ML041620392
| ML041620392 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 06/02/2004 |
| From: | Bethay S Entergy Nuclear Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 2.04.046, TAC MB8777, TAC MB8778, TAC MB8779, TAC MB8780 | |
| Download: ML041620392 (15) | |
Text
En tergy Entergy Nuclear Operations, Inc.
Pilgrim Station 600 Rocky Hill Road Plymouth, MA 02360 Stephen J. Bethay Director, Nuclear Assessment June 2, 2004 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555-0001
SUBJECT:
Entergy Nuclear Operations, Inc.
Pilgrim Nuclear Power Station Docket No. 50-293 License No. DPR-35 Pilgrim Fourth Ten-Year In-service Testing (IST) Program, IST Relief Requests: PR-03 (TAC NO. MB8779), VR-03 (MB8777),
VR-04 (MB8778), and VR-06 (MB8780)
REFERENCES:
- 1. Entergy Letter No. 2.02.109, Pilgrim Fourth Ten-Year In-service Testing (IST) Program and Request for Approval of IST Relief Requests, dated December 6, 2002.
- 2.
Entergy Letter No. 2.04.010, Pilgrim Fourth Ten-Year In-service Testing (IST) Program, Response to NRC Request for Additional Information on Pilgrim Relief Request PR-03, dated February 10, 2004.
LETTER NUMBER:
2.04.046
Dear Sir or Madam:
This letter requests NRC review and approval of the attached HPCI Pump Relief Request PR-03, Revision 1 (Attachment 1), within the scope of the fourth Ten-Year interval IST program (Reference 1). PR-03, Revision 1 replaces PR-03 included in Reference 1 and includes information previously provided to the NRC in Reference 2. Attachment 2 includes additional information requested during a subsequent conference call.
The scope of this relief applies to ASME OM Code, OMa-1 996, ISTB 5.2.3, Comprehensive Test for HPCI pumps.
Pursuant to 10 CFR 50.55a(a)(3)(i), Entergy proposes to use alternative testing to comply with ISTB 5.2.3. The proposed alternative provides an acceptable level of quality and safety because the alternative testing verifies the operational readiness of the HPCI pump.
204046 4&349
Entergy Nuclear Operations, Inc.
Pilgrim Nuclear Power Station Letter Number: 2.04.046 Page 2 Pilgrim intends to perform comprehensive HPCI surveillance testing within 90 days from receipt of an approved relief request.
The commitments contained in this letter are included in Attachment 3.
By this letter Entergy withdraws IST Relief Requests VR-03, VR-04, and VR-06, which were included in Reference 1. Entergy plans to explore the scope of VR-04 through the ASME Code committee and is considering requesting relief from specific items at a later date.
If you have any questions or require additional information, please contact Mr. Bryan Ford, Licensing Manager, at (508) 830-8403.
Sincerely, : HPCI Pump Relief Request, PR-03, Revision 1, (7 pages) : Additional Information (3 pages) : Summary of Commitments (1 page) cc:
Mr. Travis Tate, Project Manager Office of Nuclear Reactor Regulation Mail Stop: 0-8B-1 U.S. Nuclear Regulatory Commission 1 White Flint North 11555 Rockville Pike Rockville, MD 20852 Mr. Lee Licata, Project Manager Office of Nuclear Reactor Regul.
Mail Stop: 0-8B-1 U.S. NRC 1 White Flint North 11555 Rockville Pike Rockville, MOD 20852 U.S. Nuclear Regulatory Commission Region 1 475 Allendale Road King of Prussia, PA 19406 Senior Resident Inspector Pilgrim Nuclear Power Station 204046
ATTACHMENT 1 HPCI Pump Relief Recuest. PR-03. Revision 1. (7 pages)
PUMP RELIEF REQUEST PR-03. Revision I PUMP:
P-205 (Main/Booster)
SYSTEM:
High Pressure Coolant Injection (HPCI)
CLASS:
2 FUNCTION:
Provides emergency core cooling subsequent to a small break LOCA.
TEST REQUIREMENTS:
ASME OM Code OMa-1996, ISTB 5.2.3, Comprehensive Test ISTB 5.2.3(d), Vibration (displacement or velocity) shall be determined and compared with corresponding reference values. Vibration measurements are to be broad band (unfiltered). If velocity measurements are used, they shall be peak. If displacement amplitudes are used, they shall be peak-to-peak.
ISTB 5.2.3(e), All deviations from the reference values shall be compared with the ranges of Tables ISTB 5.2.1-1 and ISTB 5.2.3-1 and corrective action taken as specified in paragraph ISTB 6.2. The vibration measurements shall be compared to the relative and absolute criteria shown in the Alert and Required Action Ranges of Table ISTB 5.2.1-1. For example, if vibration exceeds either 6 Vr or 0.7 inJsec, the pump is in the Required Action Range.
RELIEF REQUESTED:
PNPS requests relief from the Code requirements of paragraph ISTB 5.2.3(d) for the HPCI Main Pump as follows:
PNPS requests relief for the Main Pump inboard (turbine side) bearing horizontal point (P3H) and the Main Pump outboard (gearbox side) bearing horizontal point (P4H) from the Code method of determining the vibration velocity (Vv) overall value for surveillance test use and for establishing reference values. PNPS proposes that vibration occurring at the discrete frequency component that is at exactly 4x Booster Pump RPM be disregarded, and not be included as part of the vibration spectrum vector summing process to obtain the main pump overall value for these points during Comprehensive pump testing. This method Is equivalent to filtering out the discrete frequency component that is at exactly 4x Booster Pump RPM from the broad band vibration spectrum. Since ISTB 5.2.3(d) requires unfiltered vibration measurements, NRC authorization is required for this deviation from the Code requirement.
PNPS requests relief from the Code requirements of paragraph ISTB 5.2.3(e) for the HPCI Main and Booster Pumps specifically from the vibration velocity (Vv) acceptance criteria specified in Table ISTB-5.2.1-1 for all Main Pump and Booster Pump vibration points except for the Booster Pump outboard horizontal axial vibration point (P8A). PNPS proposes to expand the Acceptable Range identified In Table ISTB 5.2.1-1 during Comprehensive pump testing.
BASIS FOR RELIEF:
Relief from the referenced Code requirements is based on the determination that the proposed alternative testing will provide an acceptable level of quality and safety in accordance with 1 OCFR50.55a(a)(3)(i).
Introduction/Summary:
Historic testing and analysis performed on the HPCI System by PNPS (and generically by the pump manufacturer) have consistently revealed characteristic pump vibration levels that exceed the acceptance criteria stated in Table ISTB 5.2.1-1. Observed high vibration levels during HPCI Pump testing Is a common phenomenon in the industry with similar design configurations of turbine-driven Main and Booster Pumps. The PNPS HPCI pump exhibits vibration characteristics that have been diagnosed by the pump vendor (Byron Jackson, Reference 2) as being due primarily to a hydraulic standing wave resonance in the interconnecting piping at the Booster Pump vane pass frequency (4x RPM) coinciding with structural resonances of the piping and the main pump.
The resulting high vibration appears on the Main Pump bearing housings at approximately 2x RPM in the horizontal direction, which is caused by Booster Pump excitation (at 4x RPM of the booster pump). Under normal circumstances at 4000 rpm, the vibration amplitude at the Main Pump bearings in the horizontal direction exceeds the OM Code absolute vibration Required Action Range of > 0.7 inisec. Also, under the same conditions, all of the remaining HPCI Main and Booster Pump vibration monitoring points, except for one, exceed the OM Code absolute acceptable range upper value of 0.325 InJsec.
The vibration characteristics of the HPCI Pump are predominantly a function of the pump design and should be identified as such rather than attributed to pump degradation. The high vibration has been present to the same order of magnitude since the pump was new. Although existing vibration levels of the HPCI pump are higher than the acceptance criteria provided in Table ISTB 5.2.1-1, they reflect the unique operating characteristics of the HPCI pump design configuration. It has been concluded that there are no major vibrational concerns that would result in pump degradation or would prevent the HPCI pump from performing its design safety function for an extended period of operation.
Proposed Alternate Testing to the ASME OM Code:
Pursuant to 1 OCFR50.55a(a)(3)(i), Pilgrim proposes alternative testing. The alternative testing proposes to remove the 4x Booster Pump RPM frequency component (discrete peak) from the vibration spectrum of the Main Pump since its amplitude is not related to the physical condition or rotating dynamics of the Main Pump rotor or bearing system. The Main Pump vibration spectrum, with this single 4x Booster Pump RPM frequency component removed, has been shown to be stable and more useful for monitoring actual pump condition. When this vibration frequency component at 4x Booster Pump RPM is subtracted from the Main Pump vibration spectrum the remaining vibration, which is attributed to the Main Pump, is below the OM Code Required Action Range. This corrected vibration level provides a more representative measurement of the pump condition to be used for trending.
In addition, all other discrete vibration peaks observed at the main pump horizontal vibration points will be evaluated during each Comprehensive test, and will have an Acceptable Range upper limit of 1.05 Vr and an Alert Range upper limit 1.3 Vr. These reviews of the frequency spectrum data ensure that any significant change in the vibration signature will be noted regardless of whether the severity causes the overall level to exceed its criteria. For example, if
the overall vibration level is acceptable but the 1x RPM component has increased to greater than 1.05 times the reference value overall level (Vr), then the pump will be placed in vibration alert.
This relief request also proposes to raise the upper end of the Acceptable Range for the absolute vibration level applicable to both the Booster Pump and Main Pump from the OM Code value of 0.325 in/sec to between 0.4 inJsec and 0.6 in/sec. The specific proposed absolute vibration Acceptable and Alert ranges for each HPCI Pump bearing are identified within a table as part of the Alternate Testing section of this relief request. This is requested because the Booster and Main Pump overall vibration levels (minus the discrete 4x Booster Pump RPM component for the Main Pump horizontal vibration points) are normally at or above the 0.325 in/sec value such that the pumps would always be in vibration alert. Because the discrete 4x Booster Pump RPM component for the Main Pump horizontal vibration points will be removed, the Code value of >0.7 in/sec for the Required Action Range is not proposed to be changed.
Technical Justification:
PNPS has conducted an evaluation of the HPCI pump vibration characteristics (Reference 1).
An important conclusion of the PNPS evaluation is that the mechanical condition of the Main Pump can be monitored satisfactorily by disregarding the single frequency component caused by the excitation at 4x Booster Pump RPM. The four-vane impeller of the Booster Pump generates the excitation force hydraulically. This small pressure pulsation force exists at the vane passing frequency (number of vanes times RPM) for all centrifugal pumps and is usually seen as a significant but not particularly troublesome component on the frequency spectrum for vibration measurements taken at the bearing housings. For the HPCI Pump, this vane passing frequency is a problem because it coincides with a hydraulic standing wave resonance in the cross-over piping from the Booster Pump to the Main Pump when the machine is operating at the rated speed of 4000 RPM. There is an acoustic pressure standing wave pattern, at the 4x RPM frequency, whose wavelength In water is equal to an even fraction (1/4 or 1/2) of the dimensional length inside the cross-over pipe. This Is the same principle on which an organ pipe generates a pure tone pneumatic pressure standing wave.
In addition, and exacerbating the problem, the Main Pump pedestal has a horizontal structural rocking mode of the pump pedestal at this same frequency when the main pump is operating at the rated speed of 4000 RPM. The vibration mode Is the second fundamental rocking mode, which is a torsional or twisting mode where the two end bearings move 180 degrees out of phase horizontally. The result of these coincident acoustic and structural resonances is that the Main Pump exhibits high vibration in the horizontal direction at the 4x Booster Pump RPM frequency. This is solely due to the excitation from the Booster Pump being amplified by the coincident resonances. This level of vibration at 4x Booster Pump RPM would be seen on the Main Pump bearing housings even if the Main Pump was not actually running (which is not possible as both pumps are on the same drive train).
It has also been concluded that this resonant vibration condition at the 4000 RPM operating speed is not detrimental and will not prevent the HPCI Pump from fulfilling Its function. At the 134 Hz frequency of the resonant vibration on the Main Pump, caused by the excitation at 4x Booster Pump RPM, the actual displacement amplitude at 0.7 in/sec peak velocity amplitude is 0.0017 inches peak-to-peak. This displacement imposes negligible alternating stresses on the pump pedestal, housings, and connected piping. The peak-to-peak displacement is also less than the Main Pump fluid film journal bearing clearances and would impose negligible loading to these bearings.
The purpose of the ASME OM Code for pump testing is to monitor pumps for degradation. The concept of vibration monitoring is to establish baseline values for vibration when the pump is
known to be in good working condition, such as after a maintenance overhaul. From that reference point, trending is performed to monitor for degradation based on the ratio of subsequent vibration levels relative to the reference values. The OM Code also establishes absolute vibration level criteria for Alert (>0.325 in/sec) and Required Action (>0.7 in/sec). In doing so, it was recognized that absolute vibration level limits (as opposed to relative change or ratio limits) are not always quantitatively linked directly with pump physical condition and the following remarks are stated in the ASME OMa Code 1996:
'Vibration measurements of pumps may be foundation, driver, and piping dependent.
Therefore, if initial vibration readings are high and have no obvious relationship to the pump, then vibration measurements should be taken at the driver, at the foundation, and on the piping and analyzed to ensure that the reference vibration measurements are representative of the pump and that the measured vibration levels will not prevent the pump from fulfilling its function.'
An important conclusion of the PNPS HPCI vibration evaluation is that the mechanical condition of the Main Pump can be monitored satisfactorily by disregarding the single frequency component caused by the excitation at 4x Booster Pump RPM.
A single peak frequency component can be effectively deleted from a vibration spectrum using the mean-squared subtraction method, that is, the discrete component amplitude (in/sec peak) is squared and subtracted from the spectrum overall level squared, then the square root of that difference represents the overall vibration level that exists without the energy contributed by the deleted component. It has been found that when this method is used, the remaining vibration overall level is much more consistent, stable, and trendable.
This method of vibration level correction was applied to historical spectrums. The 4x booster pump RPM component was taken out of the calculation for the main pump overall vibration level. This data shows that when the 4x booster pump RPM component is deleted from the main pump vibration, the level is below the Required Action Range (> 0.7 in./sec) but still within the Alert Range (> 0.325 inisec). It was also shown that the potential effects from the dynamic alignment of pump shaft couplings (at 2X Main Pump RPM) can still be monitored effectively.
For the HPCI Main and Booster Pumps, it has been determined that the vibration is indeed foundation and piping dependent. To reduce the HPCI Main and Booster Pump vibration down to levels that meet acceptable OM Code vibration criteria requires modifications to the HPCI Pump, mounting components, foundation and/or cross-over (interconnecting) piping.
One particular elevated vibration frequency component on the Main Pump shows the effect of the 4x Booster Pump RPM on the Main Pump horizontal vibration measurements (points P3H and P4H). Under normal circumstances at 4000 rpm, the vibration amplitude of these Main Pump vibration points exceeds the OM Code absolute vibration Required Action Range of
> 0.7 inJsec. As suggested in a Byron Jackson Tech Note (reference 2), this vibration may be improved by modifying the interconnecting piping and the Main Pump mounting pedestal. The alternative modification changes the Booster Pump impeller from four to five vanes to alter the forcing function of the standing wave resonance.
The proposed Byron Jackson modifications, other than replacing the Booster Pump impeller, are generally very difficult to implement successfully. Altering the natural frequency of a large pump installation requires either considerable additions of stiffening components or substantial additions of mass. Often the results of such design changes are unsuccessful or unfavorable due to the variable speed operation requirements.
Modification of the HPCI Booster would require replacing the current four vane impeller with an upgraded five vane impeller. The impeller modification, although yielding more predictable
results, requires extensive work to the HPCI Pump at a time when such a major rebuild of this pump is not otherwise necessary or desired. The expected result would be a modest decrease in the vibration caused on the Main Pump at 4000 RPM, although the vibration would remain above the 0.325 inch/sec Alert criteria. A small decrease in hydraulic performance is also expected when changing from a four to five vane Impeller.
It is concluded that Inservice Testing can be successfully performed for the PNPS HPCI Pump using the methods proposed in this relief request. Any significant degradation of the HPCI Pump components will be readily identified using the vibration spectral analysis methods described in the relief request. Therefore, Entergy believes that the proposed alternative testing for the PNPS HPCI Pump will provide an acceptable level of quality and safety in accordance with 10 CFR 50.55a(a)(3)(i)
ALTERNATE TESTING:
To allow for practicable monitoring of vibration levels on the HPCI pump, alternate vibration acceptance criteria are necessary. A full spectrum review will be performed for all IST vibration points during each biennial comprehensive test, utilizing the following criteria.
The tables provide the acceptance criteria that are applied to the overall vibration level for both the Main and the Booster Pumps. The note explains that for the horizontal Main Pump points, the discrete frequency component at 4x Booster Pump RPM will be removed from the overall value using the mean-squared subtraction.
MAIN PUMP**
Test Vibration Point Acceptable Rance Alert Range Reouired Action Parameter Range Vv MainPump**
2.5Vr
>2.5Vr to6Vr
>6Vr Horizontal but not or or Inboard & Outboard
> 0.60 inJsec
> 0.60 to 0.70
> 0.70 inisec (P3H & P4H) inJsec MainPump
- 2.5Vr
>2.5Vr tO6Vr
>6Vr Vertical but not or or Inboard & Outboard
> 0.40 inisec
> 0.40 to 0.70
> 0.70 in.isee (P3V & P4V) inisec MainPump S
2.5Vr
>2.5Vr to6V,
>6V, Axial but not or or Inboard (P3A)
> 0.50 inJsec
> 0.50 to 0.70
> 0.70 inJsec
_ _ _ _ _ _ _ _ _ _in is e c_
- Note for Main Pump Horizontal vibration points P3H and P4H. A frequency spectrum analysis will be performed for each comprehensive pump test, and the discrete peak at 4x booster pump RPM will be removed (using mean-squared subtraction method) from the each horizontal main pump spectrum overall value. In addition, all other horizontal main pump discrete peaks will be evaluated during each test, and will have an Acceptable Range upper limit of 1.05 Vr and an Alert Range upper limit 1.3 Vr.
BOOSTER PUMP Test Vibration Point Accentable Range Alert Ranae Reauired Action Parameter Range Vv Booster Pump
- 2.5 V,
> 2.5 Vr to 6 V,
> 6 Vr Horizontal but not or or Inboard & Outboard
> 0.50 inJsec
> 0.50 to 0.70
> 0.70 InJsec (P7H & P8H) insec Booster Pump
- 2.5 V,
> 2.5 Vr to 6 Vr
> 6 Vf Vertical but not or or Inboard & Outboard
> 0.40 inisec
> 0.40 to 0.70
> 0.70 inisec (P7V & P8V) inisec Booster Pump
< 2.5 Vr
> 2.5 Vr to 6 Vr
> 6 Vr Axial but not or or Outboard (P8A)
> 0.325 inJsec
> 0.325 to 0.70
> 0.70 inJsec inJsec SUPPORTING TECHNICAL JUSTIFICATION:
PNPS has conducted an evaluation of the HPCI pump vibration characteristics (Reference 1).
This evaluation has determined that the high vibration on the HPCI Main Pump at just over 2x RPM is due primarily to a hydraulic standing wave resonance in the interconnecting piping from the Booster Pump at the pump vane-passing frequency (4x Booster Pump RPM) coinciding with structural resonances of the cross-over piping and the Main Pump pedestal when the machine is operating at the rated speed of 4000 RPM. The Main and Booster Pumps are connected via a speed reduction gear box (1.983 to 1 ratio) such that the Main Pump rated speed of 4000 RPM corresponds to a Booster Pump speed of 2017 RPM. This results in high vibration on the Main Pump bearing housings appearing at just over 2x RPM in the horizontal direction but caused by the Booster Pump excitation at 4x Booster Pump RPM, transmitted and amplified by the interconnecting cross-over piping. This vibration at 4x Booster Pump RPM effectively dominates readings taken on the Main Pump such that the overall level would not reflect potentially significant changes In frequency components directly related to the operation and rotor dynamics of the Main Pump.
For the HPCI Pump, the potential effect from dynamic alignment of shaft couplings has been determined to be minor using measurement methods equivalent to those described in the Technical Discussion section of Byron Jackson Tech Note (reference 2). High resolution frequency spectrum analysis was performed that allowed the 2x Main Pump RPM vibration (indicative of misalignment) to be separated from the closely spaced vibration component at 4x Booster Pump RPM, and the 2x RPM component was found to have an acceptably low level.
It has been concluded that the vibration at 4x Booster Pump RPM is not physically detrimental to the Main Pump, the interconnecting piping, or the pump pedestal. The actual displacement associated with the vibration at 4x Booster Pump RPM (2017 RPM x 4 = 8068 RPM = 134.5 Hz) is very small (0.7 in/sec @ 134.5 Hz = 0.0017 inches peak-to-peak). Therefore, the resulting stresses due to such deflections are correspondingly low in the cross-over piping and pump pedestal. The Main Pump rotor is supported on oil film journal bearings. These bearings are very tolerant of pedestal and housing vibration at frequencies above the shaft running speed since the actual displacements are so small.
PNPS has also concluded that none of the possible modifications that can be performed on the HPCI pump, mounting pedestal, or cross-over piping are necessary. This is primarily due to the nature of the HPCI Pump service profile. The Byron Jackson Tech Note describes the following consideration in the Technical Discussion:
Pumping systems in which the vane passing pressure pulsations form standing waves in the attached piping are not unusual, especially If the pumps have a variable speed driver.
Standing waves are highly dependent upon water temperature. Thus, measured vibration amplitudes often vary from test to test. n The HPCI Pump service is such that the pump runs for short periods of time and at highly variable speeds. The pump inservice testing at PNPS is performed with the pump operating at or close to its rated speed (4000 RPM) and flow conditions (4250 GPM) that are unique to PNPS. For this particular pump configuration, this pump speed corresponds to the point where the acoustic resonant vibration Is typically most pronounced. In actual service for high pressure coolant injection to the reactor, the pump will operate at the speed that the flow controller requires to maintain reactor water level. The flow rate of 4250 GPM is the maximum makeup flow rate for which the HPCI system was intended to be capable of maintaining reactor water level. This flow rate is far in excess of the decay heat makeup water requirements for the reactor in the isolated condition in the absence of a major leak. The pump speed required is also dependent on reactor pressure with the required speed decreasing along with reactor pressure.
The same HPCI Pump configuration is used at other plants but often with different rated speeds and flows such that the vibration characteristics at the inservice testing point are markedly different for that reason. The vibration monitoring performed (including a frequency spectral review) to date under the IST program and the PNPS pump vibration monitoring program has shown that there has not been degradation of these HPCI Pump components.
DURATION OF PROPOSED ALTERNATIVE The proposed alternative testing shall apply for the remainder of the 4" Inservice Testing Interval at Pilgrim.
REFERENCES (1) PNPS vibration evaluation (ERM 90-445 Rev. 2 HPCI Pump Vibration Concems dated 06/17/1994) [Provided by Entergy letter 2.04.010, dated February 10, 2004].
(2) Byron Jackson Technical Service Bulletin (Tech Note) 9112-80-018 'HPCI Pump Vibration*
ATTACHMENT 2 ADDITIONAL INFORMATION (3 naaes)
Addition Information in Response NRC Questions 1 through 4 Included in Reference 2 As part of the PNPS vibration evaluation of the HPCI Pump, the Byron Jackson Technical Service Bulletin (Tech Note) 9112-8018 "HPCI Pump Vibration" was one of the basis documents referenced in the PNPS vibration evaluation (ERM 90-445 Rev. 2 "HPCI Pump Vibration Concerns dated 06/17/1994). The vibration evaluation performed in 1994 was done in anticipation of the future adoption of the ASME OM Code for the In-service Testing (IST) program that, for the first time, included vibration criteria given in absolute values in addition to values relative to baseline reference vibration data. The Tech Note recommended the following actions be performed if vibration readings exceed the ASME Code limits:
If bearing housing vibrations exceed the ALERT limits specified in the ASME Boiler and Pressure Vessel Code,Section XI, Subsection IWP-3000, the following actions are recommended:
Verify correct dynamic alignment of all shaft couplings.
Verify acoustic resonant condition from measured flow, RPM, water temperature, and vibration frequency.
Replace four vane booster pump impeller with a staggered five vane impeller. "
In addition, the Tech Note had the following potential pump modifications in the section
'Corrective Actions.:
"The following corrective actions are possible:
Change the number of vanes in the booster pump impeller Change the length of the interconnecting piping Install an acoustic filter in the interconnecting pipe After the mounted structural natural frequency of the injection pump.
The most cost-effective solution has been to replace the existing four vane booster pump impeller with a five vane impeller to raise the vane passing frequency 25 percent above the standing wave resonant frequency. "
The PNPS vibration evaluation determined that the HPCI Main Pump has an elevated overall vibration level due primarily to an acoustic resonance that, at the 4000 RPM'speed, coincides with excitation at 4x Booster Pump RPM. The potential effect from dynamic alignment of shaft couplings has been determined to be minor using measurement methods equivalent to those described in the Technical Discussion section of the Byron Jackson Tech Note. High resolution frequency spectrum analysis was performed that allowed the 2x Main Pump RPM vibration (indicative of misalignment) to be separated from the closely spaced vibration component at 4x Booster Pump RPM, and the 2x RPM component was found to have an acceptably low level.
The high resolution vibration spectrums confirmed that, for certain Main Pump vibration measurements (points P3H and P4H), there is a significant vibration frequency component at exactly 4x Booster Pump RPM. This has been attributed to the acoustic standing wave resonance of the interconnecting pipe coinciding with the first torsional rocking mode of the Main Pump on its pedestal. As suggested in the Byron Jackson Tech Note, this vibration can be addressed by modifying the interconnecting piping and the Main Pump mounting pedestal.
The alternative modification is to change the Booster Pump impeller from four to five vanes to alter the forcing function for the standing wave resonance.
PNPS concluded that these modifications are not necessary for the HPCI Pump, primarily because of the nature of the HPCI Pump service profile. The Byron Jackson Tech Note describes the following consideration In the Technical Discussion:
Pumping systems in which the vane passing pressure pulsations form standing waves in the attached piping are not unusual, especially if the pumps have a variable speed driver. Standing waves are highly dependent upon water temperature. Thus, measured vibration amplitudes often vary from test to test.
- The HPCI Pump service is such that the pump runs for short periods of time and at highly variable speeds. The pump inservice testing at PNPS is performed with the pump operating at or close to its rated speed (4000 RPM) and flow conditions (4250 GPM), which are unique to PNPS. For this particular pump configuration, this pump speed corresponds to the point where the acoustic resonant vibration is typically most pronounced. In actual service for high pressure coolant injection to the reactor, the pump will operate at the speed that the flow controller requires to maintain reactor water level. The flow rate of 4250 GPM is the maximum makeup flow rate for which the HPCI system was intended to be capable of maintaining reactor water level. This flow rate is far in excess of the decay heat makeup water requirements for the reactor in the isolated condition in the absence of a major leak. The pump speed required is also dependent on reactor pressure with the required speed decreasing along with reactor pressure. The same HPCI Pump configuration is used at other plants but with different rated speeds and flows such that the vibration characteristics at the inservice testing point are markedly different for that reason.
The vibration criteria In the ASME OM Code for pump testing uses absolute velocity values (inches/sec peak) that are generally accepted industry values used for screening vibration data to identify potentially degraded rotating equipment such as pumps, fans, motors, and turbines.
When the measured vibration exceeds the overall level criteria, the following is stated in the ASME OMa Code (1996):
"Vibration measurements of pumps may be foundation, driver, and piping dependent. Therefore, if Initial vibration readings are high and have no obvious relationship to the pump, then vibration measurements should be taken at the driver, at the foundation, and on the piping and analyzed to ensure that the reference vibration measurements are representative of the pump and that the measured vibration levels will not prevent the pump from fulfilling its function. "
For the HPCI Main Pump, it has been determined that the vibration Is indeed foundation and piping dependent and that one particular elevated vibration frequency component on the Main Pump is not related to the condition of the pump. Vibration spectrums clearly show the effect of the 4x Booster Pump RPM on the Main Pump. This one frequency component can be separated from the Main Pump vibration so that the relevant vibration (that is related to the condition of the Main Pump) can be effectively monitored and trended as required by the ASME Code, which is based on overall readings that are the sum of all frequency components on the vibration spectrum (using mean-squared summation).
It has also been concluded that this resonant vibration condition at the 4000 RPM operating speed is not detrimental and will not prevent the HPCI Pump from fulfilling its function. At the 134 Hz frequency of the resonant vibration on the Main Pump caused by the excitation at 4x Booster Pump RPM, the actual displacement amplitude at 0.70 in/sec peak velocity amplitude is 0.0017 inches peak-to-peak. This displacement imposes negligible alternating stresses on the pump pedestal, housings, and connected piping. The peak-to-peak displacement is also less
than the Main Pump fluid film journal bearing clearances and would impose negligible loading to these bearings.
The proposed Byron Jackson modifications, other than the five vane impeller, are generally very difficult to implement successfully. Altering the natural frequency of a large pump installation requires either considerable additions of stiffening components or substantial additions of mass.
Often the results of such design changes are unsuccessful or unfavorable due to the variable speed operation requirements. The impeller modification, although yielding more predictable results, Involves complete disassembly of the Booster Pump at a time when such a major rebuild of this pump is not otherwise necessary or desired. The expected result would be a modest decrease in the vibration caused on the Main Pump at 4000 RPM, although the vibration would very likely remain above the 0.325 inch/sec Alert criteria. A small decrease in hydraulic performance is also expected when changing from a four to five vane impeller.
It is concluded that In-service Testing can be successfully performed for the PNPS HPCI Pump using the methods proposed in relief request PR-03, Rev. 1 (Attachment 1 of ENO 2.04.046).
Any significant degradation of the HPCI Pump components will be readily identified using the vibration spectral analysis methods described in the relief request. The vibration monitoring performed (including a frequency spectral review) to date under the IST program and the PNPS pump vibration monitoring program has shown that there has not been degradation of these HPCI Pump components.
ATTACHMENT 3 Summary of Commitments Commitment ID Description Due Date 2.04.046-001.
Entergy will perform comprehensive HPCI pump Within 90 days upon testing in accordance with this relief request.
receipt of NRC approved PR-03,
_ Rev. 1