Engineering Paper
ITT Flygt AB Num Number Revison Sign Dept Date Page:
0200 02001 1 2 AEin WA 2003-01-27 1 (3)
Contents GENERAL GENE RAL..... ........... ............ ............ ........... ........... ............ ........ 2 DESCRIPTIO DESCRIPTION N OF THE TESTS TESTS ....... ....... 2 TEST PRINCIPLES............................ 2 Closed Closed system ........... ................. ........... ........... ......... ... 3 Motor speed control .......... ................ ........... ..... 3 Inlet restriction restriction ........... ................ ........... ............ ........ 3
Determination of NPSHR for submersible pumps Abstract: The paper describes the methods used for the tests that are used to determine the NPSHR curve. The test procedure is based developed by Flygt and is based on ISO9906, ISO2548, DIN1944 and ANSI/HI 1.6-1994. Three methods are used depending on the products being tested: With a closed system, by alternating the rotating frequency and with an inlet restriction. The paper is based on the instruction FP-98115.
Key words: NPSH, Performance test, ISO 9906
Engineering Paper
ITT Flygt AB
Determination of NPSHR for submersible pumps Page: 2 (3)
General The NPSHR ( N et Positive Suction H ead Required) is the recommended minimum inlet pressure for the pump to avoid disturbances caused by ca vitational effects. NPSHR is expressed as absolute pressure, i.e. i t is always a positive figure. Cavitation occurs if the static pressure in the pumped fluid in any point is below the evaporation pressure of the fluid. The small vapour bubbles formed at this point will disturb the flow and decrease the performance of the pump. When the bubbles implode, they will cause noise and vibrations and can also deteriorate internal surfaces of the pump. The results of a NPSHR-test are shown in a graph “NPSHR ”, where net inlet pressure (total static pressure minus evaporation pressure) is plotted against the flow.
Description of the tests The test procedure developed by Flygt is based upon official standards ISO9906, ISO2548, DIN1944 and ANSI/HI 1.6-1994, describing how performance (including NPSHR) are to be guaranteed and measured. The NPSHR test procedure include 3 different methods to match different systems and products: •
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Closed system : A closed loop system where the system pressure can be controlled. Feasible for small centrifugal and propeller pumps. Motor speed control : An open system where pump speed can be controlled. Feasible for larger pumps, especially propeller pumps. Inlet restriction: An open system where inlet pressure can be controlled. Feasible for small and midsize pumps.
All test methods are strictly described regarding details in equipment set up and preparations. The test fluid (water) is deaerated and the atmospheric pressure measured.
Test principles For all test methods the aim is to run the pump at a specified duty point and decrease the inlet pressure until the pump performance is affected by cavitation. The inlet pressure is decreased gradually and data regarding flow, pressure, pump power, water temperature and inlet pressure are collected at each step. The data obtained are plotted against inlet pressure, NPSHavailable, to form pressure drop graphs valid for this specific duty point. The procedure is then repeated for a necessary number of duty points to get a set of pressure drop graphs covering the actual performance interval.
Engineering Paper
ITT Flygt AB
Determination of NPSHR for submersible pumps Page: 3 (3)
From those graphs, an evaluation of NPSHR is made at every investigated duty point. Those values are then presented in the pump performance chart as an NPSHR –graph. Depending on size and type of pump different test methods must be applied to allow for change of duty points and operational parameters. Subsequently there will be different methods for evaluation and calculation of obtained data.
Closed system The overall system pressure can be controlled. When the settings for a specified duty point are found, the overall pressure is decreased. The inlet pressure will decrease simultaneously. The pressure is decreased gradually until pump performance is distinctly affected. Test data and calculations are collected and performed as described above.
Motor speed control For large pumps, especially propeller pumps, the most feasible way of changing operational parameters during the test are by changing the revolution speed. When the pump is working at specified duty point, the frequency of the current supply is increased stepwise to increase the flow. Typical changes are in steps of approx. 2 Hz. The pump speed is increased until pump performance is distinctly affected. Flow, pressure and power consumption is then recalculated to specified (nominal) frequency by means of the affinity laws. Test data and calculations are collected and performed as described above.
Inlet restriction The inlet restriction method is mainly used for small and midrange pumps where an inlet pipe connection is available. The test stand is an open system where the flow is controlled by valves. When the settings for a specified duty point are found, the inlet pressure is decreased by a restriction in the inlet tube. The pressure is decreased gradually until pump performance is distinctly affected. Test data and calculations are collected and performed as described above.
