tube material selection

Committed to making a better world Customer training programme Nov06Bharat Heavy Electricals Limited, Tiruchirappalli


Special operating conditions:- (Customer Specification)Constant pressure operation.

Sliding pressure operation from rated pressure down to 40% of rated pressure with as well as without any throttle reserve. The throttle reserve shall be adjustable to minimum 0% for pure sliding pressure mode of operation.

Base Load / Cycling / Two shifting operation.

Expected number of steam generator start-ups:-Type of startsNo. of startsa) Hot starts (after 8 hours 4550 of unit shut down)b) Warm start (after 36 hours 910 of unit shut down)c) Cold start (after 72 hours 455 of unit shut down)Ramp Rate:a) Minimum 3% under variable pressure operation.b) Minimum 5% under constant pressure operation.6.Etc.


CYCLING / TWO SHIFT OPERATIONCycling duty:This types of operation envisages daily bringing down the unit load down to some minimum load during periods of low demand and then raising up to the desired load when required. Considering the turndown capability of coal fired boilers, the minimum load may be approximately 40% of full load.


Two shift operation:As the name implies, this type of duty envisages shutting down the unit for a period of about 8 hours (1 shift) and then restarting and coming up on load when required. This type of duty is also referred to as cycling with the extreme case of zero minimum load.The choice of the required unit operating strategy for a new power plant, has to be critically arrived at after a detailed techno-economic analysis on a case-to-case basis. Some of the major influencing factors are:- existing grid / station capacity, generation mix (Fossil, Nuclear, Hydel, etc.), number of boilers and their individual capacities, load demand pattern on daily / monthly / annual basis, load sharing capabilities of individual units, etc.These operating requirements are normally specified by the utilities in terms of number of different load change or start up - shut down cycles as envisaged during the entire life period of power plant operation. As these information are vital for the design of various power plant equipments, these are specified at the proposal stage.


SPECIAL CONSIDERATIONS FOR BOILER DESIGN:In addition to the design and performance requirements for which base loaded units are engineered, the only additional requirement to be considered for the cycling boiler is the rapid rates of load increase under significantly greater number of start up and shut down cycles. The designer can evolve a good design by developing and adopting a set of design criteria to enable assessment of all types of loading (pressure, mechanical, thermal and cycling) to insure long term component integrity. The major factors affecting these design considerations are:-- Number of cycles- Heating and cooling rates- Component thickness, diameter and material- Operating temperature level.


SCOPECyclic loadings which are caused due to internal pressure, on the one hand, and due to radial temperature differentials during start-up (heating-up) and shut-down (cooling down), on the other hand, in the areas subjected to maximum loading thereby.The components subjected to pressure and temperature the maximum local peak stresses occur on the inside of the edges of openings or on branches in cylindrical shells. Additional forces and moments of a significant magnitude shall be separately calculated (design rules in preparation).


STRESSBasis for Establishing Stress values

a)At temperature below the range where creep and stress rupture strength govern the selection of stresses, the maximum allowable stress value is the lowest of the following:-

1)One-fourth of the specified minimum tensile strength at room temperature (ST).

One-fourth of the tensile strength at temperature (ST).

3)Two-thirds of the specified minimum yield strength at room temperature (SY).

Two-thirds of the yield strength at temperature.


b)At temperature in the range where creep and stress rupture strength govern the selection of stresses, the maximum allowable stress value for all materials is established by the committee not to exceed the lowest of the following:-

1)100% of the average stress to produce a creep rate of 0.01% / 1000 hour.

67% of the average stress to cause rupture at the end of 100 000 hours.

3)80% of the minimum stress to cause rupture at the end of 100 000 hours.


Types, classes and categories of stress

Types of stress:-

There are many names to describe types of stress.

1)Tensile10)Thermal

2)Compressive11)Tangential

3)Shear12)Load induced

4)Bending13)Strain induced

5)Bearing14)Circumferential

6)Axial15)Longitudinal

7)Discontinuity16)Radial

8)Membrane17)Normal

9)Principal



Header & Nipple Flexibility AnalysisIt is known that the headers are important components in boilers and are used for collecting or distributing water or steam or steam-water mixture. Headers are hollow cylindrical components with series of holes provided for the fixing up of the nipples.

Causes of Tube Expansion

1)Roof to Header Temperature Differential

Temperature differential occurring tube to tube across a set of nipples in an assembly.

A temperature unbalance along the longitudinal axis of the header.

4)Forces and moments from piping to the header.


INPUT

MATL: SA213 T22, SA 213 T11, SA 213 TP347

SIZE: OD63.5X10.5, OD54X12, OD51X11, OD44.5X11, OD44.5X10

MEDIUM PRESSURE:189 Kg/mm^2

MEDIUM TEMP :540 DEG CRESULT:MAX.STRESS DUE TO SUSTAINED LOAD: 1.58 Kg/mm^2

MAX.EXPANSION STRESS : 12.59 Kg/mm^2 FLEXIBILITY ANALYSIS FOR PLATEN SUPERHEATER OUTLET HEADER NIPPLES USING CAEPIPE 3.6 ANALYSIS CODE : ANSI B31.1


PACKAGE USED:CAEPIPE 3.6ANALYSIS CODE : ANSI B31.1

INPUTMATL: SA213 T22

SIZE:OD63.5X10.5

MEDIUM PRESSURE:189 Kg/mm^2

MEDIUM TEMP :540 DEG C

RESULT:MAX.STRESS DUE TO SUSTAINED LOAD: 1.58 Kg/mm^2

MAX.EXPANSION STRESS : 12.59 Kg/mm^2FLEXIBILITY ANALYSIS FOR PLATEN SUPERHEATER OUTLET HEADER NIPPLE USING CAEPIPE 3.6 Single Tube is shownDeflected shape


EXISTING ARRANGMENT RH INLET HEADERRH OUTLET HEADERROOF ELEVATIONFLOWFLOWFLOW


RH INLET HEADERRH OUTLET HEADERSROOF ELEVATION PROPOSED SAIYAMEESE ARRANGMENT FLOWFLOWFLOW


Advanced materials in supercritical boilers


Need for Advanced MaterialsTo withstand the higher steam pressure and temperatureHigh creep strength and high corrosion resistance requirementsReduced thickness for less stresses in critical components.


Effects of alloying elements in ferritic and martenistic steelsT23/T24Developed based on T22Alloyed with tungsten together with reduction of MoMicro-alloyed with Vanadium, Columbium Nitrogen and Boron Increase in creep properties come close to T/P91 Initially developed for WW panel in supercritical boilersCan be used in SH/RH in conventional boilers.


Chemical composition of T23 and T24


Comparison of allowable stress T22/T23/T24/T91

Chart1

104124131133

100117126125

80111117110

64105112102

50879595

38716783

25563966

163962546

102565028

T22

T23

T91

T24

T22

T23

T24

T91

Temperature (Deg.C)

Allowable stress (MPa)

Chart2

9512096110122

8311094106118

668888100114

46747585100

2848556880

675675405260

700700334046

T91

T92

TP 347H

TP347HFG

SUPER304H

T91

T92

TP347H

TP347HFG

SUPER304H

Temperature (deg.C)


Sheet1

STRESS VALUES IN KSI

Temp in FT22T23T91T92

50016.621.124.123.8

60016.620.923.723.2

65016.620.723.422.8

70016.620.522.922.4

75016.620.322.221.9

80016.619.921.321.4

85016.619.520.320.8

90013.618.919.120.1

95010.817.817.819.2

1000814.316.318.3

10505.711.21417.2

11003.88.410.313.6

11502.45.5710.2

12001.41.44.36.9

T23CODE CASE 2199

T92CODE CASE 2179

Allowable stresses in MPa

Temp. in deg.CT22T23T24T91

20104128146148

100104128146148

200104128139148

300104128135148

400104128131133

450100117126125

50080111117110

52574104112102

55048879595

57538716783

60025563966

625164046

650102528

Curve -1


400104124131133

450100117126125

50080111117110

52564105112102

55050879595

57538716783

60025563966

625163946

650102528

Curve-2


Temp. in deg.CT91T92TP347HTP347HFGSUPER304H

5509512096110122

5758311094106118

600668888100114

62546747585100

6502848556880

675405260

700334046

Sheet2

Sheet3


T92T/P92 developed adding tungsten to T91Excellent behaviour at elevated temperatureReduces thickness upto 30% due to higher creep strengthEffects of alloying elements in ferritic and martenistic steels


Chemical composition of T92


Effects of alloying elements in Austenitic SteelSuper 304H/347HFGReducing Ti and Nb to promote creep strengthAddition of Cu for increased precipitation strengtheningUse of Nitrogen and W for solid solution strengthening


Chemical composition of Super304H/347HFG


Comparison of allowable stress (T91/T92/TP347H/TP 347HFG/Super 304H)

Chart1

104124131133

100117126125

80111117110

64105112102

50879595

38716783

25563966

163962546

102565028

T22

T23

T91

T24

T22

T23

T24

T91

Temperature (Deg.C)


Chart2

9512096110122

8311094106118

668888100114

46747585100

2848556880

675675405260

700700334046

T91

T92

TP 347H

TP347HFG

SUPER304H

T91

T92

TP347H

TP347HFG

SUPER304H

Temperature (deg.C)


Sheet1

STRESS VALUES IN KSI

Temp in FT22T23T91T92

50016.621.124.123.8

60016.620.923.723.2

65016.620.723.422.8

70016.620.522.922.4

75016.620.322.221.9

80016.619.921.321.4

85016.619.520.320.8

90013.618.919.120.1

95010.817.817.819.2

1000814.316.318.3

10505.711.21417.2

11003.88.410.313.6

11502.45.5710.2

12001.41.44.36.9

T23CODE CASE 2199

T92CODE CASE 2179



20104128146148

100104128146148

200104128139148

300104128135148

400104128131133

450100117126125

50080111117110

52574104112102

55048879595

57538716783

60025563966

625164046

650102528

Curve -1


400104124131133

450100117126125

50080111117110

52564105112102

55050879595

57538716783

60025563966

625163946

650102528

Curve-2


Temp. in deg.CT91T92TP347HTP347HFGSUPER304H

5509512096110122

5758311094106118

600668888100114

62546747585100

6502848556880

675405260

700334046

Sheet2

Sheet3


Application in Supercritical Boilers(Evaporators)T12 : For moderate steam conditions of 545oC(SHO) / 570oC(RHO)T23 and T24 : For higher steam conditions of 600oC (SHO) /620oC (RHO) due to higher creep strengthUse of T23 : steam temperature in WW can be raised by 50oC, compared to T12.Contd...


Superheaters and ReheatersT91 : can be used for steam temperature 550oC (SHO)/570oC (RHO)Austenitic steels super 304H/TP347HFG : For steam temperature 600oC (SHO)/620oC (RHO) to be used for withstanding increased oxidation temperatures.Contd...


Headers and PipingWith increase in steam parameter, P22 limit is quickly reached.P91 can be used upto 270 bar and 580oC (SHO)P92 can be used 290 bar and 600oC (SHO)


Materials in typical 800MW


Future TrendsPromising materials 12% Cr steels namely NF12, SAVE 12Research projects going on for use of 375 bar and 700oC/720oC


ConclusionsIn the pursuit of increasing the efficiency, supercritical parameters are used in increasing manner.To suit the steam parameters, new materials are being developed.New materials such as T23 can be used in conventional boilers for reducing material content.Research is going on to realise steam parameters up to 700oC by developing materials for this high temperature.


tube material selection

Documents

minimum load

unit load

desired load

variable pressure operation

base load cycling

types of operation

shifting operation

load demand pattern