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XA0201137 Cavitation Tests

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XA0201137 Cavitation Tests - 93 - XA0201137 Cavitation Tests for "JOYO" Primary and Secondary Main Circulating Pumps M.KAMBE, M.KAMEI, PNC, Japan. ABSTRACT The paper outlines the development undertaken to determine the cavitation character- istics of mechanical sodium pumps. Test programmes on cavitation in sodium a/nd in water have been undertaken to predict the condition of cavitation onset in sodium from measurements on water mock-up. Test data show close concordance between the cavitation threshold values obtained in water and those obtained in sodium. - 94 - 1. Introduction In recent years, attempts to the reduction of FBR costs has become a matter of major concern. In this point of view, pumps as well as other FBR system components should be designed so as to contribute plant cost reduction. A pump with a small diameter will have less weight and cost less and will require less space. For the same reasons the length of the pump must be minimized. The need to reduce the dimensions of the pump leads to the demand to reduce the dimensions of the impeller. This can be achieved by increasing the speed of the pump. For a given suction pressure the maximum speed is restricted by the cavi- tation phenomenon in the impeller. Until now, sodium pump designs have included excessive safety margins and restricted operating ranges. Thus the pump dimension depends on how accurately the designer can deduce the incipient cavitation limits of the impeller. It is presently accepted that there is not much difference in the required NPSH between in-sodium and in-water, but hardly any actual case of its confirmation is available so far. The paper summarizes the results of the in-sodium and in-water hydraulic testing carried out on the full scale model of "JOYO" primary and secondary main circulating pumps. Sodium-water similitude criteria to define incipient cavitation was con- firmed. - 95 - 2. General Description of the Pump The pump which was used for the present test had been made indentical to the actual primary and secondary main circulation pump of FBR "JOYO" in size, configuration, material, etc. The cross sectional diagram of the pump's const- ructions are shown in Figs. 1 and 2 respectively. As FBR "JOYO" is a loop type, its circulation pump is of a piped suction type, and its free surface level is controlled by overflow. Its impeller is of a single suction type, and its diffuser is of a multi-vane type. Immediately above the impeller, there is provided an in-sodium bearing of hydrostatic type, and the upper section supports the radial load of the shaft by an oil lubricated ball bearing. While the weight and the hydraulic thrust of the rotor is supported by a thrust bearing on the upper part of the motor. The outer casing has a jacket and is pre- heated by circulation of heated nitrogen gas. Table 1 Pump Specifications Primary Pump Secondary Pump Model Vertical, free surface, centrifugal type | Suction 450mm 300mm Piping Size: | Discharge 300mm 300mm | Overflow 200mm 125mm Rated Flow Rate: 21.0m3/min | 20.8m3/min Total Head: 70mNa 35mNa Design Temp.: 450°C Charge Gas: Ar gas Cover Gas Pressure: lOOmmAq 100-200mmAq Pump Speed: 130-930rpm 340-975rpm Rated Output of Drive Motor: 330kW 180kW Secondary resistance Pump Speed Control: Static Scherbius Method control The Q-H characteristic curves of each pump are shown in Figs. 3 and 4. - 96 - 3. Test Facilities 3.1 Test facilities for the primary pump The in-sodium cavitation test was performed on the Sodium Pump Test Loop which is composed of the following six systems so that a series of performance and endurance tests of a large capacity sodium pump can be performed: (1) A main circuit system. (2) A sodium supplying system. (3) A sodium charging system (or a pumf> overflow system) (4) A cover gas system. (5) A drain system. (6) A nitrogen gas preheating system. Fig. 5 represents the flowsheet of the loop. The main circuit system is of a double tubular construction permitting to be preheated with nitrogen gas flow in the annular gap between an inner tube of 12in. and an outer tube of 16in., simulating the main circuit of the primary system of FBR "JOYO". Also provided are the flowmeters, pressure gauges with NaK transducers (3 gauges at the inlet and outlet of the pump), and 12in• and 6in. freeze seal type flow regulating valves. An air-cooler is installed to control the temperature of the pump test loop. The sodium supply system is to transfer the purified sodium from the separately installed purity control system to the heater by the feeding pump so that the heated sodium is charged into the main circuit via the sodium charging system, and to maintain the free surface of sodium at a given level during the loop's operation by the overflow and supply lines connected to the auxiliary tank. This sodium supply system is equipped with an electromagnetic flowmeter and a heater (lOOkW) immersed in an overflow tank. The sodium charging system (the pump overflow system) is to charge sodium from the heater into the main circuit by the sodium feeding pump, and to hold the suction pressure of the pump at a given level during the loop's operation. - 97 - The cover gas system has a role to prevent the free surface levels of the heater and the pump from oxidation and to maintain the pressure at the shaft enclosing section of the pump on the positive and thus to prevent air from mixing into there. This cover gas system is composed of an shaft enclosing argon gas feeding line, a respiratory gas line, and two vapor traps. The drain system has the function to drain the entire sodium in the loop into the auxiliary tank. The drain valves are electrically actuated so that any emergency draining is possible. The nitrogen gas preheating system has a capacity to preheat the pump and the main circuit at a time. In-water cavitaion test was carried out in the test loop of.manufactures shop. Its overall design and operating characteristics is similar to the Sodium Pump Test Loop. 3.2 Test facilities for the secondary pump In-sodium cavitation test for the secondary pump was conducted on the "JOYO" Endurance Test Loop (Fig. 6), basically a 12 inch closed loop equipped with an overflow system, a flow control valve, an electromagnetic flowmeter, and NaK transducer type pressure gauges. The water test loop was identical with that used for primary pump. - 98 - 4. Experimental procedure 4.1 Test method The following test procedure was selected for both primary and secondary pumps: 1) For in-sodium testing; - flowrate was set by regulating the flow control valve under the condition of maximum pump speed. - fluid temperature was stabilized throughout the loop at a given value. - cavitation was induced by reducing the cover gas pressure gradually from non-cavitating conditions until cavitation is detected. 2) For in-water testing; - fluid temperature was stabilized throughout the loop at normal temperature under the maximum pump speed. - cavitation was induced by gradually closing the flow control valve equipped with pump suction piping from non-cavitaing conditions until cavitation is detected. The test was conducted under the following conditions (Table 2): Table 2 Test Conditions Primary Pump Secondary Pump (1) Pump speed: 930rpm 985rpm t (2) Aperture of flow control valve: 100, 47, 36, 23 100, 72, 56, 42 | Tes and 13% and 28% \ m (3) Cover gas pressure (variable): +0.0W-1. OKg/ +0.W-1.0Kg/ | cm2G cm2G | sodiu n I (4) Sodium temperature: 370°C, 250°C 340°C, 250°C (5) Sodium purity (plug temp.): 150°C - 180°C 200°C and 120°C t (1) Pump speed: 980rpm 967rpm Tes r (2) Water temperature: Normal temp. 29°C j wate n I 1 - 99 - 4.2 Detection of cavitation Cavitaion was detected by measurement of the pump head (i.e. differential pressure between the inlet and outlet of the pump under tested). Pump head will decrease as the inception of cavitation. To define the incep- tion of cavitation, 0% or 3% head drop-off criteria is adopted. Also acoustic detection was adopted for the test of secondary pump. - 100 - 5. Results 5.1 Primary pump Test results are shown in Fig. 6 in which Net Positive Suction Head (NPSH) is plotted in respect to flow rate. NPSH was defined as the value measured at the beginning of the pump head decrease (i.e. 0% head drop-off) NPSH is estimated from the inlet pressure gauge: 2 Hsv= lO^.J^. +h22_ h"j,s+.^ _ 104. jj£. ...(5-1) where, Hsv: NPSH (m.Na) ?2: Inlet pressure (Kg/crrrabs.) IT : Specific weight of sodium (Kg/m3) hZ2*- Inlet pressure gauge elevation in respect to the suction port (m) h" : Pressure loss (m.Na) from the inlet pressure gauge to the suction port. Vs: Flow velocity in the pump suction piping (m/s) Pvp: Sodium vapor pressure ^ The pump head is obtained from the measurement of inlet and outlet pressure gauges as follows: / Pi P? ii Vo V^2 . o> H - io*A-—±-=.. —£; +- (hChzZl, -h- hZ2zo) + hVi3 + h/is + ^-rf-| --T-^ C5-^J. where, H: Pump head (m.Na) Pj: Inlet pressure (Kg/cnrabs) hz\: Elevation of the outlet pressure gauge (m) hz2: Elevation of the inlet pressure gauge (m) h's: Pressure loss (m.Na) from the discharge port to the outlet pressure gauge V: Flow velocity in the discharge piping (m/s) - 101 - Since in-sodium and in-water tests were carried out at the pump speed of 930 and 980 rpm respectively, extrapolation of the in-water test data into the same pump speed as that of in-sodium test is made as follows: (1) flow rate (delivery rate plus overflow rate) Q, where, Q: Flow rate (nvfymin) at the prescribed pump speed (930rpm) Qt: Flow rate (nr/min) at the tested pump speed N: Prescribed pump speed (930rpm) N-^: Tested pump speed (2) Head (total head and NPSH) H.
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