MASTER. CENTRIFUGAL PUMP PERFORMANCE UNDER SIMULATED TWO-PHASE FLOW CONDITIONS Clt>P\ -

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1 . DISCLAIMER MASTER This book was ptepared as on account of wort ipon»rod by an aoency ot tht Uniieg Sum Gowtnmwu Neither the United Slates Government rat any agency thereof, nor any of thtir employem.tnakmany warranty. e»piew ui implied, a> aiiumes any legal liability or responsibility tor the accuracy, completeness, cr d«jiuln«i o< any Infottwttan, apparatus, product, or. prucet* dlsclos*!, or reprt-swis that In \ix would not infringe privately cwnsi rights, fleltfencb herein to any. specific commercial oroduci. process, or service by irade Mw, tradafturk, msnufacturer, or oiherwlta; do«not nec«5i9rilv conilllute 0' Imply its endorsement, fecommcfwj»tlon, or favoring by.tha UnUod Slates Governfwni or any ajcutv ihereof. The uiews and opinions oi luihors exprnsed hefdn do not neceuacilv smie or relieci those of the United Siaiet Government or *ny ajenev thw«(. CENTRIFUGAL PUMP PERFORMANCE UNDER SIMULATED TWO-PHASE FLOW CONDITIONS Clt>P\ - Tien-Hu Chen Scientist, LOFT Test Support Branch E6&G Idaho, Inc. P. 0. Box 1625 Idaho Falls, Idaho Member, ASME William J. Quapp Manager, LOFT Test Support Branch EG&G Idaho, Inc. P. 0. Box 1625 Idaho Falls, Idaho ABSTRACT This paper presents test results from centrifugal (mixed-flow) pump performance testing conducted at the Idaho National Engineering Laboratory (INEL) using nitrogen-water flow to simulate steam-water two-phase conditions. The results are compared with the predictions of the RELAP4/&M0D6 pump performance model which is based on experimental data obtained from tests conducted on a small (radial-flow) pump in the Semiscale facility. The purpose of this investigation is to obtain the two-phase characteristics of a mixedflow pump and evaluate the prediction capability of the pump model commonly used to analyze the thermal-hydraulic response when applied to a postulated loss-of-coolant accident in a light water reactor. The test results indicate that pump two-phase degradation is much less than that predicted by the RELAP4 model. In order to obtain a more realistic prediction of pump two-phase performance, the two-phase degradation data used in the pump performance model should be based on experimental data collected from pumps having similar specific speeds and design characteristics. ^ «ousana

2 NOMENCLATURE head homogolous function = homologous head ratio/(homologous 2 speed or flow ratio), dimension less g H h hydraulic torque homologous function = homologous hydraulic torque ratio/(homologous speed or flow ratio), dimension less head, m homologous head ratio head/rated head, dimensionless M, two-phase head multiplier, dimensionless M. two-phase hydraulic torque multiplier, dimensionless N rotor speed, rad/s PC pump current, A - AP pump differential pressure, kpa Q volumetric flow rate, 1/s q homologous flow ratio = volumetric flow rate/rated volumetric flow rate, dimensionless T pump hydraulic torque, N-m t homologous hydraulic torque ratios hydraulic torque/rated hydraulic torque, dimensionless

3 total loop volume excluding the pressurizer, m loop volume occupied by nitrogen gas, m overall void fraction = V w /V, dimensionless "2 two-phase fluid density, kg/m <p> cross-section average density measured by three-beam gamma 3 dens Hometer, kg/m P H Q P N ft 2 3 water density, kg/m nitrogen density, kg/m rotor speed, rpm to homologous speed ratio = rotor speed/rated rotor speed, dimensionless SUBSCRIPTS r pump rated operating value 10 single phase value It fully degraded two-phase value (0.2 < a < 0.9)

4 rt ^...ab-wsra-opaasis^w^^ INTRODUCTION The need to understand and predict the performance of centrifugal pumps in two-phase flow stems from the requirement to design engineering safeguards to mitigate the consequences of off-normal operation or an accident in light water reactors. Pressurized water reactors (PWRs) typically have up to four circulating pumps to force the coolant around the primary recirculation loops to cool the reactor core. A major requirement in the design of PWR engineered safety systems is that sufficient cooling capability must be maintained. This cooling capacity is needed to keep the fuel rod cladding temperatures below the specified safety limits in case of the loss-of-coolant accident (LOCA). It is necessary to be able to accurately predict and analyze the reactor thermal-hydraulic response during a postulated LOCA in order to assess the consequences of such an accident. During a LOCA in a PWR, the performance of the primary coolant pump under two-phase conditions, influences the magnitude and direction of the core flow vmich critically controls the core heat transfer. Therefore, an understanding of the behavior of the'reactor coolant pump under two-phase flow conditions is needed before the thermal-hydraulic response to a PWR LOCA can be accurately predicted. 1. The term "Centrifugal pump" as used here refers to the typical reactor coolant oump which is usually a mixed-flow pump.

5 An evaluation of the RELAP4 predictions for the LOFT (Loss-of-Fluid Test) loss of coolant experiments reveal that the predictions of pump two-phase performance do not compare well with the experimental data although other thermal-hydraulic variables are reasonably well predicted. The present pump performance model generally used in thermal-hydraulic codes (e.g., RELAP4 (I) and TRAC (_2)), for LOCA calculations employs homologous flow relationships to derive the head and torque behavior. The empirical two-phase head and torque degradation multipliers commonly used in the mode 1, are based upon tests conducted on a small (radial-flow) pump in the Semiscale Facility of EG&6 Idaho, Inc. at the INEL. This paper presents test results from mixed-flow pump performance testing using nitrogen-water flow to simulate steam-water two-phase conditions. These pumps are reasonably typical of PWR coolant pumps. The predictions of the RELAP4 pump performance model, which is commonly used to predict the pump two-phase performance in calculating the thermal-hydraulic response to a LOCA, were compared against the test results. The test results were obtained from experiments carried out in the Full Area Steady-State (FAST) Loop of the LOFT Test Support Facility, of EG&G Idaho, Inc., at the INEL. As the availability of experimental data on pump two-phase performance, especially the 2. Number in parentheses refers to references at the end of this paper.

6 ^ reactor type of mixed-flow pump, is extremely limited, the data obtained from the present pump two-phase performance tests are valuable for evaluating the prediction capability of pump performance models for a larger reactor pump operating under two-phase flow conditions. DESCRIPTION OF EXPERIMENTAL SETUP The FAST Loop is a high-pressure, high-temperature and high-mass flux steady-state single phase test facility which is shown schematically in Figure 1. The fluid is normally circulated by a Westinghouse Model E/N 3000-E1 canned rotor pump (3!) which can be operated at two volumetric flow rates of 300 and 150 1/s. To create the nitrogen-water condition for testing the pump characteristics, certain loop modifications were made which included: (1) Installation of a nitrogen injection system to inject the nitrogen gas into the FAST Loop at the horizontal section to create the nitrogen-water two-phase conditions. (2) Inrtallation of a coolant drain and measurement system to measure the mass of water displaced by the nitrogen gas to determine the system overall void fraction.

7 -J TE-2 DE - Three-beam densitometer PC - Pump Current PE - Pressure Element PDE - Differential Pressure Element TE - FE - Temperature Element Flow Turbine FE-1 -DE-2 Figure 1. FAST Loop Facility Schematic

8 ^^ (3) Installation of a purge pump to circulate the cooling water through the circulating pump top vent to provide pump bearing cooling. A flow turbine (FE-3) has also been installed to monitor this flow. (4) Installation of several acceierometers (AE), and a sound speaker to monitor the pump vibrations and noise levels. The reference flow measurement instrument is a full flow turbine meter (FE-1) located at the m pipe section below the horizontal section. EXPERIMENTAL PROCEDURES AND TEST CONDITIONS The nitrogen gas was injected into the FAST Loop in increments of approximately 5% of the total loop volume (overall void fraction). Data were recorded by a MODCOMP computer at each nitrogen level up to a maximum of 35% void fraction. Data that are essential to evaluate the pump two-phase performance include the system pressure, temperature, volumetric flow rate, pump differential-pressure and current. The instrument locations are shown in Figure 1. The. mass of the drained water was measured and recorded by the loop operators separately and was used to determine the overall void fraction. Then

9 S the average two-phase density was calculated by using the water and nitrogen property tables, and the measured pressure, temperature and void fraction. The test series consisted of two tests. The specified test conditions are listed in Table I. TABLE I PUMP TWO-PHASE PERFORMANCE TEST CONDITIONS Loop Loop Maximum Pump Inlet Test Temperature Pressure Attained Void Density Number (K) (MPa)- Fraction (%) Measurement no yes The second test is a repetition of the first one with the pump inlet density measured by using a three-beam gamma densitometer. This test was terminated at a void fraction of approximately 20% due to the pump failure. The pump inlet density data measured by the densitomater were used to verify the computed two-phase density in order to ensure that no phase separation phenomena existed during the pump performance tests.

10 ANALYTICAL MODELS In this section we discuss the models for two-phase density and the RELAP4 pump performance. Determination of Overall Void Fraction and Two-Phase Density The overall void fraction for this test is defined as the fraction of total loop volume occupied by. the nitrogen gas, namely; a' = V N / V (1) N 2 where V is the loop volume which is ( ) m (excluding the volume of the pressurizer). The volume occupied by the nitrogen gas (V M ) is indirectly determined by measuring the mass of water N 2 displaced by the injected nitrogen gas. The effect of the dissolved nitrogen on the void fraction calculation is small with the maximum error somewhat less than one-tenth of a percent, with dissolved nitrogen neglected. Therefore, the amount of nitrogen dissolved in the drained water is ignored in calculating the void fraction. The pump head is determined from the measured pressuredifferential (PDE-2). The pump pressure-differential divided by the two-phase flow density yields the pump head, namely, (2)

11 where the two-phase density is assumed to be p s= apij 1" (1 a) PM Q and can be calculated when the void fraction has been determined. RELAP4 Pump Performance Made! The single-phase dimensionless pump head and hydraulic torque characteristics are usually represented by the following equations: o. 2 '= f (q/«), t/j = g (q/ u ) (3) for 0 <: q / o> <_ 1 or h/q 2 = f («/q), t/q 2 = g («/q) (4) for 0 a / q The pump performance model employed in RELAP4 calculated the pump pressure-differential as a function of fluid volumetric rate, pump rotor speed and the fluid properties. The model is designed to treat any centrifugal (that is, radial-, mixed-, or axial-flow) pump and allows for inclusion of two-phase effects. The two-phase pump head and hydraulic torque are calculated respectively as:

12 H (q, oi, a) = W lt) (q,u) - M h (a) [h^ (q, w ) - h^ (q,«) ] x Hr (5) T (q, «, a) = l u (q,«) - M t (a) [ t^ (q, u ) - t^ (q, w ) ] x Tr (6) where the hydraulic torque is calculated by T = Q X &P x 10 3 / N (7) The two-phase head and torque multipliers plus their difference curves were based on the data obtained from two-phase tests on the small Semiscale (radial-flow) pump. The relationship of the two-phc.e to single-phase behavior of the Semiscale (radial-flow) pump in RELAP4 has been assumed to be applicable to larger reactor (mixed- or axial-flow) pumps. TEST RESULTS The test results presented in this paper are sufficient to evaluate the pump two-phase performance at low void fractions. The data obtained from the first test whjch had the highest attainable void fraction and the measured pump inlet density collected from the second test are analyzed, and interpreted.

13 Hrst Test The measured values of system pressure (PE-2), system temperature (TE-2), and the total mass of drained water, plus the calculated void fraction are presented in Table II. It can be seen thai system pressure is fairly constant for all void fractions. However, the system initial temperature rises slowly and later reaches a constant value. This initial increase in coolant temperature is due to heat dissipation from the pump. The actual void fractions calculated using Equation (1) closely match the original desired values. The measured pump properties, listed in Table III, include the system volumetric flow rate (FE-1), pump pressure-differential (PDE-2), and pump current (PC), plus the calculated head and hydraulic torque, (using Equations (2) and (7), respectively). It can be seen that both the pump pressure-differential and pump current decrease monotonicafrly as void fraction builds up within the loop. The pump pressure-differential for 15 to 35% void decreases more rapidly than that for 0 to 15% void. The pump current shows similar characteristics. However, the pump head behaves differently. The pump head increases gradually at the beginning and then reduces monotonically as void fraction increases. This initial increase in pump head is due to:

14 TABLE II MEASURED LOOP PROPERTIES AND THE CALCULATED VOID FRACTION Desired Void Fraction (%) Pressure-PE2 (MPa) Temperature-TE2 (K) Total Coolant Drained (kg) Actual Void Fraction (%) TO , J *.-f?*q Note: (1) Pressure and temperature total uncertainties include systematic and random error. (2) Total coolant drained uncertainty is estimated based on the measurement scale accuracy. "(3) Void fraction uncertainty is estimated using the root means square of the nitrogen volume and total volume uncertainty based on the mass measurement uncertainty.

15 TABLE III MEASURED PIMP PROPERTIES AND THE EVALUATED HEAD AND HYORAULIC TORQUE Volumetric Flow Pump-aP Pump Hydraulic Desired Void Rate-FE-1 PDE-2 Current Pump Head Torque Fraction {%) (1/s) (kpa) PC (A) (m) (N-m) * _ _ J Note: (1) Volumetric flow rate, pump differential pressure and current total uncertainty include systematic and random error. (2) Pump head and hydraulic torque uncertainty is estimated based on the related quantity uncertainty.

16 ^ (1) the increase in void fraction which results in smaller two-phase density, (2) A.he measured single-phase (a = 0) pump pressure differential of 681 kpa (Table VI) is slightly lower than the manufacturers vasue of 710 kpa obtained from the pump characteristics curve. This i«dy be due to the differential pressure transducer which has an upper range-limit of 690 _^ 33 kpa. In summary, the test results indicate that at a void fraction of 35%, the pump pressure differential is reduced by 382, the pump current is reduced by 27%, the pump head is reduced by 7% and the hydraulic torque is reduced 47$ compared to their respective singlenphase operational values. Second Test Generally speaking, the results of this test are similar to the first test and there is no need to repeat the discussion of the results here. However, the cross-section average density obtained from the outputs of a three-beam gamma densitometer installed near the pump inlet are important to justify the assumption of homogeneous flow used in calculating the two-phase density for deriving the pump head. The densitometer measured pump inlet two-phase density are compared

17 with the calculated values presented in Table IV. It can be seen that the measured pump inlet densities agree very well with calculated overall densities. The water and nitrogen gas are mixed homogeneously during the pump two-phase performance test, thus validating the calculated densities used to obtain the pump two-phase head and torque degradations. The technique for calculating density from the gamma densitometer measurements is described in the Appendix. COMPARISON OF THE MEASURED PUMP CHARACTERISTICS WITH THE RELAP4 PREDICTIONS The measured two-phase characteristics of the FAST Loop pump are compared with those calculated by the RELAP4 pump performance model. The purpose of performing these comparisons is to determine whether test results of the FAST Loop mixed flow pump can be reproduced with acceptable accuracy by the RELAP4 pump model. The rated operating characteristics of the FAST Loop pump together with those of Semiscale, LOFT, and CE/EPRI pumps are presented in Table V. It can be seen that the FAST loop pump is very 3. The Electric Power Research Institute sponsored two-phase pump-performance program at Combustion Engineering.

18 TABLE IV COMPARISON OF MEASURED PUMP INLET DENSITY WITH THE CALCULATED OVERALL AVERAGE LOOP DENSITY Measured Void Fraction. lv\ Temperature ft* \ \ f\ ) 1Pressure (MPa) V (kg/nt) P H 2o Calculated Density p(kg/m 3 ) Density < P > (kg/m 3 ) , ^ Note: Temperature, pressure and density total uncertainty include systematic and random error.

19 similar to the LOFT primary coolant pump. Therefore, the RELAP4 pump model used for LOFT L2-2 pretest prediction (4) was employed to predict the performance of the FAST Loop pump. TABLE V PUMP DESIGN CHARACTERISTICS Specific Speed Rated Rated Rated Rated ( n/c> -5\ f~ v Pump Flow Head Speed Torque rpm * j ^ 5 * IHl*lSE Type (1/s) (m) (rpm) (W-ffl) I m p^ J I ft U ' /b Semi scale LOFT CE/EPRI FAST Loop Based on the measured volumetric flow rate obtained from the first test, the two-phase pump head, pressure-differential and hydraulic torque calculated by RELAP4 pump model used for LOFT L2-2 prediction together with their corresponding measured values are presented in Tables VI and VII. The two-phase head and torque multipliers plus the values of their respective difference curves are 4 taken directly from input data of the RELAP4 pump model used for 4. EG&G Idaho, Inc. Configuration Control No. H for the input data of the RELAP4 Pump Model.

20 TABLE VI RELAP4 PREDICTED PUMP HEAD CHARACTERISTICS COMPARED WITH TEST DATA Void Fraction ( * ) M h (a] Predicted H (a) (m) Measured (m) Predicted AP (kpa) Measured (fcpa) _

21 TABLE VII RELAP4 PREDICTED PUMP TORQUE CHARACTERISTICS COMPARED WITH TEST DATA Void Fraction M t.(a) (t -t ) (N-fli) Predicted T(a) Measured T (a) (N-fli) ,

22 LOFT L2-2 pretest predictions (4). The head and torque multipliers based on the original Semiscale pump test data have been slightly modified to better represent the larger pumps used in the LOFT experiments. Figure 2 presents the measured and calculated pump head and differential pressure degradations as functions of the void fraction. Figure 3 presents the corresponding hydraulic torque comparisons. Figures 2 and 3 clearly indicate that the loss of two-phase pump head, pressure differential, and hydraulic torque at the intermediate void fractions (i.e., 0.2 to -O.35) is much less than that calculated by the RELAP4 pump model used for LOFT L2-2 prediction. Figure 4 also presents the head degradation as a function of the void fraction for the FAST Loop pump and CE/EPRI pump (_5)' (both have approximately the same specific speed, but their geometries are not necessarily similar) compared with that used for LOFT L2-2 prediction. It is apparent that the loss of two-phase pump head for any centrifugal pump is dependent upon the pump specific speed and the particular pump design characteristics. The RELAP4 pump model used for LOFT L2-2 prediction is primarily based on the data obtained from testing the Semiscale radial-flow pump under steam-water conditions. Assuming the nitrogen-water testing is representative of steam-water testing, the discrepancies between the measured and calculated two phase pump characteristics is attributed to the different pump specific speed and design character isr

23 w*^ 1.2 Measured.Head 8 i-o (U S? 0.8 n: Predicted Pressure Void Fraction (*) Figure 2. Pump Head and Differential Pressure Comparison 1.2 [ r 1 i 1 1 i.o : Measured Torque 4) I 0.8 = ^ Predicted Torque i i Void Fraction {%) Figure 3. Pump Hydraulic Torque Comparison

24 1.2 JS l.o o T,-- -r-"- FAST Loop Pump ' / *"* < CE/EPRI pump^ <u \ LOFT L2-2 v s \ Pump Model "*, ^0.0 i i Void Fraction (25) Figure 4. Pump Head Degradation Comparison

25 CONCLUSIONS The test results clearly indicate that the pump two-phase degradation data, obtained from tests conducted on a small radial-flow pump are not universally applicable to predict the performance of a larger mixed-flow pump. Furthermore, the results of this study indicate that the two-phase characteristics of pumps with different design characteristics may behave, differently even though they have the similar specific speeds. In order to obtain a more realistic (best estimate) prediction of pump performance under two-phase flow conditions, pump degradation data (as applied in performance models such as the RELAP4 computer code) should be based on test data from pumps having similar specific speeds and design characteristics as the one to be analyzed. ACKNOWLEDGEMENTS The authors express their sincere appreciation to Dr. C. W. Solbrig of Commonwealth Edison Co., Chicago for introducing us to this problem, and subsequently freely providing continual valuable advice which made this work possible. Acknowledgements are also due to the LOFT Test Support Facility personnel, specially to Messrs. B. L. Barnes, R. L. Crumley, J. Hauth, and R. W. Stanavige for their

26 efforts in performing the tests. This study was supported as part of the Nuclear Regulatory Commission (NRC) sponsored research efforts in pressurized water reactor safety. REFERENCES 1 K. R. Kastsma, et al., "RELAP4/MOD5 - A Computer Program for Transient Thermal-Hydraulic Analysis of Nuclear Reactors and Related System-User's Manual," ANCR-NUREG-1335, Sept. 1976, Aerojet Nuclear Co., Idaho Falls, Idaho, 2 D. R. Liles, et al., "An Advanced Best Estimate Computer Program for PWR LOCA Analysis," TRAC-P1A, Dec, 1978, ; Los Alamos Scientific Laboratory, Los Alamos, New Mexico. 3 Technical Manual» A, Westinghouse Electric Corp., Pittsburg, PA. 4 W. H. Grush, et al., "Best Estimate Experiment Predictions for LOFT Nuclear Experiments L2-2, L2-3, and L2-4," LOFT-TR-1O1, Nov. 1978, EG&G Idaho, Inc., Idaho Falls, Idaho. 5 J. A. Hunter and P. A. Harris, "Performance of Small Nuclear Reactor Primary Coolant Pumps Under Blowdown Conditions," ASME Winter Annual Meeting, San Franscisco, California, December 1978.

27 APPENDIX - A TECHNIQUE FOR CALCULATING OENSITY FROM THREE-BEAM GAMMA DENSITQMETER MEASUREMENTS The configuration of the three-beam gamma densitomater used to measure the pump inlet two-phase density is shown in Figure A-l. The cross section average density was computed from the three-beam outputs using a weighted average method based on the individual beam length ; namely, < p > = p A Pg P(: (8) where P. is the density measured by A beam, P B is the density measured by B beam, P C is the dens ity measured by C beam G. Lassahn, "LOFT Three-Beam Densitometer Data Interpretation," TREE-NUREG-1I11, 1976, EGG Idaho, Inc., Idaho Falls, Idaho.

28 Source Centerline Pipe,i Source J «KT O.792cm Cm Beam Length Inside Pipe A = T3.111 Cm B = Cm C = Cm ID Cm OD * Cm A Beam Figure A-l Three-Beam Densitometer Configuration (Looking Down From Top of Loop - Flow Enters Pages)