Reference Manual

6−6
Figure 6-2. Valve with Whisper Trim I and Inline Diffuser Combination
W2618
When the pressure drop ratio exceeds 0.65, the
Whisper Trim I cage loses its effectiveness.
Diffusers, used in conjunction with the Whisper
Trim I cage to divide the overall pressure drop into
two stages, can extend the useful capability and
also improve noise performance (figure 6-2). The
diffuser provides a fixed restriction, which
increases backpressure to the valve thereby
reducing the pressure drop across the valve. This
decreases the pressure drop ratio which in turn
decreases the sound pressure level. The use of a
diffuser allows the Whisper Trim I cage to remain
within its most efficient P/P
1
range. Diffusers are
only effective for the condition they are sized for.
They are not effective in throttling applications. At
this optimum condition they can provide up to an
additional attenuation of 25 dBA.
When pressure drop ratios are high, a Whisper
Trim III cage (figure 6-3) may be used. Fluid flows
from the inside of the cage out through many
orifices. The performance of these cages is
closely tied to spacing of these orifices. As the
pressure drop ratio increases, the centerline
distance to hole diameter of these orifices also
needs to increase to prevent jet recombination.
Therefore, as the level of the Whisper Trim III
cage increases, so does the centerline distance to
hole diameter. For many applications involving
high pressure drop ratios, a baffle is installed
outside the cage. For very high pressure drop
Figure 6-3. Whisper Trim III
W9039
ratios a baffle is often used to act on the fluid jets
exiting from the cage to further reduce turbulence.
Cages similar to the Fisher Whisper Trim III cage
can reduce control valve noise by as much as 30
dBA. These cages are most effective when the
maximum downstream velocity of the fluid is equal
to or less than 0.3 of the sonic velocity of that
fluid.