Valve Magazine - Fall 2010

drop. The effective combination of advanced computational methods and basic fluid dynamics experiments with water safely showed the way to increase the capacity of the valve.

AVOIDING CAVITATION

The pressure drop produced by control valves is one source of cavitation, but far from the only one that should concern plant operators.

Cavitation frequently occurs with both centrifugal and reciprocating pumps, particularly when head loss from flow restriction exists. For example, Alden Research Laboratory has been working with the U.S. Nuclear Power Commission and nuclear plant operators to design better screens for plants’ Emergency Core Cooling Systems (ECCS). Fibers, particles and other debris generated by a pipe rupturing inside a reactor can accumulate on the ECCS intake screen, reducing the water flow, increasing the head loss and causing the pump to cavitate, which impairs its ability to recirculate cooling water.

Entrained gasses are also a concern. A fluid-handling system can be designed to discourage cavitation of the primary fluids processed, but other gasses in the fluid can cavitate with pressure changes that do not affect the main fluid. For example, gasses in lubrication oil can cavitate, causing pitting and flaking of bearing surfaces used to support power turbine rotors.

Also of concern, however, is operator error. In 1996, during a turnaround at a refinery hydrogen production unit, the water level in a tank was allowed to drop, causing the pump to cavitate. Since that pump was not immediately shut down by the operators, when the cavitation started and no cavitation sensor with an automatic pump shutoff existed, hydrogen flowed back into the tank and ignited. The explosion damaged the tank, 15 vehicles and windows on nine buildings.

Since cavitation was first described by John Strutt almost 100 years ago, the scientific understanding of this phenomenon has continued to advance. Companies also continue to develop

References:

1. Lauterborn W; Boll H (1975): Experimental investigation of cavitation-bubble collapse in the neighborhood of a solid boundary. J. Fluid Mechanics, 72, pp. 391-399.

2. Matula TJ (2000): Single-bubble sonolumi-nescence in microgravity. Ultrasonics, 38, pp. 559–565

3. Tomita Y; Shima A (1985): Mechanisms of impulsive pressure generation and damage pit formation by bubble collapse. J. Fluid Mechanics, 169, pp. 535-564

new technologies to prevent cavitation and minimize its damage. However, no product can be built to meet the needs of all the unique plant designs. Each design requires a careful analysis of the exact plant conditions so the proper solutions can be applied. VM

LUDWIG HABER is a senior flow engineer at Alden Research Laboratory. He has worked on improving performance in a wide range of fluid dynamics applications ranging from turboma-chinery to nuclear strainer systems. MARTIN WOSNIK is an assistant professor of Mechanical Engineering at the University of New Hampshire and a technical consultant to Alden. He conducts research on cavitation and bubbly flows, turbulent boundary layers and renewable energy applications.

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Allied Valve, Inc.
AVP Valve, Inc.
Bay Valve Service, Inc.
Cameron
CCI
CFM / VR-Tesco, LLC
Chalmers & Kubeck North
Crane Energy Flow Solutions
CWFC Farris Engineering
Desselle Maggard
Dowco Valve Company
Dresser, Inc. - Masoneilan
Dresser, Inc. - Consolidated
Dresser, Inc. - Direct
VRC Members

ECI / ICE Valve & Instrument
Service
Fisher Controls Int. Inc.
Flowserve Corporation
Formosa Plastics
Furmanite
Gulf Coast Modification, LP
Gulf Coast Valve, Inc.
Kirksey Machine
Metso Automation
Midwest Valve Services, Inc.
Phoenix Partners
Precision Pump & Valve Service
Quality Valve
Southeast Valve, Inc.
The Wm. Powell Company
Thorco, Inc.
Tyco Valves and Controls
United Valve
Valve Reconditioning Service Co.
Wal-Tech, Inc.