Page 76 - Fluid mechanics, heat transfer, and mass transfer

VALVES 53

. “Ball valves can create hazardous conditions in flam- . What is a foot valve? Where is it used?
mable atmospheres.” Comment. & Foot valve is used in pump suction lines to prevent
& Liquid is trapped in thevalve bore in closed condition back flow of liquids from the pump discharge side,
increasing inventory of flammable/hazardous mate- reversing the direction of movement of pump impel-
rials. Also such trapped liquid can give rise to ler, which might damage the pump, apart from
corrosion problems. Some designs of ball valves, in draining the pumped liquid back. It is a type of check
largersizes, haveprovisionto drain out trapped liquid valve.
on shutdown to overcome these problems. & In addition, when the pump is restarted, it will run dry
& Polymeric seats/packing used in ball valves can without pumping liquid due to air entering the pump
electrically insulate ball and stem from valve body. casing when it is drained.
Movement of ball relative to the polymeric film . What is a flexible valve? What are its plus and minus
between the ball and the stationary surface can give points?
rise to static electricity generation that can act as & Flexible valves consist of a flexible tube/hose with a
ignition source for flammables.
clamping device for squeezing the tube for closure,
& Use of conducting polymeric films or use of antistatic
for example, pinch clamps on plastic tubes.
device to discharge static electricity reduce these & Flow is controlled by squeezing the tube.
problems.
& The simplest form of the device is the spring
& If left open partially for extended periods under high
pinchcock.
pressure drop conditions, soft seat will flow around
& Fluid will not come in contact with any moving parts
edge of the ball opening and possibly lock the ball in
of the valve.
that position.
& It is leakproof and requires no packing.
. “Gate valve is not normally recommended for handling
& Low pressure drop.
unclean fluids/slurries.” Comment.
& Flow control is good for moderate to full flow.
& Particulates can get embedded in the valve seat
preventing complete shutoff resulting in leakages. & Used for sanitary handling of biofluids and food- and
. “It is not desirable to keep a gate valve partially open.” pharmaceutical-related fluids for which contamina-
tion is detrimental.
Comment.
& Can be used for abrasive/corrosive fluids, slurries,
& Partially open condition can lead to distortion and
and powders/granules.
bending caused by increased velocities of the flowing
fluid with attending turbulence. . What are the limitations of a flexible valve?
. What is a flush bottom valve? What is its application? & Repeated squeezing/pressing of the tube/hose results
in loss of flexibility and weakening of the tube/hose
& Fitted at the bottom of a process vessel for complete
requiring frequent replacements.
drainage of the vessel contents through the valve.
& Not suitable for high-pressure and high-temperature
& Its distinguishing feature is that the closure is flush
applications.
with the bottom of thevessel so that there is noholdup
of contents in the valve body. The outlet of the valve & Vacuum service requires use of heavy-walled hose,
discharges at a 45 angle from the vertical. which is expensive and has low levels of flexibility.

& Its disadvantage is that it requires considerable ver- . Name some applications for pinch valves in slurry
tical distance under the bottom of the vessel for service.
installation and operation. & Lime slurries, which are associated with abrasion and
. What are the salient features of a flush bottom valve? scaling problems.
& For controlling flow of liquid/slurry from bottom of a & Abrasive copper concentrate and metal minerals.
vessel. Requires changes in rubber sleeve once in 1–3 years.
& No holdup in the valve body. & Plastic beads with abrasive characteristics.
& Complete drainage of vessel contents through the & Chemical pulp stock, which is slightly abrasive.
valve. & Cement and sand, which are abrasive in nature.
& No stagnant liquid pockets in outlet nozzle. Requires changes in rubber sleeve once in 1–2 years.
& No lubricant is required on internal working parts of & Titanium dioxide. Abrasive.
the valve and therefore no contamination of process & Pulp stock rejects. Abrasive. Requires changes in
fluid. rubber sleeve once in 6 months to 1 year.