Reference Manual

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Chapter 4
Cavitation and Flashing
Severe Liquid Flow Sizing
Proper control valve sizing is important to
successful plant operation. However, sizing is not
always straightforward. At times, it involves
considering phenomena beyond that of general
service. Selecting the appropriate control valve
can be extremely critical to the complete process
loop. Liquid sizing for severe flow service,
including events involving cavitation or flashing,
must be closely examined in order to obtain
successful plant operation.
Sizing for severe flow service applications can be
explained by expanding upon base liquid sizing
knowledge. The following sections will build upon
the basic liquid sizing equations presented in
chapter 3 in order to study liquid fluid behaviors
involved with choked flow, cavitation, flashing,
viscous flow, and sizing for pulp stock. In addition,
discussion of considerations in selecting the
appropriate control valves for cavitating and
flashing services will take place.
Choked Flow
The equation illustrated below (chapter 3, equation
14) would imply that, for a given valve, flow could
be continually increased to infinity by simply
increasing the pressure differential across the
valve.
Q + C
v
P
1
* P
2
G
Ǹ
(31)
In reality, the relationship given by this equation
holds for only a limited range. As the pressure
differential is increased a point is reached where
the realized mass flow increase is less than
expected. This phenomenon continues until no
additional mass flow increase occurs in spite of
Figure 4-1. Typical Flow Curve Showing Relationship
Between Flow Rate Q and Imposed Pressure
Differential DP
A3442 / IL
increasing the pressure differential (figure 4-1).
This condition of limited maximum mass flow is
known as choked flow. To understand more about
what is occurring, and how to correct it when
sizing valves, it is necessary to revisit some of the
fluid flow basics discussed in chapter 3.
Recall that, as a liquid passes through a reduced
cross-sectional area, velocity increases to a
maximum and pressure decreases to a minimum.
As the flow exits, velocity is restored to its original
value while the pressure is only partially restored
thus creating a pressure differential across the
device. As this pressure differential is increased,
the velocity through the restriction increases
(increasing flow) and the vena contracta pressure
decreases. If a sufficiently large pressure
differential is imposed upon the device, the
minimum pressure may decrease to, or below, the
vapor pressure of the liquid under these
conditions. When this occurs the liquid becomes
thermodynamically unstable and partially
vaporizes. The fluid now consists of a liquid and
vapor mixture that is no longer incompressible.