ML12109A249
| ML12109A249 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 05/17/2012 |
| From: | Istvan Frankl Plant Licensing Branch III |
| To: | Schimmel M Northern States Power Co |
| Wengert T | |
| References | |
| TAC ME7227, TAC ME7228 | |
| Download: ML12109A249 (8) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 May 17, 2012 Mr. Mark A. Schimmel Site Vice President Prairie Island Nuclear Generating Plant Northern States Power Company - Minnesota 1717 Wakonade Drive East Welch, MN 55089-9642
SUBJECT:
PRAIRIE ISLAND NUCLEAR GENERATING PLANT, UNITS 1 AND 2 - RELIEF
[ALTERNATIVE] REQUEST 12 CONCERNING THE FOURTH 10-YEAR INSERVICE TESTING PROGRAM (TAC NOS. ME7227 AND ME7228)
Dear Mr. Schimmel:
By letter dated September 21, 2011, Northern States Power Company, a Minnesota corporation (NSPM), dOing business as Xcel Energy (the licensee), submitted a request to the Nuclear Regulatory Commission (NRC) for the use of an alternative to certain American Society of Mechanical Engineers (ASME) Code for Operation and Maintenance of Nuclear Power Plants (OM Code) requirements at Prairie Island Nuclear Generating Plant (PINGP), Units 1 and 2.
The licensee requested authorization to use an alternative range for vibration measurements for two pumps at PINGP rather than the required vibration measurement criteria per the OM Code.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(a)(3)(i),
the licensee requested to use the proposed alternative on the basis that the alternative provides an acceptable level of quality and safety.
The NRC staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that NSPM has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(a)(3)(i), and is in compliance with the ASME Code's requirements.
Therefore, the NRC staff authorizes the Relief [Alternative] Request 12 for the remainder of the fourth 1ST interval for PINGP Unit 1 and Unit 2, currently scheduled to end on December 20, 2014. All other ASME B&PV or ASME OM Code requirements for which relief was not specifically requested and approved in the subject request remain applicable.
M. A. Schimmel - 2 If you have any questions, please contact the NRC Project Manager, Tom Wengert at 301-415-4037 or via e-mail at Thomas.Wengert@nrc.gov.
Sincerely,
)
Istvan Frankl, Acting Chief Plant Licensing Branch 111-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-282 and 50-306
Enclosure:
Safety Evaluation cc w/encl: Distribution via ListServ
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO ALTERNATIVE REQUEST 12 FOR THE FOURTH 10-YEAR INTERVAL INSERVICE TESTING PROGRAM NORTHERN STATES POWER COMPANY - MINNESOTA PRAIRIE ISLAND NUCLEAR GENERATING PLANT. UNITS 1 AND 2 DOCKET NOS. 50-282 AND 50-306
1.0 INTRODUCTION
By letter dated September 21, 2011 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML112650095), Northern States Power Company (NSPM), a Minnesota Corporation, dOing business as Xcel Energy (the licensee), submitted Alternative Request 12 to the Nuclear Regulatory Commission (NRC) for two pumps at the Prairie Island Nuclear Generating Plant (PINGP). The licensee requested authorization to use an alternative range for vibration measurements rather than the required vibration measurement criteria per the American Society of Mechanical Engineers (ASME) Code for Operation and Maintenance of Nuclear Power Plants (OM Code). Alternative Request 12 is applicable to the fourth 10-year in service testing (1ST) program interval at PINGP Units 1 and 2.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(a)(3)(i),
the licensee requested to use the proposed alternative in Request 12 on the basis that the alternative provides an acceptable level of quality and safety.
2.0 REGULATORY EVALUATION
The regulation in 10 CFR 50.55a{f), "Inservice testing requirements," requires, in part, that 1ST of certain ASME Code Class 1, 2, and 3 components must meet the requirements of the ASME OM Code and applicable addenda, except where alternatives have been authorized pursuant to paragraphs (a){3)(i) or (a)(3)(ii).
In proposing alternatives, a licensee must demonstrate that the proposed alternatives provide an acceptable level of quality and safety (10 CFR 50.55a{a)(3)(i>>; or compliance would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety (10 CFR 50.55a(a)(3)(ii>>. Section 50.55a allows the NRC to authorize alternatives to ASME OM Code requirements upon making necessary findings.
The PINGP Units 1 and 2 fourth 1O-year 1ST program interval commenced on December 21, 2004, and ends on December 20,2014. The applicable ASME OM Code Edition and Addenda for PINGP Units 1 and 2 is the 1998 Edition through the 2000 Addenda.
Enclosure
- 2 The NRC's findings with respect to authorizing alternative Request 12 are provided below:
3.0 TECHNICAL EVALUATION
3.1 Licensee's Alternative Request ISTB-3300, "Reference Values", (a), states that, "Initial reference values shall be determined from the results of testing meeting the requirements of ISTB-3100, Preservice Testing, or from the results of the first inservice test."
ISTB-3300(d), states that, "Reference values shall be established at a point(s) of operation (reference point) readily duplicated during subsequent tests."
ISTB-3300(f), states that, "All subsequent test results shall be compared to these initial reference values or to new reference values established in accordance with ISTB-3310, ISTB-3320, or ISTB-6200(c)."
ISTB-3540, "Vibration", (a), states that, "On centrifugal pumps, except vertical line shaft pumps, measurements shall be taken in a plane approximately perpendicular to the rotating shaft in two approximately orthogonal directions on each accessible pump-bearing housing. Measurement shall also be taken in the axial direction on each accessible pump thrust bearing housing."
ISTB-5121, "Group A Test Procedure", (e), states that, "All deviations from the reference values shall be compared with the ranges of Table ISTB-5100-1 and corrective action taken as specified in ISTB-6200. Vibration measurements shall be compared to both the relative and absolute criteria shown in the alert and required action ranges of Table ISTB-5100-1. For example, if vibration exceeds either 6 Vr or 0.7 in/sec [inches per second] (1.7 cm/sec), the pump is in the required action range."
ISTB-6200, "Corrective Action", (a), "Alert Range", states that, "If the measured test parameter values fall within the alert range of Table ISTB-51 00-1 , Table ISTB-5200-1, Table ISTB-5300-1, or Table ISTB-5300-2, as applicable, the frequency of testing specified in ISTB-3400 shall be doubled until the cause of the deviation is determined and the condition is corrected."
Alternative testing was requested for the following centrifugal pumps:
045-591 - 121 Control Room Chilled Water Pump (Class 3) 045-592 - 122 Control Room Chilled Water Pump (Class 3)
In its September 21, 2011 request, the licensee stated:
The ASME OM Code, paragraph ISTB-3540, requires that for centrifugal pumps, vibration measurements shall be taken in a plane approximately perpendicular to the rotating shaft in two approximately orthogonal directions on each accessible pump bearing housing. Measurement shall also be taken in the axial direction on each accessible pump thrust bearing housing. [ ... ] These measurements are required to be compared with the ASME OM Code vibration acceptance criteria
- 3 as specified in Table ISTB-S100-1 [ ... ] to determine if the measured values are acceptable.
Table ISTB-S100-1 states that, if during an inservice test, a vibration measurement exceeds 2.S times the previously established reference value (Vr),
the pump is considered in the alert range. The frequency of testing is then doubled in accordance with paragraph ISTB-6200(a), until the cause of the deviation is determined and the condition is corrected and the vibration level returns to the acceptable range level. Pumps whose vibration is measured as greater than 6 times Vr are considered to be in the required action range, and must be declared inoperable until cause of the deviation has been determined and the condition is corrected. Per ISTB-3300, the vibration reference values shall be established only when the pump is known to be operating acceptably.
For pumps whose absolute magnitude of vibration is an order of magnitude below the absolute vibration limits in Table ISTB-S1 00-1, a relatively small increase in vibration magnitude may cause the pump to enter the alert or required action range. These instances may be attributed to variation in flow, instrument accuracy, or other noise sources that would not be representative of actual pump degradation. Pumps that operate in this region are typically referred to as "smooth-running." Based on a small acceptable range, a smooth running pump could be subjected to unnecessary corrective action.
Each of these pumps [listed above] has at least one Vr value that is currently less than O.OS in/sec. In order to avoid unnecessary corrective actions, a minimum value for Vr of O.OS in/sec is proposed. This minimum value would be applied to individual pump vibration locations with reference vibration values less than or equal to O.OS in/sec. Therefore, the smallest ASME OM Code Acceptable Range limit for any 1ST pump vibration measurement location would be no lower than 2.S times V r , or 0.12S in/sec. [ ... ] Likewise, the smallest ASME OM Code Alert Range limit for any 1ST Pump vibration measurement location for which the pump would be inoperable would be no lower than 6 times V r , or 0.300 in/sec. ASME OM Table ISTB-S100-1 specifies a vibration Acceptable Range of 0.32S to 0.7 in/sec for pumps ~ 600 rpm [revolutions per minute].
In addition to the requirements of ISTB for inservice testing, pumps in the PINGP Inservice Test Program are also included in the PINGP Predictive Maintenance (PdM) Program. The PINGP PdM Program currently employs predictive monitoring techniques such as: vibration monitoring and analysis beyond that required by ISTB, bearing temperature trending, oil sampling and analysis, and/or thermography analysis, as applicable.
If the measured parameters [noted above] are outside the normal operating range or are determined by analysis to be trending toward an unacceptable degraded state, appropriate actions are taken that may include: initiation of a Condition Report (CR), increased monitoring to establish a rate of change, review of component specific information to identify the cause of the condition, and removal of the pump from service to perform maintenance. Experience gathered by the PINGP PdM Program has shown that changes in vibration levels
-4 in the range of 0.05 in/sec do not normally indicate significant degradation in pump performance.
In its request, the licensee stated that, in lieu of applying the vibration acceptance criteria ranges specified in Table ISTB-5100-1, the above listed pumps that have locations with measured vibration reference values at or below 0.05 in/sec, will have subsequent test results for those locations compared to an Acceptable Range limit of 0.125 in/sec and an Alert Range limit of 0.300 in/sec. These proposed ranges will be applied to vibration test results during both Group A and Comprehensive pump tests.
The licensee further stated that, using the provisions of this relief request as an alternative to the specific requirements of ISTB identified above, this alternative will provide adequate indication of pump performance and continue to provide an acceptable level of quality and safety without unnecessarily imposing corrective actions since changes in vibration levels in the range of 0.05 in/sec do not normally indicate significant degradation in pump performance.
The licensee concluded that, using the provisions of this relief request as an alternative to the vibration acceptance criteria ranges specified in Table ISTB-5100-1 provides an acceptable level of quality and safety since the alternative provides reasonable assurance of pump operational readiness and the ability to detect pump degradation.
3.2 NRC Staff Evaluation The ASME OM Code requires that the vibration of all safety-related pumps be measured. For centrifugal pumps, paragraph ISTB-3540(a) notes that vibration measurements shall be taken in a plane approximately perpendicular to the rotating shaft in two approximately orthogonal directions on each accessible pump-bearing housing. Measurement shall also be taken in the axial direction on each accessible pump thrust bearing housing. These measurements are required to be compared with the ASME OM Code vibration acceptance criteria as specified in Table ISTB-5100-1 to determine if the measured values are acceptable.
Table ISTB-51 00-1 shows that, if during an inservice test, a vibration measurement on a centrifugal pump exceeds 2.5 times the reference value (Vr) , previously established as required by ISTB-3300, the pump is considered to be in the alert range. The frequency of testing is then doubled in accordance with ISTB-6200 until the cause of the deviation is determined, the condition is corrected, and the vibration level returns below the alert range. Pumps, whose vibration is measured at greater than 6 times V r, are considered to be in the required action range and must be declared inoperable until the cause of the deviation has been determined and the condition is corrected. Per ISTB-3300, the vibration reference values shall be established only when the pump is known to be operating acceptably.
For pumps whose absolute magnitude of vibration is an order of magnitude below the absolute vibration limits in Table ISTB-51 00-1, a relatively small increase in vibration magnitude may cause a pump to enter the alert or required action range even though the pump is operating acceptably. The pump may enter these ranges due to variations in flow, instrument accuracies, or other noise sources that would not be associated with degradation of the pump. Pumps that operate with these low vibrations are typically referred to as "smooth running." Since the acceptable range for a "smooth running" pump is so small, the pump could be subjected to unnecessary corrective action due to entry into the alert or action ranges.
-5 The NRC has previously authorized a minimum vibration level of 0.05 in/sec for "smooth running" pumps at several nuclear plants. However, monitoring only the vibration levels of "smooth running" pumps has been shown to be insufficient for determining pump degradation.
At one particular plant, the NRC staff authorized a minimum reference value of 0.1 in/sec. A pump bearing at this plant experienced a significant degradation even though the pump vibration levels were below the minimum reference value in the approved alternative. The bearing degradation was discovered during PdM program activities.
The licensee's alternative combines a minimum vibration value of 0.05 in/sec for two chilled water "smooth running" pumps with additional monitoring of the pumps using a PdM program that includes vibration analysis beyond that required by the ASME OM Code, including bearing temperature trending, oil analysis, and thermography analysis, as applicable.
The licensee notes in their alternative request that if any of the measured parameters, including predictive maintenance values, are outside of the normal operating range or are determined by analysis to be trending towards an unacceptable degraded state, appropriate actions will be taken. These actions include increased monitoring to establish a rate of change, review of component specific information to identify the cause of the condition, and removal of the pump from service to perform maintenance. The proposed alternative is consistent with the objectives of the 1ST, which is to monitor degradation in safety-related components.
Based on the minimum vibration reference value of 0.05 in/sec noted in this request and the licensee's implementation of the proposed PdM program, the NRC staff finds that the alert and required action limits specified in the request sufficiently address previously undetected pump problems and provide an adequate indication of pump performance. The licensee's PdM program is designed to detect problems involving unacceptable mechanical conditions in advance of either of the chilled water pumps reaching the overall vibration alert or action limits.
Therefore, the NRC staff finds that the proposed alternative will provide an acceptable level of quality and safety.
4.0 CONCLUSION
As set forth above, the NRC staff determines that the proposed alternative described in Request 12 provides an acceptable level of quality and safety for the 121 Control Room Chilled Water Pump, 045-591, and the 122 Control Room Chilled Water Pump, 045-592. Accordingly, the NRC staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(a)(3)(i), and is in compliance with the ASME Code's req uirements.
Therefore, the NRC staff authorizes the Alternative Request 12 for the remainder of the fourth 1ST interval for PINGP Unit 1 and Unit 2, currently scheduled to end on December 20,2014. All other ASME B&PV or ASME OM Code requirements for which relief was not specifically requested and approved in the subject request remain applicable.
Principal Contributor: M. Farnan, NRR Date: May 17, 2012
M. A. Schimmel -2 If you have any questions, please contact the NRC Project Manager, Tom Wengert at 301-415-4037 or via e-mail at Thomas.Wengert@nrc.gov.
Sincerely, IRAJ Istvan Frankl, Acting Chief Plant Licensing Branch "1-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-282 and 50-306
Enclosure:
Safety Evaluation cc wtencl: Distribution via ListServ DISTRIBUTION:
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NAME TWengert BTuily AMcMurtray* IFrankl DATE 04/30/12 04/24/12 03/28/12 05/17/12 OFFICIAL RECORD COpy