CONTROLLING CAVITATION:
HOW A DEEPER UNDERSTANDING
IMPROVES THE SOLUTION
Cavitation is a common problem in most industrial processes.
Without proper control, it can result
in equipment damage, which can be
both expensive and hazardous.
In every valve application where a
solution for control of cavitation is
sought, multiple factors must be considered. For example, the presence of
solids in a fluid stream, high temperatures or exotic fluids change which
cavitation control solution is best
suited to an application. Understanding the mechanisms that cause cavitation and what mechanisms control
it results in successful implementation of a solution. This article discusses the causes of cavitation, how
it can be predicted and the mechanisms used to control or eliminate it.
BECAUSE OF THE WIDE VARIETY OF
PROCESSES, FLUIDS AND SERVICE
CONDITIONS INVOLVED, NO ONE
MECHANISM IS BEST FOR ADDRESSING
THE PROBLEMS CAVITATION CAN CAUSE.
UNDERSTANDING HOW TO PREDICT
CAVITATION, ITS VARIOUS REGIMES AND
THE MECHANISMS OF CONTROL LEADS TO
AN OPTIMIZED SOLUTION FOR
CONTROLLING OR ELIMINATING
CAVITATION.
BY JEFF PARISH
VELOCITY AND
PRESSURE PROFILE
As a liquid travels through a simple
control valve, a “vena contracta” (a
point at which flow restriction is nar-rowest) develops directly downstream of the throttling point. The
flow area at this point is smaller than
the rest of the flow path. As the flow
area constricts, the fluid’s velocity
rises. After the fluid passes the vena
contracta, the velocity drops again.
(See Figure 1, “Velocity through a
control valve,” which demonstrates
the velocity profile through a conventional single-seated, globe-style control valve with equal flow areas
upstream and downstream.)
This velocity increase at the vena
contracta is caused by a transfer of
pressure energy to velocity energy in
the flow, which results in lower static
pressures. As the flow leaves this
high-velocity area, the velocity energy is converted back into pressure,
and static pressure partially recovers. (See Figure 2, “Pressure
through a control valve,” which shows a pressure profile through a conventional single-seated globe-style control valve.)
Each time this conversion from pressure to velocity and back again occurs, total
energy suffers a loss because of conversion inefficiencies. The initial pressure drop
may cause a large velocity increase at the vena contracta, but frictional losses and
turbulence cause the pressure to not fully recover, even though the velocity returns to
its initial value. This complies with Bernoulli’s energy equation and also satisfies the
Figure 1. Velocity through a control valve
Figure 2. Pressure through a control valve
