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P25-Special1.pdf

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PUMP CAVITATION—

VARIOUS NPSHR CRITERIA, NPSHA MARGINS, AND IMPELLER LIFE EXPECTANCY

by

Bruno Schiavello

Director Fluid Dynamics Flowserve Pump Division

Applied Technology Department Philipsburg, New Jersey

and

Frank C. Visser

Principal Design & Fluid Dynamics Engineer Flowserve Pump Division Technical Services, Central Engineering Etten-Leur, The Netherlands

Bruno Schiavello has been Director for Fluid Dynamics at Flowserve Pump Division, Applied Technology Department, in Phillipsburg, New Jersey, since 2000, and previously served in the same position with Ingersoll Dresser Pump Company. He started in the R&D Department of Worthington Nord (Italy), joined Central R&D of Worthington, McGraw Edison Company, and then Dresser Pump Division.

Mr. Schiavello was co-winner of the H. Worthington European Technical Award in 1979. He has written several papers and lectured at seminars in the area of pump recirculation, cavitation, and two-phase flow. He is a member of ASME, has received the ASME 2006 Fluid Machinery Design Award, and has served on the International Pump Users Symposium Advisory Committee since 1983.

Mr. Schiavello received a B.S. degree (Mechanical Engineering, 1974) from the University of Rome, and an M.S. degree (Fluid Dynamics, 1975) from Von Karman Institute for Fluid Dynamics.

Frank C. Visser is Principal Design & Fluid Dynamics Engineer with Flowserve Pump Division, in Etten-Leur, The Netherlands. He joined Flowserve in 1995 (at that time, BW/IP International), where he was assigned to the company’s Brite-Euram research project on advanced and preventive diagnosis of fluid-handling rotating machinery using artificial excitation and dynamic parameter identification

techniques (APHRODITE). Since then he has held several positions in research, development, and (product) engineering. His key expertise and interests relate to fluid mechanics and thermodynamics of (centrifugal) pumps and hydraulic turbines.

Dr. Visser obtained a B.S. degree (Mechanical Engineering, 1985) from Technical College Alkmaar, The Netherlands, and an M.S. degree (Mechanical Engineering, 1991) and Ph.D. degree (Technical Sciences, 1996) from the University of Twente, Enschede, The Netherlands. He is a member of the Royal Institution of Engineers in the Netherlands, and is the author of 20+ technical papers, published in journals and proceedings.

ABSTRACT

This tutorial deals with pump cavitation, discussing various net positive suction head required (NPSHR) criteria, net positive suction head available (NPSHA) margins and impeller life expectancy. It gives an introduction to the subject matter and provides insights on particulars like cavitation inception, 3 percent head drop, and 40,000 hours impeller life, as well as NPSH scaling laws. It further devotes attention to the effect of dissolved gases and thermal suppression (i.e., thermodynamic effect). With regard to numerical prediction capabilities the use of computational fluid dynamics (CFD) shall be discussed. Furthermore, guidance for cavitation damage diagnosis shall be given, including the peculiar aspects of various cavitation modes, the prediction of cavitation erosion rate, and assessment of impeller life expectancy. The tutorial will further address NPSHR criteria and NPSHA margin factors.

INTRODUCTION

Cavitation is well recognized as a phenomenon that may cause serious pump malfunctioning due to improper pump inlet conditions. It is therefore important for the pump user to understand what cavitation is, what it potentially can cause, and how it can be controlled. This tutorial aims at providing such knowledge. It is oriented toward pump users and focuses primarily on cavitation in rotordynamic (centrifugal) pumps; though much of the discussion will hold in general.

The tutorial will start with an introductory overview, discussing in broad terms the physics of cavitation, and outlining the concept of net positive suction head (NPSH); in that some cavitation flow visualization footage will be used for additional clarification and illustration. The tutorial will further focus on (pump) cavitation related phenomena, such as performance deterioration, material damage from cavitation erosion, loss of priming, and vapor lock causing complete pump failure. As inherent phenomenon, suction recirculation onset will be addressed as well.

In connection to NPSH several other characteristic cavitation parameters often found in literature will also be discussed, including suction specific speed, the cavitation number, and the Thoma cavitation number. Typical critical values of these parameters, related to distinct phenomena and criteria, such as cavitation inception, percentage head loss, and 40,000 hour operation will be explained too.

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 Supercritical Fluid Extraction P25-Special1.pdf Page 001
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