Predicting Cavitation Damage In Control Valves

Páginas: 18 (4334 palabras) Publicado: 21 de noviembre de 2012
Innovative Control Technology

SPECIAL PRINT

Predicting Cavitation
Damage in Control Valves

By:
Dr.-Ing. Jörg Kiesbauer
Dipl.-Ing. Domagoj Vnucec
Dipl.-Ing. Miriam Roth
Prof. Dr. Bernd Stoffel

Special Print from:
Hydrocarbon Processing
March 2006

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15.06.2010 10:04:05

Predicting Cavitation Damage in Control Valves
Special print from HydrocarbonProcessing · March 2006

Predicting Cavitation Damage in Control Valves
Dr. Jörg Kiesbauer, and Dipl.-Ing. Domagoj Vnucec, SAMSON AG
Dipl.-Ing. Miriam Roth, Prof. Dr. Bernd Stoffel, Darmstadt University of Technology
Cavitation can occur in control valves handling fluids, causing loud noise as well as damaging valve components and ultimately
leading to additional costs in process plants. Astandardized procedure to evaluate the destructiveness of cavitation-induced still
does not exist whereas noise emission can reliably be predicted with the new international EN 60534-8-4 standard. This article
describes a new method to solve this problem by evaluating structure-borne noise in the ultrasonic range.

1. Cavitation in control valves
The pressure of the process medium changes as itpasses
through the valve. Key pressures related to the inner flow path
include the input pressure p1, the pressure pvc at the narrowest
point (vena contracta) and the output pressure p2.
In other zones around this main flow path, the static pressure
can, however, be much higher or lower than pvc. A flow simulation of the pressure field at the vena contracta performed with
CFD (ComputationalFluid Dynamics) illustrates this behavior
(Fig. 1). At a differential pressure of 3.5 bar, the pressure field
is stable, likewise at 4.0 bar, except for a small zone at the plug
(arrow in the picture at the bottom) where the pressure is clearly lower. Small vapor bubbles start to form when the pressure
in this zone reaches the vapor pressure pv. The medium flow
carries these bubbles alongdownstream to the area of flow
exhibiting a higher pressure. At this point, the bubbles implode.
This process is defined as cavitation.
Fig. 2 contains four images taken at various differential pressures across the valve. The xFz coefficient plays a key role in this
case, being the differential pressure ratio for incipient cavitation
[1]. Together with the input pressure and vapor pressure, it definesthe differential pressure for incipient cavitation, that is
xFz•(p1–pv). The first streaks of steam are visible in Fig. 2b just
behind the plug on the left-hand side. As the differential pressure increases, the cavitation zone spreads out because the
pressure level in this area drops as a result of the increasing
flow velocities.

One consequence of this process is noise. Fig. 3 shows atypical
graph plotting noise versus the differential pressure ratio xF =
(p1–p2)/(p1–pv). The sharp rise in noise level is a typical indication for the onset of cavitation. The xFz coefficient can be recorded at this point. A cavitating flow arises between xFz and 1.
When xF < xFz, the flow is merely turbulent or laminar. When
xF > 1, no more vapor bubbles implode and the existing vapor

Fig. 1:Flow simulation (CFD) between seat and plug for p1–p2 = 3.5 bar
and 4.0 bar

2

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Fig. 2: Cavitation process in a control valve as a function of a pressure
drop between 5.5 and 9 bar (p1 = 10 bar, pv = 0.02 bar, water)

mixes with the fluid creating a two-phase flow which continues
right into the outlet pipe as the pressure p2 is lower than thevapor pressure at this point.
Noise prediction is to be covered by a new theory-based method laid down in the EN 60534-8-4 standard, which will achieve
much more precise results than previous methods [1], [6]. This
method estimates xFz, yet the most precise results are achieved
when the coefficient is determined using measured data as in
Fig. 4 [1].

Fig. 3: Typical graph plotting noise vs....
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