A SNEAK PEEK AT ASME SECTION-I, CONSTRUCTION OF POWER BOILERS

PG-27 CYLINDRICAL COMPONENTS UNDER INTERNAL PRESSURE – A Notable Difference.

y factor is introduced in Section I to take into account the reduction of stress due to redistribution when the temperature is in the creep region. 

Y factor is introduced in Section I to take into account the reduction of stress due to redistribution when the temperature is in the creep region. 

Y factor is introduced in Section I to take into account the reduction of stress due to redistribution when the temperature is in the creep region. 

Case Study 1: Shell thickness calculation
(Internal pressure) – SA-738 B

Case Study 1: Ellipsoidal dish thickness calculation
(Internal pressure)- SA-738 Gr B

Case Study 1: Ellipsoidal dish thickness calculation
(Internal pressure)- SA-738 Gr B

Case Study 1: Cone thickness calculation
(Internal pressure)

Case Study 1: Flat plate thickness

Case Study 1: Cylindrical shell thickness
(External pressure)

Case Study 1: Formed head thickness
(External pressure)

Case Study 1: Nozzle reinforcement

Case Study 1: Summary 

Code RulesDiv1Div 2 Cl 1Div 2 Cl2
Pressure (MPa)222
Temperature (ºC)250250250
Material (Shell/ dish/ cone)SA-738 Gr BSA-738 Gr BSA-738 Gr B
Allowable stress -Shell/heads/cone (MPa)168195222
Material (Nozzle)SA-335 P12SA-335 P12SA-335 P12
Allowable stress -nozzle (MPa114116116
Required shell thickness (mm)10.458.987.91
Shell weight (kg)11841077.1862.9
Required dished end thickness (mm)10.376.876.04
Dished end weight (kg)229.5155.8134.7
Vessel weight (kg)287825692314

Weight reduction

Code%
Div 1
Div 2 Cl 110.6
Div 2  Cl19.4

309kg saving If designed by Cl1

564kg saving If designed by Cl2

Code RulesDiv1 Div 2 Cl 1Div 2 Cl2
Pressure (MPa)555
Temperature (ºC)250250250
Material (Shell/ dish)SA-516 Gr 70SA-516 Gr 70SA-516 Gr 70
Allowable stress (MPa)138143144

Case Study 2: Based on SA-516 Gr 70
1500mm ID @5MPa exposed to 250ºC

Code RulesDiv1Div 2 Cl 1Div 2 Cl2
Pressure (MPa555
Temperature (ºC)250250250
Material (Shell/ dish/ cone)SA-516 70SA-516 70SA-516 70
Allowable stress -Shell/ dish/ cone (MPa)138143144
Material (Nozzle)SA-105SA-105SA-105
Allowable stress -nozzle (MPa)136143144
Required shell thickness (mm)26.7925.6925.51
Shell weight (kg)297927702770
Required dished end thickness (mm)27.2723.8123.64
Dished end weight (kg)652560560
Vessel weight (kg)459741314131

Weight reduction

Code%
Div 1
Div 2 Cl 110.1
Div 2 Cl 210.1

466 kg saving If designed by Div 2

Section I Code Case for Creep Intolerant CSEF Steels in time-dependent service for Section I Construction – Why is it important?

Section I currently, has no rules for CSEF steels that develop the onset of creep cavitation damage which results in very low creep rupture ductility (brittle and unpredictable behaviour) in service. This behavior if confirmed renders the CSEF steel damage intolerant. 

Code Case 3048 classifies CSEF steel heats based on an initial review or screening of creep rupture ductility (RofA) and performs subsequent creep testing to generate a lambda parameter to specifically address low creep rupture ductility in design if confirmed.

This Code Case also provides alternative design rules, where no rules exist, that are conservative for using intolerant CSEF steels. 

Creep strength-enhanced ferritic (CSEF) steels are a family of ferritic alloys whose high-temperature creep strength is enhanced by the creation of a precise condition of microstructure, specifically martensite or bainite, which is stabilized during tempering by controlled precipitation of temper‐resistant carbides, carbonitrides, or other stable and/or meta‐stable phases.

Creep Intolerant CSEF steels: Creep cavitation susceptibility is indicated by the limited deformation at the time of creep rupture failure. Such materials will exhibit a reduction of area (RoA) value of <70% for specimens which are creep tested at applied stress that is <60% of the yield stress at the maximum design metal temperature (Section II, Part D, Table Y-1).

Lambda Parameter for Creep Intolerant CSEF steels: Creep-intolerant CSEF steels have a high susceptibility to creep cavity formation (intolerant behaviour), minor creep deformation before rupture and high sensitivity to multiaxial stresses (e.g., trending to notch weakening behaviour). 

The Lambda parameter is calculated using the equation below 

The lambda parameter shall be calculated from either creep rupture data of the tested heats of material obtained in accordance with ASME Section II Part D Mandatory Appendix 5 or from published creep test data for the applicable CSEF steel. 

If the value of λ is less than 5, the material shall be classified as creep-intolerant material. 

If creep rupture testing was not able to be conducted to determine the value of lambda or no published creep rupture data exists for the applicable CSEF steel, the material shall be classified as creep intolerant and shall follow the requirements of this code case for elevated temperature design. 

Materials identified as creep intolerant with a component design temperature in the time-dependent regime shall use reduced allowable stress. This reduced allowable stress value is dependent on geometry using a K factor. The allowable stress value provided in ASME B&PV Code Section II, Part D, Table 1A, shall be multiplied by a factor, F, defined by the following Equation.

Author

Jayaram Vattappilly 

Hartford Steam Boiler Inspection and Insurance Company

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