ML14077A102

BWROG-TP-13-009, Rev. 0, Containment Accident Pressure Committee (344) Task 3 Pump Operation at Reduced Npsha Conditions (Cvic Pump), Attachment 10

ML14077A102
Person / Time
Site:	Peach Bottom, Browns Ferry, Boiling Water Reactor Owners Group
Issue date:	06/30/2013
From:	Kalra A BWR Owners Group, Sulzer Pumps (US)
To:	GE-Hitachi Nuclear Energy Americas, Office of Nuclear Reactor Regulation
Shared Package
ML14077A088	List: ML14077A090 ML14077A091 ML14077A092 ML14077A093 ML14077A095 ML14077A102
References
BWROG-14014 BWROG-TP-13-009, Rev. 0, E12.5.1978
Download: ML14077A102 (23)

Text

Containment Accident Pressure Committee (344) Task 3 - Pump Operation at Reduced NPSHa conditions (CVIC Pump) Authors: Ankur Kalra (Sulzer Pump)

Project Manager: Kenneth Welch (GEH)

Committee Chair: John Freeman (Exelon) BWROG-TP-13-009 Revision 0 June 2013 BWROG-TP-13-009 REV 0 INFORMATION NOTICE Recipients of this document have no authority or rights to release these products to anyone or organization outside their utility.

The recipient shall not publish or otherwise disclose this document or the information therein to others without the prior written consent of the BWROG, and shall return the document at the request of BWROG. These products can, however, be shared with contractors performing related work directly for the participating utility, conditional upon appropriate proprietary agreements being in place with the contractor protecting these BWROG products.

With regard to any unauthorized use, the BWROG participating Utility Members make no warranty, either express or implied, as to the accuracy, completeness, or usefulness of this guideline or the information, and assumes no liability with respect to its use. BWROG Utility Members CENG - Nine Mile Point Chubu Electric Power Company DTE - Fermi Chugoku Electric Power Company Energy Northwest - Columbia Comisión Federal de Electricidad Entergy - FitzPatrick Hokuriku Electric Power Company Entergy - Pilgrim Iberdrola Generacion, S.A. Entergy - River Bend/Grand Gulf Japan Atomic Power Company Entergy - Vermont Yankee J-Power (Electric Power Development Co.)

Exelon (Clinton)

Kernkraftwerk Leibstadt Exelon (D/QC/L)

South Texas Project Exelon (Oyster Creek)

Taiwan Power Company Exelon (PB/Limerick)

Tohoku Electric Power Company FirstEnergy - Perry Tokyo Electric Power Company NPPD - Cooper NextEra - Duane Arnold PPL - Susquehanna PSEG - Hope Creek Duke Energy - Brunswick SNC - Hatch TVA - Browns Ferry Xcel - Monticello BWROG-TP-13-009 REV 0 2 Executive Summary This BWROG Technical Product provides a technical evaluation of operation of the Sulzer CVIC pump model at reduced Available Net Positive Suction Head (NPSHa) conditions, including short periods of operation with the NPSHa less than the required NPSH (NPSHr). The CVIC pump model is used at the Browns Ferry, Peach Bottom, and other BWR stations. This evaluation addresses the effect on pump flow rate as well as the mechanical impact of low suction head on essential pump components.

Implementation Recommendations This product is intended for use to address (in part) issues raised in the NRC Guidance Document for the Use of Containment Accident Pressure in Reactor Safety Analysis (ADAMS Accession No. ML102110167). Implementation will be part of the BWROG guidelines on the use of Containment Accident Pressure credit for ECCS pump NPSH analyses. Benefits to Site This product provides a technical response to the NRC concerns raised in the reference above regarding the potential adverse consequences of short term pump operation with NPSHa<NPSHr.

QUALITY LEVEL SULZER PUMPS (US) INC. DOCUMENT ASME CODE SECTION Direct DOC. NO: E12.5.1978 Indirect ORDER NO:

CLASS NO. CODE EDITION (YEAR) TITLE: Task 3 - Pump Operation at Reduced NPSHa Sulzer Pumps (US) Inc.

SEASON YEAR Browns Ferry and Peach Bottom - 18x24x28 CVIC - RHR Pumps CUSTOMER GE-HITACHI Nuclear Energy Americas LLC PROJECT Browns Ferry and Peach Bottom Power Stations CUSTOMER P.O. NO.

437054820 CONTRACT NUMBER SPECIFICATION NO.

ITEM / TAG NUMBER CUSTOMER APPROVAL NUMBER: CUSTOMER APPROVAL REQUIREMENT Yes No Information Only SPACE FOR CUSTOMER APPROVAL STAMP CERTIFIED AS A VALID SULZER PUMPS (US) INC. DOCUMENT (when applicable/available)

For Outside Vendor

For Manufacture at Sulzer Pumps (US) Inc.

Risk Release Inspection Report # ________________

Other (specify)

_______________________ APPROVALS (SIGNATURE) Date Engineering 02/19/13 Quality Assurance CERTIFICATION (when applicable)

Originating Advance Engineering This Document is certified to be in compliance Dept: with THE APPLICABLE PURCHASE ORDER, By: SPECIFICATIONS, PROCEDURES, AND ADDITIONAL REQUIREMENTS LISTED IN Ankur Kalra THE APPENDICES. Title: Hydraulic Design Engineer Date: 09/04/2012

__________________________________________ Professional Engineer APPLICABLE S.O. NUMBERS: ___________ _____________________________ 270671/82 State Registration No. 270683/90 Date _______________

E12.5.1978 - Rev. DOCUMENT IDENTIFICATION Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 1 TABLE OF CONTENTS

1. PURPOSE .......................................................................................................................

............................................... 2

2. BACKGROUND ....................................................................................................................

....................................... 2

3. SCOPE .........................................................................................................................

.................................................. 6

4. ANALYSIS ......................................................................................................................

.............................................. 7 4.1 R ESULTS OF THE I N-S ITU C AVITATION T EST ON THE RHR PUMP AT L OW NPSH A. ................................................ 9 4.2 V ENDOR T ESTING OF B ROWNS F ERRY/P EACH B OTTOM AND OTHER CVIC P UMPS .............................................. 12 4.3 E XCITATION F REQUENCY AND F AILURE M ODES A NALYSIS FOR L ONG-T ERM P UMP O PERATION ........................ 13

5. CONCLUSION ....................................................................................................................

....................................... 17

6. BIBLIOGRAPHY ..................................................................................................................

..................................... 19

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 2 1. PURPOSE To evaluate the effects of operating Sulzer CVIC pumps used in the Residual Heat Removal (RHR) systems at the Browns Ferry and Peach Bottom Nuclear Plants at reduced available Net Positive Suction Head (NPSHa). This includes a short period of operation with NPSHa below the 3% head breakdown required NPSH (NPSHr). For this report, NPSH3 is synonymous with the 3% head breakdown NPSHr of the pump. RHR pump operation is required to assist in mitigation of a Design Basis Accident - Loss of Coolant Accident (DBA-LOCA). For a DBA-LOCA, RHR is assumed to have a mission time of [[

]]. It is important that during the time period when NPSHa < NPSH3, adequate core cooling flow rates are maintained by the operation of RHR and Core Spray pumps, and the pump does not experience any damage that would result in it being unable to perform its safety function for the required longer term mission time. This evaluation addresses low suction head effects on pump flow rate as well as hydraulic and mechanical impacts on essential pump components and attached piping.

2. BACKGROUND NPSH3 is the suction head at which pump discharge head performance degrades 3% compared to the non-cavitating head. Cavitation occurs when the pressure inside the pump drops below the vapor pressure of the pumpage and cavities (vapor bubbles) are formed on the impeller blades. In addition to impairing hydraulic performance, the bubbles can implode at the impeller surfaces, which in the long term can cause impeller erosion.

There are three primary factors that influence cavitation erosion: 1) hydrodynamic cavitation intensity,

2) cavitation resistance of the impeller material, and 3) the time duration over which the cavitation is acting. The hydrodynamic cavitation intensity is related to the volume of cavitation vapor (related to

bubble length) and the differential pressure (p-p v) driving the bubble implosions. The cavitation resistance is purely a function of the material mechanical properties. A detailed study of the Browns Ferry/Peach Bottom RHR CVIC pump impeller service life during operation in the maximum cavitation erosion zone has been conducted [1]. The impeller service life study shows that impeller failure due to erosion is extremely unlikely in the [[

]] of operation following a DBA-LOCA.

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 3 Depending on the relative operating point of the pump and the combined fluid system dynamic characteristics, operation with very low pump suction pressure can cause system pressure pulsations and increase system noise and vibration. According to Gulich, " During cavitation, low frequency pulsations of large amplitudes are created through large fluctuations of the cavitation zones. The compressibility of the cavities may result in cavitation surges." [2]. Cavitation induced pressure pulsations are observed in a broadband frequency range and are unrelated to the rotational frequency of the pump. The amplitude of these cavitation induced pressure pulsations tends to increase when the pump is operated at very low flows where heavy inlet recirculation is present. Operation with reduced NPSHa will also result in a decrease in pump performance in terms of discharge head and flow.

Based on centrifugal pump testing of different sizes and types, it has been observed that cavitation noise increases with decreasing NPSHa to a maximum value at a point between NPSH0 and NPSH3.

When NPSHa is decreased below NPSH3, the cavitation noise reduces substantially. These observed

characteristics are portrayed in Figure 1 and have been described in detail by Gulich [2, Chapter 6.5.2].

This phenomenon is likely due to two concurrent causes: 1) absorption or dampening of the bubble implosion energy, which is the source of the noise and vibration by increasing the vapor present at the impeller cavitation zones within the pump, and 2) attenuation of the cavitation induced pressure waves in the pumpage due to dissolved air, if present, coming out of solution resulting in formation/growth of

air bubbles in the suction line (i.e., in the region between the cavitation source on the blade surfaces

and the location of the hydrophones or pressure transducers in the inlet piping). However, it can not always be assumed that the risk of cavitation damage diminishes as the measured cavitation induced noise decreases. This is because the risk of cavitation damage is dependent on hydrodynamic cavitation intensity which increases with bubble volume and increasing differential pressure.

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 4 Figure 1: Influence of Cavitation Coefficient (s) on Cavity Volume (L cav), Cavitation Noise (NL), and Erosion (E R), (Gulich)

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 5 A pump performance characteristic observed when NPSHa drops below NPSH0 is the new head performance resulting from cavitation and the system resistance characteristics. Namely, the cavitation vapor bubble blockage will limit the pump flow at the point where the pump head drops to the system

head curve. Hence, the new operating point is the intersection point of the reduced head curve (cavitation characteristic curve) and the system head curve. Figure 2 illustrates a general head performance curve and the cavitation characteristic curves (97% and 95%) interacting with the system

curve. Appendix A of Sulzer report [3] provides a detailed discussion on the steady-state interaction between the pump characteristics and the system characteristics as well as a methodology to determine the pump steady-state operating point at reduced NPSHa.

150 250 350 450 550 650 750 850 950250035004500550065007500850095001050011500 Flow Rate (gpm)

Head (ft)0.05.010.015.020.025.030.0 NPSH (ft)Head Head 97%Head 95%System ResistanceNPSH3 Figure 2: Pump Cavitation Characteristics

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 6 3. SCOPE As discussed in the previous section, pump cavitation can result in increased impeller erosion, system noise and vibration, and reductions in pump performance. Experience gained through in-situ testing of a Browns Ferry RHR pump, testing of similar CVIC pumps, and operation of Browns Ferry/Peach Bottom RHR pumps in the field is used to assess pump operation under reduced NPSHa conditions in

the presence of heavy cavitation.

Typically, new pumps undergo performance acceptance tests (flow, head, efficiency, and NPSHr determination) at the manufacturer's test facility to ensure that the pump performance characteristics

are acceptable. The standard NPSHr characterization test establishes a 3% NPSHr curve by incrementally reducing the NPSHa until a 3% reduction in pump discharge head is measured. The analysis scope of this report also includes evaluation of an in-situ cavitation test performed on an RHR CVIC pump at Browns Ferry and an evaluation of the RHR pump operating data. This data and pump operating data are used in conjunction with the CVIC pump's mechanical design features to assess in-situ operation of the Browns Ferry and Peach Bottom RHR pumps under short-term operation when NPSHa < NPSH3 and long-term operation where NPSHa > NPSH3.

Specifically, the following test data and information is used: a) A cavitation test [4,5] was performed on an RHR CVIC pump at the Browns Ferry Nuclear Plant in May 1976. The results from this test correlated noise and vibration values with the RHR pump running at low NPSHa. This information has been used to further assess the capability of the Browns Ferry and Peach Bottom RHR pumps to operate under conditions

following a DBA-LOCA event. b) During typical NPSHr characterization on a test bed, the suction head at the pump inlet is reduced until approximately 10-15% head degradation is recorded at each tested flow rate. By virtue of this testing process, all pumps that underwent such testing have been operated with NPSHa equal to or less than NPSH3. Generally, one pump in a set of pumps with same hydraulics (impeller and casing combination) is tested for NPSHr. Therefore, one pump, at maximum impeller diameter, from the Browns Ferry/Peach Bottom RHR pumps (S/O

270671/82 and 270683/90) underwent NPSHr characterization at a Sulzer facility prior to shipment.

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 7 c) Excessive system vibrations caused by various hydraulic excitations can lead to damage which ultimately could result in failure of pump components including bearings, impellers, mechanical seals, etc. An evaluation of possible failure modes of these components due to cavitation induced vibration is performed.

4. ANALYSIS The following input assumptions, including NPSHa information, were provided by BWROG for use in evaluating short-term pump operation with NPSHa < NPSH3 and during the long-term mission time of

a typical DBA-LOCA event.

a) NPSHa can be less than NPSH3 only during the first [[

]] following DBA-LOCA event. This is because in the short-term LOCA phase, the RHR pump operates at maximum flow rates where NPSH3 is high. For this analysis, it is assumed that NPSHa stays within

[[ ]] of the NPSH3 when RHR is at maximum flow. If NPSHa is more than [[

]] below NPSH3 then the pump will operate at reduced flow rate as shown in Appendix A of Sulzer report [3]. Figure 3 shows a representative trend of NPSHa with Containment Accident Pressure (CAP) credit and NPSH3 for short-term LOCA. NPSH3 with a [[

]] uncertainty adder is also plotted. b) In the DBA-LOCA analysis, RHR flow is reduced in the long-term phase to where NPSH3 values are lower. NPSHa will then decrease as suppression pool temperature increases until the pool temperature peaks. Following the temperature peak, NPSHa will recover as the

suppression pool cools. Figure 4 shows a typical plot of NPSHa without CAP credit versus time for the first [[

]]. Further improvement in NPSHa would be realized as duration extends to [[

]] as the suppression pool is cooled in the long term. c) The pump is required to operate without experiencing a mechanical or a hydraulic failure for the [[ ]] following a DBA-LOCA at long-term cooling water flow rates. d) A large volume suppression pool maintains a continuous flooded RHR pump suction (water supply elevation is above the pump suction).

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 8 [[ ]] Figure 3: Short-Term LOCA NPSHa Timeline [[

]] [[ ]] Figure 4: Long-Term LOCA NPSHa Timeline [[

]]

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 9 4.1 Results of the In-Situ Cavitation Test on the RHR pump at Low NPSHa.

NPSH tests were performed in May 1976 [4,5] on the installed Browns Ferry Unit 3 RHR pump 3A to determine if additional NPSH margin was available in the vendor NPSH curves. Since reliable pump operation and not loss of head is the principle concern at high pump flow conditions, tests were performed to determine the NPSH at which the onset of unacceptable pump vibration and audible cavitation noise could be detected.

The NPSH tests were performed with the RHR pump operating at [[

]] gallons per minute (gpm) in suppression pool cooling mode. Reduced suction pressures were achieved by throttling the suction valve. Pump motor vibrations were monitored by two accelerometers at the top of the motor; one in line and the other at right angles to the pump nozzle orientation. The pump suction throttling was terminated before the "breakout point" (sudden and severe loss of discharge head) of the pump was reached. Severe audible cavitation noise was present but the motor vibration was still within acceptable limits. The test results are

shown in the table below.

Summary of Test Results

[[

]]

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 10 Notes: 1. Data recording started when the suction pressure went negative because the only points of interest were low NPSHa condition data points.

2. "Start cavitation" means that an audible change in the sounds being emitted from the pump could be heard. Classical "pumping marbles" sounds were starting to be emitted from the pump. However, pump performance and pump vibration levels were still very smooth.

3. "Some cavitation" means that the "pumping marbles" sounds being emitted from the pump were somewhat louder. However, the pump was still operating smoothly with little change in

vibration levels since the start of the test.

4. "Cavitating" means that cavitation ("pumping marbles") sounds were very audible. However, pump performance from all indications was still normal and pump vibration levels were still within acceptable limits.

Vibration Results: The following information has been extracted from the strip chart vibration data recorded during the tests.

[[

]] Data Analysis, Results, and

Conclusions:

The tests were performed to determine the NPSH at which the onset of unacceptable pump vibrations and audible cavitation noise could be detected. The vibration levels were recorded on a strip chart recorder and comments were recorded with respect to the noise levels. The time

of operation at each reduced NPSH condition was also recorded.

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 11 a) Test results show that at [[

]], pump noise and vibration levels remained constant for NPSHa greater than [[

]] and then the noise level increased to reach a peak between [[

]] of NPSHa. At [[

]] noise and vibration levels remained constant for NPSHa greater than [[

]] and then the noise level increased to reach a peak between [[

]] of NPSHa.

b) At both flow conditions under significant cavitation conditions with nearly [[

]] head degradation, the pump ran smoothly with minimal increase in vibrations.

c) Change in noise levels and change in vibration levels with decreasing NPSHa were recorded . The field observations were consistent with expected pump behavior based on

industry experience and also consistent with research work.

d) Although the displacements due to vibration appear high, the frequency is very low. Therefore, the vibration velocity is also very low. Velocity of vibration is the more important parameter as it directly relates to the energy of excitation. Since Browns Ferry data above shows a very low level vibration velocity (maximum of [[

]]) premature damage to components, especially bearings, will not occur e) In total, the pump operated for over [[

]] under very low NPSHa conditions, including operation for [[

]] at NPSHa values well below NPSH3. Following operation under these conditions there was no evidence of damage to any of the pump components including the pump mechanical seals.

The above in-situ NPSH tests of the Browns Ferry RHR pump clearly demonstrated that the pump can operate satisfactorily under severe cavitation conditions without sustaining damage or suffering from deleterious effects. Furthermore, in the DBA-LOCA event, the time period when NPSHa could be less than NPSH3 is limited to the first [[

]] (Fig. 3) after which NPSHa > NPSH3 increases in the long-term (Fig. 4).

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 12 4.2 Vendor Testing of Browns Ferry/Peach Bottom and other CVIC Pumps Reduced NPSHa conditions were created during NPSHr characterization tests performed on the RHR pumps at the manufacturer pump test facility. During these tests, pumps are held at NPSHa conditions corresponding to a full range of inlet conditions from no cavitation present to NPSHa < NPSH3. These conditions are maintained for a few minutes at each test point for the purpose of loop stabilization and data collection. The Browns Ferry and Peach Bottom RHR pumps underwent similar NPSH testing and no failures or unreasonable levels of vibrations were reported. Hence, the Browns Ferry/Peach Bottom CVIC pumps were shown to

satisfactorily withstand cavitation induced noise (pressure pulsations under low NPSHa) and

vibrations arising during brief periods of low NPSHa operation (including periods where NPSHa < NPSH3) that might be encountered during the short-term DBA-LOCA. Moreover, similarly designed CVIC pumps (listed in Table 1) of similar configuration to the Browns Ferry/Peach Bottom pumps have undergone NPSH tests at Sulzer test facilities without any

reported failures or unacceptable levels of vibrations.

Table 1: Pump Test List

[[

]] Differences between the vendor test and field configuration set-up can impact the pump system vibration levels for the same excitation frequencies. For instance, pump/piping rigidity determines how the system responds to a given force amplitude at a particular excitation frequency. Table 2 below shows a comparison between the factory verification test and field set-up for some of the factors that can impact system vibrations.

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 13 Table 2: Sulzer Test Facility Set-up versus Browns Ferry/Peach Bottom RHR Field Set-up Elements Test Field Note

[[

]] This comparison shows that the test facility set-up and field configuration of Browns Ferry/Peach Bottom RHR CVIC pumps are similar in several important aspects. Therefore, it is reasonable to expect that the magnitude of cavitation induced vibrations observed during factory tests and field operation will be similar. The in-situ field test conducted on the Browns Ferry RHR pump confirmed this assertion.

4.3 Excitation

Frequency and Failure Modes Analysis for Long-Term Pump Operation Typical vibration spectra applicable to a wide range of pumps under various flow and speed conditions have been provided by Gulich [1, Chapter 10]. Vibrations observed during the normal operation of a pump include rotational frequency and vane passing frequency. Both of these frequency components are speed dependent.

In the case of cavitation induced vibrations, the excitation frequencies are not speed dependent and tend to be broadband above 500Hz. The amount of cavitation and corresponding vibration

will depend on NPSHa, speed (related to energy level), and relative operating flow rate. At

very low flows with inlet recirculation present, fluctuating vapor cavities entrained in the

recirculating flow will typically result in low frequency excitation in the range of 0.5 Hz to about 0.2 times rotational frequency [[

]] [1, Chapter 10, Table 10.9 (3)/Spectrum 6]. For the Browns Ferry/Peach Bottom RHR pump speed this frequency range is:

[[

]]

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 14 These frequencies are dependent on the flow rate (degree of inlet recirculation and NPSHa). For long-term DBA-LOCA service, the RHR pumps are expected to be operated at BEP or

above so low flow inlet recirculation is not a consideration.

Cavitation induced pressure pulsations typically impact the impeller blades causing both axial and radial excitation forces. Hence, primary pump components that could be affected by cavitation induced vibrations are the impeller blades, wear rings, radial and axial bearings, mechanical seal faces, and suction and discharge piping. Table 3 below lists the possible failure modes for these components from cavitation induced pressure pulsations.

Table 3: Potential Pump Failure Modes Component Function Failure Mode Cause of Failure a) Mechanical Seal Controls leakage from the pump Excessive leakage Axial vibration damages seal faces b) Motor Bearing Provides rotor support and stability.

Controls deflection

at the mechanical

seal. Severe wear or rupture of bearing Excessive loading due to axial

vibrations c) Suction and Discharge

Piping Transport pumpage Bending, crack, or rupture Axial vibrations d) Impeller Impart kinetic energy to fluid Crack/break High vibration e) Wear Ring Limit leakage flow between high

pressure impeller

discharge and

impeller eye Increased leakage flow due to

increased

clearances from

contact/wear Contact between rotating and

stationary parts due

to high vibration f) Pump Bearing Support cantilevered rotor Loss of bearing support due to

increased

clearances from

contact/wear Contact between rotating and

stationary parts due

to high vibration Although the vibration amplitudes are not expected to reach damaging levels for long-term pump operation at NPSH3, the CVIC type RHR pumps have additional features that improve the reliability of these components:

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 15 a) Mechanical Seals - The magnitude of seal leakage will be insignificant compared to the pump flow rate and, therefore, increased seal leakage will not adversely affect pump

operation.

b) Motor Bearing - The hydraulic damping forces in the pump axial direction are very large. The pumpage that is present between the impeller shroud and the case sidewalls acts as a squeeze film damper absorbing the energy of vertical vibrations. Additionally, the axial motor bearings have a high dynamic load capacity and are capable of withstanding axial loads due to cavitation induced vibration. Below is a calculation that compares the motor

bearing thrust load capacity with the expected pressure pulsation load acting on the bearings.

The suction pressure pulsation amplitude under normal operating conditions (based on tests conducted on similar pumps) is 1 psi. Based on numerous pump tests and EPRI GS-6398 [6], the maximum pressure pulsation amplitude under the worst cavitation condition is 4 to 5 times the value at normal conditions. Therefore, the maximum amplitude of pressure pulsations under

the worst cavitation condition is expected to be in the range of:

= 5 x 1 psi = 5 psi The increase in dynamic axial thrust load acting on the pump under the worst cavitation

condition:

Axial Thrust = Pulsation Pressure x Impeller Wear Ring Frontal Area

= (5 - 1) x x d 2 / 4 lbf [1, Chapter 9, Eq 9.2.10] Where d = impeller wear ring outer diameter (OD) obtained from Sulzer Wear Ring drawing

= 4 x x 18.224 2 / 4 lbf = 1043 lbf

From SKF Catalog the L10h bearing life is given by the following equation:

L10h = 1,000,000 / 60 / n x (C/P) 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />

Where, L10h = Life at which 10% of the bearings can be expected to have failed due to fatigue failure.

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 16 C = basic dynamic loading, lbf P = equivalent dynamic bearing load, lbf n = rotational speed, rpm

The L10h bearing life under a normal maximum axial thrust load of [[

]] (Per TVA GE Motor Outline Dwg.992C43OAE, Motor Bearing Information [7])

Therefore;

[[

]] ---. (1)

Similarly, the bearing life under an increased thrust load condition:

[[

]]-.(2) Eq. (2) divided by Eq. (1) yields:

[[

---.---.. (3)

]]

The calculated L10h bearing life under conservative axial thrust conditions due to cavitation is

[[ ]], which is significantly greater than the required operation time of

[[ ]]. Therefore, the motor bearings will not fail due to increased dynamic loading during worst case cavitation conditions.

c) Pump Suction and Discharge Piping - Piping in the field is Seismic Category I, which is designed to withstand forces of greater magnitude than cavitation pressure pulsations.

d) The CVIC pump impellers are of a robust single suction shrouded design. Thousands of pumps using similar impeller design have accumulated millions of hours of field operation.

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 17 e) Wear rings used in the Browns Ferry/Peach Bottom CVIC type RHR pumps provide a squeeze film damping effect that absorb small radial vibrations. Radial bearing clearances are smaller than wear ring clearances further reducing the possibility of wear ring contact and failure. Also, these components are made of non-galling materials so that even if accidental contact was to occur, they will not sustain any damage.

f) The long length over diameter (L/D) of the lubricated radial bearing located between the impeller and the mechanical seal in the Browns Ferry/Peach Bottom CVIC type RHR pump acts like a squeeze film dampener to significantly reduce the transmission of dynamic forces from the rotor to the pump case. Moreover, the carbon bushing used in the construction of these bearings has self-lubrication properties that minimizes damage potential due to galling

should contact occur.

5. CONCLUSION Operating experience gained through in-situ testing, testing of similar pumps, and operation of the pump in the field are reliable methods for evaluating the expected performance of CVIC pumps including the Browns Ferry and Peach Bottom RHR pump assemblies under different operating modes and cavitation regimes.

During actual in-situ NPSH testing, a Browns Ferry RHR pump was operated under severe

cavitation at low NPSHa values without any reported failures or unreasonable level of vibrations.

Based on the vibration magnitudes observed during these tests, the vibration levels that will be reached by the RHR pumps during operation with NPSHa < NSPH3 are expected to be well within the acceptable limits for these pumps. The fact that there was no damage to any of the pump components shows that the cavitation induced pressure pulsations will not result in pump component failure during short-term operation under reduced NPSHa conditions.

As discussed previously, the time during which NPSHa could be less than NPSH3 is short [[

]] at the beginning of a DBA-LOCA. These Browns Ferry and Peach Bottom RHR pumps were subjected to operation under severe cavitation (NPSHa < NPSH3) conditions in the vendor test facility during NPSHr characterization tests. Since these pumps and their Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 18 components underwent the NPSH shop tests without sustaining damage or experiencing unreasonable levels of vibrations under similar test facility set-up as the in-situ field set-up, it is

reasonable to expect that a short period of low NPSHa operation will not adversely affect the operation of the Browns Ferry and Peach Bottom RHR pumps for a long-term DBA-LOCA mission. This conclusion is also valid for CVIC pumps of similar frame size, hydraulics and mechanical configuration operating under similar conditions as the Browns Ferry and Peach Bottom RHR pumps. It is important to note that the Browns Ferry and Peach Bottom RHR pumps have a flooded suction that is continually fed by the suppression pool; therefore, the pumps will

always have a positive suction head available.

The information presented in this report provides ample evidence that cavitation induced vibration in CVIC pumps of similar frame size, hydraulics and mechanical configuration as the Browns Ferry and Peach Bottom RHR pumps, when installed under similar conditions as the Browns Ferry and Peach Bottom RHR pumps, are not expected to experience pump component failure

during the [[ ]] of operation under DBA-LOCA conditions. Further testing of similar CVIC pumps is not expected to yield results different from those reported herein or change the basic conclusion with respect to survivability of the Browns Ferry and Peach Bottom CVIC pumps during the postulated DBA-LOCA event.

Task 3 - Pump Operation at Reduced NPSHa E12.5.1978 18x24x28 CVIC 19 6. BIBLIOGRAPHY

[1] Sulzer Report - E12.5.1948 - Task 4-Operation in Maximum Erosion Rate Zone, Browns Ferry/Peach Bottom 18x24x28 CVIC RHR Pump

[2] J. Gulich Centrifugal Pumps (2008), Springer-Verlag publishers, ISBN 978-3-540-74410-8, Section 6.5 "Cavitation-induced noise and vibration"

[3] Sulzer Report - E12.5.1911 - Task 3-Pump Operation at Reduced NPSHa, Monticello -12x14x14.5 CVDS RHR Pump

[4] C. Michelson, H. L. Jones, and T. G. Tyler, "Tenessee Valley Authority-Browns Ferry Nuclear Plant Units 1 RHR Pump Protection Against Operation in Excess of Design Runout", May 17, 1976.

[5] C. Michelson, H. L. Jones, and T. G. Tyler, "Tenessee Valley Authority-Browns Ferry Nuclear Plant Units 1 Additional Information Requested by NRC Concerning RHR Pump Protection

Against Operation in Excess of Design Runout", July 21, 1976

[6] EPRI GS-6398 - Guidelines for Prevention of Cavitation in Centrifugal Feedpumps.

[7] GE Induction Motor - Outline Drawing for TVA 1, 992C430AE, Revision 5.

[8] ASME OMA Code - 1996 Addenda to ASME OM Code - 1995, Code for Operation and Maintenance of Nuclear Power Plants

[9] Bolleter, Ulrich, Diether Schwarz, Brian Carney, and Earl A. Gordon, "Solution to Cavitation Induced Vibration Problems in Crude Oil Pipeline Pumps", 8 th International Pump Users Symposium.

[10] I. S. Pearsall, "Acoustic Detection of Cavitation", Proceedings of Institution of Mechanical Engineers 1966-67.