ML20057D764
| ML20057D764 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 09/27/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20057D759 | List: |
| References | |
| NUDOCS 9310050330 | |
| Download: ML20057D764 (5) | |
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NUCLEAR REGULATORY COMMISSION ENCLOSURE g+v f WASWNGTON D C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO INSERVICE TESTING PROGRAM RELIEF RE0 VEST 51-VRR-2 LIMERICK GENERATING STATION. UNITS 1 AND 2 PHILADELPHIA ELECTRIC COMPANY DOCKET NOS. 50-352 AND 50-353
1.0 INTRODUCTION
The Code of Federal Regulations,10 CFR 50.55a, requires that inservice testing (IST) of certain American Society of Mechanical Engineers (ASME) Code Class 1, 2, and 3 pumps and valves be performed in accordance with Section XI of the ASME Boiler and Pressure Vessel Code and applicable addenda, except where relief has been requested and granted or proposed alternatives have been authorized by the Commission pursuant to 10 CFR 50.55a(f)(6)(i), (a)(3)(i), or (a)(3)(ii).
In order to obtain authorization or relief, the licensee must demonstrate that:
(1) conformance is impractical for its facility; (2) the proposed alternative provides an acceptable level of quality and safety; or
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(3) compliance would result in a hardship or unusual difficulty without a i
compensating increase in the level of quality and safety. Section 50.55a (f)(4)(iv) provides that inservice tests of pumps and valves may meet the requirements set forth in subsequent editions and addenda that are incorporated by reference in 10 CFR 50.55a(b), subject to the limitations and modifications listed, and subject to Commission approval.
NRC guidance l
contained in Generic Letter (GL) 89-04, " Guidance on Developing Acceptable Inservice Testing Programs," provided alternatives to the Code requirements i
determined to be acceptable to the staff and authorized the use of the alternatives in Positions 1, 2, 6, 7, 9, and 10 provided the licensee follows the guidance delineated ii the applicable position. When an alternative is l
proposed which is in accordance with Gereric Letter (GL) 89-04 guidance and is documented in the IST program, nn further evaluation is required; how9ver, implementation of the alternative is subject to NRC inspection.
Section 50.55a authorizes the Commission to grant relief from ASME Code requirements or to approve proposed alternatives upon making the necessary fir. dings.
The NRC staff findings with respect to granting or not granting the relief requested or authorizing the proposed alternative as part of the licensee's IST program are contained in this Safety Evaluation (SE).
In rulemaking to 10 CFR 50.55a, effective September 8, 1992, (see 57 FR 34666), the 1989 edition of ASME Section XI was incorporated in 10 CFR 50.55a(b). The 1989 edition provides that the rules for IST of pumps and valves shall meet the requirements set forth in ASME Operations and
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. Maintenance Standards Part 6 (OM-6), " Inservice Test 3ng of Pumps in Light-Water Reactor Power Plants," and Part 10 (0M-10), "Iriservice Testing of Valves in Light-Water Reactor Power Plants." Pursuant to 'O CFR 50.55a(f)(4)(iv),
portions of editions or addenda may be used provited that all related requirements of the respective editions or addends are met, and subject to Commission approval. Because the alternatives reet later editions of the Code, relief is not required for those inservico tests that are conducted in accordance with OM-6 and OM-10, or portions thereof, provided all related requirements are met. Whether all related requirements are met is subject to NRC inspection.
The licensee submitted relief request 51-VRR-2 to their pump and valve IST program in a letter dated May 18, 1993. This relief request required evaluation by the staff.
2.0 RELIEF RE0 VEST 51-VRR-2 The licensee is requesting relief from the Code exercise procedure requirements of ASME Section XI, Paragraph IWV-3522, for the RHR pump min-flow check valves 51-1F046 A thru 0 and 51-2F046 A thru D.
The licensee is proposing to exercise these valves in the forwari direction by indirect verification that the RHR system is passing the minimum design flow through the min-flow line. This flow will be determined by using a pump reference curve.
3.0 LICENSEE'S BASIS FOR RE00ESTING RELIEF t
The licensee states:
These valves are located on the RHR min-flow line and have an active safety position in the opcn direction to permit low flow recirculation to the suppression pool in order to protect the RHR pumps from potential damage associated with low pump flow.
Full stroke exercising these valves in the forward direction requires confirmation that the disc moves away from the seat per ASME Section XI Code, Subsection IWV, Article IWV-3522(b). These valves are neither equipped with a position indicating device nor provided with an observable mechanical means (i.e. lever) to confirm disc position. Therefore, full flow test verification of these valves by visual means is not possible.
Two other methods exist to confirm full stroke of check valves to the open position. The first method is to verify that the design flow rSte is passed through each valve as discussed in NRC Generic Letter 89-04, Attachment 1, Position 1.
The second method is to employ "other positive means" such as acoustic monitorina of the 3
valve disc position.
Both of these methods satisfy the uaent of i
IWV-3522(b).
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, Three alternatives were investigated related to the flow verification method.
Each of these alternatives quantifies flow through the RHR min-flow line.
The alternatives are (1) indirect flow measurement via conversion of a pressure signal utilizing a reference pump curve, (2) addition of test taps and associated flow indication devices immediately upstream and downstream of the RHR min-flow orifices, and (3) use of ultrasonic flow meters.
Alternative (1), indirect flow measurement via conversion of a pressure signal utilizing a reference pump curve is the most practical and cost effective option and provides a level of quality and safety equivalent to the testing prescribed in ASME IWV-3522(b) and NRC Generic Letter 89-04, Attachment 1, Position 1.
An RHR pump is run with its respective min-flow isolation valve HV-51-1(2)F007 A,B,C,0 in the open position and all other RHR pump discharge piping paths isolated.
- ssurance that all flow is diverted to the min-flow line is provided since all other discharge paths are either equipped with double isolation valves, are local leak rate tested (LLRT) or are boundary LLRT valves, or are equipped with isolation valves that are opposed by higher (reactor) pressure, except for the RHR cross-tie line. The RHR cross-tie line is equipped with a single isolation valve HV-51 (2)82A,B that is locked closed and rarely stroked which provides a high confidence that negligible leakage passes through the valve.
The design minimum flow rate for RHR is 1000 gpm which is 10% of rated RHR system flow.
The preservice testing pump curve, which is field-validated in the low flow region, is utilized to determine the corresponding pump differential pressure associated with the minimum flow value.
Next, the pump suction pressure which is determined from suppression pool level (Ref GPRR-4) is added to the min-flow differential pressure to obtr.in the maximum allowable discharge pressure corresponding to achieving the min-flow rating. This value is then converted to a voltage value via calculation based on the pressure-voltage ranges of the RHR discharge pressure transmitter PT-51-1(2)N055 A,C,E,G.
The resultant voltage is the maximum allowable discharge pressure-voltage associated with the min-flow rating. Therefore, this voltage value becomes the check valve exercise test acceptance criteria.
The actual pump discharge pressure is determined by measuring the output voltage at pressure transmitter PT-51.1(2)N055A,C,E,G with a voltmeter. This voltage is compared to the aforementioned maximum allowable discharge pressure-voltage to verify the passing 1
of the design flow rate through the valve. The actual pressure-voltage must be equal to or less than the maximum allowable pressure-voltage in order for an acceptable exercise test.
Otherwise, the exercise test is unacceptable.
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, The preservice testing pump curves will be re-validated prior to the next RHR min-flow test. The curves will be verified (a) from zero flow (shut-off head) to approximately 2 times the minimum flow rating using a total of 3 data points and (b) at the reference full flow condition.
Pump curve verification will be accomplished using pressure instruments PT-51-1(2)N055A,C,E,G for pressure readings and flow instruments FY-51-1(2)K600A,B,C,D and FT-51-1(2)N052A,B,C,D for flow readings. These instruments meet IWV-4100 accuracy requirements. The pump curve is also partially field-validated quarterly at the reference full flow value during the pump inservice test.
Alternative (2), addition of test taps and associated flow indication devices immediately upstream and downstream of the RHR min-flow orifices requires a plant modification for implementation. This option would impose a capital burden on the facility without a compensating increase in the level of quality and safety since alternative (1) provides a method which satisfies IWV-3522(b).
Alternative (3), use of ultrasonic flow meters has becn attempted and yielded erratic and erroneous results. The flow characteristics of the fluid in the RHR min-flow line are not amenable to ultrasonic flow measurement. The combination of high l
flow velocities and pipe configuration produce turbulent flow conditions which are not compatible with ultrasonic flow instrumentation. The optimum pipe location (straight pipe section) for mounting the ultrasonic flow instrument was utilized during testing. This method is used in other applications at LGS but has not been successful with the RHR min-flow test.
An alternate exercise test method utilizing "other positive means" was also considered. This alternative employs acoustic monitoring as a non-intrusive test method to record an acoustic signal of the check valve disc achieving full open position.
PEco has established a check valve monitoring program. This program, although in place is still evolving. A high confidence level exists in the closed direction check valve acoustic signatures.
However, the confidence level in the open direction check valve signatures is insufficient to assure that the design flow rate and/or full open position has been achieved.
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t 4.0 ALTERNATE TESTING The licensee proposes:
Full stroke exercise testing in the forward direction will be accomplished by indirectly verifying that the RHR design min-flow rate is passed through each valve.
Flow will be obtained via conversion of a pressure signal utilizing a reference pump curve.
5.0 EVALUATION The RHR minimum flow check valves are 4-inch valves located in each of the four RHR recirculation (min-flow) lines which direct RHR pump flow from the pump discharge back to the suppression pool. They have a safety function in the open direction to facilitate operation af the RHR pump in low flow conditions. None of the recirculation line:; currently contain flow or pressure instrumentation.
ASME Section XI, Paragraph IWV-3522(b), requires that valves which are i
normally closed and have a safety function in the open direction be tested by proving that the disk moves promptly away from the seat when there is flow through the valve or by using a mechanical exerciser. Since the recirculation lines in the RHR system at Limerick are not instrumented, direct flow measurement is not available to verify design flow through these valves.
The licensee has proposed to verify minimum design flow through the RHR recirculation check valves by verifying that the RHR discharge pressure corresponds to a pump discharge flow rate of at least 1000 gpm on a reference curve generated for each RHR pump. This test will be conducted when the RHR discharge line is isolated and all flow from the RHR pump is directed through the recirculation line and into the suppression pool.
The licensee stated in their submittal that the reference curve would be generated from three data points from pump shutoff head to approximately 2000 gpm. Figure 5.4-15 of the Updated F;nal Safety Analysis Report shows that the pump flow curve for a i
typical RHR pump is not flat in the low flow range, thereby providing better results in the conversion.
In a phone conversation with the licensee on 1
August 17, 1993, the licensee stated that from a recent test of the "D"
RHR pump on Unit 2, the total pump head for data points taken at shutoff head, 1000 gpm, and 2000 gpm, was 835 feet, 800 feet, and 760 feet.
Open exercise verification of the RHR recirculation line check valves by measuring flow through the recirculation lines is currently impractical because no flow instrumentation is installed in the lines. The licensee has investigated the use of an ultrasonic flow meter but stated in the relief request that this type of flow meter had yielded erratic and erroneous results for flow through the RHR recirculation lines during testing. Therefore, this i
alternative is not a viable option.
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. Another option suggested by the licensee would be to install pressure taps on either side of a flow orifice in the recirculation line to measure the pressure drop across the orifice, and ' convert to flow through the line.
However, this alternative would require a modification to the RHR system.
The alternative to determine flow rate through the valves by measuring pressure and using a pump reference curve to correlate the pressure to a flow rate is the best option of those evaluated by the licensee. The Code does not specify instrument accuracies or data reduction methods for determining flow through check valves. With no means of measuring flow directly, the proposed alternative enables the licensee to determine the flow rate and ensure adequate flow through the valves.
The licensee's proposed method of flow verification meets the intent of the Code and provides a reasonable assurance of full-stroke of the check valves because the discharge flow of the RHR pump can be approximated from the pump reference curve.
Installing flow instrumentation is not necessary to meet the intent of the Code to full-stroke the check valves. Requiring the licensee to install flow instrumentation when an alternate method exists which satisfies the Code intent would be an undue burden on the licensee.
6.0 CONCLUSION
Relief is granted to verify the RHR recirculation flow check valves open to pass design flow by determining flow rate using a pump reference curve pursuant to 10 'FR 50.55a(f)(6)(i) based on the impracticality of measuring flow rate dir-ly, in consideration of the licensee's proposed alternative and the burden on the licensee if the Code requirements were imposed.
Principle Contributor:
J. Colaccino Date:
September 27. 1993 l
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