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Creep of high temperature steam piping: EDF experience with fossil-fired power plants from 1955 to 1987
Nuclear Engineering and Design 116 (1989) 389-398 North-Holland, Amsterdam
389
CREEP OF H I G H T E M P E R A T U R E S T E A M PIPING: E D F E X P E R I E N C E W I T H F O S S I L - F I R E D P O W E R P L A N T S F R O M 1955 T O 1987 G. T H O R A V A L Maintenance, Department, Electricit~ de France, Nuclear and Fossil Generation Division, Quartier Michelet, 13-27 Esplanade Charles de Gaulle, F-92060 Paris La Defense - C E D E X 57 - France
Received March 1989
Since 1955, EDF has maintained a constant policy of the systematic inspection of its high temperature piping. As these pipes are made of the same material, the measures taken in many plants are comparable. EDF can now, after 30 years of operation, analyse the availability of its means of measurement, the choice of the materials and of the calculation rules, the accuracy of the measurements, and finally the good behaviour of most of our pipes. Some particular cases of abnormal creep rate have been the subject of a particular study.
Introduction When Electricit6 de France (EDF) was created, in 1946, it took over the operation of old plants, designed before the Second World War, with low steam characteristics, therefore no creep problems arose. The first plants with steam temperatures above 500°C were of American design, built under the Marshall plan. Since the fifties, to meet a very big increase in electricity consumption, E D F has pursued a specific policy of building several series of standardized plants. Thirty eight fossil-fired plants of 125 MW capacity (125 bar, 540°C), 37 plants of 25 MW (163 bar, 565°C), 4 plants of 600 MW (163 bar, 565 o C), and 4 plants of 700 MW burning oil, producing reduced characteristics steam (163 bar, 540°C), were started up successively just before the oil crisis. The last series of standardized fossil-fired plants is comprised of 3 × 600 MW (163 bar, 565°C) once-through plants, the design of which was different from the previous plants, as the contribution of nuclear plants to French electricity generation was increasing, and the possibilities of load following by the nuclear reactors were still not developed. Therefore, the purpose of these last 3 new fossil plants was to provide the capability of rapid load following. Until the eighties, most of the fossil-fired boilers (particularly coal-fired ones) were run at constant maximum power throughout the year, accumulating high temperature hours. The situation has now completely changed over the last five years, with the development of the French nuclear programme, and many obsolete
fossil-fired plants have been definitively stopped (or cocooned) while the most recent ones were used for only a few hours a year. During these 30 years, the invariability of the materials used in the design, and the regularity of inservice follow-up of piping, have enabled us to assemble significant results of creep behaviour on piping in 76 plants, for time durations between 10 000 and 200000 h, mostly at high temperature. Before giving the results of our steam piping behaviour analysis, we will address the choice of material, the design rules, the inservice follow-up means, and the policy applied in cases of abnormal creep rates.
1. Choice of materials Since the beginning of the fifties, close collaboration between E D F and the main French steel companies has been set up, which has resulted in a wide ranging agreement on steel characteristics (scales of chemical composition, heat treatments, room temperature mechanical features...), and on drawing up rules for creep test procedures. Moreover, systematic long-duration tests were performed in the laboratories of the different companies concerned, the conclusion of which was the approval of 87 types of steel, manufactured by 22 French and foreign steel companies. Since the building of the 125 MW plants series, one particular grade of steel has been systematically used for high temperature steam piping subject to creep:
0 0 2 9 - 5 4 9 3 / 8 9 / $ 0 3 . 5 0 © Elsevier Science P u b l i s h e r s B.V.
G. Thoraval / Creep of high temperaturesteam piping
390
chromium molybdenum steel A F N O R 10 CD 9-10 (2¼% C r - l % Mo), with heat treatment at 925 to 950°C depending on the steel companies, followed by a slow cooling and by a reheating to about 700 ° C. This gave the steel a room tensile strength between 520 and 670 MPa, and a minimum stress producing a creep elongation of 1% in 100000 h of 59 MPa at 550°C. The work performed at that time is the origin of the setting up of French standards A F N O R N F A 36206 [2] and A F N O R N F A 49213 [3], which standardized the properties of creep resistant steels used in boilers, pressure vessels, and piping.
etail a
2. Design rules Detail a
2.1. Calculation of piping thickness
• 25
Specific calculation rules for 125, 250 and 600 MW plants were issued from the CET (Thermal Studies Committee). For calculation of piping thickness, this committee chose a formula derived from the ASME POWER P I P I N G ANSI B 31.1:
e
PDe 2(fz + y P ) + C (see notations in Appendix 1).
The same formula was used in more recent E D F standards: CPC (Common Specifications Book) for 700 MW plants, and CST (Technical Specifications) for new fossil-fired and nuclear plants. An E D F study [4] enabled us to verify that this formula, derived from a limited development of LAME fundamental formulas, was slightly conservative at creep temperatures. The value of f , maximum allowed stress, is determined by application of the rule detailed in Appendix 1. In practice, the result of the calculation leads, in most cases, to using f = 0.6 x average stress to rupture in 100000 h, which leads to a calculated thickness higher than that which would be obtained by f = stress producing a creep elongation of 1% in 100000 h. That could explain the low creep rate results that we will see later on.
2.2. Measurement sections Since 1960, any piping submitted to creep (i.e. whose thickness is due to maximum allowable stress for creep, and which would thus have been thinner by application of f = ~ x yield strength in the standard formulas) has been systematically provided with at least one specially equipped measurement section. This rule was extended
\
%f----.2
/
Material : Stainless steel Values in m m
Fig. 1. Location of the humps.
to boiler collectors in 1966, by a regulation (circulaire du 25/08/66). The measurement sections are at least 3 m from the nearest elbow, and access to them is relatively easy. Each section (fig. 1) is equipped with 8 stainless steel hemispheric humps, welded on 4 diameters at 45 ° intervals. They are covered with a detachable metallic collar, as protection against external impact loads. The performed according to a specific operating method, including preheating to 350°C.
2.3. Design of supports E D F rules on spacing and the design of supports, laid down to ensure correct natural frequencies and low
G. Thoraoal / Creep of high temperature steam piping
391
Table 1 Example of results (10600 j-old pipe, superheated steam) Diameter number
1
2
3
4
Room temperature (° C) Pipe temperature (* C) Diameter read (nun) Maximum variations from average (mm) Temperature corrected length (ram) Initial measurement (ram) Variation (ram) Relative variation (10 -4 )
9 20 287.105 + 0.006 - 0.004
9 20 285.780 + 0.003 - 0.002
9 20 286.773 + 0.004 - 0.006
9 20 287.01 + 0.003 - 0.002
287.066 286.699 + 0.367 13.74
285.742 285.514 + 0.228 8.53
286.734 286.470 + 0.264 9.88
286.971 286.738 + 0.233 8.72
sag, lead to a m a x i m u m b e n d i n g stress due to the weight below 10 M P a . Consequently, the p i p i n g subm i t t e d to creep does n o t require a n y particular rule for s u p p o r t design.
Average
286.628 286.355 -0.273 10.22
old pipes, or w h e n m e a s u r e h u m p s have b e e n damaged, or w h e n a precise area is suspected of a b n o r m a l creep, it is possible to p e r f o r m m e a s u r e m e n t s of diameters directly o n the pipe. This is precise e n o u g h for forgeda n d - b o r e d pipes w h o s e f a b r i c a t i o n tolerances n e v e r exceed 1 ram: this is the case of s u p e r h e a t e d steam piping. It could b e j u s t a n i n d i c a t i o n for fusion welded pipes (reheated steam).
3. Inservice follow-up m e a n s 3.1. Measurement locations
3.2. Operating rules for measurements
Each time there is a special m e a s u r i n g section, the m e a s u r e m e n t s are p e r f o r m e d there. However, for very
T h e m e a s u r e m e n t s are t a k e n with a m i c r o m e t e r g r a d u a t e d e a c h "r~ ram. T h e m i c r o m e t e r is adjusted o n
Table 2 Determination of ' a ' and ' n ' parameters. Plant
125 MW
Pipes
Superheat
Reheat
250 MW Superheat
Reheat
600 MW Superheat
Reheat
700 M Superheat
Reheat
Number of pipes Steamtemperature ( o C) Steampressure (bar) Outside diameter (nun) Inside diameter (nun) ' a ' average (ram) Dispersion of ' a ' value (ram) ' n ' average Dispersion of ' n ' value
49
48
33
37
7
7
8
8
540
542
567
567
567/542
567/542
542
541
127
30
167
35
167
36/37
166
35
267
419
508
702
674/618
1044/1000
618
770
!87
379
330
630
450/440
930
440
700
0.118 0.05 to 0.40 0.58 0.35 to 0.77
0.108 0.05 to 0.55 0.53 0.15 to 0.75
0.13 0.05 to 0.27 0.45 0.32 to 0.66
0.134 0.05 to 0.30 0.41 0.2 to 0.8
0.243 0.12 to 0.45 0.52 0.26 to 0.67
0.142 0.10 to 0.30 0.338 0.20 to 0.48
0.15 a
0.15 a
0.5 a
0.5 a
-
-
a) The 700 MW plants have no more than 20000 h operation, so the results are not yet available. The values given here are the average creep elongations which will be reached if ' n ' has an average value of 0.5.
392
G. Thoraval / Creep of high temperature steam piping
the spot. Room and piping temperature are noted. Whenever possible, the measurements are taken when pipes have been cooled. Otherwise, a temperature adjustment must be calculated. In order to avoid systematic errors, 2 operators perform 3 measurements each. The noted length is the arithmetic average, and the variabions are also noted. If they are excessive, the measurements are taken again. The final section diamter is the average of the four measured diameters. Examples of results are given tables 1 and 2. 3.3. Accuracy of the measurements - Micrometer accuracy: very good, better than 1 # m with the sliding gauge and adjustment on the spot. - Operator: the accuracy is estimated at 10 #m, adding reading and position erors; a statistical study of the measurement variations (variation around the average) revealed variations always less than 40/~m, and less than 20 # m with a probability of 0.82. - Temperature adjustment: variations can occur due to lack of homogeneity of the temperatures of the material when the pipe is not completely cooled. For a variation of 5 ° C in half of the diameters, the measurement variation is 15 #m. Geometric modifications: the stainless steel humps may have been hit or the welding may have failed. Generally, they cause such a step in the results graph that they are immediately detected. Direct evaluation of accuracy: by reading the results of some measurements of pipes creeping slowly and regularly, for long periods, we can point out abnormal results, with negative or zero evolution between two successive measurements, followed by new measurements going back to the initial curve. When no significant variation of operation occurred during this period, we can evaluate in this way a maximum measurement error, of about 0.1 mm (100 #). This accuracy of about 0.1 mm is considered to be acceptable: the target creep rate is about 1%, i.e., for pipes of 500 mm in diameter, there is a diameter variation of 5 mm.
4.
Inservice
follow-up
policy
Creep is an evolution of material submitted to stress and high temperature combined, which is characterised by an evolution of material structure, and by a permanent deformation rate. Its typical evolution can be divided into 3 stages:
- a first stage creep, characterized by a high rate that is progressively decreasing, - a second stage creep, characterized by a constant low rate, - a third stage creep, corresponding to an increasing creep rate ending in rupture. - The purpose of the inservice follow-up is to be sure that the pipe evolution always remains in the second stage creep. The frequency of inspections is: - For pipes equipped with measurement sections, one inspection every two years and then, if the creep rate is normal after 35000 h, the interval between two consecutive measurements is increased to five years. Besides, we must consider that some pipes submitted to creep may not have been identified as such by the designer, or that some pipes, designed for short periods of high temperature, have in practice been heated for a long time (by-pass of heaters for instance). Consequently, the diameter of these pipes is checked every 10 h. The pipes concerned are the following: - Carbon steel pipes, the temperature of which exceeds 400° C, - 0.5-1.25% Cr-0.5 Mo steel pipes, the temperature of which exceeds 500 o C, 2.25% C r - l % Mo steel pipes, the temperature of which exceeds 525 ° C. The main steam pipes of power plants are submitted, by the nature of the process, either to a constant temperature, or to temperatures with limited and known values. In these conditions, the diameter increase measurement is a good indication of the condition of the pipe, and enables us to evaluate, by extrapolation, the deformation rate after 100000 h. Any measurement of pipe lengthening would be useless, as the longitudinal pressure stress is half of the circumferential one, which leads to a creep rate divided by about 26. As long as the creep rate does not exceed 1% for 100000 h, we take no action. If the rate exceeds 1% before the end of the component life, a close study is performed with, if possible, estimation of a usage factor (function of temperature and pressure history), sample tests of accelerated creep (at higher temperatures or stresses), microstructure analysis of material and economical approach. Then we can decide whether to remove the component, sooner or later, or to modify the operating conditions. In practice, the usage factor, which is defined at any time by the ratio of the creep rate already established divided by the creep rate allowed, is often difficult to calculate. The typical sample test we can use is the
G. Thoraval / Creep of high temperature steam piping
.11C
"l'mle scale
' a ' is the diameter elongation in 100000 h, measured or estimated ' n ' is an exponent in the formula: d = at" where d is the noted deformation (%), and t the time (hours × 10-5). This formula is a mathematical definition of the curve of creep, third stage excluded.
/t,cl"al
ea~
725
50
25.5
712
100
39
691
300
187
669
1000
526
650
3000
1336
393
6. Comments
thermal one, with a wide range of temperatures (cf, fig. 2): indeed, the mechanical test, with various stresses, and constant temperature is not perfectly linear for damaged steels, although the constant stress one remains linear. Another advantage of thermal accelerated creep tests is that they give a curve (fig. 2) with temperature as a parameter, so it is easy to estimate, if necessary, the modification of steam temperatures suitable for the state of the material. This is in order to work out the economic choices: namely, lowering the steam temperatures or removing the pipe. In fact, we use both mechanical and thermal tests, the first one for new pipes, to avoid distorted results due to inetallurgical modifications of the material under very high temperatures, the second one for damaged pipes, to avoid distorted results due to the evolution of micro-eracks under very high stresses.
- Creep rate in 100000 h varies from 0.05 to 0.55%, which is quite satisfactory. This result is logical as the design criteria is 0.6 × stress to rupture in 100000 h in nearly every case. Generally speaking for these particular pipes, the ratio between 0.6 × stress to rupture in 100000 h and stress to 1% elongation in 100000 h is about 0.9, and the application of classical laboratory results allows us to estimate an elongation in 100000 h of: 1% x (0.9) 6 = 0.5%. - The disparity in the results has several causes: - Manufacturing: varied ratios of elements included in the composition of the steel, within the allowed range; variations of heat treatments, performed in the piping factory or on the power plant building site. - Geometry: the minimum thicknesses are checked, following allowed tolerances of 12.5% for fusion welded pipes, and of about 1 ram for forged-andbored ones. However, the real thickness is often much higher, particularly for fusion welded pipes for which the chosen plate was the nearest standard one, which could be thicker by 10 to 20% than the calculated minimum value. Let us note that a 12% thicker pipe, made of 10 CD9.10 steel, reaches 1% elongation after about 1 × 126:2 times later than the reference one. Operation: the temperature has sometimes been lowered on purpose, in order to protect the turbine. When it has, it corresponds to a clear change of slope of the creep rate curve. - The average value of the parameter n is about 0.5. However, as many results concern pipes whose creep rate is very low, a value of about 0.7 or 0.8 would be more probable (fig. 1), if we had to estimate the real creep rate of a rapidly creeping pipe without any initial diameter value.
5. Reading the results (Appendices 4 to 10)
6.1. Ovalisations by creep effect
In order to give a complete description of the creep rate, we have determined for each pipe two parameters: a and n (table 2).
Many reheated steam pipes, which are fusion welded, show a clear distorsion of reference dimensions which decreases during the first few thousand operating hours:
10
10 a.
10 ~
10 500
550
600
650
700
750
000
TEMPERATURE ( degnm C )
Fig. 2. Creep accelerated thermal test; constant stress = 41.8 MPa. Superheated steam pipe sample; conclusion: for T = 600°C, residual life = 22000 hours.
-
G. Thoraval / Creep of high temperature steam piping
394
o
2O
x
+
X
+ O •
+
0
x
[] (9 "/rk,
0.5
~0
°
0.1
02 0
I
AVERAGE DIAMETER ELONGATION
i 2
3
J
1
;
I
L
I
I
;
=
~
r
4
5
6
7
8
9
10
11
12
13
14
Fig. 3. Correlation ovalisation/creep elongation. t~(t) ' w i t h o = ¼ i-4~ I D , - D m l 0~* ~
W (0)
i = 1
OPERATIC~N HOURS x 10,000
Fig. 4. Champagne Plant 1. Creep measurement campaign no 13. Superheated steam pipe.
Dm
Di = measured diameter, Dr. = average valve.
elongation causes excessive pipe bending, it is spectacular enough to be noticed.
7.2. Second case (fig. 1 O) when the second measurements are taken, the larger diameters are found to be smaller than at the first readings. A good correlation has been established between the average pipe elongation and its ovalisation rate (fig. 3). Let us note that this phenomenon could be worrying if its rate was excessive: local elongation, particularly if the largest diameter corresponded to the longitudinal weld, could be high in the root of the weld, while the total measured elongation could be quite low.
7. Noteworthy
cases
Three noteworthy cases deserve a particular study. The first one is interesting because of the lessons we can draw from it. The other cases are the only two pipes which have presented a really high creep rate in our plants, which means that, for an average elongation value of 0.14% in 100000 h for other pipes, these had values of about 2%.
The follow-up of the increase of the diameter of a pipe showed first an abnormally high slope of the curve, and then reached 1% after 60000 h. As this plant had run partially at low power, the conventional limit of 1% in 100 000 h was greatly exceeded. When the creep rate value was about 1.4%, several actions were undertaken: - Research into manufacturing records. - Enforcement of local creep follow-up. - Extension of diameter measurements all along the pipe, by direct measurements on the machined pipe wall. 15 o
,,=,
5
7.1. First case (figs. 4-9) During a periodic inspection of supports, an abnormal bend of a pipe was detected. A hanger was broken, and the sag was quite visible. The complete calculation showed a local elongation of less than 0.2%. The lesson we learnt from it was that, before creep
o
f 1
I
I
I
I
A
I
J
=
2
3
4
5
6
7
8
9
,,
I
I
=
I
10
11
12
13
OPERATION HOURS x 10.000
Fig. 5. Albi Plant. Creep measurement campaign. Reheated pipe.
G. Thoraoal / Creep of high temperature steam piping Series (MW~
Number of plants
125
4 4 17
Steam
Superheated
3 Reheated
125
18 6
250
32 1 1
Superheated
250
3~1
Reheated
6O0
4 3
Superheated
6O0
4 3
Reheated
700
4
Superheated
700
4
Reheated
Steam tempecature-wessum (*C) (MPa) 545 540 540 542
- 9.0 - 12,5 - 12.7 - 12.7
TotaJ number
Diameter (ram)
oqp~x~ 49
3,0 3,0
2 (419/379) 2 (406/370)
48
567 - 16.7 ~ - 14.4 568 - 16.9
5O8/330 495/335 2 (363t237)
35
567 568 -
3.5 4.0
702/630 2 (368/322)
37
567 - 16.7 568 - 16.7
674/450 618/440
7
3.6 3,7
674/450 1000/930
7
542 - 16,7
672/460
4
1087/990
4
567 542 -
542 *
3,4
14 13
409/325 2(243/20~) 2(267/167) 370/274
542 542 -
395
0.10 0.20 0.30 0.40 DIAMETER ELONGATION ( %. ) IN 100,000 HOURS
Reheated steam pipes
2.2.4
TOTAL
Fig. 6. Piping characteristics. - Analysis of micrographic structure. - Cutting of a ring for accelerated creep tests. It appeared that the problem was limited to several
•
_9 7
0
VS-T3
~
r--1
0.10 0.20 0.30 0.40 0.50 DIAMETER ELONGATION ( %. ) IN 100,000 HOURS
0.60
Fig. 8. 125 MW series. Superheated steam pipes. 0.2
0.1
0.05
i 10'
I HOURS
10 Is
Fig. 7. Examples of curves log/log d = F(t).
parts of the pipe, which only just passed the creep test (1000 h) and had the m i n i m u m carbon content. Microanalysis of the structure showed abnormal evolution on these parts, while the other parts were normal. Consequently, we concluded that the abnormal structure was due to the manufacturing or installation processes, and was not an inservice evolution. It was due to bad metallurgical heat treatment producing a material whose room tensile strength was less than 45 MPa. The accelerated creep tests performed by the E D F Laboratory showed a residual life of 60 000 h, which was less than the estimated residual life of the plant. The pipe was removed. Let us note two points: - First, the close inspection of the pipe, performed after its removal, revealed the absence of any cracking in spite of 2% creep elongation. - Secondly, the replacement of the pipe was carried out in two steps: one part first, the second part a year after. The second part, the year after, was found to
G. Thoraval / Creep of high temperature steam piping
396
--7 7
7
1
0.05
0,1
0.15
0.20
0.25
J
0.30
DIAMETER ELONGATION (%) IN 100,000 HOURS
determined. Its value was 0.41, to be compared with the value of a normal pipe usage factor calculated also: 0.018. Two possibilities were then considered: - To continue operation as before, to follow the 'usage factor' and to change the pipe when the usage factor got close to 1. Or to save the pipe by lowering the steam temperature, during all the plant's residual life: 545°C (corresponding to 950 000 h). The first solution was chosen, for two reasons: first the high price of coal and then the estimation that, in the future, this plant would be operated only a few hours each year.
Reheated steam pipes 8.
Conclusion
12
The first conclusion is our satisfaction with chromium molybdenum steel, which remains safe when it gets old: its creep behaviour follows the predicted evolution (which is quite limited), and, for the particular cases of abnormal creep elongation, we have never found any cracks, in the welds or in the pipe steel.
7
3
1
0.10
0.20
0.30
DIAMETER ELONGATION (%) IN 100,000 HOURS
Fig. 9. 250 MW series. Superheated steam pipes. 1.5
be highly elongated near the circular weld, in the section which had been submitted to the heat treatment when the new part had been connected. The evolution of this damaged steel during heat treatment seems to have seriously lowered its creep resistance capacity.
7.3 Third case (fig. 11) The first conclusions of the report described above led to an investigation in several plants to find any similar pipes. In one of the plants, the creep rate of some parts of a pipe was abnormally high. As the accelerated creep test had demonstrated (for the second case) that these pipes had a remaining life of several ten thousand hours, the immediate removal of the third case pipe was not decided. A close study of the history of this pipe was performed, by reading the plant records, and the 'usage factor' of the abnormal pipe was
O.5
1
2
3
4
s
6
7
8
9
OPERATIONHOURSx 10,000
Fig. 10. Particular case No. 2 (St. Ouen). Creep measurements. Superheated pipe (16.7 MPa, 565°C). Initial outside diameter = 508 mm.
G. Thoraval / Creep of high temperature steam piping d%
d°/,
+
2
+
1.9
1.8
1.7 0.20
397
additional elements, some additional special measurement sections should be provided in the pipe in question. Finally, even if the low values of the creep rate measurements carried out over these last 30 years make us think that our calculation rules are too conservative, the two abnormal cases we found are sufficient to convince us that the margins of our standards are necessary.
Acknowledgements
/ 1,6
100
110 0.10
It is acknowledged that many of the points described in this paper are conclusions of an earlier study [1] performed by the author's predecessor at E D F , A. Marchand, before he retired.
Appendix 1
EDF Calculation rules (1982 - CST) normal part
Thickness of pipes The pipe thickness is never lower than: 0.03
e
i I0'
10 s
HOURS
Fig. 11. Third remarkable case.
- The second conclusion is that the follow-up of high temperature piping appears necessary, although these components are static ones. The method used is accurate and readily available to perform a close follow-up which remains acceptable for operating plants. The third conclusion is that, even for approved steel, with a correct chemical composition, slight modifications of additional elements within the allowed range, possibly associated with heat treatment, can modify the behaviour of the steel. So we think that only several long duration creep tests (1000 and 10000 h for instance) can give a good indication of the future hehaviour of a new pipe, and that, in the case of 'limit values' of
e P De f
PDe 2(fz + y P )
+ C,
= = = =
m i n i m u m required wall thickness (mm), internal design pressure (MPa), outside diameter (ram), allowable stress (MPa) due to internal pressure, the value of which is the lowest obtained from: ½ of the specified m i n i m u m tensile strenght at r o o m temperature - 1 / 1 . 5 of the m i n i m u m expected yield strength for 0.25[ offset at temperature 100% of the stress to produce a creep rate of 1% in 100000 h at temperature 60% of the average stress to produce rupture at the end of 100 000 h at temperature 805[ of the m i n i m u m stress for rupture in 100 000 h at temperature. Only E D F approved steels are allowed. z = joint efficiency ( < 1), -
-
-
-
Table A1 Design temperature ( ° C) Ferritic steels Austenitic steels
480 0.4 0.4
510 0.5 0.4
540 0.7 0.4
565 0.7 0.4
590 0.7 0.5
620 0.7 0.7
G. Thoraval / Creep of high temperaturesteam piping
398 c y
= a d d i t i o n a l t h i c k n e s s for c o r r o s i o n , - a coefficient h a v i n g values as given in table A1.
References [1] A. Marchand, Fluage des tuyauteries h haute teml~rature Exp&ience EDF, E D F report, ref. EDF-SPT D562 N ° 12 167 octobre 1983. [2] N O R M E A F N O R N F A 36206 - T61es pour chaudi~res et appareils h pression - Aciers allies au Mo, au M n - M o et au Cr-Mo.
[3] N O R M E A F N O R N F A 49213 - Tubes. [4] V. Cereser, Calcul de l'~'paisseur des tuyauteries h haute pression et h haute temp6rature, EDF report, ref. EDFSEPTEN - Note technique GV 70 N ° 8 - F6vrier 1970. [5] P. Mousset, Mesure du fluage en service des collecteurs et tuyauterie de vapeur, E D F report, ref. E D F - S P T D 5 5 1 / P C M 74403. [6] P. Poncin and P. Mousset, Brian des 6tudes et exp6rience d'Electricit6 de France eta mati6re d'emploi des mat6riaux travaillant h haute temp6rature dans les centrales thermiques traditionelles, Annales de la ehimie Paris, - 1981.
389
CREEP OF H I G H T E M P E R A T U R E S T E A M PIPING: E D F E X P E R I E N C E W I T H F O S S I L - F I R E D P O W E R P L A N T S F R O M 1955 T O 1987 G. T H O R A V A L Maintenance, Department, Electricit~ de France, Nuclear and Fossil Generation Division, Quartier Michelet, 13-27 Esplanade Charles de Gaulle, F-92060 Paris La Defense - C E D E X 57 - France
Received March 1989
Since 1955, EDF has maintained a constant policy of the systematic inspection of its high temperature piping. As these pipes are made of the same material, the measures taken in many plants are comparable. EDF can now, after 30 years of operation, analyse the availability of its means of measurement, the choice of the materials and of the calculation rules, the accuracy of the measurements, and finally the good behaviour of most of our pipes. Some particular cases of abnormal creep rate have been the subject of a particular study.
Introduction When Electricit6 de France (EDF) was created, in 1946, it took over the operation of old plants, designed before the Second World War, with low steam characteristics, therefore no creep problems arose. The first plants with steam temperatures above 500°C were of American design, built under the Marshall plan. Since the fifties, to meet a very big increase in electricity consumption, E D F has pursued a specific policy of building several series of standardized plants. Thirty eight fossil-fired plants of 125 MW capacity (125 bar, 540°C), 37 plants of 25 MW (163 bar, 565°C), 4 plants of 600 MW (163 bar, 565 o C), and 4 plants of 700 MW burning oil, producing reduced characteristics steam (163 bar, 540°C), were started up successively just before the oil crisis. The last series of standardized fossil-fired plants is comprised of 3 × 600 MW (163 bar, 565°C) once-through plants, the design of which was different from the previous plants, as the contribution of nuclear plants to French electricity generation was increasing, and the possibilities of load following by the nuclear reactors were still not developed. Therefore, the purpose of these last 3 new fossil plants was to provide the capability of rapid load following. Until the eighties, most of the fossil-fired boilers (particularly coal-fired ones) were run at constant maximum power throughout the year, accumulating high temperature hours. The situation has now completely changed over the last five years, with the development of the French nuclear programme, and many obsolete
fossil-fired plants have been definitively stopped (or cocooned) while the most recent ones were used for only a few hours a year. During these 30 years, the invariability of the materials used in the design, and the regularity of inservice follow-up of piping, have enabled us to assemble significant results of creep behaviour on piping in 76 plants, for time durations between 10 000 and 200000 h, mostly at high temperature. Before giving the results of our steam piping behaviour analysis, we will address the choice of material, the design rules, the inservice follow-up means, and the policy applied in cases of abnormal creep rates.
1. Choice of materials Since the beginning of the fifties, close collaboration between E D F and the main French steel companies has been set up, which has resulted in a wide ranging agreement on steel characteristics (scales of chemical composition, heat treatments, room temperature mechanical features...), and on drawing up rules for creep test procedures. Moreover, systematic long-duration tests were performed in the laboratories of the different companies concerned, the conclusion of which was the approval of 87 types of steel, manufactured by 22 French and foreign steel companies. Since the building of the 125 MW plants series, one particular grade of steel has been systematically used for high temperature steam piping subject to creep:
0 0 2 9 - 5 4 9 3 / 8 9 / $ 0 3 . 5 0 © Elsevier Science P u b l i s h e r s B.V.
G. Thoraval / Creep of high temperaturesteam piping
390
chromium molybdenum steel A F N O R 10 CD 9-10 (2¼% C r - l % Mo), with heat treatment at 925 to 950°C depending on the steel companies, followed by a slow cooling and by a reheating to about 700 ° C. This gave the steel a room tensile strength between 520 and 670 MPa, and a minimum stress producing a creep elongation of 1% in 100000 h of 59 MPa at 550°C. The work performed at that time is the origin of the setting up of French standards A F N O R N F A 36206 [2] and A F N O R N F A 49213 [3], which standardized the properties of creep resistant steels used in boilers, pressure vessels, and piping.
etail a
2. Design rules Detail a
2.1. Calculation of piping thickness
• 25
Specific calculation rules for 125, 250 and 600 MW plants were issued from the CET (Thermal Studies Committee). For calculation of piping thickness, this committee chose a formula derived from the ASME POWER P I P I N G ANSI B 31.1:
e
PDe 2(fz + y P ) + C (see notations in Appendix 1).
The same formula was used in more recent E D F standards: CPC (Common Specifications Book) for 700 MW plants, and CST (Technical Specifications) for new fossil-fired and nuclear plants. An E D F study [4] enabled us to verify that this formula, derived from a limited development of LAME fundamental formulas, was slightly conservative at creep temperatures. The value of f , maximum allowed stress, is determined by application of the rule detailed in Appendix 1. In practice, the result of the calculation leads, in most cases, to using f = 0.6 x average stress to rupture in 100000 h, which leads to a calculated thickness higher than that which would be obtained by f = stress producing a creep elongation of 1% in 100000 h. That could explain the low creep rate results that we will see later on.
2.2. Measurement sections Since 1960, any piping submitted to creep (i.e. whose thickness is due to maximum allowable stress for creep, and which would thus have been thinner by application of f = ~ x yield strength in the standard formulas) has been systematically provided with at least one specially equipped measurement section. This rule was extended
\
%f----.2
/
Material : Stainless steel Values in m m
Fig. 1. Location of the humps.
to boiler collectors in 1966, by a regulation (circulaire du 25/08/66). The measurement sections are at least 3 m from the nearest elbow, and access to them is relatively easy. Each section (fig. 1) is equipped with 8 stainless steel hemispheric humps, welded on 4 diameters at 45 ° intervals. They are covered with a detachable metallic collar, as protection against external impact loads. The performed according to a specific operating method, including preheating to 350°C.
2.3. Design of supports E D F rules on spacing and the design of supports, laid down to ensure correct natural frequencies and low
G. Thoraoal / Creep of high temperature steam piping
391
Table 1 Example of results (10600 j-old pipe, superheated steam) Diameter number
1
2
3
4
Room temperature (° C) Pipe temperature (* C) Diameter read (nun) Maximum variations from average (mm) Temperature corrected length (ram) Initial measurement (ram) Variation (ram) Relative variation (10 -4 )
9 20 287.105 + 0.006 - 0.004
9 20 285.780 + 0.003 - 0.002
9 20 286.773 + 0.004 - 0.006
9 20 287.01 + 0.003 - 0.002
287.066 286.699 + 0.367 13.74
285.742 285.514 + 0.228 8.53
286.734 286.470 + 0.264 9.88
286.971 286.738 + 0.233 8.72
sag, lead to a m a x i m u m b e n d i n g stress due to the weight below 10 M P a . Consequently, the p i p i n g subm i t t e d to creep does n o t require a n y particular rule for s u p p o r t design.
Average
286.628 286.355 -0.273 10.22
old pipes, or w h e n m e a s u r e h u m p s have b e e n damaged, or w h e n a precise area is suspected of a b n o r m a l creep, it is possible to p e r f o r m m e a s u r e m e n t s of diameters directly o n the pipe. This is precise e n o u g h for forgeda n d - b o r e d pipes w h o s e f a b r i c a t i o n tolerances n e v e r exceed 1 ram: this is the case of s u p e r h e a t e d steam piping. It could b e j u s t a n i n d i c a t i o n for fusion welded pipes (reheated steam).
3. Inservice follow-up m e a n s 3.1. Measurement locations
3.2. Operating rules for measurements
Each time there is a special m e a s u r i n g section, the m e a s u r e m e n t s are p e r f o r m e d there. However, for very
T h e m e a s u r e m e n t s are t a k e n with a m i c r o m e t e r g r a d u a t e d e a c h "r~ ram. T h e m i c r o m e t e r is adjusted o n
Table 2 Determination of ' a ' and ' n ' parameters. Plant
125 MW
Pipes
Superheat
Reheat
250 MW Superheat
Reheat
600 MW Superheat
Reheat
700 M Superheat
Reheat
Number of pipes Steamtemperature ( o C) Steampressure (bar) Outside diameter (nun) Inside diameter (nun) ' a ' average (ram) Dispersion of ' a ' value (ram) ' n ' average Dispersion of ' n ' value
49
48
33
37
7
7
8
8
540
542
567
567
567/542
567/542
542
541
127
30
167
35
167
36/37
166
35
267
419
508
702
674/618
1044/1000
618
770
!87
379
330
630
450/440
930
440
700
0.118 0.05 to 0.40 0.58 0.35 to 0.77
0.108 0.05 to 0.55 0.53 0.15 to 0.75
0.13 0.05 to 0.27 0.45 0.32 to 0.66
0.134 0.05 to 0.30 0.41 0.2 to 0.8
0.243 0.12 to 0.45 0.52 0.26 to 0.67
0.142 0.10 to 0.30 0.338 0.20 to 0.48
0.15 a
0.15 a
0.5 a
0.5 a
-
-
a) The 700 MW plants have no more than 20000 h operation, so the results are not yet available. The values given here are the average creep elongations which will be reached if ' n ' has an average value of 0.5.
392
G. Thoraval / Creep of high temperature steam piping
the spot. Room and piping temperature are noted. Whenever possible, the measurements are taken when pipes have been cooled. Otherwise, a temperature adjustment must be calculated. In order to avoid systematic errors, 2 operators perform 3 measurements each. The noted length is the arithmetic average, and the variabions are also noted. If they are excessive, the measurements are taken again. The final section diamter is the average of the four measured diameters. Examples of results are given tables 1 and 2. 3.3. Accuracy of the measurements - Micrometer accuracy: very good, better than 1 # m with the sliding gauge and adjustment on the spot. - Operator: the accuracy is estimated at 10 #m, adding reading and position erors; a statistical study of the measurement variations (variation around the average) revealed variations always less than 40/~m, and less than 20 # m with a probability of 0.82. - Temperature adjustment: variations can occur due to lack of homogeneity of the temperatures of the material when the pipe is not completely cooled. For a variation of 5 ° C in half of the diameters, the measurement variation is 15 #m. Geometric modifications: the stainless steel humps may have been hit or the welding may have failed. Generally, they cause such a step in the results graph that they are immediately detected. Direct evaluation of accuracy: by reading the results of some measurements of pipes creeping slowly and regularly, for long periods, we can point out abnormal results, with negative or zero evolution between two successive measurements, followed by new measurements going back to the initial curve. When no significant variation of operation occurred during this period, we can evaluate in this way a maximum measurement error, of about 0.1 mm (100 #). This accuracy of about 0.1 mm is considered to be acceptable: the target creep rate is about 1%, i.e., for pipes of 500 mm in diameter, there is a diameter variation of 5 mm.
4.
Inservice
follow-up
policy
Creep is an evolution of material submitted to stress and high temperature combined, which is characterised by an evolution of material structure, and by a permanent deformation rate. Its typical evolution can be divided into 3 stages:
- a first stage creep, characterized by a high rate that is progressively decreasing, - a second stage creep, characterized by a constant low rate, - a third stage creep, corresponding to an increasing creep rate ending in rupture. - The purpose of the inservice follow-up is to be sure that the pipe evolution always remains in the second stage creep. The frequency of inspections is: - For pipes equipped with measurement sections, one inspection every two years and then, if the creep rate is normal after 35000 h, the interval between two consecutive measurements is increased to five years. Besides, we must consider that some pipes submitted to creep may not have been identified as such by the designer, or that some pipes, designed for short periods of high temperature, have in practice been heated for a long time (by-pass of heaters for instance). Consequently, the diameter of these pipes is checked every 10 h. The pipes concerned are the following: - Carbon steel pipes, the temperature of which exceeds 400° C, - 0.5-1.25% Cr-0.5 Mo steel pipes, the temperature of which exceeds 500 o C, 2.25% C r - l % Mo steel pipes, the temperature of which exceeds 525 ° C. The main steam pipes of power plants are submitted, by the nature of the process, either to a constant temperature, or to temperatures with limited and known values. In these conditions, the diameter increase measurement is a good indication of the condition of the pipe, and enables us to evaluate, by extrapolation, the deformation rate after 100000 h. Any measurement of pipe lengthening would be useless, as the longitudinal pressure stress is half of the circumferential one, which leads to a creep rate divided by about 26. As long as the creep rate does not exceed 1% for 100000 h, we take no action. If the rate exceeds 1% before the end of the component life, a close study is performed with, if possible, estimation of a usage factor (function of temperature and pressure history), sample tests of accelerated creep (at higher temperatures or stresses), microstructure analysis of material and economical approach. Then we can decide whether to remove the component, sooner or later, or to modify the operating conditions. In practice, the usage factor, which is defined at any time by the ratio of the creep rate already established divided by the creep rate allowed, is often difficult to calculate. The typical sample test we can use is the
G. Thoraval / Creep of high temperature steam piping
.11C
"l'mle scale
' a ' is the diameter elongation in 100000 h, measured or estimated ' n ' is an exponent in the formula: d = at" where d is the noted deformation (%), and t the time (hours × 10-5). This formula is a mathematical definition of the curve of creep, third stage excluded.
/t,cl"al
ea~
725
50
25.5
712
100
39
691
300
187
669
1000
526
650
3000
1336
393
6. Comments
thermal one, with a wide range of temperatures (cf, fig. 2): indeed, the mechanical test, with various stresses, and constant temperature is not perfectly linear for damaged steels, although the constant stress one remains linear. Another advantage of thermal accelerated creep tests is that they give a curve (fig. 2) with temperature as a parameter, so it is easy to estimate, if necessary, the modification of steam temperatures suitable for the state of the material. This is in order to work out the economic choices: namely, lowering the steam temperatures or removing the pipe. In fact, we use both mechanical and thermal tests, the first one for new pipes, to avoid distorted results due to inetallurgical modifications of the material under very high temperatures, the second one for damaged pipes, to avoid distorted results due to the evolution of micro-eracks under very high stresses.
- Creep rate in 100000 h varies from 0.05 to 0.55%, which is quite satisfactory. This result is logical as the design criteria is 0.6 × stress to rupture in 100000 h in nearly every case. Generally speaking for these particular pipes, the ratio between 0.6 × stress to rupture in 100000 h and stress to 1% elongation in 100000 h is about 0.9, and the application of classical laboratory results allows us to estimate an elongation in 100000 h of: 1% x (0.9) 6 = 0.5%. - The disparity in the results has several causes: - Manufacturing: varied ratios of elements included in the composition of the steel, within the allowed range; variations of heat treatments, performed in the piping factory or on the power plant building site. - Geometry: the minimum thicknesses are checked, following allowed tolerances of 12.5% for fusion welded pipes, and of about 1 ram for forged-andbored ones. However, the real thickness is often much higher, particularly for fusion welded pipes for which the chosen plate was the nearest standard one, which could be thicker by 10 to 20% than the calculated minimum value. Let us note that a 12% thicker pipe, made of 10 CD9.10 steel, reaches 1% elongation after about 1 × 126:2 times later than the reference one. Operation: the temperature has sometimes been lowered on purpose, in order to protect the turbine. When it has, it corresponds to a clear change of slope of the creep rate curve. - The average value of the parameter n is about 0.5. However, as many results concern pipes whose creep rate is very low, a value of about 0.7 or 0.8 would be more probable (fig. 1), if we had to estimate the real creep rate of a rapidly creeping pipe without any initial diameter value.
5. Reading the results (Appendices 4 to 10)
6.1. Ovalisations by creep effect
In order to give a complete description of the creep rate, we have determined for each pipe two parameters: a and n (table 2).
Many reheated steam pipes, which are fusion welded, show a clear distorsion of reference dimensions which decreases during the first few thousand operating hours:
10
10 a.
10 ~
10 500
550
600
650
700
750
000
TEMPERATURE ( degnm C )
Fig. 2. Creep accelerated thermal test; constant stress = 41.8 MPa. Superheated steam pipe sample; conclusion: for T = 600°C, residual life = 22000 hours.
-
G. Thoraval / Creep of high temperature steam piping
394
o
2O
x
+
X
+ O •
+
0
x
[] (9 "/rk,
0.5
~0
°
0.1
02 0
I
AVERAGE DIAMETER ELONGATION
i 2
3
J
1
;
I
L
I
I
;
=
~
r
4
5
6
7
8
9
10
11
12
13
14
Fig. 3. Correlation ovalisation/creep elongation. t~(t) ' w i t h o = ¼ i-4~ I D , - D m l 0~* ~
W (0)
i = 1
OPERATIC~N HOURS x 10,000
Fig. 4. Champagne Plant 1. Creep measurement campaign no 13. Superheated steam pipe.
Dm
Di = measured diameter, Dr. = average valve.
elongation causes excessive pipe bending, it is spectacular enough to be noticed.
7.2. Second case (fig. 1 O) when the second measurements are taken, the larger diameters are found to be smaller than at the first readings. A good correlation has been established between the average pipe elongation and its ovalisation rate (fig. 3). Let us note that this phenomenon could be worrying if its rate was excessive: local elongation, particularly if the largest diameter corresponded to the longitudinal weld, could be high in the root of the weld, while the total measured elongation could be quite low.
7. Noteworthy
cases
Three noteworthy cases deserve a particular study. The first one is interesting because of the lessons we can draw from it. The other cases are the only two pipes which have presented a really high creep rate in our plants, which means that, for an average elongation value of 0.14% in 100000 h for other pipes, these had values of about 2%.
The follow-up of the increase of the diameter of a pipe showed first an abnormally high slope of the curve, and then reached 1% after 60000 h. As this plant had run partially at low power, the conventional limit of 1% in 100 000 h was greatly exceeded. When the creep rate value was about 1.4%, several actions were undertaken: - Research into manufacturing records. - Enforcement of local creep follow-up. - Extension of diameter measurements all along the pipe, by direct measurements on the machined pipe wall. 15 o
,,=,
5
7.1. First case (figs. 4-9) During a periodic inspection of supports, an abnormal bend of a pipe was detected. A hanger was broken, and the sag was quite visible. The complete calculation showed a local elongation of less than 0.2%. The lesson we learnt from it was that, before creep
o
f 1
I
I
I
I
A
I
J
=
2
3
4
5
6
7
8
9
,,
I
I
=
I
10
11
12
13
OPERATION HOURS x 10.000
Fig. 5. Albi Plant. Creep measurement campaign. Reheated pipe.
G. Thoraoal / Creep of high temperature steam piping Series (MW~
Number of plants
125
4 4 17
Steam
Superheated
3 Reheated
125
18 6
250
32 1 1
Superheated
250
3~1
Reheated
6O0
4 3
Superheated
6O0
4 3
Reheated
700
4
Superheated
700
4
Reheated
Steam tempecature-wessum (*C) (MPa) 545 540 540 542
- 9.0 - 12,5 - 12.7 - 12.7
TotaJ number
Diameter (ram)
oqp~x~ 49
3,0 3,0
2 (419/379) 2 (406/370)
48
567 - 16.7 ~ - 14.4 568 - 16.9
5O8/330 495/335 2 (363t237)
35
567 568 -
3.5 4.0
702/630 2 (368/322)
37
567 - 16.7 568 - 16.7
674/450 618/440
7
3.6 3,7
674/450 1000/930
7
542 - 16,7
672/460
4
1087/990
4
567 542 -
542 *
3,4
14 13
409/325 2(243/20~) 2(267/167) 370/274
542 542 -
395
0.10 0.20 0.30 0.40 DIAMETER ELONGATION ( %. ) IN 100,000 HOURS
Reheated steam pipes
2.2.4
TOTAL
Fig. 6. Piping characteristics. - Analysis of micrographic structure. - Cutting of a ring for accelerated creep tests. It appeared that the problem was limited to several
•
_9 7
0
VS-T3
~
r--1
0.10 0.20 0.30 0.40 0.50 DIAMETER ELONGATION ( %. ) IN 100,000 HOURS
0.60
Fig. 8. 125 MW series. Superheated steam pipes. 0.2
0.1
0.05
i 10'
I HOURS
10 Is
Fig. 7. Examples of curves log/log d = F(t).
parts of the pipe, which only just passed the creep test (1000 h) and had the m i n i m u m carbon content. Microanalysis of the structure showed abnormal evolution on these parts, while the other parts were normal. Consequently, we concluded that the abnormal structure was due to the manufacturing or installation processes, and was not an inservice evolution. It was due to bad metallurgical heat treatment producing a material whose room tensile strength was less than 45 MPa. The accelerated creep tests performed by the E D F Laboratory showed a residual life of 60 000 h, which was less than the estimated residual life of the plant. The pipe was removed. Let us note two points: - First, the close inspection of the pipe, performed after its removal, revealed the absence of any cracking in spite of 2% creep elongation. - Secondly, the replacement of the pipe was carried out in two steps: one part first, the second part a year after. The second part, the year after, was found to
G. Thoraval / Creep of high temperature steam piping
396
--7 7
7
1
0.05
0,1
0.15
0.20
0.25
J
0.30
DIAMETER ELONGATION (%) IN 100,000 HOURS
determined. Its value was 0.41, to be compared with the value of a normal pipe usage factor calculated also: 0.018. Two possibilities were then considered: - To continue operation as before, to follow the 'usage factor' and to change the pipe when the usage factor got close to 1. Or to save the pipe by lowering the steam temperature, during all the plant's residual life: 545°C (corresponding to 950 000 h). The first solution was chosen, for two reasons: first the high price of coal and then the estimation that, in the future, this plant would be operated only a few hours each year.
Reheated steam pipes 8.
Conclusion
12
The first conclusion is our satisfaction with chromium molybdenum steel, which remains safe when it gets old: its creep behaviour follows the predicted evolution (which is quite limited), and, for the particular cases of abnormal creep elongation, we have never found any cracks, in the welds or in the pipe steel.
7
3
1
0.10
0.20
0.30
DIAMETER ELONGATION (%) IN 100,000 HOURS
Fig. 9. 250 MW series. Superheated steam pipes. 1.5
be highly elongated near the circular weld, in the section which had been submitted to the heat treatment when the new part had been connected. The evolution of this damaged steel during heat treatment seems to have seriously lowered its creep resistance capacity.
7.3 Third case (fig. 11) The first conclusions of the report described above led to an investigation in several plants to find any similar pipes. In one of the plants, the creep rate of some parts of a pipe was abnormally high. As the accelerated creep test had demonstrated (for the second case) that these pipes had a remaining life of several ten thousand hours, the immediate removal of the third case pipe was not decided. A close study of the history of this pipe was performed, by reading the plant records, and the 'usage factor' of the abnormal pipe was
O.5
1
2
3
4
s
6
7
8
9
OPERATIONHOURSx 10,000
Fig. 10. Particular case No. 2 (St. Ouen). Creep measurements. Superheated pipe (16.7 MPa, 565°C). Initial outside diameter = 508 mm.
G. Thoraval / Creep of high temperature steam piping d%
d°/,
+
2
+
1.9
1.8
1.7 0.20
397
additional elements, some additional special measurement sections should be provided in the pipe in question. Finally, even if the low values of the creep rate measurements carried out over these last 30 years make us think that our calculation rules are too conservative, the two abnormal cases we found are sufficient to convince us that the margins of our standards are necessary.
Acknowledgements
/ 1,6
100
110 0.10
It is acknowledged that many of the points described in this paper are conclusions of an earlier study [1] performed by the author's predecessor at E D F , A. Marchand, before he retired.
Appendix 1
EDF Calculation rules (1982 - CST) normal part
Thickness of pipes The pipe thickness is never lower than: 0.03
e
i I0'
10 s
HOURS
Fig. 11. Third remarkable case.
- The second conclusion is that the follow-up of high temperature piping appears necessary, although these components are static ones. The method used is accurate and readily available to perform a close follow-up which remains acceptable for operating plants. The third conclusion is that, even for approved steel, with a correct chemical composition, slight modifications of additional elements within the allowed range, possibly associated with heat treatment, can modify the behaviour of the steel. So we think that only several long duration creep tests (1000 and 10000 h for instance) can give a good indication of the future hehaviour of a new pipe, and that, in the case of 'limit values' of
e P De f
PDe 2(fz + y P )
+ C,
= = = =
m i n i m u m required wall thickness (mm), internal design pressure (MPa), outside diameter (ram), allowable stress (MPa) due to internal pressure, the value of which is the lowest obtained from: ½ of the specified m i n i m u m tensile strenght at r o o m temperature - 1 / 1 . 5 of the m i n i m u m expected yield strength for 0.25[ offset at temperature 100% of the stress to produce a creep rate of 1% in 100000 h at temperature 60% of the average stress to produce rupture at the end of 100 000 h at temperature 805[ of the m i n i m u m stress for rupture in 100 000 h at temperature. Only E D F approved steels are allowed. z = joint efficiency ( < 1), -
-
-
-
Table A1 Design temperature ( ° C) Ferritic steels Austenitic steels
480 0.4 0.4
510 0.5 0.4
540 0.7 0.4
565 0.7 0.4
590 0.7 0.5
620 0.7 0.7
G. Thoraval / Creep of high temperaturesteam piping
398 c y
= a d d i t i o n a l t h i c k n e s s for c o r r o s i o n , - a coefficient h a v i n g values as given in table A1.
References [1] A. Marchand, Fluage des tuyauteries h haute teml~rature Exp&ience EDF, E D F report, ref. EDF-SPT D562 N ° 12 167 octobre 1983. [2] N O R M E A F N O R N F A 36206 - T61es pour chaudi~res et appareils h pression - Aciers allies au Mo, au M n - M o et au Cr-Mo.
[3] N O R M E A F N O R N F A 49213 - Tubes. [4] V. Cereser, Calcul de l'~'paisseur des tuyauteries h haute pression et h haute temp6rature, EDF report, ref. EDFSEPTEN - Note technique GV 70 N ° 8 - F6vrier 1970. [5] P. Mousset, Mesure du fluage en service des collecteurs et tuyauterie de vapeur, E D F report, ref. E D F - S P T D 5 5 1 / P C M 74403. [6] P. Poncin and P. Mousset, Brian des 6tudes et exp6rience d'Electricit6 de France eta mati6re d'emploi des mat6riaux travaillant h haute temp6rature dans les centrales thermiques traditionelles, Annales de la ehimie Paris, - 1981.