Flowserve 50165 Nordstrom Figure - Manual (Page 5)

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Nordstrom Figure 50165 and 50169 Dynamic Balance Iron Plug Valve FCD NVENIM2006-00 – 09/05
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Bending Strength
Piping systems are subject to mechanical constraints at fixed support points such as rigid nozzles,
anchors, etc. Cold springing at assembly and system temperature changes, together with gravity,
possible inertia loads, landslides, non-uniform subsidence in buried lines, etc. all potentially affect the
bending moment at various points in the piping.
Valves are also subjected to the bending moment occurring in the adjacent pipe that is in addition to the
normal pressure loading. Bending loads can cause deformation in valve bodies that can be detrimental
to valve functional performance. It is therefore a recommended design practice to avoid locating valves
at points of large bending loads.
In many cases, normal valve design practice results in a body strength greater than the strength of the
adjoining pipe thereby providing inherent protection against valve damage. In other cases, piping condi-
tions or systems designs may actually increase the possibility of harmful valve body deformation.
The following are examples of possible problems.
a) Basic “standard” valves that are made into “venturi” type valves by providing enlarged end connec-
tions on the smaller standard basic valves.
b) Any “standard” valve installed in heavy-wall “overweight” piping where the extra thickness may e
the pipe to be stiffer and stronger than the valve.
Valve designs having a high body bending strength should be used if there is reason for concern
regarding possible high bending loads.
Pressure Surge
Closure of a valve in a flowing fluid line causes the velocity of the fluid to be reduced to zero. If the fluid
is a relatively incompressible liquid, the inertia of an upstream column produces a pressure surge at the
valve whose mag nitude is inversely proportional to the time required for closure. The surge pressure
is also proportional to the length of the upstream fluid column and the fluid velocity prior to closure
initiation. If the application involves a long upstream line, a long downstream line, high velocity, and/or
rapid closure, singly or in any combination, the possibility of an unacceptable pressure surge should be
investigated.
Also to be considered are condensation-induced pressure surges which occur when a fluid velocity
change is caused by rapid condensation or when a slug of water is accelerated by contact with steam.
An example would be when condensate collects on one side of a closed valve that has steam on the
other side. Opening the valve causes collapsing steam voids, sharp pressure surges and acceleration
of condensate slugs. Condensation-induced pressure waves can result in pressure pulses that are
significantly higher than those produced by a sudden valve closure. In such events, non-shock rated
gray iron valves installed in steel piping systems are particularly vulnerable to catastrophic failure. Traps
are required to prevent condensate accumulation and “blow off’ valves, located at the low point in the
system, are needed to ensure condensate drainage. Operation and maintenance personnel must be
aware of the function of these devices in relationship to the “shutoff’ valve operation and the necessity
for them to function properly.