ML19341C842

TMI-2 Low Pressure Disc Ultrasonic Insp Rept

ML19341C842
Person / Time
Site:	Crane, North Anna
Issue date:	02/02/1981
From:	WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML19260G657	List: ML19341C839 ML19341C842
References
NUDOCS 8103040274
Download: ML19341C842 (2)

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,=_----g LOW PRESSURE DISC ULTRASONIC INSPECTION REPORT l

INTRODUCTION O

This report covers the ultrasonic inspection of the Metropolitan Edison Co.,

l Three Mile Island #2 unit.

This unit is of a shrunk on disc design with each disc keyed to the shaft at the disc bore area.

The disc keyways are located such that standard NDT techniques cannot be used.

The U.T. inspec-tion was perfonr.ed to detect possible cracking in the disc keyway area.

The need for this inspection is a result of recent industry experience where stress corrosion cracking has occurred in machines of shrunk on disc design.

Cracking in this disc bore area, if allowed to propagate, could eventually lead to a disc burst.

}

DISCUSSION Ultrasonic inspection techniques have been developed for detecting keyway cracks without the need for unshrinking discs.

Two techniques are used; one for detecting crack indications and the other for verifying crack indi-cations and measuring their depths.

The initial technique used in the tangential aim, where an ultrasonic trans-ducer is mounted on a plexiglass block that sits on the disc hub.

The plexi-glass block is contoured so that it is in complete contact with the disc hub.

The ultrasonic waves are directed tangentially towards the keyway so that any cracks above the keyway area will be perpendicular to the sound beam and 3

reflect the sound. A careful analysis of the time difference between the echoes from the keyway and the crack allows discrimination of false indica-tions.

This time differential procedure is accomplished by circumferentially scanning (see Figure 1).

Since the disc profile is composed of a series of compound radii a mathematical model was developed for transducer positioning and alignment. The model was computerized to allow calculation of:

(1)all needed information for positioning and aiming the transducer for detection anywhere on the disc bore, (2) metal path length, elapsed time to keyway echoes, and time between keyway echo and crack echo, (3) time between keyway echo and suspected spurious indications, (4) various angles with interrelated ultrasonic central ray direction to disc and transducer axis.

Also because of the disc profile and the need for good sonic coupling between

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the disc metal and the plexiglass blocks (called compound angle wedges) a large number of blocks are required (see Figure 2).

If a crack indication is picked up sonically, the location is determined by the transducer location and its rela,tionship to keyhole echoes and indication echoes (see Figure 3 and 4).

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i 1

Since the t'angential aim technique is not suitable for determining depth of crack, a complementary technique called radial aim is used to:

(1) verify cracking detected by the tangential aim technique, and (2) to estimate the depth of crack.. In this method, the ultrasonic waves are directed perpen-dicular to the keyway.. The radial aim nethod works because a crack undergoes branching and. spreading while it grows. Thus, the branching part of the

. crack includes components which reflects ultrasound back to. the transducer.

Depth of the crack is estimated by measuring the time between the keyway echo and the flaw echo, and then coverting to round trip distance by multiplying by shear wave propagation velocity (125,000 in./sec).

ANALYSIS 4

After ultrasonic inspection has indicated a possible disc crack, an analysis is made to determine what the indicated crack size is in relationship to the critical crack size as determined by fracture mechanics.

This relationship is used to detbrmine Westinghouse recommendations regarding j

continued operation with a disc crack indication.

EQUIPMENT 1

Electronic Instruments A) Krautkramer USIP ll-Ultrasonic Equipment (for recommended test settings see Appendix A).

B) Tektronix Model 455B or Model 465 Portable Oscilloscope.

(Model 465B is preferred).

(For recommended test settings see Appendix B).

Transducers Transducers ranging in diameter from 1/4 inch to 1-1/8 inch and in frequency from 2-1/4 MHZ to 10 MHZ as detailed in the specific tables for each of the BB discs. Other frequencies may be used to further evaluate indications.

Wedges Numerous plexiglass wedges varying in entrance angle, rotational angle, radius of curvature, physical size and rotational direction are provided, as required for each of the BB discs, as detailed in the specific tables, a

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, -,, -, -.. ~.. - - - - -.,.-- -

. _. _ _ _ _ _ _ _. ~... _ _.

s Transducer Manipulators Transducer manipulators are used whenever hand scanning cannot be conducted.

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A) Wedge Extensions B) Transducer Manipulators (W R&D Drawing No. 12222J20) j C) Charlotte Manipulator,

(

j Camera j

Tektronix C-5A Oscilloscope Camera, or equivalent.

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i 3

Couplant t

Ultragel II. Westinghouse M-54410LF.

1 Calibration Standards A) 3/4 inch x 8 inches long cylindrical rod,

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B) Plexiglass Block 1

Conversion Factors j

Ultrasonic measurements of distance are presented as time interval t

f-measurements on the 465B oscilloscope.

Thus, time measurements in s/ division on the oscilloscope must be multiplied by 0.113 in/ sec L

for shear wave travel to convert them to linear neasurements in inches.

INSPECTION RESULTS Three Mile Island - LP #1 and LP #2 No. 1 LP - No indications were r,eported on this rotor.

No. 2 LP - No indications were reported on this rotor.

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REC 0t'MENDATI0fiS No reportable indications were found on any of discs on LP #1 or LP #2.

Based on our erigineering judgement, the rotors are suitable for satisfactory opera tion.

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FIG. 4-Transducer Position (1,b) and Alignr.ent Parameters (S6'I) to Exar.ine Keyway Target ( c,' h).

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USIP ni SERIAL NO _2 //h'? ~ Gb9S OSCILLISCOPE SERIAL NO. _

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GOVERNOR END GENERATOF FND

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