Cavitacion

Severe Service Journal
Volume 5 Issue 2

February 2003

For Internal Use Only
RESEARCH • PRODUCTS • PRICING • NEWS • STEAM CONDITIONING

CHOKED FLOW IS NOT
ALWAYS BAD, JUST
MISUNDERSTOOD
(By Chris Hammond)
There has been a recent trend with some
customers to become alarmed when valve sizing
indicates choked flow. While choked flow does
bring the possibility of valve damage, it isnot
always a given. In order to educate your
customer in regards to the phenomena, we need
to examine what choke flow is.
The maximum or limiting flow rate, qMAX,
commonly called choked flow, is manifested by
no additional increase in flow rate with
increasing pressure differential with fixed
upstream conditions. In other words, when
looking at the simplified sizing equation

q = CVP1 − P2
SG

continuing to increase the pressure drop by
reducing the downstream pressure, P2, should
always give an increase in flowrate, q, for a
given Cv. But that would be a perfect world.
For our reality based valve world, the
downstream pressure will eventually drop low
enough to where the vena contracta pressure will
be less than the liquid flowing media’s vapor
pressure. This willcause vapor to form. This
vapor will take the place of liquid. Eventually, a
point is reached where enough vapor is
produced and the liquid flow rate no longer
increases. The flow is choked.

The main concern should be the Cavitation
Coefficient, KC, value versus the Application
Ratio, AR. As mentioned in an article in the June
2002 issue of the Severe Service Journal,
comparing thesetwo numbers will tell whether
the choked flow is non-damaging (AR < KC), has
damaging cavitation (AR > KC), or that the liquid
is flashing (AR = 1 or more). If the valve meets
the CV requirements in a choked flow condition
and the KC is greater than the AR, then the valve
will work fine without damage – even though
the flow is choked!

WHAT DOES
“CHARACTERIZED” MEAN TO
YOU?
(By RonFunk)
CAVITATION TRIM
With cavitation trim the word "characterized"
means that you will have various levels of
cavitation protection as the valve opens further
and further. The highest level of cavitation
protection will be at the lowest valve opening
with progressively decreasing levels of
protection as the valve opens further. It may be
that the application requires three-stage
protectioninitially, but at some opening no
cavitation protection is necessary. What may
happen is that you will end up with a cage that
starts with three stage protection, then has two
stage protection, then one stage protection, then
zero staging. The reason the cage would be
designed this way is to allow for a smooth
characteristic.

Severe Service Journal
WHISPERIII or WHISPERFLO TRIM
Theterm "characterized" when applied to noise
attenuating trim means that the shape of the
curve has been altered, but the same level of
noise attenuation will apply to the entire curve.
Whisper trim is normally very linear. Many
users require equal percentage trim, thus
"characterized" noise attenuation trim only
means the shape of the characteristic. There are
a few noise trims that utilize adesign that allows
for different levels of noise attenuation. These
are easily identified by a nomenclature such as
C,B,A, or X,Y,Z, etc. These trims are not to be
identified as "characterized".

MORE ON THE NEW DST-G
(By Ted Geelhart)
Outgassing generally causes two major
problems. One problem is damage due to high
velocity jets coming out of solution, which carry
small liquidparticles. These liquid particles
impinge on internal surfaces at very high
velocities causing damage. Note that outgassing
fluids often carry solid particulate that can
further exacerbate this damage. Secondly, the
high velocity jets coming out of solution tend to
hit against the inside of the body wall and trim
parts causing vibration to occur. These jets are
defined by the hole/window size...