 years exactly knowwhat they are doing." Preignition is a serious threat tooperators' lives and health. High-temperature alarmsin ammonia/air mixers should be equipped with high-temperature trips. Externally insulated mixers andreactor cones can develop preignition hot spots thatare not noticed in time. Backflow platinum after anincident makes the exposed equipment surfaces extravulnerable for preignition.

There are three methods to stay out of preignition:cleaning, cleaning, and cleaning again.

DISCUSSIONBrent Heimann, Arcadian: How often do you nowclean the ammonia air mixer? Do you clean it at everygauze change?

Verduijn: Yes, we clean the mixer after every gauzechange. We clean and we repassify with nitric acid.


1995: Catastrophic Failure of Ammoni/Air Mixture

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