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Sr. Manager(Prod)National fertilizers Ltd, India,An Expert for www.ureaknowhow.comFellow of Institution of Engineers India

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INTRODUCTIONINTRODUCTION Materials plays very important role in any

industrySelection of material is vital at design

stage itselfWrong selection of material may lead to

catastrophic failures and outage of plants & even loss of Human lives

Right selection of material leads to long life of plant

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MAIN TOPICS

CLASSIFICATION OF MATERIALSTYPES OF MATERIALS USED AT

VIJAIPURTYPES OF STEELSSPECIAL MATERIALS IN USE NEW DEVELOPMENTMATERIALS FOR UREA SERVICE

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CLASSIFICATION OF MATERIALS

C AR BO N S TEE LLO W TEM P E R A TU R E C AR B O N S TEE LALLO Y S TE E LSTA IN LESS STE E L

F E R R O U S

ALU M IN IU MZIR C O N IU MC O PP E RW H ITEM ETA L

N O N -F ER R O U S

M E TA LLIC

TEF LO NR U B B ERW O O D

N O N -M E TA LLIC

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MATERIALS

• There is a wide variety of materials use to construct process equipment. The type of material selected depends on its compatibility with process conditions and cost.

• The materials of choice can be divided into the following general categories

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METALS

• The metals used in the process industries can be generally divided into two groups, ferrous and nonferrous. Ferrous metals are arbitrarily defined as those containing at least 50% iron.

• As a group they far exceed any other group of metal alloys used in the process industry.

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FERROUS ALLOYS

• The reason for popularity of ferrous alloys is their relatively low cost, ready availability and good workability. Ferrous alloys are subdivided into cast irons, carbon steels , low alloy steels, and stainless steels.

• There are also tool steels, but these will not be discussed as they are of limited used in the process industry.

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CAST IRON• Cast iron are iron alloys that have greater than 1.5%

Carbon content. These are four types.1 .GRAY CAST IRON.-This type is used in applications

such as construction equipment requiring vibration damping and wear resistance. Its gray appearance is caused by the graphite particles spread through out its mass. Gray cast iron has a low tensile strength and is brittle, and it should not be used in high pressure process vessels.

2. WHITE CAST IRON.-White cast iron has a much lower silicon content than gray cast iron. No graphite particles exist in its microstructure, and any carbon in the cast iron is combined with iron in the form of iron carbide(Fe3C). The metal is hard, extremely abrasive, brittle .

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Contd. 3. DUCTILE CAST IRON.-

When ductile cast iron is manufactured, small spherical nodules of graphite are produced in its microstrure.This cause it to be ductile rather than brittle. Ductile cast iron is used for high strength pipes, valve bodies, pump casing, compressor casings, crankshaft and machine tools.

4..WROUGHT CAST IRON

Prior to the nineteenth century, wrought iron was the only form of iron used. It is basically pure iron low in the carbon but with a few percent of slag remaining in the form of an iron silicate. The slag form a fibrous structure within the wrought iron that gives it excellent shock, vibration, and corrosion resistance. wrought iron can be used for water pipe, air brake pipes, and engine bolts.

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IRON CARBON EQUILIBRIUM DIAGRAM

TEM

PE

RA

TUR

E

TEM

PE

RA

TUR

E

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IRON CARBON EQUILIBRIUM DIAGRAM

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PROPERTIES

STRENGTHHARDNESSDUTILITYTOUGHNESS

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STRENGTHTHE RESISTANCE OFFERED BY

MATERIAL ON APPLICATION OF EXTERNAL FORCE IS CALLED STRENGTH

HARDNESSHARDNESS IS RESISTANCE

OFFERED BY MATERIAL TO INDENTATION

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HARDNESS & ITS MEASURMENTSHARDNESS & ITS MEASURMENTS

• Several indentation method, give hardness value in terms of absolute units, briefly a given load is applied to a specimen for a specific time ,removed and the area of indentation is then measured.

• HARDNESS = Load• Area(kg/mm2).1.Brinnel Hardness Test.2. Vickers or Diamond pyramid hardness(DPH).3. Rockwell Hardness Test.

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Indenter

Specimen surface

h1 h2

Pre loadPre load

Main Load

+

dD

I

BrinnelVickers Or

DPHKnop

Indentation shapes for various indentation hardness test method

Measuring sequence in Rockwell Hardness Test

HV=1.8544F/D2.BH= F

( ΠD)(D-√D2-d2)F=Load

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DUCTILITY

• IT IS A MEASURE OF THE AMOUNT OF PLASTIC DEFORMATION A MATERIAL CAN TAKE UNDER TENSILE FORCE WITHOUT FRACTURE

• RATIO OF ELONGATION OF MATERIAL AT FRACTURE TO ORIGINAL LENGTH

• BRITTLENESS IS THE PROPERTY OPPOSITE TO DUCTILITY

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TOUGHNESS• THE AMOUNT OF ENERGY ABSORBED

BY A MATERIAL AT THE TIME OF FACTURE UNDER IMPACT LOADING

• IT IS CAPACITY TO TAKE IMPACT LOAD

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ROLE OF DIFFERENT ELEMENTS USED IN STEEL ALLOYS

1.CHROMIUM(Cr):“It is strong Carbide former and form complex series of carbides compound of chromium and iron.” (a)It is essential for formation of passive film to resist corrosion. (b)It helps to raise the critical Temperature of steel. (c)It improves hardness and wear resistance.

2.NICKEL:

(a)It improves toughness,ductility and ease in welding.(b)It improves resistance to Fatigue failure.(c)It is austenitic stabilizer.

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3.MOLYBDENUM:

(a)It increases corrosion resistance in presence of chlorides.(b)It prevents Pitting and Crevice corrosion.

4.CARBON:

(a)It permits the hardenability by heat treatment.(b)It gives the strength at high temperatures.

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5.NITROGEN:

(a)It enhances pitting resistance.(b)It is a supplement of Carbon in Low Carbon Steels.(c)It improves Pitting and Crevice Corrosion resistance.

6.MANGANESE:

(a)It prevents to form Iron Sulphide which is harmful in steels.(b)It promotes forgeability of steel.

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Copper

• Increases strength up to about 12 %. Higher concentration cause brittleness. Copper improve the elevated temperature properties and machinability. Used in concentration of about 0.5% or less, produces a tenacious self sealing oxide film on a metal surface.

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7.PHOSPHORUS: 1.IT IS ADDED IN LOW CARBON STEELS UPTO 0.12%.IT DISSOLVES IN FERRITE AND INCREASES STRENGTH AND HARDNESS. 2.IT IMPROVES THE MACHINABILITY.

8.SILICON

1.IT IS ADDED IN STEELS AS ADEOXIDISER DURING CASTING OF IGNOTS.2.IT DISSOLVES IN FERRITE INCREASING STRENGTH AND HARDNESS WITHOUT LOWERING THE DUCTILITY

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CARBON STEEL

• The characteristic difference between cast iron and carbon steel is the carbon content.

• Carbon steel has less than 1.5 % carbon.• It is easy to fabricate and has better

strength than cast iron.• It is easier to weld than cast iron.• The main disadvantage in using carbon

steel is its susceptibility to corrosion.

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TYPES OF CARBON STEEL

LOW CARBON STEEL

MEDIUM CARBON STEELHIGH CARBON STEEL

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LOW CARBON STEELCONTAIN UPTO 0.3% CARBON

KNOWN AS SOFT OR MILD STEEL

NOT RESPONSIVE TO HEAT TREATMENT

SCREW, NAIL, NUTS,BOLTS, WASHERS, WIRE FRAMES

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MEDIUM CARBON STEEL CONTAINS CARBON 0.3 TO 0.6%

HARDER AND MORE TENSILE STRENGTH THEN LOW CARBON STEEL

BETTER MACHINIG QUALITIES AND MORE RESPONSIVE TO HEAT TREATMENT

SHAFTS, CONNECTING RODS, SPINDLES, MACHINE PARTS

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HIGH CARBON STEELCONTAINS 0.6% TO 1.7% CARBON

HIGHER TENSILE STRENGH AND HARDER

READILY RESPOND TO HEAT TREATMENT

HAND TOOLS, CHISELS, PUNCHES, FILES, CUTTING TOOLS, RAIL ROAD WHEELS

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MATERIALS FOR VARIOS NETWORKS

NETWORK MATERIAL GRADE

COOLING WATER, STEAM CONDENSATE

CARBON STEEL

API 5L Gr.B

STEAM UP TO 3750C

CARBON STEEL

ASTM A 106Gr.B

STEAM MORE THAN 400 0C

ALLOY STEEL

ASTM A335 Gr.P11 & P22

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STAINLESS STEEL• “A steel that has 12% or more chromium is

considered a stainless steel.”• “Another criterion defining a stainless steel is its

Passivity.”• PASSIVITY.- “Passivity is the ability of a metal

to form an impervious surface coating which inhibits corrosion resulting from the electrochemical reaction of the metal with the surrounding environment.”

• “Stainless Steels exhibit passivity in oxidizing environment.”

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Professional organization and their Scope of Concern

Abbreviation

organization Scope of Concern

ASTM America Society of Testing & Material

Provides testing procedures for a wide variety of material

ASME American society of mechanical engineers

Provides a list of recommended metal & welding material

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AISI American iron & steel Institute

Provides standard descriptions for cs & alloy.

SAE Society of automotive engineers

Provides a four digit system and alloy steels used in high production item

CDA Copper development association

Instituted a numerical system for identifying copper.

ANSI AmericanNational Standard Institute

Provides safety & design criteria

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Corrosion as function of Chromium

• “Following figure shows the corrosion rate as a function chromium content in an iron chromium alloy.”

Corrosion rate of iron-Chromium alloys exposed to an intermittent water spray at room temperature

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Effect of Chromium

Stainless steels are chromium containing steel alloys. The minimum chromium content of the standardized stainless steels is 10.5%. Chromium makes the steel 'stainless'  this means improved corrosion resistance, as can be seen in the chart.

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IRON-CHROMIUM PHASE DIAGRAM

• Following figure shows the phase diagram for iron chromium in all proportions.

• With chromium contents between 20 and 70% ,the “sigma” microstructure is formed.

• SIGMA MICROSTRUCTURE.- “This microstructure is hard, brittle and poor corrosion resistance.”

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IRON CHROMIUM PHASE DIAGRAM

TEM

PE

RA

TUR

E IN

0 C

205

426

640

8711

09 0

1315

1538

176

0

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TYPES OF STAINLESS STEEL

• There are three general types of stainless steel of interest to the process engineer.

1. FERRITIC STAINLESS STEEL2. AUSTENITIC STAINLESS STEEL3. MARTENSITIC STAINLESS STEEL

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FERRITIC STAINLESS STEEL

• Ferritic stainless steel has a• carbon content of 0.2% or less.• Chromium content 11-18% .• Although it can not be heat treated and

has poor tensile and impact strength, it better corrosion resistance than martensitic stainless steel.

• It is suitable for use with strong oxidizing acids such as Nitric acid.

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IRON CHROMIUM PHASE DIAGRAM WITH CARBON 0.2%

FERRITIC S.S. Far from sigma phase

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FERRITIC STAINLESS STEEL

HIGHLY RESISTANT TO STRESS CORROSION CRACKING SHOW AN INCREASES AS RESULT OF COLD WORK

THEY ARE MEGNETIC, HAVE GOOD DUTILITY READILY WELDABLE, NON MEGNETIC

WELDING IS DIFFICULT SS 430(17% Cr) USED FOR AUTOMOTIVE TRIMS

AISI % C % Mn % P % S % Si % Cr

430 0.12 1.0 0.04 0.03 1.0 14 to 18

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MARTENSITIC STAINLESS STEEL

• Farritic stainless steel has a• Carbon.-1.2% or less. and• Chromium.-12-18%.• It has better hardenability and strength

than does ferritic stainless steel.• It is used as cladding to carbon steel for

some process vessels .

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IRON –CHROMIUM PHASE DIAGRAM FOR 1.0% CARBON

Martensitic S.S.Far from sigma phase

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MARTENSITIC STAINLESS STEEL (Chemical Requirements)

AISI % C % Mn % P % S % Si % Cr

410 0.15 1.0 0.04 0.03 1.0 11.5 to 13.5

420 0.15 1.0 0.04 0.03 1.0 12 to 14

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AUSTENITIC STAINLESS STEEL

• “Austenitic stainless steel is a more complex material because the addition of NICKEL (3.5 to 22%) allows it to retain its austenitic microstructure at all temperature

• It has a high tensile strength and best impact strength, ductility and corrosion resistance of all the stainless steel over a very wide range of temperatures.

• All stainless steels are susceptible to pitting from exposure to high chloride concentration. However,austanitic steel with high Molybdenum content(1 to 3 %)have improve resistance to pitting.

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Austenitic Steels in High Temperature Service

The austenitic alloys have a face centered cubic structure which has a better corrosion resistance compared to the ferritic steels. The austenitic structure is normally not stable in irons below 700 C, but adding nickel to the steel makes the austenitic phase stable down to room temperature.

These alloys are basically chromium nickel steels. Chromium is used in these alloys to make the steel corrosion resistant, whereas nickel stabilizes the even more corrosion resistant austenitic structure. Silicon and aluminum are added to increase the oxidation resistance.Titanium and niobium, as well as boron, nitrogen, tungsten, vanadium and cobalt can be added to increase the creep strength due to precipitation strengthening. Manganese can be used to substitute nickel as an austenite former.

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Fe-Cr-Ni Ternary diagram for an 18% Cr alloy with the austenitic S.S

Austenitic S.S.

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AUSTENETIC STAINLESS STTEEL(Chemical Requirements)

AISI % C % Mn

% Si % Cr % Ni % Mo

304 0.08 2 1.0 18 to 20 8 to 10.5 -

304L 0.03 2 1.0 18 to 20 8 to 10.5 -

309 0.2 2 1.0 22 to 24 12 to 15 -

310 0.2 2 1.0 24 to 26 19 to 22 -

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AUSTENETIC STAINLESS STTEEL(Chemical Requirements)

AISI % C % Mn

% Si % Cr % Ni % Mo

316 0.08 2 1.0 16 to 18 10 to 14 2 to 3

316L 0.03 2 1.0 16 to 18 10 to 14 2 to 3

321 0.08 2 1.0 17 to 19 9 to 12 Ti >5xC

347 0.08 2 1.0 17 to 19 9 to 12 Nb >5xC

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AUSTENETIC STAINLESS STTEEL

AISI USES

304 DM Water, Lube oil, waste water, CO2

304L VACUUM SECTION

309 Its filler wire & electrodes are used to weld Carbon Steel with Austenitic Stainless Steel

316L M.P. & L.P. SECTIONS, HYDROLYSER SECTION

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MATERIALS FOR CARBAMATE SERVICE

CARBAMTE IS HIGHLY CORROSIVE IN NATURE AT HIGH PRESSURE & TEMPERATURES

ALL OVER THE WORLD LICENSERS OF UREA PLANTS ARE SEARCHING OR COST EFFECTIVE & BETTER CORROSION RESISTANT MATERIALS

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MATERIALS FOR CARBAMATE SERVICE

TITANIUM

ZIRCONIUM

316l MOD OR 316L UG OR 3RE60

2RE69 OR 25Cr.-22Ni.-2Mo. (UNS S31050)

DUPLEX STEEL

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TITANIUMUSED FOR LINING & TUBE MATERIAL OF

AMMONIA STRPPER IN SNAM PROGETTI’S UREA PLANTS

USED FOR LINING OF UREA REACTORS OF TOYO PLANTS AT PANIPAT & BHATINDA

ADVANTAGES :

PASSIVATION AIR IS NOT REQUIRED

STRIPPER BOTTOM TEMPERATURE CANBE KEPT UPTO 210 0 C

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TITANIUM DISADVANTAGES :IS NOT MAINTENANACE FRIENDLY

ERROSION OF TUBES ENDS RESULTING IN BYPASSING

“Titanium & Zirconium Hexagonal

closed packed”

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316L MOD & 2RE69

THESE ARE TAILOR MADE GRADES OF AUSTENETIC STAINLESS STEEL TO SUIT REQUIRED SPECIFICATIONS OF PURCHASER

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USES OF 316L MOD & 2RE69316L MOD

• REACTOR LINING• CARBAMTAE

SEPERATOR LINING• H.P. SECTION PIPING• CARBAMATE

CONDENSOR LINING IN UREA-I

• TUBES OF CARBAMTAE CONDENSOR

2RE69• AMMONIA STRIPPER

LINING• CABAMATE

SEPERATOR LINING & TUBES IN UREA-II

• ADDITIONAL REACTOR TRAYS IN UREA-I PLANT

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CHEMICAL COMPOSITION 316L MOD & 2RE69

316L MOD• CARBON : < .02 %• CHROMIUM : 18 % • NICKLE : >13 %• MOLY : 2-2.6 % • MAGNESE : 1.5-2 %• SILICON : 0.4 %• SULPHUR : 0.01 %• PHOSPHORUS : 0.015 %• N MAX : 0.10 %

2RE69CARBON : < .02 %CHROMIUM : 24-26 % NICKLE : 21-23 %MOLY BEDNUM : 2-2.6 % MAGNESE : 1.5-2 %SILICON: 0.4 %SULPHUR : 0.015 %PHOSPHORUS: 0.02 %N MAX : .1-.15%

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316L MOD & 2RE69

METALLOGRAPHIC PROPERTIES : MATERIAL IN CONTACT WITH PROCESS FLUID

SHALL HAVE AUSTENITIC STRUCTURE

FERRITE CONTENT SHALL NOT EXCEED 0.6% EXCEPT FOR MANUAL WELDS, FOR WHICH 1% IS ALLOWED

“SIGMA PHASE SHALL BE ABSENT” CHROMIUM CRABIDES MAY BE PRESENT IN

MINIMUM AMOUNTS ONLY

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316L MOD & 2RE69METALLOGRAPHIC PROPERTIES : MATERIAL SHALL PASS HUEY TEST & SHALL BR

PERFORMED ACORDING TO ASTM A262 PRACTICE ‘C’ AND MAXIMUM CORROSION RATE ALLOWED SHALL BE:

TYPE OF MATERIAL

CORROSION RATEMM/YEAR

DEPTH OF ATTACKMICRON

316L MOD25-22-2

0.60.3

9070

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RATE OF CORROSION( as per Huey Test)

MATERIAL CORROSION RATEMM/YEAR

ZIRCONIUM 0.005

TITANIUM 0.06

25-22-2 0.3

316L MOD 0.6

HUEY TEST-Metal sample is boiled in 65% HNO3 for 48 Hrs(5times) to estimate corrosion rate

1mm=40mils

(mpy)

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ADVANTAGES

EXCELLENT ERROSION AND CORROSION RESISTANT

COST EFFECTIVE

MAINTENANACE FRIENDLY

LIMITATIONS: TEMPERATURE IS LIMITED TO 207 0 C

PASSIVATION AIR IS REQUIRED

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MATERIAL OF CONSTRUCTION FOR MAJOR EQUIPMENTS IN UREA PLANT

1.REACTOR:

2.STRIPPER: LINE 1 (Retired)

(a)Liner--------------Ti(b)Ferrules-----------2RE-69(c)Tubes --------------Ti(3.5mm)(d)Internals ----------2RE-69(e)Shell --------------C S

(a)Shell ------------CS.

(b)Liner------------316LM

(c)Trays----------316L.

(d)Addl & Replaced Trays--------2 Re-69

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STRIPPER LINE- 1&2

(a)Top&Bottom Domes ----------2RE-69(b)Tubes ----------------------------Bimetallic 2.0mm 2RE69 & 0.7mm Zr (c)Ferrules --------------------------2RE-69(d)Internals -------------------------2RE69(e)Shell ------------------------------C S

62Poor heat transfer

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3.CARBAMATE CONDENSER ( E-5)

(a) Tubes -----------------2 RE-69(b)Dome ----------------Liners---316L Liners(Replaced)---2RE-69

4.MV-2 --------------316L

5.MV-3 --------------304L

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Construction :-1. Kettle type construction2. Carbamate + vapors inlet through a mixer

condenser ,Inlet and outlet passes separated by Partition plates

3. Material of Construction– Channel /tube sheet liner - 2 RE 69– Tubes - 2 RE 69– Partition plate - SS 316 L M– Changed new petal - 2 Re -69

Carbamate Condenser

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7.MV-6& -------------------304L MV-7

8.Hydrolyser ---------------316L

9. C-2 -------------------------------304L

Contd.

6. Pre-Concentrator

E14 A –(Tube &Cell)----------------316L

E-14B(Tube)---------------------------304L

(Cell)----------------- ------------------CS.

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ALLOY C P S Si Mn Cr Ni Mo N304 0.08 0.045 0.030 1.0 2.0 18-20 8-10.5

304L 0.03 0.045 0.030 1.0 2.0 18-20 8-12

316 0.08 0.045 0.030 1.0 2.0 16-18 10-14 2-3

316L 0.03 0.045 0.030 1.0 2.0 16-18 10-14 2-3

316LN 0.02 0.015 0.010 0.40 1.70 17.5 14 2.6 0.18

2RE-69 0.02 0.015 0.010 0.40 1.70 25 22 2.1 0.12

HVD-1 0.05 CU-1.5 0.02 0.038 1.25 25.16 8.5 2.5

MATERIALS OF CONSTRUCTION(COMPOSION IN Wt%)

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“DUPLEX” STAINLESS STEEL

• “A DULEX steel is characterized by a microstructure containing both Ferritic phase with a BCC crystallographic structure and an Austenitic phase with a FCC structure.”

• The Ferritic phase is normally 40-60%, mainly introduced in the wrought alloys by a careful balance of the critical alloying elements.

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BCC.-Body centered cubic “ High strength low ductility.”

e.g.-Ferrite(α-iron),Cr,V,Mo,W etc.FCC-(Body centered cubic). “Low strength high ductility.”

e.g. Austenite( γ-iron),Al,Cu,Pb,Ag,Au,Ni,Pt etc.

HCP-(hexagonal closed packed) HCP metal are not ductile as FCC metal

e.g.-Be,Mg,Zn,Cd,Co,Tl,Zr. Etc

The HCP cell consists three layers of atom The top and bottom layers contain six atom at the corner of the hexagonal and one atom at the centre of each hexagonal. The middle layer contains three atoms nestled between the atom of the top & bottom layers hence the name close packed.

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DUPLEX STEELS

MIXTURE OF AUSTENITE & FERRITE

HIGHER STRENGTH & BETTER RESISTANCE TO CHLORIDES

Cr: 18-27%, Ni: 4-7%, Mo: 2-4%

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DUPLEX STTEEL(Chemical Requirements)

% C

% Mn

% Si % Cr % Ni % Mo

HVD-1(EJ-1)

0.08

2 1.0 26 to 27

7 to 9 2 to 3

FR-255

0.03

2 2.0 26 5 3

Cu 2 %

USE SPINDLES & SEATS OF UREA GRADE VALVES( in Urea Plant EJ-1)

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Stress Corrosion CrackingA particular problem for the common austenitic

grades (e.g. 304 and 316) is stress corrosion cracking (SCC). Like pitting corrosion this occurs in chloride environments, but

it is possible for SCC to take place with only traces of chlorides, so long as the temperature is over about 60°C,

and so long as a tensile stress is present in the steel, which

is very common. The ferritic grades are virtually immune from this form of attack, and the duplex grades are highly

resistant. If SCC is likely to be a problem it would be prudent to specify a grade from these branches

of the stainless family tree.

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Types of Stress• “Stresses occur in any material that is subject to a load

or applied force, Stress is the internal resistance, or counterforce, of a material to the restoring effect of an external force or load.” There are many types of stresses ,but they can all be generally classified in in one as six categories-

1 .Residual stress, 2. structural stress.3. pressure stress, 4. Flow stress,5. Thermal and 6. Fatigue stress.

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Residual Stresses

• “Residual stress are due to manufacturing process that leave stress in a material. Welding leaves residual stresses in the metal welded. stress associated in welding are further discussed later in this module.”

• STRUCTURAL STRESSES- “Structural stresses are produced in the structural member because of the weights they support. The weight provide the loadings. These stresses found in the building foundation and frame works, as well as machinery parts.”

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PRESSURE STRESSES• “Pressure stresses are stresses induced in vessels

containing pressurized material. The loading is provided by the same force producing the pressure. In a Reactor facilities. The Reactor vessel is a prime example of pressure vessel.”

• FLOW STRESSES -“Flow stresses occur when a mass of flowing fluid induces a dynamic pressure on a conduit wall. The force of the fluid striking the wall acts as the load. This type of stress may be applied unsteady fashion when flow rate fluctuate. Water hammer is a example of transient flow.”

• FATIGUE STRESSES- “Fatigue stresses are due to cyclic application of a stress." The stress could be due to vibration or thermal cycling.”

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‘THERMAL STRESSES”THERMAL STRESSES”• “Thermal stresses exist whenever temperature

gradient are present in a material.” “Different temperature produce different expansion and subject material to internal stress." This type of stress is particularly noticeable in mechanism operating at high temperature that are cooled by cold fluid.

• Example-Compressors intercoolers. Practically in 41 stream E-27 tubes thermal stresses cracking corrosion found in 1997.All the tubes have been changed, after this SCC.

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STRESS CORROSION CRACKING

• Practical experience of 41-E-27 Tube leakage was found in 1997,due to SCC.

• Based on practical experience in combination with laboratory test a stress corrosion cracking has been compiled to make to make selection easier .,following fig.

• It shows that even a very small amount (a few ppm) of chloride may result in cracking of austenitic grade like 304L and 316L.

•

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Stress corrosion cracking (SCC) Resistance of various SS.in oxygen bearing Neutral chloride solution . Testing time 1000 hrs. applied stress equal to yield strength. Open circle mean no SCC for SAF 2507.

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Advantages of Duplex S.S.• Good resistance to “chloride stress corrosion

cracking.”(CSCC).• The duplex stainless steel also offer

resistance to general and pitting corrosion.• Good resistance to erosion and abrasion.• There are numerous cases where plant

equipment properly fabricated from duplex SS has operated with full immunity in chloride containing environment where types 304,304L, 316,316Lhave failed due to stress corrosion cracking.

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NEW DEVELOPMENT IN DUPLEX

• First generation duplex.-The first generation duplex containing.-

• Cr-25 %.• Ni-5 %, and• Mo-1.5 % and Nitrogen ---Nil• There is no Nitrogen. Because Carbon Content

upto 0.2%.There is a considerable loss in corrosion resistance during welding

• Therefore ,a post weld heat treatment is required to assure good prosperities.

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SECOND GENRATION DUPLEX

• The second generation duplexes have low carbon levels, assuring resistance to irregular attack(IGA) The nitrogen contents are usually more than 0.1%.in addition to improving pitting and crevice corrosion.

• Cr.----25 %,Ni.—5 %, Mo.-1.5 %.N- 0.1%.

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THIRD GENERATION DUPLEX

• The third generation duplex contains about 0.2% copper.

• Cr.-25 % Ni.-4.0 % Mo-Nil. Cu.-0.2%• “A third generation developed in

SWEDON, has recently been introduced Alloy 2304.” and SAFUREX.

• HVD-1 is also 3rd generation Duplex S.S. and developed by Snampogetti.

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SUPERFERRITIES

• These are highly resistant to chloride pitting and crevice corrosion. They have found extensive applications as tubing for power plant condensers, and heat exchanger handling chloride solution, such as Sea water. Having a ferrite microstructure. They have a very high resistance to CSCC. In addition they are suitable for use in organic acids, dilute reducing acids.

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contd

• Alloy 29-4C(UNS S44735) containing- • Cr -20 %• Mo -4.0 %• Alloy Sea –cure (UNS S44660)• Cr-26 %• Mo 3.0 %• Ni1.0 %

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DUPLEX DEVELOPED BY STAMICARBON &TEC(ACES)

DP3 UREA GRADE(DP12)

R4 UREA GRADE(R5)

Cr 24.8-26 24-26

Ni 6.5-7.5 6.0-6.7

Mo 2.5-3.2 1.2-2.0

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SAFUREX(STAMICARBON)

• Safurex is jointly developed by SANDVIK & STAMICARBON and designated SafurexTM.can allow lower Oxygen content for passivation.

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Chemical composition and PRE No for various Duplex & S.S.

Grade Cr Ni Mo N PRE Microstructure

2 Re 60(UG) 18.5 4.9 2.7 0.07 28 Duplex

SAF 2304 23 4.5 - 0.1 24 Duplex

SAF 2205 22 5.5 3.2 0.18 35 Duplex

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Grade Cr Ni Mo N PRE Microstructure

SAF 2507 25 7 4 0.3 43 Duplex

AISI 304L 18.5 10 - - 18 Austenitic

AISI 316L 17.5 13 2.1 - 24 Austenitic

Sanicro 28 27 31 3.5 - 38 Austenitic

Contd.

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“Selection for Fabrication”Again it is usually the case that grades are selected for

corrosion resistance and then consideration is given to how the product can be fabricated. Fabrication should be considered as early as possible in the grade selection process, as it greatly

influences the economics of the product. Table lists some common

grades and compares their relative fabrication characteristics.

These comparisons are on arbitrary 1 to 10 scales, with 10 indicating excellent fabrication by the particular method.

Table . Common Grades and Their Relative Fabrication Characteristics

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Grade Formability Machinability Weldability303 1 8 * 1

304 8 5* 8

316 8 5* 8

416 1 10 1

430 4 6 2

2205 5 4 5

3CR12 5 6 6

* Improved Machinability versions of these grades offer higher machinabilities in some products.

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CORROSION RATE IN STRIPPER

1000 10000ppm O2

0.01

0.1

1

.0C

orro

sion

rate

mm

/yr

DUPLEX

316LS.S.

2RE 69

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HOW TO GAUGE RESISTANCE TO PITTING

• The resistance of a particular grade of stainless steel to pitting and crevice corrosion is indicated by its Pitting Resistance Equivalent number or PRE, as shown in table . The PRE can be calculated from the composition as:

PRE = %Cr + 3.3 %Mo + 16 %NPRE is also known as LCR-Localized Corrosion Resistance

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A higher PRE value , hower,cannot be used as a direct selection criterion. For selection of material in chloride-containing solution a diagram showing the critical pitting temperature. Has to be used

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Table. Pitting Resistance Equivalent Number or PRE for Various Grades

Grade Class PRE

3CR12 ferritic 11

430 ferritic 17

303 austenitic 18*

304/L austenitic 18

316/L austenitic 24

2205 duplex 34

904L austenitic 34

S31254 austenitic 43

S32750 duplex 43

S32520 duplex 43

*The calculated PRE for 303 is wrong, due to sulphur in the composition.

94