cmi1.whitepaperpdf.asme & en

7/24/2019 Cmi1.Whitepaperpdf.asme & En

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POWERGEN EUROPE 2012 Koln, Germany Page 1 of 18

Comparison Study Between ASME code and

EuroNorm 12952 for HRSG Design

By

Ir. Pascal Fontaine

Tendering Manager

CMI Energy

[email protected]

Av Greiner 1, 4100 Seraing

BELGIUM

Ir. Robert Bonsang

Technical Project Manager

CMI Energy

[email protected]

Av Greiner 1, 4100 Seraing

BELGIUM

Table of Contents

1. Introduction .....................................................................................................................2

2. Heat exchanger tubes .......................................................................................................3

3. Piping and headers design ...............................................................................................3

4. Piping weights .................................................................................................................4

5. Calculated thickness of ASME versus EN .......................................................................6

6.

Piping routing and stress analysis ....................................................................................9

7. Pipe fittings ................................................................................................................... 10

8.

Drum design .................................................................................................................. 11

9. Cycling fatigue analysis ................................................................................................. 11

10. Valves design............................................................................................................. 13

11. Safety devices ............................................................................................................ 15

12. Pump design .............................................................................................................. 16

13. Quality control of welds ............................................................................................. 16

14. Conclusions ............................................................................................................... 18

15. References ................................................................................................................. 18


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1. Introduction

In Europe, boilers that are part of Combined Cycle Power Plants, have to be designed

according to an harmonized code as per the mandatory Pressure Equipment Directive

97/23/EC (PED). National design codes are progressively replaced by the EuroNorm 12952

as it is used in Europe and even abroad, even though the ASME code is still prevailing. These

various design codes must be selected according to PED. Recently, CMI has completed a

Heat Recovery Steam Generator for a 400 MW standard block CCPP in France, for which

ASME code was used for the HRSG design. For that project, CMI has conducted an internal

review based on the EuroNorm instead of the ASME code. The purpose of this exercise was

to study and compare advantages and design features of ASME code versus EuroNorm. This

report presents the conclusions of this review, which will be of interest to the European power

market because EuroNorm is more and more prescribed by specification for CCPP.

Figure 1 Standard horizontal HRSG, 3 pressure levels and reheater

HRSG behind the Siemens SPG 4000F gas turbine.

.


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2.

Heat exchanger tubes

Obviously, thermodynamic rules apply the same way, regardless of the design code applied.

In other words, heating surfaces and the number of tubes required, remain the same should the

boiler be calculated based on ASME code or Euro Norm. The original boiler design totalling

9000 tubes of 22 meters long, for a total heating surfaces of 364267 m.

For this particular project, the original ASME tube selection was as follows: welded tubes for

carbon steel and seamless tubes for alloyed steel. Following our market analysis, it appeared

that welded tubes are not commonly used in case of Euro Norm. Therefore, we selected

seamless tubes throughout for this EN design, as shown in Table 1 below:

Exchanger ASME material

(welded and seamless)

EN material

(all seamless)LP,IP economizer and

evaporator

SA178A EN 10216-2/P235GH

HP economizer and

evaporator

SA178C EN 10216-3/P355NH

LP evaporator SA178A

SA213 T11(*)

EN 10216-2/P235GH

EN 10216-2/13CrMo4-5

LP , IP superheater SA178A EN 10216-2/P235GH

Reheater and HP

Superheater

SA213T22(*)

SA213T91(*)

EN 10216-2/10CrMo9-10

EN 10216-2/ X10CrMoVNb9-1

(*) seamless

Table 1 Finned tubes material used for this particular plant

Following materials selection as per Table 1, and considering a life time of 200000 h for EN,

tubes thickness of finned tubes are the same between codes on LP and IP circuits. However,

the HP circuit is lighter in EN. The weight reduction is as follows: 10.3 t on carbon steel, 5.7 t

on 10CrMo9-10, and 3.2 t on X10CrMoVNb9-1 tubes.

3.

Piping and header design

EN10253-2 defines eight series of normalized pipe thicknesses from Series 1 to Series 8.

Similarly, ASME B16.10 defines standard piping schedules, from SCH 20 to SCH 160. Table

2 compares thicknesses of ASME schedules and EN series, and shows that available thickness

is similar in both design codes. For standardisation purpose, CMI uses in-house piping

specifications, which comprise series of maximum design pressure and temperature. Such

specifications define material selection, pipe normalized thickness, fittings rating of valves


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and flanges according to diameters. As each main line has its defined piping specifications,

the 3D model used for the isometric generation picks up the correct rating and thickness of all

accessories located in this pipe or connected pipes subject to the same design conditions. CMI

has developed such piping specifications adapted for either ASME or EN codes (Table 2).

Series 4 to 8 are comparable to SCH 40 to SCH XXS. These are most usual for piping design.

mmTHICKNESS

Series 2 Series 3 Sch.40 Series 4 Sch.80 Series 5 Sch.120 Series 6 Sch.160 Series7 Sch.XXS Series 821,3

26,7(26,9)

33,4(33,7)

42,2(42,4)

48,3

60,3

73(76,1)

88,9

114,3

141,3(139,7)

168,3

2

2,3

2,6

2,6

2,6

2,9

2,9

3,2

3,6

4

4,5

2,6

2,6

3,2

3,6

3,6

3,6

3,6

4,0

4,5

5,0

5,6

2,77

2,87

3,38

3,56

3,68

3,91

5,16

5,49

6,02

6,55

7,11

3,2

3,2

4

4

4

4

4,5

5,6

6,3

6,3

7,1

3,73

3,91

4,55

4,85

5,08

5,54

7,01

7,62

8,56

9,53

10,97

4,0

4,0

4,5

5,0

5,0

5,6

7,1

8,0

8,8

10,0

11,0

11,13

12,7

14,27

4,5

5,6

6,3

6,3

7,1

8

8,8

11

12,5

14,2

4,78

5,56

6,35

6,35

7,14

8,74

9,53

11,13

13,49

15,88

18,26

5,0

5,6

6,3

8,0

8,0

8,8

10,0

11,0

14,2

16,0

17,5

7,47

7,82

9,09

9,70

10,15

11,07

14,02

15,24

17,12

19,05

21,95

7,1

8,0

8,8

10,0

10,0

11,0

14,2

16,0

17,5

20,0

22,2

Sch.20 Sch.ST Series2 Sch.30 Series 3 Sch.40 Series 4 Sch.60 Series 5 Sch.80 Series 6 Sch.120 Series 7 Sch.160 Series 8219,1273

323,8(323,9)

355,6

406,4

457

508

559

610

660

711

762

6,35

6,35

6,35

7,92

7,92

7,92

9,53

9,53

9,53

12,70

12,70

12,70

9,53

9,53

9,53

9,53

9,53

9,53

9,53

6,3

6,3

7,1

8,0

8,8

10,0

10,0

10,0

10,0

10,0

10,0

10,0

7,04

7,80

8,38

9,53

11,13

12,70

12,70

14,27n.a.

7,1

8,8

8,8

10,0

10,0

11,0

11,0

n.a.

12,5

12,5

12,5

12,5

8,18

9,27

10,31

11,13

12,70

14,27

15,09

n.a.

17,48

8,0

10,0

10,0

12,5

12,5

12,5

12,5

12,5

17,5

17,5

25,0

25,0

10,31

12,70

14,27

15,09

16,66

19,05

20,62

22,23

24,61

12,5

12,5

12,5

16,0

17,5

17,5

17,5

20,0

25,0

12,7

15,09

17,48

19,05

21,44

23,83

26,19

28,58

30,96

16,0

16,0

17,5

20,0

22,2

22,2

25,0

28,0

30,0

18,26

21,44

25,40

27,79

30,96

34,93

38,10

41,28

46,02

17,5

22,2

25,0

28,0

30,0

32,0

36,0

n.a.

45,0

23,01

28,58

33,32

35,71

40,49

45,24

50,01

53,98

59,54

22,2

30,0

32,0

36,0

40,0

45,0

50,0

n.a.

60,0

Table 2 Comparison between ASME schedules and EN Series

4.

Piping weights

Weights of large bore piping have been compared between ASME and EN codes. For our

specific case study; total piping weight was 165 t in case of EN, against 157,9 t for ASME.

Looking beyond our specific case study, we have tried to answer whether such 4,9%

difference could be systematic for all projects between ASME and EN code. Going into

details of each pipe section, the answer to this question is clearly negative, as explained on

Figure 3. Please read this graph as follows: each large bore piping is represented by a

rectangle, which height is the weight ratio EN versus ASME. As such, area of each rectangle

is proportional to its corresponding weight. This graph clearly shows that most of the piping

have the same weight between ASME and EN codes (ratio is between 0,95 to 1,05). However,

a limited number of pipes accounts for most differences. Refer to table 3, pipes A, B, C, are

much lighter in EN, while on the other side, X,Y, Z pipes are much heavier in EN. One can

see that Z pipe (RHT outlet pipe) has a weight difference larger than the total difference.


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Figure 3 Weight ratios between ASME and EN codes

Ref Designation O.D

mmThickness

ASMEmm

Thickness

ENmm

Material

ASME

Material

EN

Weight

difference

(EN-

ASME)

A RHT inlet manifold 508 15.09 12.5 SA335P11 13CrMo4-5 -0.4 t

B Cold reheat pipe 508 15.09 12.5 SA335P11 13CrMo4-5 - 1.3 t

C IP vapo downcomer 273 15.09 12.5 SA106B P265GH - 1.3 t

X LP steam pipe 457 9.53 12.5 SA106B P265GH + 2.5 t

Y RHT21 outlet 610 30.96 45 SA335P22 10CrMo9-10 + 2.1 t

Z RHT21 outlet pipe 610 30.96 45 SA335P22 10CrMo9-10 + 8.9 t

Table 3 Pipes featuring the most important weight differences

Table 4 gives weight differences of these 6 pipes without normalized thickness. Difference

becomes less than 0,5%, which proves that piping weight is mostly related to Series applied.

Ref Designation O.D. Calculated

thickness

ASME

Calculated

thickness

EN

Weight difference

(EN ASME)

A RHT inlet manifold 508 10.05 8.69 -0.22 t

B Cold reheat pipe 508 10.05 8.69 -0.69 t

C IP vapo downcomer 273 4.70 4.59 -0.06 t

X LP steam pipe 457 2.12 2.24 +0.09 t

Y RHT21 outlet manifold 610 26.35 28.91 + 0.45 t

Z RHT21 outlet pipe 610 26.35 28.91 + 1.18 t

Table 4 Weight differences with calculated thicknesses


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5.

Calculated thickness of ASME versus EN

Let us compare the EN design stress versus the Maximum Allowable Stress of ASME. Except

for carbon steel between 240 and 400C, and in high temperature ranges, the Maximum

Allowable Stress (M.A.S.) according to ASME is usually lower than the design stress

according to EN. The formulas used to calculate the thickness do not really compensate this

difference:

EN Formula ASME Formula

Thickness = (P * D ) / (2* f + P ) Thickness = (P * D ) / (2* MAS + 2 * Y * P )

P is the calculation pressure

D is the outside diameter

f is the design stress

P is the design pressure

MAS is the Maximum Allowable Stress

Y is a coefficient varying from 0.4 to 0.7 according to

temperature (0.4 for t


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Figure 5

Figure 6


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Figure 7

Globally, calculated thicknesses can be summarized as indicated in Table 5 here below:

Material Useful zone Pipes thinner

with EN

Pipes thinner

with ASME

SA106B or

EN10216-2/P265GH

100C/400C 100C/240C 240C/400C

SA335P11 or

EN10216-2/13CrMo4-5

300C/525C 300C/525C n.a.

SA335P22 or

EN10216-2/10CrMo9-10

400C/570C 400C/540C 540C/570C

SA335P91 or

EN10216-2/X10CrMoVNb91

500C/600C 500C/600C n.a.

Table 5 Comparison of calculated pipe thicknesses

Considering that temperatures of pipes connected to LP, IP and HP drums are 188C, 250C,

and 342C respectively, and that those of the steam lines are 260C,580C, and 582C, we

can see that pipes are sometimes heavier in EN, and sometimes heavier in ASME code.

For HP superheaters and reheater made of P91 material, we note that thicknesses are lower in

EN compared to ASME Code, which is beneficial for cycling stress and material fatigue.


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6. Piping routing and stress analysis

The pipe stress analysis, according to EN 13480-3, considers two sustained cases, including

either the normal snow, or the normal wind. While, ASME code considers one sustained case

with neither snow nor wind. Moreover, EN considers separately exceptional wind, and

exceptional snow, defined as 1.75 times the normal wind and the normal snow. This can result

in additional or sturdier guiding supports, or possibly additional hangers.

The design code does not much influence pipe routings, except possibly for piping around

control valves. Control valve suppliers, which are usually working according to ASME code,

proposes valves which are straight in-line type. Other suppliers, which are usually working

according to DIN standards, usually offer control valves based on angle or Z types, even ifthey can propose sometimes straight in-line type also. This can lead to very different pipe

routings, as shown on Figure 8. That has little impact on the pipe length and weight, but this

can hold the designer at the early project phase, as the valve supplier is not selected yet, and

no detailed drawings are available.

Fig. 8 Comparison of pipe routing including control valves

HP feedwater control valve relocated in pipe routing

from in-line (ASME), to angle (EN) configuration


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7.

Pipe fittings

Regarding pipe fittings, there is a big difference between ASME and EN codes because:

- EN 12952-3 does not allow socket welding above 20 bar G or 350C. Even if socket

could still be use on the LP circuit, CMI decided to apply full penetration weld for all

welds.

-

ASME code proposes standardized fittings (such as sockolets and weldolets),

which do not exist in EN code. Reinforced nozzles, used for lateral connections, such

as vents, drains or instruments, must be designed specifically.

CMI has developed a catalog of EN fittings, based on butt weld ends (Figure 9), to be

manufactured according to specific design conditions and drawings. This applied solution is

more expensive than ASME standard fittings out of the shelf, but this is necessary for piping

design according to EN code. Otherwise reinforcement pads must be used locally.

Fig. 9 Example of fitting to be manufactured

For low pressure carbon steel piping, we have identified that some fittings can be up to 10

times more expensive to source in EN. However, for high pressure alloyed steel piping like

P91, prices of fitting are comparable.


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8. Drum design

For this particular project, the HP drum was manufactured in SA302 GrB material, resulting

in a calculated drum thickness of 114 mm. For the EN version of this boiler, CMI selected the

material EN 10028-2/15NiCuMoNb5-6-4. Currently, this material has no equivalent in

ASMEI. Although more expensive, it is more resistant. The calculated thickness was only 75

mm in EN, instead of 114 mm in ASME. It is to be noted that this material is well known by

CMI, as its DIN equivalent has been used for numerous HP drums fabricated in CMI

workshops. Also, this EN 10028-2/15NiCuMoNb5-6-4 requires special welding procedure.

Regarding IP and LP drums, plate thicknesses were the same, as material selected was EN

10028-2/P295GH equivalent to SA516 Gr70. Also, let us note that EN 12952-7 requests that

the internal side of the shell must remain visible for inspection. Consequently, the standard

design of drum internals was adapted.

9. Cycling fatigue analysis

ASME I considers continuous operation at design conditions, but it does not mandate

assessment for fatigue analysis. Even though the boiler is designed according to ASME code,

CMI typically uses EN code to make its cycling fatigue analysis. The allowable number of

cycle is calculated as per the Euro Norm EN 12952-3 (Fig. 10). Practically, cold, warm and

hot cycle stress numbers are considered as per plant cyclic service informed by specification.

Other stress cycles can also be accounted, such as partial cycle when the first unit is started on

a 2-2-1 arrangement, or even Low Cycle Fatigue (LCF) due to attemperation in operation.

Then, the Palmgren-Miner Linear cumulative fatigue damage theory (also known as The

Miners Rule) is used to account each of those fractions of cumulative fatigue damage.

Application of the norm shows that a cold start is up to 20 times more damaging than a warm

start, and that the stress range resulting from a hot start is typically below the fatigue limit and

not contributing to the total fatigue damage (except for the damaging quenching issue). The

fatigue damage is very sensitive to stress range because of its logarithmic nature (see the

double logarithmic scale of Fig. 11). A small variation in stress amplitude largely impacts the

corresponding number of cycles. Fatigue calculation does not precisely establish the line

between a crack and a no-crack initiation, but it is rather a statistical probability of crack

occurrence under a certain number of cycles Na, with the corresponding stress amplitude fa,

representing a percentage of risk of failure.


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The sensitivity and the probabilistic nature of fatigue results in an uncertainty in fatigue

lifetime analysis. Some uncertainties come from simplified the Stress Induced Factors.

Compared to the former TRD301 German code, from which this EN part is derived largely,

finite element analysis can be used to determine SIF. Euro Norm appears to be less

conservative than its former TRD 301.

Fig.10 Whler curve from EN 12952-3 showing material fatigue for symmetric stress range

(amplitudefa) versus allowable number of cycles Na for various material tensile strengthRm.

Fig. 11 Application of EN 12952-3 for determination of acceptable HP drum gradients.


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EN fatigue analysis is applied to thick HP drum walls, to outlet headers of reheater and HP

superheater. An interesting point is the ramp rate variation versus operating pressure (Fig.

11): as pressure increases, the allowable pressure ramp rate also increases. As noted, the

allowable gradient can be optimized as pressure is building up. These calculated

temperature/time gradients are converted into pressure/time gradients as these are more

accessible and controllable parameters during transients. This feature is used to optimize the

start-up by applying progressive pressure ramp rates (Fig. 12), which allows optimisation of

the overall boiler start-up time, without consuming any extra lifetime of the boiler. Such

progressive pressure gradients are implemented into the plant DCS as set points applied on

the HP steam turbine by-pass valve.

10.

Valve design

Regarding the on-off valves, EN12516 proposes two methods for valve selection:

- a tabulation method similar to ASME B16.34 from the ASME code, or

- a calculation method similar to DIN 3840, for which the designer must calculate the

wall thickness.

Fig. 12 Optimized pressure gradients set points resulting from application of EN 12952-3


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Table 6 shows a typical table which can be found in the tabulation method. Ratings are noted

as Bxx (similar to PNxx in DIN), or as CLxxx (similar to ANSI). Ratings can be standard or

special ratings, and materials are presented by groups (Table 6 features group 1C1).

Some groups are specially devoted to ASTM materials, and some other groups are devoted to

EN materials. As the EN12516-1 gives wall thicknesses close to those of ASME B16.34, it is

easy for valves suppliers to switch from ASME to EN codes.

Table 6 Typical table from EN12516-1

EN give much more flexibility in valve selection than ASME code does. As an example, there

are up to four different body lengths for each valve. In practice, the valve manufacturer selects

one of the allowed lengths, as available. As the valve suppliers will be selected in due course

of project execution, isometric pipe drawings are initially drawn with the maximum valve

lengths, in order to avoid interferences. Once valves are ordered, isometric drawings are

updated, and the requested pipe material is checked to avoid any shortage for pipe spools.


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11.Safety devices

While ASME code relies only on spring loaded safety valves as safety devices on each

circuits, EN propose several safety systems for pressure relief, mostly grouped as follows:

- based on safety valves, which can either be spring loaded (similar to ASME), or

assisted, or supplementary loaded , or pilot operated.

- without safety valves, using a Controlled Safety Pressure Relief System (CSPRS).

Safety devices are categorized as follows in EN:

Spring loaded safety valves (type similar to ASME code).

Assisted safety valves can be lifted by means of a powered assistance mechanism,

but it can work also properly without such assistance. The purpose of this mechanismis to limit overpressures, and allow to slightly increase the closing time in case of risk

of valve chattering.

Supplementary loaded safety valves have an additional force increasing the sealing

force. This is designed in a way that if it is not released, the safety valve will reach the

certified capacity at a pressure not greater than 1.1 times of the maximum allowable

pressure of the boiler.

Pilot operated safety valves are operated by the fluid discharged from a pilot valve

which is itself a direct loaded safety valve.

CSPRS is a safety system consisting in an assisted valve combined with control units

(Figure 13).

When using one of the three first safety valve types, at least 75% of the required discharge

capacity must be on drum and the rest at superheater outlet; when using the pilot operated

safety valves or CSPRS, at least 25% of this capacity must be on the drum, or even 0% if at

least one pressure signal is transmitted to the control unit from the drum. This means that the

HP safety relief valves can be replaced by a secured steam by-pass between HP circuit and

cold reheat line. This design is quite common with the DIN design, and expected by German

plant operators. It has the advantage of removing safety valves on the HP drum and HP

superheater, but needs a secured bypass, with a local sophisticated hydraulic enclosure. As the

assumption of failure in the supply of desuperheating water must be considered, the design of

the cold reheat line must include the possibility of temporary high temperature.


16/18


Fig. 13 Safety devices as per Euro Norms

12.

Pump design

Pumps are designed according to EN 5199, and the design code used for the HRSG has no

influence on pump design. Concerning the flow and the head of the feedwater pumps, the EN

12952-7 mentions two points of design, similar to those of German TRD. But, it also states

that these margins do not need to be met if two water level limiters cut off the heating, should

water level falls below the lowest permissible drum water level. HRSGs are always designed

with these monitoring and conditions either based ASME or EN codes. However, EN code

requires independent level switches on drums, in additional to ASME requirements. Apart

from these mentioned adaptations above, PIDs are not impacted otherwise by codes.

13.

Quality control of welds

On this particular project case study, the horizontal HRSG heat exchangers comprise 18000

welded tube-to-header connections, 670 circumferential tube-to-tube welds, and 1000 welds

on piping. Although it is not the purpose of this paper to review all differences in quality

controls between ASME and EN, we have tried to highlight the most of them in table below.

Although EN is usually slightly more stringent than ASME, it is to be noted that CMI has

already requested additional controls beyond ASME code, similar to EN requirements.


17/18


Type of connection EN requirements ASME requirements

Heat exchangers:

Tube to header( tube thk


18/18

POWERGEN EUROPE 2012 K l G P 18 f 18

14.Conclusions

Following this detailed comparison study on a recent HRSG design, we can conclude that the

most significant differences between EN and ASME design codes are as follows:

1. Both EN and ASME codes resulted in similar pressure parts and piping weights.

2. Compared to ASME code, EN allows to calculate pressure parts in more details. EN

provide therefore further possibilities for boiler optimisation. In particular, finite

element analysis is more broadly used on EN.

3.

Compared to ASME, the HP drum thickness is much lower on EN. As such, pressure

gradients and cycling capabilities of EN boilers are enhanced compared to ASME.

4. Euro Norm shows that allowable pressure gradient varies with pressure. Therefore,

boiler start-up time can be optimized without consuming fatigue lifetime by using

progressive pressure gradients. This interesting feature, derived directly from a fatigueanalysis, is used by CMI on a standard basis even for ASME boiler

5. Unlike ASME code, EN allows to use by-pass systems as safety devices, giving more

plant operation flexibility, while saving safety valves and associated costs.

6.

Unlike ASME, EN fittings must be designed and manufactured for specific pipe cases.

7. Valves sourcing for EN code is more difficult than ASME, because numerous valve

suppliers are not yet ready to supply valves according to EN. Consequently, EN valves

are more expensive than ASME valves.

Compared to ASME code, and based on the same boiler performances, the EN design iscurrently about 3% more expensive, and the project planning is extended by about 2 months

longer. However, this difference tends to reduce. EN is a more sophisticated code, which

allows further design optimisation, and providing greater flexibility in plant operation.

As HRSG designer, CMI can design and supply equally according to either ASME or EN.

15.

References

[1] Euro Norm NBN EN 12952, February 2002 Water-tube boilers and auxiliary

installations, design and calculation for pressure parts[2] ASME Boiler and Pressure Vessel Code Section I and Section VIII, Rules for

construction of Pressure Vessels, ASME, New York , 2001 Edition , 2002 Addenda

[3] TRD 301 Code, April 1979, Zylinderschalen unter innerem Uberdruck

[4] HRSG optimization for cycling duty based on Euro Norm EN 12952-3, Power Gen

2007, Jean-Franois Galopin and Pascal Fontaine

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