ML19350D920
| ML19350D920 | |
| Person / Time | |
|---|---|
| Site: | Waterford  |
| Issue date: | 05/14/1981 |
| From: | WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML19262F455 | List: |
| References | |
| 10PRO3A, PROC-810514, NUDOCS 8105260454 | |
| Download: ML19350D920 (14) | |
Text
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ENCLOSURE I 4
CALCULATION METHOD FOR CRITICAL CRACK SIZES i
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10PR03a 8105260ffy
During August,1979, Westinghouse discovered the occurrence' of disc keyway cracking on a nuclear low pressure turbine. As a result of this experience, Westinghouse has developed a procedure to rank uninspected operating units for purposes of developing inspection schedules that allow for inspection of units prior to expected occurrence of disc fracture.
l The ranking of uninspected operating units is done on the basis of an index which is defined by the ratio of maximum expected crack depth, a to effective critical crack depth, ace (eff).
Several factors must be taken into accoant in the computation of the maximum expected crack depth. The most important of these factors are material composition and [
] b,c the [ ~ ~ ~~ '~ ~ ~ -
_ _.1 b,c operating time, steam quality. These factors are used as the basis for ranking. The composition and operating temperature have been incorporated into a g'raph describing the crack growth rate based on actual disc cracking experience.
The expected crack depth is determined l
by multiplying the crack growth rate taken from this graph by the total operating hours of the unit, or by the elapsed time until the next scheduled inspection of the unit. To determine the maximum expected
, crack depth, the growth rate described by the upper bound of this graph is multiplied by the time:
l a*(
) xt I
where: t = operating time
~
10PR03a t
(h)
= maximum crack growth rate at temperature T The material strength factor is taken into account for high strength discs by means of a correction to the maximum growth rate.
Since experience shows that under normal conditions cracks are not formed in dry regions of a turbine, steam quality is taken into account by assuming that the crack growth rate in dry steam is negligible. The 4
final factor, steam purity, is taken into account whenever an event 4
involving corrosive impurities is known to have occurred.
In the event significant corrosion problems become evident, an early inspection may be necessary.
The critical crack depth, a is calculated using the following cp, fracture mechanics expression:
X 2
0 IC a
cr 1,21n c
l where: Q = flaw shape parameter. Depth to length ratio of flaw was flaw was ic'servatively assumed to be [
] a,e IC = fracture toughness ?f disc l
e=
maximum bore stress [
] b,c l
10PR03a I
T Finally, the effective critical crack depth is calculated as follows:
cy (eff) a
-[
]ac a
c7 where: [
] a,c
= depth of keyway Using the parameters described above, the ratio of [
] b,c is calculated for each of the first four discs in a turbine. So long as this ratio is less than [
] a,c there is reasonable assurance i
that the turbine can be operated without a rupture of the disc occurring.
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WSES-FSAR-UNIT-3 10.2.3 TURBINE DISK INTEGRITY
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As a result of Westinghouse testing, new criteria have evolved for predicting the missile containing at ility of the low pressure turbine structures.
The previous calculations have been redone using these new criteria and the results show the original position on the containment of disk fragments within *he turbine casing can no longer be maintained.
The Turbine Overspeed Protection System is completely independent of the normal turbine governing and mechanical overspeed protective devices. In the event of a turbine trip, this system ensures that the turbine generator I
unit will not exceed the design overspeed which is 120 percent of rated s pe ed.
Present manufacturing and inspection techniques for turbine rotor and dis'c forging makes the possibility of an undetected flaw extremely remote. Forg-ings are subject to inspection and testing both at the forging suppliers and at Westinghouse. Current design procedures are well established and conser-vative, and analytical tools such as finite element and fracture mechanics techniques allow in depth analysis of any potential trouble spots such as area of stress concentration or inclusions which could give rise to crack propagation.
10.2.3.1 Materials Selection 10.2.3.1.1 High Pressure Turbine The high pressure turbine element, as shown in Figure 10.2-7 is of a double flow design. Flow design is thrust balanced. Steam from the four control s
valves enters the turbine element through four inlet pipes. These pipes feed four double-flow nozzle chambers flexibly connected to the turbine casing. Steam leaving the nozzle chambers passes through the rateau control stage and flows through the reaction blading. The reaction blading is mounted in the blade rings shown in Figure 10.2-8 which in turn are mounted in the turbine casing.
The high pressure rotor is made of NiCrMoV alloy steel. The specified j
minimum mechanical properties are as follows:
Tensile strength, psi, min.
100,000 Yield strength, psi, min. (0.2 percent offset) 80,000 Elongation is 2 in. percent, min.
18 Reduction of Area, percent, min.
45 Impact S*.rength, Charpy V-Notch, f t.
Ibe.
60 (min at room temperature) 50 percent fracture appearance transition temperature, 50 F, maximum I
4 10.2-5
WSES-FSAR-UNIT-3 The main body of the rotor weighs approximately 100,000 pounds. The approximate values of the transverse centerline diameter, the maximum diameter, and the main body length are 36 in., 66 in., and 138 in.,
respectively.
The blade rings (ASIM A356-68) and the casing cover and base (both ASTM A216-66) are made of carbon steel castings. The specified minimum mechanical properties are as follows:
Tensile Strength, psi, sin.
70,000 Yield Strength, psi, min.
36,000 Elongation in 2 in. percent, sin.
22 Reduction of Area, percent, sin.
35 The bend test specimen is capable of being bent cold through an angle of 90 degrees and around a pin one inch in diameter without cracking on the outside of the bent portion.
The approximate weights of the four blade rings,. the casing cover, and the casing base are 80,000 lbm., 115,000 lbm. and 115,000 lbm. respectively.
The casing cover and base are tied togethar by means of more than 100 studs. The stud material is a hot rolled alloy steel (ASTM A193-66 Cr.B16) having the following mechanical properties:
2 1/2 In.
Over 2 1/2 Over 4 and Less to 4 In.
To 7 In.
Tensile Strength, psi, min.
125,000 115,000,
110,000 Yield Strength, psi, min.
105,000 95,000 85,000 (0.2 percent offset) l Elongation in 2 in.,
15 16 16 percent, min.
Reduction of Area, percent min.
50 50 45 The studs have lengths ranging from 17 to 66 in, and diameters ranging from 2.75 to 4.5 in.
About 90 percent of them have diameters ranging between 2.75 and 4 in. The total stud cross-sectional area is approximately 900 sq. in.
and the total stud free length volume is approximately 36,000 cu. in.
10.2.3.1.2 Low Pressure Turbines The double flow low pressure turbines incorporate high efficiency blading and diffuser type exhaust design. The low pressure turbine cylinder is fabri-cated from carbon steel plate to provide uniform wall thickness, reducing thermal distortion to a minimum. The entire outer casing is subjected to low temperature exhaust steam.
10.2-6 l
WSES-FSAR-UOIT-3 The temperature drop of the steam from its inlet to the LP turbine to its
[-
exhaust f rom the last rotating blades is taken across three walls, an inner cylinder number 1, a thermal shield, and an inner cylinder number 2.
This precludes a large temperature drop across any one wall, except the thermal shield which is not a structural element, thereby virtually eliminating thermal distortion. The fabricated inner cylinder number 2, is supported by the outer casing at the horizontal centerline and is fixad transversely at the top and bottom and axially at the centerline of the steam inlets, thus allowing freedom of expansion independent of the outer casing.
Inner cylinder number 1 is, in turn, supported by inner cylinder number 2, at the horizontal centerline and fixed transversely at the top and bottom and axially at the centerline of the steam inlets, thus allowing freedom of expansion independent of inner cylinder number 2.
Inner cylinder number 1 is surrounded by the thermal shield. The steam leaving the last row of blades flows into the diffuser where the velocity energy is converted to pre.sure energy.
The disks are made of NiCrMoV alloy steel.
There are two identical sets of five shrank-on disks, one set for each of the two flows. Each disk in a set is numbered; the disk closest to the transverse centerline is desig-nated number 1.
When the turSice is in operation each disk experiences a dif ferent stress and is, consequently, machined from a suitable grade of I
alloy steel.
D' 4k number 2 expetisaces the highest stress, while disk number 5 experiences the lowest. The specified mechanical properties disk materia is are shown in Table 10.2-3.
The ou';er cylinder and the two inner cylinders are fabricated mainly of ASTM A515-65 material. The specified minimum mechanical properties are shown in Table 10.2-3.
4 The rotors are made of NiCroMoV alloy steel.
Tha specified minimum mechani-cal properties are shown in Table 10.2-3.
10.2.3.2 Fracture Toughness l
Fracture of the disks into 90,120 and 180 degree segments was considered as criteria in selecting number of disk segments.
A 120 degree segment has an initial translational kinetic energy 12.5 per-cent greater than that of a 90 degree segment, however it also has a 3,3 percent greater rim periphery resulting la greater energy loss while pene-trating the turbine casing. This results in nearly equal kinetic energy of the 90 and 120 degree segments leaving the turbine casing.
However, since the 90 degree segments have the smaller impact areas they represent a more severe missile.
The initial translational kinetic energy of a half disk is equs ? to that of a quarter disk. Because of kinematic considerations, a half sea ent will m
always impact with the rotor after fracture. The 180 degree segment, due i
I to its larder size, will subject the stationary parts to greater def;.ma-tion.
As a result the 180 degree segment will leave the turbine casing with lower energy than the 90 degree segment.
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10.2-7 Amendment No. 4, (6/79)
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WSES-PSAR-UNIT-3 For the purpose of evaluating the missile containing ability of the turbine structure, the shrunk-on disks have been postulated to fail in four quarters.
To evaluate the missile containing ability of its steam turbines, Westing-house conducted a test program at its research laboratories.
The tests involved spinning alloy steel disks to failure within various carbon steel containments. The disks were notched to ensure failure in.
given number of segments at the desired speed. Test results were corre-laced with various parameters descriptive of the missile somentum and energy and the geometry of the missile and containment.
The containments, were of varying geometry but all were axisynetric and concentric with the rotation axis of the disk. They ranged in complexity from a circular cylinder to containments which approximated actual turbine construction.
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From these tests, logical criteria has evolved for predicting the missile containing ability of various turbine structures.
In addition, the taats also served to detensine the mode of failure which certain structural shapes common to turbine construction undergo when impacted by a missile.
This is important since the mode of failure has a great influence on the amount of energy absorbed by the containment.
Normal operating temperatures of each key point at high pressure and low pressure turbines are shes in Figure 10.2-2.
The minimum throttle stese l
temperature required for rolling the HP turbine from a cold start is 388 F with a corresponding pressure of 200 psig. The maximum LP turbine exhaust hood temperature is limited to 175 F.
A detail analysis of brittle rupture probability of low pressure turbine disks is presented in Reference 1.
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'O.2 9 Amendment No. 4, (6/79)
WSES-FSAR-UNIT-3 10.2.3.3 High Temperature Properties Calculations for effects of a postulated failure of the HP turbine rotor at design overspeed (120 percent of the rated speed) show that all f ragments.
generated by any postulated failure of the HP turbine rotor would 'be con-tained by the HP turbine blade rings and casing.
Since the steam at the turbine stop valve only has a temperature of 526 F ~
for Waterford-3, it is not considered that creep processes will occur on 6
the HP rotors and therefore, stress rupture properties are not relevant.
Probability and fatigue crack growth rate data and stress rupture data of the high pressure rotor are given in Reference 1.
10.2.3.4 Turbine Disk Design The turbine is designed to withstand normal conditions, anticipated tran-sients, and accidents resulting in a turbine trip without loss of struc-tural integrity. A record search for compliance with Branch Technical Position HTEB10-1 has not been performed, because the turbine has been manuf actured and placed in storage prior to issuance of the Branch Posi-tion.
However, the turbine is designed to the following criteria:
a)
The highest anticipated speed from loss of load is less than 110 percent of rated speed. The turbine is designed for 120 percent of rated speed. The Branch Position requires design overspeed five percent above the highest anticipated speed.
b)
At 115 percent of rated speed, the average tangential stress in low pressure discs or high pressure rotors due to centrifugal force, 6
interference fit, and thermal gradients do not exceed 0.75 of the minimum specified yield strength of the materials.
c)
The rotors are designed so that the response levels at the natural critical frequency of the turbine shaft assemblies are controlled l
between 0 speed and 20 percent overspeed, so as to cause no l
distress to the unit during operation.
l d)
The bore of the high pressure rotors can be inspected in-service with the rotors removed from the cylinder. The rims of the low-I pressure discs can also be inspected. There are, however, no re-liable methods for in-service inspection keyways and bores without disassembling.
10.2.3.5 Preservice Inspection l.
The preservice inspection and test methods applied during the manufactur-Amendment No. 6, (8/79)
I 10.2-10
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WSES-FSAR-UNIT-3 ing process assure that.the product comp 1tes with the specitications.
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The low pressure turbine rotor body and disk are heat treated nickel-chromium-molybdenum-vanadium alloy steel procured to specitications that define the manufacturing method, heat treating process, and the test and inspection methods. Specific tests and test documentation, in addition to dimensional requirement s, are specified for the forging manuf acturer.
Inspection and tests for the low pressure turbine rotor body are conducted at the forging manufacturer's plant.
a)
A ladle analysis ot each heat of steel for chemical composition is to be within the limits defined by the specification.
b)
Following preliminary machining and heat treatment for mechanical propercies but prior to stress reliet, all rotor diameters and faces are subjected to ultrasonic tests defined in detail by a Westinghouse specification which exceeds the requirements of AS1h A 41a-64.
c)
After all heat treatment has been completed, the rotor forging is subjected to a thermal stability test defined by a Westinghouse speci-Lication which is more restrictive than the requirements of ASTM A 472-69.
d)
The end faces of the main body and the fillet areas joining the body to the shaft ends of the machined forging are subjected to a magnetic particle surface inspection as defined by ASTM A 275-71.
e)
After the bore of the rotor is finished machined, the bore is given a visual examination followed by a wet magnetic particle inspection de-fined in detail by a kestinghcuse specification which exceeds the re-quirements of ASTM A 275-71.
f)
Utilizing specimens removed from the rotor forging at specified loca-tions, tensile, Charpy V Notch impact and FATT properties are deter-mtnod following the test methods defined by ASTM A 370-67.
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Af ter the rotor body is finiehed machined at Westinghouse, the rotor sur-l tace is given a fluorescent magnetic particle examination as detined by a Westinghouse specification which is similar to ASTM E 138-63.
Inspection and tests for the low pressure turbine rotor disks are conducted at the forging manutacturer's plant.
l a)
The ladle analysis of each heat of steel is to be within the composi-tion limits defined by the specification, b)
After all heat treatment, rough machining and stress reliet opera-tions, the hub and rim areas of the completed disk forging are sub-jected to ult rasonic examinations. These ultrasonic tests are defined by a Westinghouse specification which exceeds the requirements of ASTM A 418-64.
c)
The tensile, Charpy V Notch i;npact and FATT properties are determined l
4 10.2-11 l
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WSES-FSAR-UNIT-3 from specimens removed from the disks at specific locations. The test method used for determining these mechanical properties are defined by ASTM A 370-67.
Af ter the disks are finished machined at Westinghouse, :he disk surfaces except blade grooves are given a fluorescent magnetic ', article examination as defined by a Westinghouse specification which is sisilar to ASTM E 138-63.
After the prehcated disks are assembled to the rotor body to obtain the specified interference fit, holes are drilled and reamed for axial locking pins at the rotor and disk interface. These holes are given a fluorescent penetrant inspection defined by a Westinghouse specification which is similar to ASTM E 165-65.
I Prior to shipping, each fully bladed rotor is balanced and tested to 120 percent of rated speed in a shop heater box.
The high pressure turbine rotor has the same basic material composition as the low pressure rotors. This nickel-chromium-molybdenum-vanadium alloy steel forging is procured, processed, and subjected to test and inspection requirements the same as the low pressure rotor which includes:
a)
Ladle analysis b)
Ultrasonic tests c)
Magnetic particle inspection d)
Thermal stability test l
e)
Bore inspection f)
Tensile and impact mechanical properties j
g)
Fluorescent magnetic particle inspection h)
Heater box and 120 percent speed test 10.2.3.6 In-service Inspection j
Various parameters for the turbine generator and accessories are recorded and alarmed in tha main contral room and logged on the plant computer. A full compliment of controls and instruments are provided in order that the l
i turbine generator may be started, operated, tested, and shutdown from the main control room.
Periodic turbine generator inspections, including inspections and tests of the main steam stop and control valves and reheat stop and control valves, 6
will be performed on Waterford-3 with the goal of maximizing turbine generator reliability and ef ficiency and thus minimizing long-term power generation costs.
10.2-12 Amendment No. 6, (8/79)
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WSES-FSAR-UNIT-3 Factors that determine the timing of inspections include:
a)
Operating symptoms: vibration, abnormal pressures and temperatures, loss of capability, increase in heat rate, etc.
b)
Mode of Operation: number of start ups, cyclic or base loading, etc.
c)
Findings from prior inspections both in-house and at other utili-3
- ties, a
3 d)
Recommendations of the turbine generator manufacturer.
The purpose of these inspections are to search for, correct, and minimize I
the causes of items, such as:
a)
Wear:
bearings, gears, linkages, valve parts, packings, spill strips, hydrogen seals, collector rings, etc.
b)
Erosion: solid particle in dry regions, moisture in wet regions.
c)
Depositions: collections in the stream path that result in loss of capability or efficiency and possible exposure to undesirable chem-icals.
d)
Distortions and misalignment.
6 e)
Cracking:
thermal or fatigue.
f)
Mechanical damage: buckets, diaphragas, stator core, etc.
g)
Contamination of fluid systems.
h)
Reduction in integrity of insulation on stator bars, field winding, or core laminations.
1)
Loosening of generator hardware, blocking, supports, core, etc.
j)
Generator contamination (oil or dirt) blocking of ventilation passages.
k)
Excessive heating in electrical systems.
1)
Excitation system electrical and mechanical problems *
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When the turbine is disassembled, a visual,and-4 magnetic particle examina-4 tion is made externally on accessible areas of the high pressure rotor, low pressure turbine blades and low pressure discs. The coupling bolts are visually examined.
S% ER_% A ---7 a)
Throttle, gove.rnor, reheat stop and interceptor valves are inspected af ter initial start-up of a turbine. As per the following program some valves are inspected 12-15 months a f ter start-up, others 24-27 months, and the remainder 36-39 months so that all valves are
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inspected at least once in the 39 months of operation following ini-10.2-12a Amendment No. 6, (8/79)
t m >Ecy A
The met.todology used to calculate the sizes of the critical cracks in the Jow pressure rotor discs and the frequency of inspection is described in Westinghouse's document 10 PRO 3 entitled " Calculation Method for Critical Crack Size" and.its attachment "L.
P. Turbine Disc Information."
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1 WSES-FSAR-UNIT-3 1
i tial start-up and throttle and reheat stop valves are inspected twice in this period. Af ter this initial inspection program is completed, all valves are inspected at least once every 36-39 months.
b)
Functional test of the turbine steam inlet valves are performed weekly. This test can be made while the unit is carrying load. The 6
purpose of the test is to insure proper operation of. throttle, go-ve nor, reheat stop and the interceptor valves. The operation of i
these valves are observed during the test by an operator stationed at the valves. Movements of the valves should be smooth and free.
i Jerky or intermittent motion may indicate a buildup of deposits on shafts.
These frequent in-service inspections, coupled with a comprehensive moni-toring program during operation, will assure ef ficient and reliable tur-bine generator performance over the life of the plant.
10.2.4 EVALUATION The steam generated in the two steam generators is not normally radio-active. Only in the event of primary-to-secondary system leakage (due to steam generator tube leak) is it possible for the SPCS to become radio-actively contaminated. In this event, monitoring of condenser air discharge will detect any contamination. A full discussion of the radiological as-pects of primary-to-secondary leakage, including anticipated operating con-centrations of radioactive contaminants, means of ~ detection ci radioactive contamination, anticipated releases to the environment, and lia: ting condi-l tions for operation, are included in Chapters 11 and 12.
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A description of the protection provided by bypassing and dumping main steam to the condenser and atmosphere in case of sudden load rejection by the turbine generator is included in Subsection 10.4.4.
A description of the protection provided by exhausting steam to the atmosphere through the safety valves in the event of a turbine generator trip and coincident failure of the SBS is given in Section 10.3.
An internal energy method as outlined in Power Test' Code PTC20.2 was utilized to determine the expected speed rise upon full load dump. The results are with full load dump of 1199.91 MW and successful operation of the OPC, the expected speed rise is 1.065 percent; with failure of OPC and operation of 4
the mechanical trip weight the expected speed rise is 1.18 percent.
J The overspeed protection devices showing provided redundancy is shown in Table 10.2-2.
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4 10.2-13 Amendment No. 6, (8/79)
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