Page 74 - Fluid mechanics, heat transfer, and mass transfer

VALVES 51

check valve, tilting disk check valve, diaphragm & Opening characteristics, that is, velocity versus disk
check valve, and foot valve. position data.
& Swing check valves are often used in piping systems & Velocity required to fully open and firmly backseat
with gatevalves. Can be installed in horizontal piping the disk. For most applications, it is preferable to size
or in vertical piping with upward flow. One difficulty the check valve so that the disc is fully open and
with these valves is in piping systems with pulsating firmly backseated at normal flow rates.
flow. Does not assure tight shutoff and back flow & Pressure drop at maximum flow.
leakage may cause contamination of upstream fluid, & Stability of the disk at partial openings. Disk stability
which might also create hazards.
varies with flow rate, disk position, and upstream
➢ Swing check valves are commonly used in larger
disturbances and is an important factor in determin-
pipes.
ing the useful life of a check valve.
& Typical swing, ball, and lift check valves are illus-
& Sensitivity of disk flutter to upstream disturbances.
trated in Figures 3.11, 3.12 and 3.13.
& One of the worst design errors is to oversize a check
& Ball check valve is rugged in construction and can
valve that is located just downstream from a distur-
stand repeated or cyclic operation. The ball revolves bance such as a pump, elbow, or control valve. If the
in the flowing fluid so that a different part of its disk does not fully open, it will be subjected to severe
surface rests in the seat each time the valve closes, motion that will accelerate wear. To avoid this prob-
thus distributing the wear on the ball. The heavy ball lem, it may be necessary to select a check valve that is
ensures reliability of seating in clean service. In- smaller than the pipe size.
volves high DP to the piping system. With low & Speed of valve closure compared with the rate offlow
viscosity fluids, it can give rise to water hammer.
reversal of the system.
➢ Ball check valves are frequently installed on tanks
& The transient pressure rise generated at check valve
and prevents back flow when pump is stopped.
closure is another important consideration. The pres-
➢ One limitation of ball check valves is its flow
sure rise is a function of how fast thevalve disk closes
restriction feature. The ball is directly in the flow
compared with how fast the flow in the system
stream and the fluid flows around it.
reverses. The speed that the flow in a system reverses
➢ Susceptible for damage from abrasive solids. depends on the system. In systems where rapid flow
& Lift check valve—globe and vertical types. It is a reversals occur, the disk can slam shut causing a
type of check valve used on pump suction lines to pressure transient.
prevent liquid draining out of the suction lines. Not & The closing speed of a valve is determined by the
satisfactory for unclean or viscous fluids. mass of the disk, the forces closing the disk, and the
& Other types of check valves include tilting disk, distance of travel from full open to closed.
restrained, butterfly, and spring-loaded types. & Fast closing valves have the following properties: the
. What is the other name for a check valve? disk (including all moving parts) is lightweight,
closure is assisted by springs, and the full stroke of
& Nonreturn valve.
the disk is short.
. What are the considerations involved in the selection of
& Swing check valves are the slowest closing valves
check valves?
because they have heavy disks, no springs, and long
disk travel.
& The nozzle check valve is one of the fastest closing
valves because the closing element is light, spring
loaded, and has a short stroke.
& The silent, duo, double door, and lift check valves
with springs are similar to nozzle valves in their
closing times, mainly because of the closing force
of the spring.
& Systems where rapid flow reversals occur include
parallel pumps, where one pump is stopped while the
others are still operating, and systems that have air
chambers or surge tanks close to the check valve.
FIGURE 3.11 Swing check valve. For these systems, there is a high-energy source