1996: Successful Use of Stainless Steels in Nitrogen-Based

Successful Use of Stainless Steels inNitrogen-Based Fertilizer Plants

In ammonia plants, high-strength, chloride-resistant duplex grades are a good solution to watersidecorrosion at a low cost. For the urea process, the fully austenitic grade 2RE69 offers an expected

service life of more than 15 years in the CO2 stripping process.

Knut TersmedenAB Sandvik Steel, S-811 81 Sandviken, Sweden

Introduction

The high investment cost for an ammonia-ureacomplex automatically implies that a long andsafe service is needed to gain a satisfactory

return on the investment. A common reason for a pre-mature replacement is corrosion. As corrosion can be acomplex phenomena where a small change in theprocess parameters or some minute impurities canchange the performance of the alloy completely, theselection of material must take this into account. Bystudying past performance of old plants, it is possibleto spot areas where corrosion is likely to appear. Heatexchangers suffering from chloride induced corrosionis an example of one of these areas hi ammonia plants.Another one is the high pressure urea strippingprocess. Both areas require a different solution, butthey have in common that a longer service life can beachieved at no or very little extra cost. Costs can oftenbe insignificant in comparison to one or two days ofstandstill caused by an unexpected material failure.

Ammonia Plants

Over the years a substantial number of heatexchangers operating below 300°C have been retubedwith new and more corrosion resistant materials. Evenif it can be done with excellent results, there will be anextra cost for the plant operator, avoidable from thebeginning if proper materials selection would havebeen carried out.

The most common corrosion problem is caused bypolluted cooling water containing small amounts ofchlorides. Carbon steel tubing will suffer from generalcorrosion while austenitic stainless steel grades likeAISI304L, 316L, or 321 may fail due to pitting corro-sion or stress corrosion cracking depending on temper-ature. The duplex stainless steels with a ferriticaustenitic structure offers a much improved corrosionresistance to chlorides.

AMMONIA TECHNICAL MANUAL 239 1997

Duplex stainless steels

Duplex stainless steels are the two-phase microstruc-ture containing roughly equal amounts of austeniteand ferrite. The structure offers two very importantadvantages compared with austenitic grades: higherstrength and better resistance to chlorides (Figure 1).

The ferritic-austenitic structure means that the mate-rial temperature has to be limited to the approximaterange -100°C to +300°C (-150°F to +600°F). At moreelevated temperatures, embrittlement of the materialwill occur and below -100°C the impact strength willbe low.

Sandvik 3RE60 was introduced to solve stress cor-rosion cracking in heat exchangers for preheating ofliquor in pulp and paper plants. Based on experiencegained from the successful application of 3RE60, anewer range of duplex grades followed, with generallya higher chromium content and different applicationprofiles. Table 1 shows the chemical composition ofsome common duplex stainless steels compared withaustenitic stainless steels.

Mechanical and physical properties

Mechanical strength for duplex stainless steels ismuch higher than for austenitic grades. If the proofstrength is used for calculating the minimum thicknessfor certain internal pressure then the thickness can bereduced by about 50% for duplex stainless steels com-pared with stainless steels like 304L or 316L.

Pitting corrosion

The pitting resistance equivalent (PRE) is a simpli-fied way to compare the resistance to pitting corrosionfor different grades. A higher PRE value generallymeans a better resistance to pitting. The PRE valuecannot, however, be used as a direct selection criteria.For selection of material in chloride containing solu-tions, a diagram showing the critical pitting tempera-ture (CPT) is a great help. The CPT curves are basedon a rapid method to test at what critical temperatureslocalized pitting will take place. A test solution of dis-tilled water and NaCl is used to give a certain chlorideconcentration. The temperaure is then increased with

5°C increments until localized pitting occurs. Theresults expressed in the temperature-chloride diagramsgive a good correlation to practical experience. Even ifthe temperature and the chloride concentrations are themost important factors to explain pitting corrosion,other factors like low pH, aggressive impurities, orpoor welds can contribute to lower the actual pittingtemperature. Therefore, a safety margin is recom-mended especially when the service conditions cannotbe controlled in a satisfactory way.

Stress corrosion cracking

Based on practical experience in combination withlaboratory tests, a stress corrosion cracking (SCC) dia-gram has been compiled to make selection easier. Ascan be seen in the diagram in Figure 5, even a verysmall amount like a few ppm of chlorides may resultin cracking of austenitic grades like 304L and 316L.

CO2 section

Another corrosion problem is found in the sectionfor CO2 removal. Where sour gas is removed byabsorption in media-like aqueous aminé (DEA/MEA),sulfinol or potassium carbonate solutions such asBenfield and Catacarb, problems can appear in cool-ers, condensers, and reboilers. Today 304L and 316Lare used, as carbon steel has an unsatisfactory servicelife. It happens that even 304L and 316L can fail. It isthen important to establish the type of failure. Whencorrosion can only be seen from the gas side, thenchloride induced corrosion from the water side cannormally be excluded. A general recommendation isthat if the service life has been over 3 years then SAP2304 can replace 304L and SAP 2205 can replace316L. If the service life is much shorter, then localizedareas with very low pH are likely to be the cause. Thegrade Saniere 28 is then recommended in view of itsexcellent resistance to acidic solutions.

Retubing of carbon steel heat exchangers

If carbon steel tubing is failing after some time inoperation, a common practice is to use SAF 2304 forretubing. Grades like 304L and 316L can, of course,


Table 1. Chemical Composition and PRE for Various Duplex and Austenitic Stainless Steels (PRE: PittingResistance Equivalent = %Cr + 3,3%Mo + 16%N)

Grade UNS-No

3RE60 S3 1500SAF2304 S32304SAF2205 S31803SAF2507 S32750AISI304L S30403AISI316L S31603Saniere 28 N08028

Cr

18.523222518.517.527

Ni

4.94.55.57101331

Mo

2.7-3.24-2.13.5

N PRE*

0.07 280.1 240.18 350.3 43

- - 182438

Microstructure

DuplexDuplexDuplexDuplexAusteniticAusteniticAustenitic

*Nominal values, not guaranteed.

Figure 1. Duplex microstructare.

Rp0.2MPa (ksi)500-1(72.5)

400-(58)

300-(43.5)

200-(29)100-(14.5)

0-AISI316L SAP 2304 SAF2205 SAF2507

Figure 2. Comparison of minimum yield strength,0.2% offset, of the duplex steels and the austenitic

316L.


Figure 3. Pitting corrosion attack on a tube(above) and an unattacked tube (below).

OPT, c100(210)

80(175)

60(140)

40(105)

20(68)

0(32)

C (°F), 300 mV SCE

I

\

\\\\

>

No f

\\SAI

AISI 31i

\\\

XAISI 304L

litting

= 2304

V^'L\"--^v--.

\SAF 220

*%«̂

Pitting

X5

^

6941 b

"""

%•

0.01 0.02 0.05 0.10 0.20 0.50 1.0 2.0Cl", weight-%

Figure 4. Critical pitting temperature for variousstainless steels in neutral chloride solutions, deter-

mined potentiostaticaily at +300 mV SCE.

Tempe300(570)

250(480)

200(390)

150(300)

100(210)

50(120)

0(32)

rature,°C (°F)/-••

\\\\

904L\1

V*v

X

No SCC

II\

\V•*s

•̂^-— . -«.

\^

SCCf

\^«-.u

"^x.SÂF23Ô4

AISI 304/C

AISI 316/a

•̂ | 6877b

SAF 2507No cracking

^)8£28/Sani

*'"*>«^S^

Î04L

16L

)

era 28

'2205**«-,

0.0001 0.001 0.01 0.1 1 10CI", weight-%

Figure 5. SCC resistance of various stainless steelsin oxygen-bearing neutral choloride solutions.

Open circles means no SCC for SAF 2507.

Figure 6. Microstructure of a material with stresscorrosion cracks.


be used if the risk for stress corrosion cracking can beeliminated. SAP 2304 and the other duplex stainlesssteels have, in addition to higher strength, a muchlower thermal expansion than austenitic stainlesssteels. In fact the thermal elongation is very close tothat of carbon steels, which makes it safe to combinethe two types of materials. Figure 7 shows the thermalelongation for carbon steel, duplex stainless steel, andaustenitic stainless steel.

If the carbon steel tube sheet is in good condition,then it can be retubed with SAP 2304. The wall thick-ness can be reduced as carbon steel tubing has a corro-sion allowance and also a lower strength than SAP2304. At the maximum, internal pressure according toASME Sec VHI Div I at 300°C (600°F) is 200 bar (2.9ksi) for 19.05 x 1.65 mm (3/4 in. x 16 BWG) and 150bar (2.1 ksi) for 25.4 x 1.65 mm (1 in. x 16 BWG).Few pressure related restrictions are likely to occur.Other standards like the German DIN 2413 will evenallow a higher internal pressure.

Expanding and welding of SAP 2304 tubing to car-bon steel or stainless steel tube sheet can be donewithout problems.

Seawater cooling

Some ammonia plants are using seawater cooled incertain heat exchangers. Saniere 28 has successfullybeen used to replace failed tubing in CuNi, Al-Brassand other alloys. Even if copper containing alloyshave excellent resistance to seawater, a fairly lowvelocity of 2-3 m/s (7-10 ft/s) can cause erosion dam-age. Also, leakages of ammonia into the cooling watercan cause corrosion. Sanicro 28 has been successfullyused for seawater cooling at velocity over 1.5 m/s (5ft/s) with a limited metal temperature. The first instal-lations were done in 1981, and after more than 10years in operation the result has been very good.Today, SAP 2507 offers an even better alternative as itcan be used with metal temperatures of up to 80-100°C (175-210°F). A recent investigation proves thatretubing with SAP 2507 is possible in tube sheetsmade of carbon steel, Cu-Ni, Al-Brass, Naval Brassand, of course, high strength stainless steel. The use ofexisting tube sheets will definitely cut the cost. Whenchanging from Cu-alloyed material to stainless steel,

the design parameters have to be reviewed withrespect to chloride content, chlorination, flow rate andmetal temperature to optimize the performance. Asseawater is a good electrolyte, contact between differ-ent types of material can result in galvanic corrosion.In such cases, sacrificial anodes can be an acceptablemethod to control the activity.

Installations in ammonia plants

In the list of references - special stainless steels -auxiliary systems in ammonia/urea plants - the follow-ing applications are described.

3RE60 Waste heat boilersShift conversion heatersFeed water heatersHeat exchangers

SAP 2304 Converter effluent coolerSyngas compressor cooler

S AF 2205 Raw gas compressor coolersCompressor coolers

Sanicro 28 Ammonia condensers - SeawatercoolingProcess coolers - Seawater coolingBenfield solution cooler - SeawatercoolingMethanated gas cooler - SeawatercoolingProcess condensate cooler -Seawater coolingHigh-pressure absorber cooler -Seawater coolingCO2 condenser - Seawater cooling

In addition to these applications SAP 2304 has alsobeen selected for interstage coolers and water coolers.

Study of material costs

A complete calculation of the life cycle cost is com-plicated as many cost factors have to be compiled.Fortunately when comparing heat exchangers with dif-


SAF 2304

Carbon steel

AtS!316L

6832b

10 15 (X10-6)

Figure 7. Thermal elongation, per °C (20-100°C). Figure 8. Bimetallic tubing with outer tube in2KE69 with lining of zirconium.

Figure 9. Fabrication of urea stripper.Stamicarbon at BSL, France.


ferent materials it becomes much easier. A recent costcomparison between five different materials solutionsfor a new heat exchanger proved a very small differen-tial cost.

The original heat exchanger in carbon steel for pro-cessing of hydrocarbons in a refinery had to bereplaced due to chloride containing cooling water.Since the corrosion took place on the tube side corro-sion, resistant material was only required for the tubeand the tube sheets. In Table 2 the different alterna-tives are presented

mainly because 316Ti had failed in similar serviceconditions and hence a much better grade was needed.

The cost comparison shows a small difference incost when using high strength material with good cor-rosion properties. The savings by using a corrosionresistant material is such that it will always be morecostly to start with a heat exchanger with unsatisfacto-ry corrosion resistance and later replace it with a newmore corrosion-resistant alternative.

Table 2. Cost Comparisons: Floating Head Type (265 m2), Carbon Steel Shell, Seamless Tubes

Tube Material Dimension

C-SteelAISI316T1SAP 2205904LSAP 2507

25 x 2.5 mm25 x 2.0 mm25 x 2.0 mm25x2.0 mm25 x 1.65mm

Tube Sheet

C-SteelAISI316T1SAP 2205904LSAP 2205

70mm70mm40mm70mm40mm

Comparative Cost

0.51.01.131.261.2

* 688 pcs - 25 x wt. x 5,000 mm, tubes are welded and expanded into the tube sheets.

As SAP 2507 (UNS S32750) has both a higherstrength and also better corrosion resistance comparedwith the other alternatives, the thickness of tubing wasreduced from 2.5 mm to 1.65 mm. As the skin temper-ature of the tube sheet is lower than for the tubing, itwas decided to use SAP 2205 (UNS S31803) type ofmaterial. This material is also a high strength materialand therefore the thickness was reduced from 70 mmto 40 mm. The reduced thickness contributes to thevery low total cost for the most corrosion resistantalternative. In fact SAP 2507 turned out to be only22% more expensive than 316TL Compared to the car-bon steel alternative, the difference is greater.Although the SAP 2507 alternative was more expen-sive than the carbon steel, SAP 2507 paid-off in lessthan 2.5 years, as the carbon steel alternative lastedless than one year. If other costs are included, such asreplacement and loss of production the pay-off timewill be even shorter.

SAP 2507 was selected for the new heat exchanger

Stainless Steel for Urea High PressureService

Material and composition

The urea process adopting stripping technologymainly uses two types of special austentitic stainlesssteel to combat the corrosion problems caused byammonium carbamate. As ammonium carbamate canonly exist hi a certain temperature and pressure range,the testing and evaluation of material is complex. Thetwo types of steel can, under certain process condi-tions, give a long service life. It is, however, extremelyimportant to use the right material in the right place.As neither of these grades are standardized by interna-tional organizations, it means the material has to beordered according to the licenser's specifications toensure the right properties.


Table 3. Chemical Composition (in wt.%) Sandvik Composition

Sandvik

3R60 Urea Grade2RE69

"Reference name"

3 16L Urea Grade25/22/2

C max.

0.0200.0

Cr

17.520

Ni

14252

Mo

2.62.1

Sandvik 3R60 Urea Grade is often referred to as316L Urea Grade because it is a modified 316L com-position. From time to time, it happens that standard316L is installed by mistake instead of 316L UreaGrade. The consequence might be a disaster as 316Lwill have a much higher corrosion rate. The differenceis ASTM allows for the 316L composition, a large tol-erance range, while 3R60 Urea Grade is produced to aspecial and well defined composition. The welldefined composition means a maximum corrosion rateof 0.6 mm/year can be guaranteed for 3R60 UreaGrade in the Huey test according to ASTM 262Practice C. The Huey test is an accepted method with-in the urea industry to evaluate the corrosion resis-tance of austenitic stainless steels for urea service. Ingeneral, material with a low corrosion rate is expectedto perform well also in service.

The more alloyed Sandvik 2RE69 offers a muchimproved corrosion resistance compared with 3R60Urea Grade. In the Huey test a maximum general cor-rosion rate of 0.12 mm/y can be guaranteed. The2RE69 is commonly referred to as 25/22/2. It mightlook like a well defined grade, but in fact it only refersto the nominal amount of chromium/nickel/molybde-num. As the Sandvik name for the 2RE69 weldingconsumable has the designation 25.22.2LMn, there isan obvious risk for confusion. By referring to thelicensers, applicable specification or to the Sandvik'sgrade name 3R60 Urea Grade and 2RE69, which ful-fils the latest requirements from the licensersStamicarbon and Snamprogetti, the risk can beavoided.

Service experience with 2RE69

VEB Stickstoffwerk, referred to as SKW Piesteritz,was the first urea plant to install 2RE69 tubing in thestripper using the Stamicarbon process. The same tub-

ing are still in operation after more than 20 years. It isa remarkably good result when considering the mostaggressive corrosion conditions prevail in the stripper.A long service life for 2RE69 is not only experiencedby SKW Piesteritz, but also a large number of otherplants. 2RE69 tubing was supplied to about 30 strip-pers before 1977 confirming a long and reliable ser-vice. Today, the predicted service life for 2RE69 iswell over 15 years.

Even under normal service conditions there is a veryslight corrosion rate often in the range of 0.05 to 0.09mm/year. A very good control of the service condi-tions is required to achieve such a long service life. Inan attempt to further reduce the variation in the corro-sion rate in service, a maximum corrosion rate in theHuey test of 0.12 mm/year can now be guaranteed for2RE69 compared with the 0.18 mm/year which nor-mally is specified.

The excellent corrosion resistance of 2RE69 hasresulted in a wider usage in the high pressure sectionof the urea plant. Several plant operators are now reg-ularly replacing corroding 316L Urea Grade with2RE69 to achieve a longer service life. An improvedavailability of tube, plate and bar in the frequent sizesfrom stock makes even urgent repairs possible.Reactor lining and trays, nuts and bolts, carbamatecondensers with tubing hi 316L Urea Grade, and highpressure piping are items where upgrading to 2RE69has been done by plant operators.

From a life cycle point of view, it will certainly pay-off for new plants to include the upgrading from thebeginning even if the initial material cost will beslightly higher.

Bimetallic tubing in NH3-stripper

The use of bimetallic tubing in the stripper is a newpatented method developed and introduced by


Snamprogetti. The bimetallic tubing consists of anouter tube in 2RE69 with an inner seamless lining ofZirconium 702. The bimetallic tubing is fabricatedwith a mechanical bond between the two components.This ensures a firm location of the inner lining andalso good heat transfer through the tube wall. Beforethe first full-scale installation in 1991, the bimetallictubing was tested by Snamprogetti for 40 h in service.The tubes after the test were reported to be as if theywere new.

In the older design titanium tubing was used gener-ally with good performance in an environment knownto be very severe. Localized erosion inside the tubingcould appear, resulting in a premature replacement ofthe stripper. Zirconium has better corrosion and ero-sion properties in this type of environment than titani-um or 2RE69. Therefore, a thin inner lining of zirconi-um is sufficient as the strength comes from the outertube in 2RE69. The outer tube in 2RE69 is also weld-ed to the tube sheet, weld surfaced with 2RE69 type ofmaterial. This is a cost saving design as fabrication intitanium is more complicated and more costly than2RE69. The manufacturing and testing of bimetallictubing is more complex, which Snamprogetti hastaken care of in the specification for the bimetallictubing. Sandvik, with an integrated production frommelting to final product, produces both the lining ofzirconium and the outer tube in 2RE69, which is agreat help in meeting the requirements.

The new design offers several advantages comparedwith the old design, and perhaps the best indication of

the acceptance of the new design is that, within thelast two years, Sandvik has received 8 orders forbimetallic tubing to 14 strippers.

Welding in urea plants

Surfacing. As the process solution is on the tube sidein the stripper/decomposer and condenser sections of aurea plant it is necessary to have a tube sheet withgood corrosion resistance as well. For reasons ofstrength and economy a carbon or low alloyed steeltube sheet is weld surfaced. The heads need also to beweld surfaced or lined with an appropriate stainlesssteel grade.

Weld surfacing is most frequently carried out byelectroslag surfacing (ESS) or submerged arc surfac-ing (SAW) using strip electrodes. To a lesser extent,gas shielded processes such as plasma arc welding isused. The weld deposit that has gained internationalacceptance is of the 2RE69 type. In both electroslagand submerged arc welding, a strip electrode of theSandvik grade B25.22.2.LMn and the dimensions 60 x0.50 mm is used in a two or more layer welding proce-dure. The fluxes used with both welding methods arebasic in order to give optimum corrosion properties inthe weld deposit. The shell of the reactor may be linedwith a modified 316L plate, but the corrosionallowance needs to be large due to the severe serviceconditions. As service conditions become moreaggressive due to increases, temperatures, and pres-sures, it may be advantageous to either use 2RE69

Table 4. Summary of Material Used in High Pressure Urea Service

Process Use Steel Type

Stamicarbon

Snamprogetti

jer 2RE69Condenser 2RE69 (3R60 Urea Grade)*H P Piping 3R60 Urea Grade (2RE69)*

Bimetallic Tubing 2RE69/Zirconium 7022RE693R60 Urea Grade

^ DerCondenserH P Piping

* The alternative in brackets is less frequent, but it can be supplied.


plate lining or weld surfacing with SandvikB25.22.2.LMn.

Tube-to-tube sheet welding

The most frequently used methods for tube-to-tubesheet welding are manual or mechanized TIG weldingand manual metal arc welding. Tubes of 2RE69 shouldbe welded into the 22Cr/22Ni/2MoN weld surfacetube sheet using either Sandvik W25.22.2.LMn TIGwelding consumables or Sandvik E25.22.2.LMnBcovered electrodes.

Most tube-to-tube sheet designs utilize a protrudingtube to allow for the attachment of ferrules in the strip-per. A fillet weld is used.

Repair welding

Many older vessels have been lined with 316L U.G.and now after many years of service, it is necessary toweld repair or replace parts of the lining. As the weld-ing is carried out in-situ, the most widely used weldingmethod is manual metal arc welding. For both 316LU.G. and 2RE69 types of lining, the recommendedcovered electrode is Sandvik E25.22.2.LMnB. Thisbasic electrode is suitable for welding in all positions.The electrode is suitable for attaching the lining tosteel parent metal due to its high alloying content.

Welding recommendations

Sandvik 25.22.2.LMn gives a fully austenitic weldmetal, similar in composition to 2RE69. Alloying with

4.5% manganese and nitrogen, and strict control of theimpurity elements, result in a resistance to hot crack-ing that is far superior to most other fully austeniticweld metals. In order to successfully pass the qualifi-cation tests and maintain a high standard in produc-tion, it is necessary to observe a number of criteriawhen deciding upon the welding procedure. The mostcritical stages of the qualification tests are the simulat-ed weld repairing and the tube-to-tube sheet welding.

Our recommendations are:Strip Surfacing.(1) For both ESS and SAW, use a flux which gives a

low pickup of silicon and impurity elements, and alow burn-off of manganese and chromium. Suitablefluxes are 37S for ESS and 3IS for SAW.

(2) Use B25.22.2.LMn for both layers. It is not nec-essary to use a buffer layer of 309L.

(3) Use welding parameters that give low dilutionwith the parent metal and low pickup from the flux.Examples of welding parameters for strip electrodeswith dimensions 60 x 0.50 mm are for ESS: 1,200-1,250 A, 25-27V, 160-180 mm/min.

For SAW use: 700-750A, 28-30V, 110-130mm/min., the same parameters can be used for all lay-ers.

Tube-to-Tube Sheet Welding.TIG welding:

(1) Use low heat input. Keep the current below 90Aand the heat input per bead below 1 kJ/mm.

Pulsed TIG welding:(1) Use a low ratio between pulse current and back-

ground current, preferably below 1.5.(2) Use the highest possible frequency, preferably

Table 5. Corrosion Properties of Weld Deposits

Average Corrosion Rate, mm/yearHueyTestR25.22.2.LMn R25.22.2LMn E25.22.2.LMnB

TIG all-weldmetal

0.08**

TIG weldedjoint*

0.10**

MMA weldedmetal

0.11**

E25.22.2.LMnB

MMA weldedjoint*

0.10**

B25.22.2.LMn/ B25.22.2LMn/31Stwo layeroverlay

0.12

37Stwo layeroverlay

0.08

* ASTM A262 Practice C (boiling 65% HNO3,5 x 48h).** For conversion to jum/48h multiply by 5.48.


above 1 Hz.(3) Use a high ratio between pulse time and back-

ground time, preferably above 1.Manual metal arc welding:

(1) Use an electrode diameter of max 2.5 mm, and aheat input of max 1 kJ/mm per bead.

Repair by Manual Metal Arc Welding.(1) Use covered electrodes with diameter of maxi-

mum 4 mm.(2) Weld with stringer beads.(3) Weld symmetrically, i.e., alternate the beads

from one side of the repair to the other.Weld Metal Properties.A summary of corrosion properties of 25.22.2.LMn

is given in Table 5.

Conclusion

Corrosion problems can vary from plant to plantbecause of local service conditions. The performanceof older plants give indications of where the problemsusually turn up. The initial cost for better materialmight be higher, but it will pay off as it is always morecostly to replace it later with a more corrosion resis-tant material.

In ammonia plants duplex grades have proven to bea good solution to waterside corrosion at a low cost. Inthe urea process the fully austenitic grade 2RE69 hasan expected service life of more than 15 years in CO2

stripping process from Stamicarbon. The good corro-sion properties have meant an increased usage of thegrade as replacement of 316L Urea Grade. Thebimetallic tubing in 2RE69, with an inner lining of zir-conium, is still a rather new product. The rapid switchfrom the older design using titanium to the new one isperhaps the best confirmation of a successful design.

Further Reading

Bemhardsson, S., R Mellström, and B. Brox, SandvikR&D Lecture S-51-36, presented at the 8thInternational Congress on Metallic Corrosion,Mainz, Germany (Sept. 1981).

Gustafsson, Per, Gunter Bindl, and ChristerMartensson, "Sandvik SAF2507 - The HighPerformance Duplex Stainless Steel. Theory andPractice," Sandvik R&D Lecture S-51-44-ENG,presented at ACHEMA91, Frankfurt/Main,Germany, (June 9-15,1991).

Larsson, Bertil, and Berthold Lundquist, "Fabricationand Practical Experience of Duplex StainlessSteels," Sandvik R&D Lecture S-51-39-ENG

(1987)."Sandvik Special Stainless Steel - Auxiliary Systems

in Ammonia/Urea Plants," Reference list S-12311(May 1993).

DISCUSSION

H. F. Perree, Stamicarbon: Mr. Tersmeden, I have aquestion on the cost of the materials. What is the pricedifference between bimetallic tubes and 25/22/2tubes?Tersmeden: The 2RE69 is 20%-30% more expensive.It might sound like a lot because when we had a lookat the composition of the 316L urea grade, it lookedvery similar to 316L; however, we must do specialmelts. To get very low carbon and sulfur phosphatecontent to be able to pass the specification, we add aspecial melting procedure.Perree: I suppose that is the difference between 316L

and 2RE69, but what about the difference between25/22/2 and bimetallic?Tersmeden: The bimetallic tubing is undoubtedlymore expensive. Zirconium is roughly two to threetimes more expensive than the 2RE69, but on the otherhand, we are using a very thin layer with only about0.7 mm zirconium. However, basically, we producetwo tubes and put them together. The bimetallic tubingwill always be more expensive. Fortunately, inSnamprogetti, they have found a very smart way ofmaking the whole unit less expensive than the old onein titanium.


Documents

1996: Successful Use of Stainless Steels in Nitrogen-Based