Safety Improvements in a Methanation Reactor

Safety Improvements in a Methanation Reactor

A 35 year old Methanator vessel required replacement due to High Temperature Hydrogen Attack.The opportunity was taken to upgrade the Over temperature Protection system to meet the

requirements of IEC 61511. An additional Layer of Protection was added to reduce demand on theHigh Temperature Trip system.

Mike Walton, Tony Southerton and Paul SharpKemira GrowHow UK, Ince, Chester, UK

Introduction

emira GrowHow UK operates a 1050Te/d Ammonia Plant as part of itsfertiliser manufacturing complexlocated at Ince near Chester in the

North West of England.

The Ammonia plant was originallycommissioned in 1970. The plant has beenupgraded over the years to improve energyefficiency and increase Production. The latestupgrade was a major Energy Efficiency Projectimplemented in the winter of 2005/6.

During this turnround a separate project toreplace and improve the safety and reliability ofthe Methanation reactor was implemented. TheMethanator (V1205) was replaced and at thesame time the instrumented protection system(trip system) was upgraded following a detailedSIL assessment in accordance with theinternational standard IEC 61508/61511“Functional safety - Safety instrumentedsystems for the process industry sector".

Vessel History

The Ammonia plant was designed andconstructed by Chemico and the original designwas generally typical of plants of the late1960s/early 1970s. In common with manyplants of the period, the selected material formuch of the equipment in hydrogen service wasCarbon - ½ % Molybdenum steel (Carbon ½Moly). This was based on the Nelson curvespublished in API 941 at the time.API have revised the Nelson curves severaltimes since their original publication in 1967.Due to numerous examples of HighTemperature Hydrogen Attack (HTHA) onCarbon ½ Moly steels in service, the curve forCarbon ½ Moly was withdrawn from the maindiagram. This material is no longerrecommended for Hydrogen service and isconsidered as only equivalent to carbon steel inthe later editions of API 941. In appendix A ofAPI 941, which deals exclusively with Carbon½ Moly and Mn ½ Moly materials onlymanganese alloyed ½ Moly materials aredescribed as unaffected below the C-½Molycurve.

Most of the equipment in the front end of theInce Ammonia Plant from the Waste HeatBoiler after the Secondary Reformer through to

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2712007 AMMONIA TECHNICAL MANUAL

the Low Temperature Shifts and Methanatorwere originally constructed in Carbon ½ Moly.Over the years much of this equipment had beenreplaced as part of upgrading projects.

By the early 2000s the most significant Carbon½ Moly equipment on the plant operating abovethe recommend Nelson curve for Carbon steelwere the Waste Heat Boiler shell, TheMethanator (V1205) and the Methanator outletline (RFG-123). The Waste Heat Boiler wasscheduled for replacement as part of the EnergyEfficiency project but the Methanator areawould be left unchanged. The Methanator vesselV1205 was the original vessel V1205manufactured in 1967.

The normal operating conditions for theMethanator vessel were compared with theNelson curve in API 941 Appendix A. The inlettemperature is right on the limit for Carbon steeland the temperature rise across the catalyst tookthe outlet conditions above the carbon steel line,but still well below the old Carbon ½ Molycurve.

The vessel had been subject to regularinspections during its working life. Though, asthe lifetime of the Methanator catalyst is long,internal inspections were infrequent.Some cracking was initially detected during aninternal inspection in 1992. At the time,Metallography indicated the cracks could beoriginal plate defects so it was unclear whetherHTHA was occurring. However, following

increasing indications of internal crackingpicked up by external ultrasonic testing in 1996,and 2000, a further (unplanned) internalinspection had to be undertaken during the 2000turnaround. This required removal of thecatalyst under an inert atmosphere, sieving andreloading and top up of the catalyst. The costs ofthis removal, top up and internal inspectionwere around £47,000. (approx. $90,000 US)

The cracking found during the internalinspection in 2000 extended either side of the1992 cracking for over five times the length,exceeding ½ the circumference of the vessel(>2700 mm). The height of the band hadextended from 105 to 600 mm. Isolated crackswere ground out to determine their depthFollowing ultrasonic examination in February2003 it is concluded that cracking within theabove band was present in excess of 10 % of thewall thickness of the vessel. It was clear that thevessel was suffering from HTHA and that eithermajor vessel repairs or replacement would berequired during the shutdown scheduled for2005. Other Ammonia plants with Carbon ½Moly Methanator vessels have reported similarproblems with Hydrogen attack (Ref 1).

Repair was rejected for the following reasons.

• The internal cracking required to be groundout. In grinding out the cracks and blendingto remove stress concentrators, significantareas of the shell would be rendered belowthe calculated design thickness.

• To reinstate the wall thickness of the vesselto an acceptable level would require cuttingout and replacing a circumferential band ofthe vessel.

• The vessel would need to be removed andtaken to a vessel manufacturer for repair.

• Obtaining suitable C ½ Mo material for arepair would probably not be possible. Thiswould require use of a Chrome/Molymaterial for the repair. This wouldcomplicate the welding procedures.

272 2007AMMONIA TECHNICAL MANUAL

• A complete vessel post weld heat treatmentwould be required. The heat treatmenttemperature required for a Cr/Mo materialexceeds that for C ½ Mo, and would risklosing mechanical strength in the C ½ Momaterial.

• Repair could probably not be achievedwithin the timescale of a plant shutdown.

• An interim plant shutdown and internalinspection would be required after 2 years offurther service. The shutdown, inspectionand start-up costs would be large.

• Further internal monitoring of the repairsperformed would be required from the insideof the vessel at subsequent plant overhauls.Each inspection would require removal,reloading and top up of the catalyst, with therisk of catalyst damage and shortened life.

It is therefore concluded that a replacementvessel was required. It was also agreed that theoutlet line should be replaced at the same time

New Vessel design

The new vessel was designed by JohnsonMatthey Catalysts/ABB Eutech to moderndesign standards.

Several improvements in design were made.• The material of construction was changed to

1.25% Cr 0.5%Mo to eliminate the risk ofHigh Temperature Hydrogen Attack.

• The Design Temperature was increased to500 oC (932 oF) from 427 oC (800 oF) so thatthe vessel was better able to handle any hightemperature excursions in the event of aprocess upset.

• The inlet and outlet connections and theconnecting pipework were increased indiameter to give reduced pressure drop atcurrent plant operating rates.

• The catalyst is now supported directly onsupport balls in the base of the vessel.

Previously a support grating supported by awelded ring inside the vessel was used. Thiswelded ring promoted stresses and crackingin the vessel.

• The inlet nozzle and manway werecombined into a single nozzle on the top ofthe vessel. This provided a 24” manway,providing improved access to the vessel,whilst minimising the number ofconnections to the vessel shell. A removabletop elbow is provided in the inlet line to giveaccess in to the vessel.

Many of these improvements simplified thevessel design and construction, with a furtherbenefit that looking forwards to the future thereduction of internal features and attachments tothe vessel shell mean that requirements forinternal inspection of the vessel may be reduced.

The new Methanator vessel and outlet line


Catalyst Performance

We had experienced low levels of slip of COand CO2 from the original vessel and wanted tomaintain this with the new vessel to maximiseprotection of the Synthesis converter catalyst.

Johnson Matthey Catalysts recommended acharge of their KATALCOJM 11-4 catalyst withthe upper 20% of the bed installed as the pre-reduced KATALCOJM 11-4R catalyst. Thebenefits of the pre-reduced catalyst in the upperportion of the bed are increased activity andshorter catalyst reduction time during the initialplant start-up.

The catalyst reduction went smoothly with arapid drop in exit CO and CO2 levels, which fellbelow 1 ppm within 6 hours.

The vessel is normally operated with an inlettemperature of 270-280 °C, but a low tem-perature trial was carried out at the request ofthe catalyst supplier.

The inlet temperature was reduced graduallyand the exit CO and CO2 levels were monitored.Initially both were below the limit of detection(0.3 ppm). The CO2 level rose slightly once thetemperature reached 260 °C (500 °F) , but evenat 250 °C (482 °F it was well below 1 ppm. NoCO slip was detected on any analysis.

CO2 SLIP FROM METHANATORAT REDUCED INLET TEMPERATURES

00.10.20.30.40.50.60.70.80.9

1

245 250 255 260 265 270 275INLET TEMP (°C)

(PPM)

CO2 ppm

3 per. Mov. Avg. (CO2 ppm)

ALL CO Results below detection limit (0.3 ppm)

Review of Existing HighTemperature Protection

The Methanation reactions are highlyexothermic with the potential for a very largetemperature rise in the event of breakthrough ofeither CO2 or CO from upstream equipment.

CO + 3 H2 CH4 + H2OH = -206 KJ/mol

CO2 + 4 H2O CH4 + 2H2OH = -165 KJ/mol

These equate to a temperature rise ofapproximately 74 °C (165 °F) for 1 % CO in theinlet or 60 °C (140 °F) for 1 % CO2.

With the inlet to the CO2 removal systemcontaining 18 % CO2 and the inlet to the HTShift containing 13 % CO, the potential exists togreatly exceed the design temperature of theMethanator vessel should either the CO2removal or CO conversion systems suffer asignificant upset leading to gross breakthrough.

Temperatures in excess of 600 °C (1112 °F)(c.f. design temp of 427 °C (800 °F)) have beenrecorded during upsets at Ince. At Ince theseupsets have occurred following electrical powerfailure and consequent loss of amine solutioncirculation in the CO2 Removal system.

However other causes of major upsets canoccur. A major upset at the UKF Pernis(Holland) plant followed a sudden failure of awaste heat boiler and consequent quenching ofthe HT shift inlet temperature with consequentgreatly increased CO slippage (Ref 2). A studyinto this incident concluded that peaktemperatures in the bulk of the Methanationcatalyst had reached at least 750 °C (1382 °F)and wall temperatures had reached approx. 525°C (977 °F).

In both the upsets at Ince and at Pernis theextreme high temperatures resulted even when


the High Temperature Trip systems had workedcorrectly.

In the event that the trip systems had notoperated then the temperatures inside the vesselscould have been significantly higher, potentiallyputting the vessel integrity at risk. The reasonfor the high temperatures (well above the hightemperature trip point) being reached even whenthe trip systems worked correctly, was the veryrapid rise in temperature that was occurring andthe time lag in measuring the temperature riseand then closing the trip valves. This isdiscussed extensively in the J. Blanken (Pernis)paper.

TEXE

2 oo 3

FROM CO2REMOVALSYSTEM METHANATOR

Having experienced high temperatures in theMethanator in the past we were very keen toprevent a reoccurrence and we were aware ofthe safety critical nature of the Methanator inlettrip valve. The replacement of the vessel gaveus a clear opportunity to review the currentprotective systems and try to improve on them.At the same time the authorities were asking usto review our instrumented trip systems againstthe new international standard for SafetyInstrumented Protection Systems IEC61508/61511.

Clearly the Methanator high temperature tripsystem was one of the key priorities for us toaddress in this review.

SIL Assessment

The Hazard to be protected against was grossoverheating of the vessel due to an uncontrolledexothermic reaction, with consequent potentialfor catastrophic failure, and release of hotflammable gas and catalyst.

Clearly any such incident was highly likely toresult in serious injuries/fatalities to personnel.

The requirements of IEC 61508/61511 mandatethat the required Safety Integrity Level (SIL) ofthe protective systems to reduce the risk offatality to an acceptable level need to bedetermined. Several different risk assessmentmethodologies are allowed in the standard todetermine the risk reduction required, including“risk graph” and “Layer of protection” methods.

In this case, the simple risk graph methodindicated that a high level of risk reductionwould be required from the over temperatureprotection system. A more sophisticatedanalysis was required to quantify the variousrisks which could potentially cause overheatingof the vessel.

As there are several causes of CO or CO2breakthrough into the vessel, a Fault Treeanalysis was carried out to identify the rootcauses (see Fault Tree 1). For each caseidentified, a qualitative analysis was carried outto determine whether there was a real risk of thevessel design temperature being exceeded. Forthe most significant cases, the likelihood of theevents was estimated. The demand on the HighTemperature trip system was determined.

There had been several incidents of powerfailure to the site during the plants life time.

Loss of amine circulation due to power failureor failure of either control loop on the solventflow to the CO2 absorbers was likely to result ina significant high temperature excursion in theMethanator It was clear that the frequency of


demands on the High Temperature trip systemwas likely to be fairly high. A preliminary SILassessment based on the estimated risksindicated that this system would need a SILlevel of 3 or even 4. (i.e. a risk reduction ofgreater than 99.9 % or even 99.99 %). This wasclearly greater than the existing system, withonly a single trip valve and slow closingelectrically operated isolation valve couldachieve and was beyond the level that we feltcould be achieved and maintained satisfactorilyon our site.

It was clear that the existing High TemperatureTrip system was inadequate to provide thedesired level of protection and thatimprovements were required. It did not seempracticable to upgrade the High TemperatureTrip system on its own to achieve this kind ofrisk reduction.

We decided that we needed to provideadditional independent protection to try to stopan incident earlier, i.e. before contaminated gasentered the Methanator vessel. This wouldreduce the demand on the high temperature tripsystem.

Additional “Layer of Protection”

Reviewing the causes of failure, loss ofcirculation in the CO2 Removal systempresented the greatest risk of large scalebreakthrough.

In particular, failure of the site power supplycould result in total loss of circulation wellbefore the feeds to the plant were tripped,resulting in the potential for gross breakthroughand overheating.

If we could detect a loss of circulation in theCO2 removal system, then we could initiate theMethanator trip system to isolate the vesselbefore CO2 containing gas entered. This wouldprovide an additional “Layer of Protection” and

significantly reduce the demand on the hightemperature trip system.

If successful this would avoid exposing thereactor to elevated temperatures and allow forquicker plant restart after an incident.

In order to protect the Methanator from loss ofcirculation in the CO2 removal system. Thefollowing changes to the trip system wereimplemented.

• Provide new independent Very Low flowtrips on the MDEA supplies to both the 1st

and 2nd stage CO2 Removal Absorbers.Operation of either of these trips initiatesclosure of the Methanator trip system toclose the inlet trip valves and electricallyoperated isolation valve.The trip initiators are fitted with short timedelays to minimise the risk of spurious trips,during pump changeover.The Very low flow trips are initiated by newdP flow transmitters, independent of theexisting flow control transmitters.

• An auto-start of the standby MDEA pumpson Low flow to the absorber has been added,to reduce the risk of a Very low flow trip inthe event of a pump problem.

By adding these Very Low flow trips the risk ofa gross breakthrough of CO2 into theMethanator itself was much reduced, so that theanticipated demand on the High TemperatureTrip system was significantly reduced. (seeFault Tree 2)

In the event of power failure to the site, thecirculation pumps in both stages of the CO2removal system would stop and hence bothVery Low Flow trips would normally operate.In this case both Very Low Flow trips wouldhave to fail to cause a demand on the HighTemperature trip system.


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Temperature Measurements

In the event of breakthrough of high levels ofCO or CO2 in to the Methanator vessel then it isclearly desirable that this is detected and theshutdown system initiated as quickly aspossible, to minimise the temperature that thevessel shell is exposed to.The plan for the bed temperature measurementwas to have 2 vertically mounted multi-thermocouple thermowells providing tem-perature measurement in the upper, middle andlower portions of the bed. This would allow thebed temperature profile to be monitored and anycatalyst de-activation detected. However whilstthese multipoint temperature indicators wouldbe excellent for normal plant monitoring, therewere concerns that they might not respondquickly enough to the very rapid change in gastemperature following a gross breakthrough.The reasons for delay in response would include– time for the temperature profile to move downthe bed due to the large heat capacity of thecatalyst itself, heat transfer limitations to thethermowell and the thermal inertia of the largediameter multi-point thermowells. (Again thesereasons are discussed in the J. Blanken paper).

It was decided to improve the speed of responseto a high temperature excursion by the provisionof two additional standard thermowells,containing a single thermocouple. Thesethermocouples being located close to the top ofthe catalyst bed. (Obviously careful checking ofthe actual catalyst level was required duringfilling to ensure adequate coverage of thethermocouples)

After review, it was decided that the hightemperature trip system should be a 2 out of 6initiators.The inputs from the 2 standard thermocouplesshould be the first react to a high temperatureevent.It was also decided to include the upper 2thermocouples from each of the 2 multi-point

thermocouples as additional inputs to the tripsystem to provide redundancy and to provideprotection if gross de-activation of the top layerof catalyst were to move the initial reactionfront lower into the catalyst bed.

This provided us with a trip system with 6initiators covering the top half of the catalystbed.

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The remaining two points from the lower half ofthe bed were not included in the trip system, toavoid excessive complexity in testing the systemand because of the time lag before they wouldsee the start of a high temperature excursion.They are however included in the pre-alarm.

All 8 thermocouples are displayed on the plantDCS system and the temperature profile isregularly reviewed as part of routine catalystperformance checks. The plant has a history ofvery stable temperature profiles in this reactor,as the MDEA used in the CO2 removal systemis not considered a catalyst poison.

Panel alarms are provided for 1 out of 8 points High temperature Pre-alarm. 1 out of 6 points Very High Temperature Alarm 2 out of 6 points Very High Temperature Trip

To assist in providing early response to a hightemperature excursion, the High Temperaturealarm setting was reduced to 320oC (608oF).This is approximately 25oC above the normaloutlet temperature, which is sufficient to preventspurious alarms but gives the operator time totake corrective action in the event of graduallydeveloping problems such as poor absorption inthe CO2 removal system.

The High Temperature trip setting was reducedto 400oC (752oF). i.e. 100oC below the vesseldesign temperature. This means that the hightemperature trip will be initiated well before thevessel design temperature is reached andprovides time for the operation of the inlet tripvalves.

Logic Solver

The plant had initially been built with a standardrelay based trip system. However in 2000 thetrip system for the reforming section of the plantincluding the Methanator had been replacedwith a Hima-Sella Safety PLC, which gave us areduced risk of failure to danger of the logicsolver system.

Final Elements (Trip Valves)

The SIL assessment of the High temperature tripsystem was reviewed to take credit for the newlow flow trips. The required Safety IntegrityLevel for this system was reduced to SIL 2 onthe basis that the low flow trips would be to aSIL 1 standard.

The SIL Assessment was carried out using“Layer Of Protection Analysis” in the ABB“TRAC” software package

The bulk of the trip system could readily meetthe criteria for a SIL 2 system, but the singlefinal element trip valve was inadequate.Although the electrically operated isolationvalve was also closed by the trip system, nocredit was taken for this in the calculation of thesystem reliability, as its closure time was toolong in the event of gross breakthrough. Inaddition it would fail to operate in the powerfailure condition. It was therefore decided toreplace the existing single trip valve with 2 tightshut-off, fast closing trip valves. (Tomoe Tritectriple offset butterfly valves fitted with Bettisactuators).

The supplier Emerson Process Management(Valve Automation Division) was able to helpus with failure rate data for the valves, actuatorsand solenoids, which were required to calculatethe “average Probability of Failure on Demand”(PFDaverage) of these final elements.

Calculation of the fractional dead time of thesystem demonstrated that we could meet theoverall required PFDaverage with annual testing ofthe trip valves.

This was a major improvement from the originalsituation, but clearly we did not wish toshutdown the ammonia plant every 12 monthsin order to prove the operation of the trip valves.


On-Line Final Element PartialStroke Testing

A facility for on-line partial stroke testing of thefinal elements was designed to allow for 4 yearintervals between full testing, in line with theexpected interval between plant overhauls. Thepartial stroke test frequency was set at 12months.

The partial stroke check will ensure the valve isable to move from its normal running fully openposition by 10 degrees, thus giving confidencethat the actuator is functional and the valve isnot jammed in its open position. The tripsolenoid valve is also tested to ensure that it willoperate electrically from the trip system and thatthe solenoid valve is functional and movesbetween the normal energised (supply air tovalve) and de-energised (solenoid air to vent)position. It is possible to test the trip solenoid byblocking the air into the actuator via a ball valvebefore tripping the solenoid with a key switchinput into the logic solver.

Each valve is partial stroke tested independentlyby lowering the air supply pressure to a presetminimum, using a pneumatic pushbutton switchwhich must be held down by the operator duringtesting. Once the pushbutton is released, thevalve moves back to its normal fully openposition. There are two independent tripsolenoid enclosures mounted locally to thevalves, which are locked and house the partialstroke test pushbutton, along with associatedregulators and valves. A colour coded key isrequired for the trip solenoid enclosure andsolenoid trip key switch to prevent the wrongvalve being worked on and potentially trippingthe valves closed.

2 oo 6

CO2REMOVALSYSTEM

METHANATOR

MDEA

MDEA

2nd

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1st

Stage

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Sketch of Final trip system

Methanator inlet trip valves installed on theplant

Conclusions

Due to the highly exothermic nature of theMethanation reactions and the potentially largequantities of CO and CO2 that can be suppliedto the reactor, we believe that the Methanationreactor is potentially one of the more significanthazards in a conventional Ammonia plant.

In our plant, and probably many others, theprotective system that was originally installeddid not reflect the relatively high risk ofoverheating this vessel with potentiallycatastrophic consequences.


In addition many older plants had Methanatorvessels that were constructed of a material thathas proven unsuitable for long term operation inhigh temperature Hydrogen service and as aresult are suffering HTHA and consequentcracking.

Prior to replacement, it was consideredunsatisfactory to be operating a vessel withknown cracks in a service where it was knownthat there was potential for the vessel to beheated beyond its design temperature.

We feel that the combination of :

• Replacing the vessel with a new vessel ofimproved metallurgy and designed tominimise stresses in the shell.

• Providing additional layers of protection inboth the initiating and final elements of theInstrumented protection system.

has greatly reduced the risk of a serious failureof this vessel and contributed to a saferAmmonia plant.The SIL assessment process requires a thoroughconsideration of the potential causes of ahazardous event. In this case we felt that thenumber of potential causes of overheating andhigh potential consequence justified a full faulttree and qualitative risk assessment to identifythe key risks to the vessel, prior to embarking onthe quantified SIL assessment process.

The SIL assessment process quickly highlightedthat the single final element was the weak linkin the system, particularly as there was nomethod of testing this between plant shutdowns.

Once we had rectified the deficiencies in thefinal element, the fault tree enabled us to homein on the most likely cause of a demand on thehigh temperature trip system and put in anindependent layer of protection to reduce thedemand on this system.

We have now embarked on a process ofcarrying out SIL assessments for all of our highconsequence safety instrumented protectionsystems on the plant.

Acknowledgements

We would like to thank all Kemira GrowHowpersonnel involved in the project to replace theMethanator vessel. In particular Keith Wilson,our metallurgist who provided information onthe Hydrogen attack problem which helped tojustify the vessel replacement and PowysThomas the project engineer for this project.

We would like to thank the management ofKemira GrowHow for authorising theexpenditure on this Safety and reliability projectat a time when they were already investingheavily in energy efficiency improvement of theplant.

We would like to thank John Brightling and hiscolleagues at Johnson Matthey Catalysts andABB for their help in design of the new vessel,and Emerson Process Management for theirassistance with the provision of the SIL ratedtrip valves.

References

1. “Retrofit Experience with a 32 Year OldAmmonia Plant”, V.Gupta and B. Borserio,AIChE Symposium on Safety in Ammoniaplants, 2002.

2. “Temperature Runaway of a Methanator”J.M. Blanken, AIChE Symposium on Safetyin Ammonia plants, 1980.


Safety Improvements in a Methanation Reactor

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