Page 53 - Fluid mechanics, heat transfer, and mass transfer

FLUID FLOW
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& The vortex shedding frequency can be an exciting & The pressure spike (acoustic wave) created at rapid
frequency when it matches the natural frequency of valve closure can be as high as five times the system
the tube and vibration results. working pressure.
& With tube motion the flow areas between the & Incorrect flow direction through valves can induce
tubes are being expanded and contracted in con- pressure waves, particularly as the valve functions.
cert with the frequency of vibration. This in turn & The pressure wave or spike can travel at velocities
changes the flow velocity that controls the fre- exceeding 1400 m/s for steel tubes.
quency of the vortex shedding. Since tubes vibrate & Rapid changes in flow, operation of positive displace-
only at unique frequencies, the vortex shedding
ment pumps, entrained or separated gases, and in-
frequency can become locked in with a natural
creased temperatures induce damaging pressure
frequency.
changes.
& When a vapor bubble collapses as in the case of
cavitation, water rushes into this space from all
directions causing hammer.
2.2 WATER HAMMER
& Lack of proper drainage ahead of a steam control
. What is water hammer? valve. When thevalve is opened, a slug of condensate
& Water hammer is generally defined as pressure wave enters the equipment at high velocity impinging on
or surge or momentary increase in pressure or hy- the interior walls, causing hammer.
draulic shock, caused by the kinetic energy of a & Improper operation or incorporation of surge protec-
nonviscous fluid in motion when it is forced to stop tion devices can do more harm than good. An
or change direction suddenly. example is oversizing the surge relief valve or im-
& Kinetic energy of the moving mass of liquid upon properly selecting the vacuum breaker–air release
sudden stoppage or abrupt change of direction is valve.
transformed into pressure energy. & Mixing of steam with relatively cool water in a pipe
& Slow or abrupt start-up or shutdown of a pump or confined space also results in water hammer.
system, pipe breakage, turbine failure, or electric & If not controlled, the transients in pressures cause
power interruption to the motor of a pump are some damage to pipes, fittings, valves, and steam traps,
examples. Surge is a slow motion mass oscillation of causing leaks and shortening the life of the system.
water caused by internal pressure fluctuations in the Liquid for all practical purposes is not compressible
system. and any energy that is applied to it is instantly
& Waterhammeroccursduring the start-up or ener- transmitted. Neither the pipe walls nor the liquid
gizing of a steam system. If the steam line is will absorb the created shocks. Such transients can
energized too quickly without proper warm up also lead to blown diaphragms, destruction of seals,
time and the condensate created during the start- gaskets, meters, gauges, and steam trap bodies, heat
up is not properly removed, water hammer will be exchanger tube failures, and injuries to personnel.
the result. . What are the conditions that cause water hammer?
& When a rapidly closed valve suddenly stops water
& Hydraulic Shock: Start, stop, abrupt change in the
flowing in a pipeline, pressure energy is transferred to speed of a pump, power failure, or rapid closure of a
the valve and pipe wall. Shock waves are set up valve (usually a control valve, which can slam shut-
within the system. Pressure waves travel backward down in 1 or 2 s). Sudden stoppage generates a shock
until encountering the next solid obstacle, then for- wave similar to that of stopping a hammer with a
ward, and then back again. The velocity of the bang. Pressures generated can be as high as 40 bar
pressure wave is equal to the speed of sound. It (600 psi).
generates damaging noise as it travels back and forth,
& Differential Shock: Develops in two-phase systems,
until dissipated by frictional losses.
for example, whenever steam and condensate flow in
& Successive reflections of the pressure wave between
the same line with different velocities (steam veloc-
pipe inlet and closed valve resulting in alternating ities will be 10 times the liquid velocities). When
pressure increases and decreases often result in se- condensate fills the line, steam flow is stopped and on
vere mechanical damage. the downstream side pressure drops. Steam pushes
& A valve closing in 1.5 s or less depending on valve the condensate slug like a piston/hammer.
size and system conditions causes an abrupt stoppage & Thermal Shock: Steam bubbles may become
of flow. trapped in pools of condensate in a flooded main,