Reference Manual

4−14
backpressure provided may allow cavitation to
occur.
D If the backpressure device becomes worn,
the backpressure will decrease and cavitation in
the valve may occur.
Another disadvantage that is rarely mentioned
occurs when a valve is opened against a high
upstream pressure. Until the flow reaches the
backpressure device and stabilizes, the valve will
experience the entire pressure drop of the system.
Although this may only occur for a short period of
time, the potential for damage exists.
In the instance of rotary valves, air injection
(known as aspiration) also can be used to
minimize the effects of cavitation in a system. With
this method, air is injected upstream of the vena
contracta. The dispersed air acts as a buffer when
the vapor bubbles implode so that the intensity of
the cavitation is lowered. Unfortunately, the
location of the vena contracta, the amount of air to
be injected, etc. are hard to quantify.
Because air is being injected into the system, this
method of cavitation control is usually used on
large valves dumping to a tank or pond or where
solids in the system prevent the use of other
cavitation control devices.
Cavitation is an interesting but destructive
phenomenon. Preventing cavitation is the most
acceptable way of limiting potential for damage.
Proper application of available products, based
upon sizing equations and field experience, will
provide long term success.
Summary
The past two chapters have indicated that a
fundamental relationship exists between key
variables (P
1
, P
2
, P
v
, G, C
v
, Q) for flow through a
device such as a control valve. Knowledge of any
four of these allows the fifth to be calculated or
predicted. Furthermore, adjustments to this basic
relationship are necessary to account for special
considerations such as installed piping
configuration, cavitation, flashing, choked flow,
and viscous flow behavior. Adherence to these
guidelines will ensure correct sizing and optimum
performance.
It is important to understand that pulp stock flow
exhibits characterizations that closely resemble
those of water. Guidelines for hindering the effects
of cavitation are based upon process testing using
water. One must consider that a pulp stock
multi-phase flow may result in less severe damage
when compared to that of water for flashing,
cavitation, or turbulent flow. However, it must be
noted that pulp stock can lead to other issues
such as erosion and corrosion, depending on
process make-up and the materials used in the
process. Therefore, it is important to understand
the process media, as well as firm process
conditions, in order to ensure the correct valve is
properly sized and selected for the given severe
service application.
As noted throughout the chapter, it is evident that
severe flow phenomena through a control valve
can occur under the proper conditions. In general,
the most common liquid severe service
applications involve either cavitation or flashing. It
is important to have a basic understanding of both
liquid service incidents as presented in this
chapter.