Flowserve 50165 Nordstrom Figure - Manual (Page 16)

Nordstrom Figure 50165 and 50169 Dynamic Balance Iron Plug Valve FCD NVENIM2006-00 – 09/05
16
In contrast, the inertia of the fluid column in a liquid pipeline is not so easily overcome. Its relative
incompressibility provides no such cushion or proximity-limiting mechanism. The entire upstream fluid
mass is required to be decelerated at once by the closing valve and the resulting pressure surge may be
of sufficient magnitude to cause structural damage.
An additional potential problem can occur downstream from the closing valve. This may be described as
fluid column rupture and involves the inertia of the fluid column carrying it away from the closed valve
with the proximate space being occupied by a bubble of the fluid vapor or, simply, a substantial vacuum.
If there is sufficient backpressure in the line, the fluid column will reverse its velocity and close the void
created by the fluid column rupture and causes another pressure surge when it reaches the valve.
It should be recognized that pressure surge intensity is roughly proportional to the length and velocity of
the fluid column upstream of the closing valve and inversely proportional to the time taken to close the
valve. Fluid column rupture and return surge intensity is proportional to the same condition on the other
side of the valve in addition to the back pressure in that section of piping. Therefore, a slow closing is
helpful in limiting the magnitude of the pressure surge phenomena.
In large long distance liquid pipelines it is critically important to evaluate pressure surge possibilities
and to establish limits on the speed of closure of the flow shutoff valves. In operating such valves or
setting the speed of operation of power actuated valves, design limits on speed of closure should be
conscientiously observed.
Rapid closure of a valve in any flowing liquid pipeline can cause a substantial pressure surge that may
manifest itself in a sharp “bang” or possibly a series of “bangs”. This is frequently referred to as water
hammer. This phenomenon can occur in any flowing liquid line and is not limited to waterlines. Rapid
closing of a shutoff valve in a flowing liquid line should be avoided especially during the last part of the
stem travel.
Noise
There are many different valve-operating conditions that can result in noise. Such noise may be
“normal” considering the nature of the fluid and the pressure, temperature and velocity of flow. There
may be a “wind” noise in a flowing gas line. There may be clear or hoarse whistling sounds resulting
from the shape of the flow passage, including the flow path through a valve. Cavitating conditions in a
liquid line can cause a “white noise” that ranges from a whisper to a sound like rocks and gravel to a
deafening roar. There may also be mechanical noises as a result of movement of internal parts acted on
by the flowing fluid. Some of these noises may be relatively harmless insofar as system integrity and
performance are concerned. Mechanical damage in lines with compressible fluid is generally limited to
points of sonic or supersonic velocity, or where a vortex resonance with an internal component causes
movement and wear or breakage.
Vortex resonance with an internal component may also cause problems in liquid service. In addition,
noise may be evidence of cavitation which has the potential for causing mechanical, damage, including
massive erosion of the metal walls of a valve or pipe walls and/or other internal components.
A full technical discussion of all of the sound-generating mechanisms is beyond the scope of this docu-
ment. Nevertheless, it is recommended that an evaluation be made of any condition of remarkable noise
in a piping system at least to the point of understanding its cause. If a valve is involved, a determination
should be made as to whether the valve is the source or just happens to be the location of the noise.
Usually, if the valve is the source, the noise can be “tuned” by slightly “throttling” the valve.