Principle Of Operation - Fisher Design CAV4 Instruction Manual

Design CAV4
A2922-1/IL
Figure 2. Recommended Minimum Seat Load
for All Constructions
2. Be certain the valve body and adjacent pipelines
are free of foreign materials that may damage valve
seating surfaces.
3. If continuous operation of the system will be re-
quired during valve inspection and maintenance, install
a conventional three-valve bypass around the point of
valve installation.
For longer service life and more effective
operation, the process liquid must be
clean. Impurities or entrained solids in
the process liquid may cause irreparable
erosion damage to the seating surfaces
and may plug cage holes, causing cavi-
tation damage. During valve installation
or the plant cleaning cycle, install a
strainer upstream from the valve to help
free pipelines of foreign material.
4. Flow through the valve must be as indicated by the
flow direction arrow on the valve body.
5. Use accepted piping practices when installing the
valve in the pipeline. For flanged valves, remove the
protective covering from the outlet flange studs, and
use a suitable gasket between the valve and the pipe-
line flanges.
4
Note
WARNING
Personal injury could result from pack-
ing leakage. Valve packing was tight-
ened prior to shipment; however, the
packing might require some readjust-
ment to meet specific service condi-
tions.
Flushing the Pipeline
Before flushing the piping system, install the Design
CAV4 flushing trim (figure 11), if available, as de-
scribed in the Use of Flushing Trim procedure in the
Maintenance section.

Principle of Operation

With Cavitrol IV trim, liquid flow enters the valve
through the side connection and enters the cage
through the top set of holes as shown in figure 3. No
significant pressure drop occurs across the set of
holes because the flow area is relatively large. As the
liquid flows down through the cage, it undergoes four
staged pressure drops by flowing through four addi-
tional sets of holes. All significant throttling action oc-
curs in the four sets of holes down-stream of the seat-
ing surfaces. Flow leaves the valve through the bottom
connection.
When the valve is partially open, the valve plug blocks
some of the holes above the seating surfaces. A small
amount of flow enters the blocked holes and flows out
through the clearance between the cage and plug. Be-
cause all significant pressure drop is taken down-
stream of the seating surfaces, the clearance flow
does not cavitate or cause erosion of the seating sur-
faces. Each of the four stages has a successively larg-
er flow area, resulting in higher pressure drops across
the first stages where there is no danger of the liquid
pressure falling low enough to allow the formation of
vapor bubbles that can cause cavitation.
Because more than 90 percent of the total valve pres-
sure drop is across the first three stages, both the inlet
pressure to the final stage and the pressure drop
across the final stage are relatively low. A low inlet
pressure and pressure drop at the final stage result in
a pressure at the vena contracta (the lowest pressure
reached in the flow stream) that is above the liquid va-
por pressure. Maintaining this higher pressure at the
vena contracta ensures that the liquid pressure does
not fall below the vapor pressure, and thus cavitation
does not occur.